Small Magnetic Loop Antenna Project - QRPBuilder.com

WA4MNT Small Transmitting Magnetic Loop Antenna Project 

30m, 20m, 17m, 15m, 12m, 10m Bands 

With the solar cycle improving, I wanted to build an efficient HF antenna for the upper bands that I 

could easily throw in the trunk of the car and operate quickly with a minimum of setup time. I 

decided on a small magnetic loop antenna after researching the subject on the web and reading the 

following statement from VK5KLT’s article on small magnetic loop antennas. 

“A properly designed and constructed small loop of nominal 1m diameter will 

outperform any antenna type except a tri-band beam on the 10m/15m/20m bands, 

and will be within an S-unit (6db) or so of an optimised mono-band 3-element 

beam that’s mounted at an appropriate height above ground.” 

There is a wealth of information on small magnetic loop antenna construction on the web. One of 

the best sources I found was by AA5TB, http://www.aa5tb.com/loop.html, and VK5KLT, 

http://www.brisdance.com/vk4amz/files/Download/UnderMagLoop.pdf . The antenna I built is a 

compilation of ideas I borrowed from many sources, gleaned from the work of others, and a little 

redesign.

This project resulted in a three foot diameter copper loop mounted on a small pedestal, with 

continuous coverage of 30m through 10m bands (10MHz-30MHz). I have been able to obtain an 

SWR of 1:1 - 1.2:1 over the entire range. Based on the advice of the most successful builders, I 

chose copper as the metal of choice and all joints are silver soldered to reduce the interconnection 

resistance. I chose to design my own trombone style capacitor (~10pF - ~110pF), and shielded 

Faraday Loop input. I used copper tubing, readily available low loss dielectric materials, PEX (cross 

linked polyethylene) tubing for the capacitor insulator, UHMW plastic (Polypropylene) for all other 

RF exposed parts, and non magnetic hardware for all mechanical fastening. My design was based 

on AA5TB’s on-line calculator, http://www.aa5tb.com/aa5tb_loop_v1.22a.xls, and my dielectric 

spacing exceeds a 2KW rating. I chose to mount my portable loop, a little less than one diameter, 

off the ground, from the bottom of the loop, with six radials, two loop diameters long, from the base. 

These antennas are high-Q resonant circuits. Many kilovolts can be present across 

the capacitor, and produce concentrated electro-magnetic radiation even at low 

power levels. For safety, maintain a minimum of 6 feet away from the antenna, while 

transmitting. 

I have access to milling and lathe equipment, so my exact approach may not be suitable for many 

amateurs; however many good designs are available using butterfly or vacuum capacitors and 

easier available tools. Even simpler monoband designs may be more appropriate. I incorporated a 

motor drive for remote tuning with a wired control cable. With the antenna bandwidth being so 

narrow, tuning for maximum receiver noise yields almost optimum SWR. I use no antenna tuner 

between the radio and antenna. 

I am not an antenna theoretician; my expertise is in mechanical design. I have attached all my 

detailed .pdf’s and defer to the complete VK5KLT “An Overview of the Underestimated Magnetic 

Loop HF Antenna” article at the end of this document for the theory of operation. 

Results using my MFJ-259B antenna analyzer: 

10m - 28.700 SWR 1.2 : 1 R=53, X=10 

28.200 SWR 1.1 : 1 R=46, X=8 

12m – 24.900 SWR 1.2 : 1 R=56, X=9 

15m – 21.300 SWR 1 : 1 R=47, x=0 

21.060 SWR 1 : 1 R=43, X=0 

17m – 18.150 SWR 1.2 : 1 R=43, X=7 

20m – 14.250 SWR 1.1 : 1 R=43, X=0 

14.060 SWR 1.1 : 1 R=44, X=0 

30m – 10.125 SWR 1.1 : 1 R=50, X=8 

Small magnetic loops typically have 5 dBi gain when used with two loop diameter length radials. 

They exhibit a vertically polarized signal at the horizon and horizontally polarized signal overhead. 

Thank you to all the amateurs that have shared their wisdom and made the information public on 

the internet to make this project a success. 

You may consider joining the Yahoo groups, MagneticLoopAntenna or MagLoop 

Ken - WA4MNT 

www.qrpbuilder.com 

E-mail – kloc@swiftwireless.com

Trombone style capacitor ~10pF – ~110pF 

Shielded Faraday input loop

Trombone capacitor gear drive 

Using a www.mpja.com, #16816MD, 24 vdc, 40 rpm gearmotor 

Frequency scale

Base with thumbscrews for 6’ radials, and surplus fiberglass mast section. 

Motor controller and cables 12v gel cell transceiver supply, 

with 12v to 24v switching supply 

DC-DC module, www.lightobject.com 

for the gearmotor drive

Set up for operation 

Small enough to fit in the trunk of my Toyota Corolla

12v -24v switcher for gearmotor 

Motor controller, direction and speed 

www.mpja.com, #16816MD, 24v, 40 rpm gearmotor

Common Plastics Dissipation Factor Chart 

This will aid you in selecting suitable plastics for loop construction. Look for plastics with low dissipation 

factors. G10 /G11 glass epoxy board have a poor dissipation factor, 0.018, similar to PVC. UHMW 

(Polypropylene) is one tenth the cost of PTFE (Teflon).

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

SEE DETAIL B 

TROMBONE STYLE 

CAPACITOR 

`15pF - 80pF 

SEE DETAIL C 

SEE DETAIL D 

SEE DETAIL A 

1.825" O.D. FIBERGLASS MAST SECTION 

(MILITARY SURPLUS) 

ALUMINUM BASE PLATE 

2 

CAUTION !!! 

1 3 4 7 8 

5 

REV 

ECO 

6 

DIMENSIONS - INCHES 

UNLESS SPECIFIED 

TOLERANCES ARE: 

ANG. # 0.5 

X # 0.25 

X.X # 0.1 

X.XX # 0.01 

X.XXX # 0.001 

6 

INITIAL RELEASE 

THE INFORMATION CONTAINED IN THIS 

DOCUMENT IS THE PROPERTY OF 

BENT RIVER MACHINE AND SHALL NOT BE USED 

OR DISCLOSED OUTSIDE OF BENT RIVER MACHINE 

WITHOUT WRITTEN AUTHORIZATION 

ENG: WA4MNT 

DO NOT SCALE 

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DESCRIPTION 

THESE ANTENNAS ARE HIGH-Q RESONANT CIRCUITS. 

MANY KILOVOLTS CAN BE PRESENT ACROSS THE CAPACITOR, 

AND PRODUCE CONCENTRATED ELECTRO-MAGNETIC 

RADAITION EVEN AT LOW POWER LEVELS. FOR SAFETY, 

MAINTAIN A MINIMUM OF 6' AWAY FROM THE ANTENNA 

WHILE TRANSMITTING. 

NOTES: 

ALL COPPER ANTENNA ELEMENTS JOINED BY SILVER SOLDER 

ANY FASTENERS USED MUST BE 300 SERIES S.S. 

OPTIMUM HEIGHT IS TWO LOOP DIAMETERS ABOVE GROUND 

OPTIMUM RADIALS ARE ~2 LOOP DIAMETERS LONG 

FORMAT - B 

SHEET 1 OF 3 

WA4MNT 3' SMALL MAGNETIC 

LOOP ANTENNA 30m-10m 

DWG. NO. 

WA4MNT 

P.O. BOX 956 

CLARKDALE, AZ 86324 

928-639-3481 (VOICE) 

WWW.QRPBUILDER.COM 

LOOP_ASSY 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

POINTER 

2 

1 3 4 7 8 

5 

2EA. STATOR SPACER 

DETAIL B 

SCALE 0.750 

2 

2 EA. MCM 7297K15 BEVEL GEARS & 2EA. 

.093 X .62 L. SS ROLL PINS 

ROTATION INDICATOR 

SHAFT 

SUPPORT 

2EA. ADJUSTMENT 

SPACER 

ANTENNA RING 

4EA. STATOR 

INSULATOR 

LOWER STATOR 

SUPPORT 

GEARSHAFT 

1 3 4 7 8 

5 

REV 

ECO 

6 

DETAIL A 

SCALE 0.750 




ANG. # 0.5 

X # 0.25 

X.X # 0.1 

X.XX # 0.01 

X.XXX # 0.001 

6 







ENG: WA4MNT 

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TITLE 

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FORMAT - B 

SHEET 2 OF 3 



DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


LOOP_ASSY 

DATE 

08/08/10 

WWW.MPJA.COM 

MOTOR #16816 MD 

12 VDC, 40 RPM 

BY 

KL 

MOTOR MOUNT 

ADJUSTMENT 

PLATE 

CONNECTOR 

BRACKET 

GEARSHAFT 

SPACER & 

SHORT 

GEARSHAFT 

UPPER STATOR 

SUPPORT 

2EA. STATOR 

ASSEMBLY 

2EA. STATOR 

SPACER 

REV 

D 

C 

B 

A

D 

C 

B 

FARADAY LOOP 

ASSEMBLY 

A 

2 

1 3 4 7 8 

5 

SO-239 

INPUT FLANGE 

2 

DETAIL D 

SCALE 0.500 

1 3 4 7 8 

5 

REV 

SCALE 

ROTOR ASSEMBLY 

SILVER SOLDER THE INPUT FLANGE TO 

THE ANTENNA RING ON THE BACKSIDE 

RING SUPPORT 

ANTENNA RING 

ECO 

6 

NUT 




ANG. # 0.5 

X # 0.25 

X.X # 0.1 

X.XX # 0.01 

X.XXX # 0.001 

6 


SCALE 

SPACER 

LEADSCREW 






ENG: WA4MNT 

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SCALE: 0.048 

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TITLE 

DESCRIPTION 

DETAIL C 

SCALE 0.750 

FORMAT - B 

SHEET 3 OF 3 



DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


LOOP_ASSY 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

SILVER SOLDER AND DRILL PLUG TO ACCEPT 

RG-8 CENTER CONDUCTOR, SOLDER CENTER 

CONDUCTOR TO PLUG 

SILVER SOLDER BOTH PLATES TO SHELL 

2 

.50 REF. 

SCALE 

1.000 

INSIDE CONDUCTOR AND 

INSULATOR OF RG-8 COAX 

SLIP FIT INTO SHELL 

AT FINAL ASSEMBLY SOLDER RG-8 CENTER 

CONDUCTOR TO CENTER OF SO-239 CONNECTOR 

1.00 REF. 

.72 

1 3 4 7 8 

5 

REV 

FARADAY FLANGE 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


FARADAY SHELL 

ENG: WA4MNT 

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SCALE: 0.500 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 






FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


FARADAY_LOOP 

DATE 

08/07/10 08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

4EA. ROTOR - 1/2" RIGID COPPER TUBING 

PRESS IN 

4EA. ROTOR PLUG 

UPPER LEADSCREW WASHER 

& .25 SNAP RING 

2 

NUT W/4 EA. 6-32 

FH SCREWS 

ROTOR UNION 

LEADSCREW 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


LOWER LEADSCREW WASHER 

& 6-32 SCREW 

SILVER SOLDER 4 PLACES 






ENG: WA4MNT 

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SCALE: 0.250 

3RD ANG 

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TITLE 

DESCRIPTION 


FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

ROTOR_ASSEMBLY 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

12.50 

5/8" I.D. "PEX" TUBING (ACE HDWR) 

SLIP FIT IN STATOR TUBE 

STATOR 


1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


2 PIECES REQUIRED 






ENG: WA4MNT 

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SCALE: 0.333 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

STATOR UNION 

FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

STATOR_ASSEMBLY 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

SCALE 

1.000 

1.00 

.50 

1 3 4 7 8 

5 

REV 

ECO 

6 

6.50 

NOTE: THIS DIAMETER IS DETERMINED BY EXPERIMENTATION 

AND IS APPROXIMATELY 1/5 LOOP DIAMETER 




ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

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SCALE: 0.500 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

MATERIAL - 3/8" O.D., 5/16" I.D. SOFT COPPER TUBING 






FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


FARADAY_SHELL 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.03 

2 

1.00 

.14 

.72 

.50 

.625 

.86 

4X R.06 

1 3 4 7 8 

5 

.50 

2X .125 

REV 

ECO 

.625 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

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SCALE: 3.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

MATERIAL - COPPER 

2 REQUIRED 






FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

FARADAY_FLANGE 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.50 

6X .257 

2 

4.000 

3.750 

2.750 

1.750 

.750 

1 3 4 7 8 

5 

.250 

.000 

.76 

.000 

.500 

1.250 

2.000 

2.500 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


MATERIAL - UHMW 






ENG: WA4MNT TITLE 

DO NOT SCALE 

SCALE: 1.000 FORMAT - B 

SHEET 1 OF 1 

3RD ANG 

PROJECT 

DESCRIPTION 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

ADJUSTMENT_PLATE 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

1.500 

.750 .281 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 







ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

MATERIAL - UHMW PLASTIC 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

FORMAT - B DWG. NO. 

SHEET 1 OF 1ADJUSTMENT_SPACER 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

FLATTEN ~2.00 

BOTH SIDES 

1.50 

2 

5.50 

2X .281 

3.00 

SCALE 

0.200 

1 3 4 7 8 

5 

.625 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


36" TO CENTERLINE OF TUBING 






ENG: WA4MNT 

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SCALE: 0.059 

3RD ANG 

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TITLE 

DESCRIPTION 

FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

SCALE 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


ANTENNA_RING 

DATE 

08/08/10 

0.100 

MATERIAL - 5/8" O.D. FLEXIBLE COPPER TUBING (ACE HWD) 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.06 

2 

2.50 

.25 

.75 

1.88 

1.50 

1.00 

2X .201 

1 3 4 7 8 

5 

.25 

.625 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 







ENG: WA4MNT 

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3RD ANG 

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TITLE 

DESCRIPTION 

MATERIAL - G10 GLASS EPOXY BOARD 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 


SHEET 1 OF 1CONNECTOR_BRACKET 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.120 

.310 

1 3 4 7 8 

5 

REV 

.093 

ECO 

6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

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SCALE: 8.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

MATERIAL - BRASS OR COPPER 









FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


FARADAY_PLUG 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.188 

1.500 .250 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 







FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


GEAR_SPACER 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

.500 

2 

1 3 4 7 8 

5 

2 

.250 

6-32 UNC - 2B TAP 

0.260 #36 DRILL ( 0.107 ) 0.320 -( 1 

) HOLE 

3.550 

.093 

.125 

.844 

1 3 4 7 8 

5 

.335 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


.197 

ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

6-32 UNC - 2B TAP 0.180 

#36 DRILL ( 0.107 ) 0.320 -( 1 ) HOLE 

MATERIAL - 316 SS 






FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


GEARSHAFT 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.02 

2 

1.375 

.439 

.75 

.094 

.02 

.250 

1 3 4 7 8 

5 

.200 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


MATERIAL - 300 SERIES S.S. 

ENG: WA4MNT 

DO NOT SCALE 

SCALE: 2.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 






FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/09/10 

GEARSHAFT_SHORT 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.125 

2 

1.875 

.265 

.359 

.718 

.25 

.359 

1.750 

1.00 

1 3 4 7 8 

5 

.718 

2.375 

.50 

REV 

.125 

.625 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 







FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


INPUT_FLANGE 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

14.000 

8-32 UNC - 2B TAP 0.330 

#29 DRILL ( 0.136 ) 0.410 -( 1 ) HOLE 

1 3 4 7 8 

5 

.210 

.640 

6 

REV ECO 

DESCRIPTION 

DATE BY 


03/10/10 08/08/10 KL 




ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 

ENG: WA4MNT 

DO NOT SCALE 

SCALE: 0.666 

3RD ANG 

PROJECT 

TITLE 

SHEET 1 OF 1 

.030 

FORMAT - B 

1.685 

.76 






DWG. NO. 

.250 

MATERIAL - 3/8-16 S.S. THREADED ROD 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


LEADSCREW 

.192 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.750 

2 

.250 

BOTH ENDS 

.375 

12.250 

1 3 4 7 8 

5 

REV 

ECO 

8-32 UNC - 2B TAP 0.330 

#29 DRILL ( 0.136 ) 0.410 -( 1 ) HOLE 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 








ENG: WA4MNT 

DO NOT SCALE 

SCALE: 0.333 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 


SHEET 1 OF 1LEADSCREW_COUPLER 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.060 

2 

.750 .136 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.005 

6 


MATERIAL - S.S. 






ENG: WA4MNT 

DO NOT SCALE 

SCALE: 3.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/09/10 


SHEET LEADSCREW_LOWER_WASHER 

1 OF 1 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.060 

.75 .257 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.005 

6 


MATERIAL - S.S. 






ENG: WA4MNT 

DO NOT SCALE 

SCALE: 3.000 

3RD ANG 

PROJECT 

TITLE 

DESCRIPTION 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/09/10 


SHEET LEADSCREW_UPPER_WASHER 

1 OF 1 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

3X .257 

R1.00 

1.850 

6.750 

2 

5.000 

2.750 

1.750 

4X .975 

.750 

.000 

4.500 

4.000 

3.750 

2.250 

.750 

.500 

.000 

1 3 4 7 8 

5 

.500 

.250 

.000 

REV 

4.250 

.250 

.000 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 







ENG: WA4MNT 

DO NOT SCALE 

SCALE: 0.500 

3RD ANG 

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TITLE 

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WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

10-24 UNC - 2B TAP 0.360 

#25 DRILL ( 0.150 ) 0.450 -( 2 ) HOLE 



SHEET 1 LOWER_STATOR_SUPPORT 

OF 1 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.50 

2 

.144 

.279 X 82° 

1.500 

1.093 

.220 

.000 

.500 

14-20 UNC - 2B TAP 0.460 

#10 DRILL ( 0.194 ) 0.580 -( 2 ) HOLE 

.000 

.844 

1.250 

1.500 

1.656 

1 3 4 7 8 

5 

.197 

2.500 

R.50 

REV 

.472 

ECO 

6 

.100 




ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

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FORMAT - B 

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MOTOR_MOUNT 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

1.500 

2 

.740 

.250 

1.440 

1 3 4 7 8 

5 

.750 

REV 

A 

ECO 

- 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

.750 

6 


4X .144 THRU, .279 X 82° 

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DESCRIPTION 






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NUT 

NUT 

DATE 

04/24/10 

3/8-16 UNC - 2B TAP THRU 

5/16 DRILL ( 0.313 ) THRU -( 1 ) HOLE 


BY 

KL 

REV 

A 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.196 

THRU, .385 X 82° 

2 

4.500 

4.500 

4.250 

3.005 

2.875 

1 3 4 7 8 

5 

1.625 

1.495 

.000 

.250 

.000 

REV 

ECO 

6 

.000 

.128 

.375 

.500 

.250 

.000 




ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 

.070 

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MATERIAL - CLEAR ACRYLIC 






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D 

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B 

A 

2.000 

1.000 

.000 

2 

1 3 4 7 8 

5 

.000 

.250 

.500 

14-20 UNC - 2B TAP 0.460 

#10 DRILL ( 0.194 ) 0.580 -( 2 ) HOLE 

2 

3.000 

2.375 

1.500 

.625 

.000 

5.310 

2.630 

1 3 4 7 8 

5 

1.750 

REV 

.000 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


3RD ANG 

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1.850 

R1.00 

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C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.06 

2.0 .140 

1 3 4 7 8 

5 

REV 

ECO 

PAINT WHITE, W/ BLACK CROSS 




6 

ANG. ± 0.5 ° 

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X.XX ± 0.01 

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6 







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MATERIAL - G10 GLASS EPOXY BOARD 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 


SHEET 1 OF 1ROTATION_INDICATOR 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

12.50 

SCALE 

0.750 

1 3 4 7 8 

5 

REV 

ECO 

6 

.625 O.D. 

.565 I.D. 




ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


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MATERIAL - 1/2" RIGID COPPER WATER PIPE 

4 REQUIRED 






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ROTOR 

DATE 

08/08/10 

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C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.660 

.750 

.558 

1.250 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 



ENG: WA4MNT 

DO NOT SCALE 

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ROTOR_PLUG 

DATE 

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BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

4.000 

3.500 

2.375 

2.000 

1.625 

.500 

.000 

.000 

.500 

2 

1.125 

1.500 

1.875 

2.500 

3.000 

.620 

.750 

1 3 4 7 8 

5 

.250 

.125 

.000 

6-32 UNC - 2B TAP 0.260 

#36 DRILL ( 0.107 ) THRU -( 4 ) HOLE 

2.500 

1.500 

.000 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 







ENG: WA4MNT TITLE 

DO NOT SCALE 

SCALE: 1.000 FORMAT - B 

SHEET 1 OF 1 

3RD ANG 

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DESCRIPTION 

6-32 UNC - 2B TAP 0.260 

#36 DRILL ( 0.107 ) 0.320 -( 2 ) HOLE 


DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


ROTOR_UNION 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.062 

.144 

1.500 

1.250 

.250 

.000 

1 3 4 7 8 

5 

14.250 

.125 

.000 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


MATERIAL - G10 

ENG: WA4MNT 

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928-639-3481 (VOICE) 


SCALE 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

.250 SQ. 

.144 

1.500 

1.000 

1 3 4 7 8 

5 

REV 

.250 

ECO 

.125 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


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3RD ANG 

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MATERIAL - ALUMINUM 






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DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.02 

2 

1.375 

.439 

.75 

.094 

.02 

.250 

1 3 4 7 8 

5 

.200 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


MATERIAL - 300 SERIES S.S. 

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FORMAT - B 

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DATE 

08/09/10 

GEARSHAFT_SHORT 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

SCALE 

12.00 

0.750 

1 3 4 7 8 

5 

REV 

ECO 




6 

.962 .882 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

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3RD ANG 

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MATERIAL - 3/4" COPPER REPAIR TUBING 

4 REQUIRED 






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WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


STATOR 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

10.000 

1/4-20 UNC - 2B TAP 0.480 

#7 DRILL ( 0.201 ) 0.600 -( 1 ) HOLE 

.75 

1 3 4 7 8 

5 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.005 

6 


ENG: WA4MNT 

DO NOT SCALE 

SCALE: 0.500 

3RD ANG 

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FORMAT - B 

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928-639-3481 (VOICE) 


STATOR_SPACER 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

.25 

2 

1.500 

3.500 

2.000 

.960 

.257 

1 3 4 7 8 

5 

.750 

.750 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 



ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

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TITLE 

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FORMAT - B 

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DWG. NO. 

WA4MNT 

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STATOR_UNION 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

2 

1.500 

1 3 4 7 8 

5 

1.560 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


ENG: WA4MNT 

DO NOT SCALE 

SCALE: 1.000 

3RD ANG 

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TITLE 

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3/8-16 UNC - 2B TAP 0.750 

5/16 DRILL ( 0.313 ) 0.940 -( 1 ) HOLE 







FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


TUBE_PIVOT 

DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

B 

A

D 

C 

B 

A 

2 

1 3 4 7 8 

5 

1.750 

1.000 

.25 

2 

.50 

.375 

R1.00 

2X .257 

6.750 

5.000 

4X .975 

2.750 

1.750 

.750 

1 3 4 7 8 

5 

.000 

10-24 UNC - 2B TAP 0.360 

#25 DRILL ( 0.150 ) 0.450 -( 2 ) HOLE 

.760 

4.500 

4.000 

3.750 

2.250 

.750 

.500 

.000 

REV 

ECO 




6 

ANG. ± 0.5 ° 

X ± 0.25 

X.X ± 0.1 

X.XX ± 0.01 

X.XXX ± 0.001 

6 


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1/4-20 UNC - 2B TAP 0.480 


1/4-20 UNC - 2B TAP 0.480 








FORMAT - B 

SHEET 1 OF 1 

DWG. NO. 

WA4MNT 

P.O. BOX 956 


928-639-3481 (VOICE) 


DATE 

08/08/10 

BY 

KL 

REV 

D 

C 

UPPER_STATOR_SUPPORT 

B 

A

An Overview of the Underestimated Magnetic Loop HF Antenna 

It seems one of the best kept secrets in the amateur radio community is how well a small 

diminutive magnetic loop antenna can really perform in practice compared with large 

traditional HF antennas. The objective of this article is to disseminate some practical 

information about successful homebrew loop construction and to enumerate the loop’s key 

distinguishing characteristics and unique features. A magnetic loop antenna can very 

conveniently be accommodated on a table top, hidden in an attic / roof loft, an outdoor 

porch, patio balcony of a high-rise apartment, rooftop, or any other space constrained site. 

A small but efficacious HF antenna for restricted space sites is the highly sort after Holy 

Grail of many an amateur radio enthusiast. This quest and interest is particularly strong 

from amateurs having to face the prospect of giving up their much loved hobby as they 

move from suburban residential lots into smaller restricted space retirement villages and 

other communities that have strict rules against erecting elevated antenna structures. In 

spite of these imposed restrictions amateurs do have a practical and viable alternative 

means to actively continue the hobby using a covert in-door or portable outdoor and 

sympathetically placed small magnetic loop. This paper discusses how such diminutive 

antennas can provide an entirely workable compromise that enable keen amateurs to keep 

operating their HF station without any need for their previous tall towers and favourite 

beam antennas or unwieldy G5RV or long wire. The practical difference in station signal 

strength at worst will be only an S-point or two. 

Anyone making a cursory investigation into the subject of magnetic loop antennas using 

the Google internet search engine will readily find an overwhelming and perplexing 

abundance of material. This article will assist readers in making sense of the wide diversity 

of often times conflicting information with a view to facilitate the assimilation of the 

important essence of practical knowledge required to make an electrically-small loop work 

to its full potential and yield very good on-air performance. 

A few facts: 

A properly designed and constructed small loop of nominal 1m diameter will outperform 

any antenna type except a tri-band beam on the 10m/15m/20m bands, and will be within an 

S-point (6dB) or so of an optimised mono-band 3-element beam that’s mounted at an 

appropriate height above ground. 

Magnetic loops really come into their own on the higher HF bands from say 40m through 

to 10m; oftentimes with absolutely stunning performance rivalling the best conventional 

antennas. Easily field deployable and fixed site tuned loops have been the routine antenna 

of choice for many years in professional defence, military, diplomatic, and shipboard HF 

communication links where robust and reliable general coverage radio communication is 

deemed mandatory. On 80m and 160m top-band the performance of a small loop antenna 

generally exceeds that achievable from a horizontal dipole, particularly one deployed at 

sub-optimal height above ground. This is a common site limitation for any HF antenna. 

So where’s the catch; if the small loop is such a good antenna why doesn’t everyone have 

one and dispense with their tall towers? The laws of nature and electromagnetics cannot be 

violated and the only unavoidable price one pays for operating with an electrically-small 

antenna is narrow bandwidth. Narrow instantaneous bandwidth rather than poor efficiency 

is the fundamental limiting factor trade-off with small loops. 

1

Any small antenna will be narrow band and require tuning to the chosen operating 

frequency within a given band. Users of magnetic loops must be content with bandwidths 

of say 10 or 20 kHz at 7 MHz or a little more than 0.2%. They are content as long as the 

antenna can be easily tuned to cover the frequencies that they wish to use. For a remotely 

sited or rooftop mounted antenna implementing this tuning requires just a modicum of that 

ingenuity and improvisation radio hams are renowned for. 

A small transmitting loop (STL) antenna is defined as having a circumference of more than 

one-eighth wavelength but somewhat less than one-third wavelength which results in an 

approximately uniform current distribution throughout the loop and the structure behaves 

as a lumped inductance. The figure-8 doughnut shaped radiation pattern is in the plane of 

the loop with nulls at right angles to the plane of the loop. The loop self-inductance can be 

resonated with a capacitance to form a high-Q parallel tuned circuit. The attainment of a 

high-Q tells us that the loop antenna is not lossy and inefficient. When power is applied to 

the loop at its resonant frequency all of that power will be radiated except that portion 

absorbed in the lumped I 2 R conductor and capacitor losses manifesting as wasteful heat. 

With proper design these series equivalent circuit losses can be made negligible or at least 

sufficiently small compared to the loop’s radiation resistance that resultantly high intrinsic 

radiation efficiency and good antenna performance can be achieved. 

Current through the loop’s radiation resistance results in RF power being converted to 

electromagnetic radiation. However, since the small loop’s radiation resistance is very 

small compared to that of a full sized resonant ½ λ dipole, getting this favourable ratio of 

loss to radiation resistance is the only “tricky” and challenging part of practical loop design 

and homebrew construction. Through utilizing a split-stator or a butterfly style air variable 

capacitor construction or preferably a vacuum variable capacitor, low loss can be achieved 

in the tuning capacitor. Conductor loss can then be controlled by optimal choice of the 

diameter of copper tubing used to form the loop element and paying very careful attention 

to low ohmic interconnections to the capacitor such as welded or silver soldered joints, etc. 

With 100 Watts of Tx drive power there are many tens of Amperes of RF circulating 

current and Volt-Amps-Reactive (VAR) energy flowing in the loop conductor and tuning 

capacitor. 

In the case of an air variable, capacitor losses are further minimised by welding the rotor 

and stator plates to the stacked spacers to eliminate any residual cumulative contact 

resistance. When connected across the loop terminals the butterfly construction technique 

inherently eliminates any lossy rotating contacts in the RF current path. The configuration 

permits one to use the rotor to perform the variable coupling between the two split stator 

sections and thus circumvent the need for any lossy wiper contacts to carry the substantial 

RF current. Since the fixed stator plate sections are effectively in series, one also doubles 

the RF breakdown voltage rating of the composite capacitor. In view of the fact the loop 

antenna is a high-Q resonant circuit, many kilovolts of RF voltage can be present across 

the tuning capacitor and appropriate safety precautions must be taken. Small transmitting 

loop antennas capable of handling a full 400 Watts PEP or greater are readily achievable 

when appropriate construction and tuning components are selected. 

Feeding and matching: 

Although loop antennas have deceptively simple appearance, they are complex structures 

with radiation patterns and polarisation characteristics dependent on whether they’re fed in 

a balanced or unbalanced fashion. The method of feeding and matching the loop resonator, 

2

ground plane configuration, as well as the geometric form factor and physical proportions 

of the loop element itself are all fertile ground for experimentation. Various matching 

methods include series capacitor, transformer coupled subsidiary shielded-Faraday loop, 

and gamma-match, etc; each with their respective merits. 

The choice really boils down to personal preference as both the gamma and Faraday feed 

techniques work well. However, the Faraday shielded auxiliary loop located at the bottom 

central symmetry plane yields better loop electrical symmetry and balance that can in turn 

provide sometimes beneficial deeper front-to-side ratio and pattern nulls. In addition to 

imparting slight pattern asymmetry the Gamma match method can also result in some 

deleterious common-mode current flow on the outer braid of the feed coax that might need 

choking-off and isolating with ferrite decoupling balun to prevent spurious feeder radiation 

and extraneous noise pick-up on Rx. Much also depends on the site installation set up in 

respect of conductive objects in the loop’s near field that can disturb symmetry. 

With the elegantly simple transformer-coupled Faraday loop feed method the 50Ω signal 

source merely feeds the auxiliary loop; there’s no other coupling / matching components 

required as there are no reflected reactive components to deal with (the main loop appears 

purely resistive at resonance with just the core Rrad and Rloss components in series). 

The impedance seen looking into the auxiliary feed loop is determined solely by its 

diameter with respect to the primary tuned resonator loop. A loop diameter ratio of 5:1 

typically yields a perfect match over a 10:1 or greater frequency range of main loop tuning. 

Simple transformer action occurs between the primary loop and the feed loop coupled 

circuit due to the highly reactive field near the resonant primary loop which serves to 

greatly concentrate magnetic flux lines which cut the small untuned feed loop. The degree 

of magnetic flux concentration is a function of the Q of the tuned primary which varies 

with frequency, i.e. the highest Q occurring at the lowest frequency of operation and the 

lowest Q exhibited at the highest frequency. This variation in Q results from the variation 

in the sum of the loss resistance and the complex mode radiation resistances of the primary 

radiator loop as a function of frequency. The effective feed impedance of the secondary 

loop is controlled by its diameter / ratio of area and by the number of flux lines cutting it; 

thus the impedance seen looking into the secondary loop will be essentially independent of 

frequency. One can intuitively see this because when the feed loop is extremely small in 

relation to a wavelength at the lowest frequency of operation, the number of magnetic flux 

lines cutting it is large because of the very high Q, whereas when the feed loop becomes a 

larger fraction of a wavelength as the frequency of resonance is increased, the 

concentration of flux lines is reduced due to the lower Q. 

If one seeks mode purity and figure-8 pattern symmetry with deep side nulls, the fully 

balanced Faraday transformer coupled subsidiary broadband impedance matching loop 

with its 5:1 diameter ratio would be the preferred choice of feed structure. 

Loop balance is also important for rejecting local electric E-field conveyed noise; whereas 

the small loop is predominantly H-field responsive, any electrical imbalance results in 

common-mode currents on the feeder that will impart deleterious E-field sensitivity which 

may contribute to additional local noise pickup. That inherent loop imbalance and 

asymmetry is one of the slight trade-offs associated with a Gamma feed compared to an 

auxiliary Faraday loop transformer feed. This aberration is not an issue with Tx mode of 

course. 

3

Loop radiation characteristics: 

Small loop antennas have at least two simultaneously excited radiation modes; a magnetic 

and an electric folded dipole mode. When the ratio proportions of loop mode and dipole 

mode radiation are juggled to achieve equal strengths some radiation pattern asymmetry 

results and a useful degree of uni-directionality can be achieved with a typical front to back 

ratio of about 6dB or so. 

The small loop with its doughnut shaped pattern exhibits a typical gain of 1.5 dBi over 

average ground and a gain of 5 dBi when deployed with either short radials (the length of 

each radial need only be twice the loop diameter) or mounted over a conductive ground 

plane surface. By comparison a large ½ λ horizontal dipole mounted ¼ λ above average 

ground has a gain of 5.12 dBi and a ¼ λ Vertical with 120 radials each ¼ λ long has a gain 

of 2 dBi over average ground. The front to side ratio of a well balanced loop is typically 

20 to 25 dB when care is taken to suppress spurious feeder radiation due to common-mode 

currents flowing on the coax braid. 

However the small loop has one very significant advantage over any other antenna due to 

its unique radiation pattern. If the vertically oriented loop’s figure-8 doughnut pattern 

radiation lobe is visualised standing on the ground the maximum gain occurs at both low 

and high angles, radiating equally well at all elevation angles in the plane of the loop, i.e. 

radiation occurs at all vertical angles from the horizon to the zenith. Because the loop 

radiates at both low and high angles, a single loop can replace both a horizontal dipole and 

a Vertical. This is particularly beneficial on 160, 80 and 40m where the loop will provide 

outstanding local / regional coverage and easily match and often outperform a tall ¼ λ 

Vertical for long haul DX contacts, i.e. an exceptionally good general purpose antenna. 

Energy radiated by the small loop is vertically polarised on the horizon and horizontally 

polarised overhead at the zenith. It will be quickly realised that a loop has the distinctive 

property of providing radiation for transmission and response for reception over both long 

distances and over short to medium distances. This is achieved by virtue of low angle 

vertically polarised propagation in the former case and by means of horizontally polarised 

oblique incidence propagation in the latter case. In contrast, a Vertical monopole is useful 

only for low angle vertically polarised propagation since it exhibits a null overhead and 

poor response and radiation at angles in excess of about 45 degrees. Such antennas are of 

course very useful for long distance communication by means of low angle sky wave skip 

propagation, or for short range communication via the ground wave propagation mode. 

In further contrast, a horizontal ½ λ dipole (or beam arrays comprising dipole elements) at 

a height above ground of a just a fraction of a wavelength (as opposed to idealised free 

space or mounted very high) exhibits maximum polar response directly overhead (good for 

NVIS) with almost zero radiation down near the horizon. Such popular “cloud warmer” 

antennas in residential situations as the surreptitiously hung ubiquitous G5RV, End-feds, 

dipoles, inverted-V, etc. are thus most useful for short to medium range communication in 

that portion of the HF radio spectrum where oblique incidence propagation is possible. 

Importantly it should be noted when comparing small loops with conventional antennas 

that a 20m Yagi beam for example must ideally be deployed at a height above ground of at 

least one wavelength (20m) in order to work well and achieve a low take-off angle tending 

towards the horizon for realising optimal no compromise long-haul DX operation. 

4

Unfortunately such a tower height is impractical in most residential zoning rule situations 

imposed by municipal councils and town planners. If the Yagi beam is deployed at a lower 

10m height then a diminutive loop will nearly always outperform the beam antenna. This 

writer never fails to be amused by folks who acquire a potentially high performance Yagi 

HF beam and sacrilegiously deploy it in suboptimal installations in respect of height above 

ground or proximity to a metal roof. The problem worsens on the lower bands below 20m 

where the resultant high angle lobe pattern direction is not at all very conducive to 

facilitating good DX communication. 

In comparison to a vertically mounted / oriented loop, the bottom of the loop does not need 

to more than a loop diameter above ground making it very easy to site in a restricted space 

location. There is no significant improvement in performance when a small loop is raised 

to great heights; all that matters is the loop is substantially clear of objects in the immediate 

surrounds and the desired direction of radiation! Mounting the loop on a short mast above 

an elevated roof ground-plane yields excellent results. 

A good HF antenna for long haul DX requires launching the majority of the Tx power at a 

low angle of radiation; things a good, efficient and properly installed vertical, a properly 

sited small magnetic loop, and a big multi-element beam atop a very tall tower do very 

well. 

Receiving properties: 

In a typical high noise urban environment a loop will nearly always hear more than a big 

beam on the HF bands. The small magnetic loop antenna (a balanced one) responds 

predominately to the magnetic component of the incident EM wave, while being nearly 

insensitive to the electric field component; which is the basic reason why loops are so 

impressively quiet on receive; often times dramatically so. They will pull in the weak 

signals out of the ambient noise and you will very likely receive stations that you’d never 

hear when switching across to a vertical, dipole or beam antenna. 

In a propagating radio wave the magnitude of the electric vector is 120π or 26 dB greater 

than the magnitude of the magnetic vector, the difference being due to the intrinsic 

impedance of free space (377 Ohms). On the other hand the induction fields associated 

with man-made noise have electric E-field components many times greater than a normal 

radiation field (radio wave). While a dipole or vertical antenna is sensitive to both the 

electric and magnetic components of a wave, the small loop is responsive only to the 

magnetic H-field component and it will be substantially “blind” and offer a high degree of 

rejection to pickup of undesired man made noise and atmospheric disturbances. 

Hence the widely used term “magnetic loop” antenna to signify this field discrimination to 

the components of the incoming incident EM wave. Antenna theory treats the loop as the 

electrical conjugate of the dipole, i.e. the loop is a “magnetic dipole” while an ordinary 

dipole is an “electric dipole”. 

Significantly, a small loop antenna will typically produce a signal-to-noise ratio / SNR that 

is some 10 to 20 dB greater than a horizontal dipole in a noisy urban environment and an 

even greater improvement in SNR when compared to a vertical antenna as a result of the 

man-made noise comprising a strong electric field component and being largely vertically 

polarised. 

5

The most important criterion for reception is the signal to noise ratio and not antenna gain 

or efficiency. In the HF band, particularly at the low-mid frequency portion, external manmade 

and galactic / atmospheric noise is dominant. 

The magnetic loop antenna has one other important practical advantage in receive mode. 

The aforementioned high-Q resonator imparts a very narrow band frequency selective 

bandpass filter ahead of the Rx front-end stages. Such an incidental preselector comprising 

the antenna itself imparts greatly improved receiver performance on the congested lower 

HF bands with high power broadcast stations and particularly when lightning strikes and 

atmospheric electrical discharges are present in the regional area. Unwanted overload 

causing and adjacent-channel QRM interference signals are rejected or heavily attenuated. 

As well as eliminating strong-signal overload and intermodulation effects, the filtering 

dramatically reduces the amount of lightning induced broadband impulse energy fed to the 

Rx front-end and weak signals can still be heard when reception under such adverse 

conditions was previously impossible. 

It is these collective characteristics of small loop antennas that enable them to often very 

significantly outperform their large dipole, Yagi or Quad beam counterparts during direct 

A/B comparative testing. Conversely in Tx mode the antenna’s inherent filter action 

selectivity causes any transmitter harmonics to be greatly attenuated and not radiated. This 

can help with eliminating some forms of TVI. 

Effects of ground on loop antenna performance: 

When a dipole antenna is placed horizontally above ground, its electrical “image” in the 

ground is of the opposite phase. As a consequence, if the height above ground of a 

horizontal dipole is reduced to less than ¼ wavelength, fairly high system losses develop 

due to a rapid decrease in radiation resistance concurrent with a rapid rise in loss resistance 

resulting from dissipation of power within a less than perfect ground. This represents a 

classic double-whammy scenario and deleterious performance for dipoles deployed at 

insufficient height above ground. 

By way of contrast, the oscillatory RF currents associated with the image of a small 

vertical oriented loop antenna above ground are “in-phase” with those of the loop. 

Therefore the effect of ground on the performance of a vertically oriented loop is relatively 

small. In fact, because the magnetic component of an electromagnetic wave is maximum at 

the boundary between the ground and the space above, loop performance is usually best 

when the loop is located near the ground at a distance outside of the loop’s close-in 

induction field (just a loop diameter or two). However, if nearby conductive objects such 

as power lines or buildings exist in the direction of transmission / reception; it is normally 

preferable to choose a height above ground which will provide the loop with a clear and 

unobstructed view of the intended signal path. 

In comparing the performance of a vertical whip and a small vertical loop located atop of a 

building, it may be said that the loop will generally be the clear winner with respect to 

vertical and horizontal radiation patterns. This is because the pattern of a whip antenna 

driven against the top of a building is usually not predictable with any accuracy at all 

because vertical currents will flow all the way up and down the several conductive paths 

between the antenna and the earth; each path contributing to the total radiation pattern in 

the form of multiple lobes and nulls. 

6

A balanced loop antenna, however, is inherently immune to such problems because the 

ground below the antennas does not form the missing half of the antenna circuit in respect 

of supporting ground-return currents as it does with a vertical whip / monopole antenna. 

Therefore the multiple current paths to ground (earth) are eliminated with the loop. Of 

course both the loop and the whip are subject to the well-known wave interference effects 

in elevation due to height above the ground (or water). 

Reflective metal objects having a size greater than about 1/3 of a wavelength and at a 

distance of less than about 2 wavelengths from the loop antenna can produce standing 

wave “nulls” in a given direction at various frequencies. If the antenna is to be mounted 

atop a metal roof, diffraction interference from the edge of the building roof should be 

considered if undesirable nulls in certain directions at some frequencies are to be avoided. 

Usually the best location is near the edge of such a conductive roof, in the direction of the 

desired signal or signals. 

Loop Directivity: 

It is commonly believed that a vertically oriented loop antenna exhibits a bi-directional 

pattern with maximum reception occurring in the plane of the loop. Although this is true 

for vertically polarised sky-wave signals arriving at very low elevation angles (less than 

about 10 degrees) and for ground-wave signals, it is certainly not true for reception of high 

angle sky-waves (greater than about 30 degrees) whose polarisation usually rotates from 

vertical to horizontal at a fairly random rate due to “Faraday Rotation” of free-electrons 

within the ionosphere. At angles exceeding 45 degrees, the loop response shifts to a 

preference for horizontal polarisation arriving at an azimuth angle of 90 degrees with 

respect to the plane of the loop. Thus, for short-range communication links, i.e. less than 

about 500 km, best reception will usually occur with the loop rotated 90 degrees, that is, 

the plane of the loop perpendicular to the azimuthal arrival angle. 

It is not easy to predict which azimuthal bearing will provide the best night-time reception 

with a loop over paths of less than about 500 km at frequencies of less than about 7 MHz. 

This is due to the prevalence of both sky-wave and ground-wave signals which randomly 

combine to produce rather serious fading. Usually, trial and error is the best solution for 

determining which antenna orientation will produce the most favourable compromise 

between the highest average signal-strength and the least troublesome fading. Generally for 

distances exceeding 500 to 1000 km, the best orientation is with the plane of the loop in the 

direction of the arriving signal. Further, the side nulls exhibited by the loop at low 

elevation angles may be used to “null-out” the ground-wave signal to reduce fading when 

sky-wave propagation exists simultaneously. In comparison a vertical whip has a null 

overhead and thus is ineffective for short and medium distances. A vertical loop antenna 

located less than about 0.15 λ above ground exhibits excellent coverage from the zenith 

down to almost zero degrees in the elevation plane making the loop useful over almost any 

distance range. At elevation angles higher than about 20 degrees, a loop is almost 

omnidirectional in azimuth when receiving sky-wave signals. 

For a loop above average ground, as opposed to ground having perfect conductivity, the 

response at very low vertical angles e.g. less than about 5 degrees, is typically 10 dB or 

more below the achievable response above perfect ground. It is perhaps worthy to note that 

the ground immediately below the loop principally affects the response at high vertical 

angles while the properties of the ground at a large radius distance from the antenna tends 

to characterise the performance of the loop at low vertical angles in the plane of the loop. 

7

Construction and siting issues: 

Without a good quality low-loss split stator or butterfly or vacuum variable capacitor of 

adequate RF voltage and current rating, it is quite futile building a magnetic loop antenna 

and expecting it to yield the impressive results it’s potentially capable of. The minimisation 

of all sources of loss is particularly important in Tx mode. By virtue of the shorter rotor, 

the butterfly style capacitor has slightly lower rotor loss than the split-stator construction 

style. The tuning capacitor is undoubtedly the single most critical component in a 

successful homebrew loop project. Although more expensive and harder to find, vacuum 

variable capacitors have a large capacitance range in respect of their min/max ratio and 

allow a loop to be tuned over a considerably wider frequency range than that achievable 

with an air variable capacitor. Vacuum capacitors also have lower intrinsic losses than 

most air variables. Good quality Jennings vacuum variable capacitors and a multitude of 

Russian made equivalents can be readily found on the surplus radio parts markets and 

eBay, as can their associated silver-plated mounting and clamp hardware to ensure a low 

contact resistance connection to the loop antenna conductor. A very low contact resistance 

interface is essential between the capacitor terminals and the copper loop conductor. 

Other creative means can also be used to fashion a high VAR rated low-loss capacitor such 

as trombone, piston, or interdigitated meshing plate configurations. Air is always the 

preferred dielectric as most other materials have high loss tangents and dissipation factors. 

Whether a vacuum or air variable or homebrew capacitor is chosen, their mechanical shafts 

can be readily interfaced to a reduction gearbox and motor drive to facilitate easy remote 

tuning of a roof top or covert loft mounted loop. The antenna tuning can be manual or 

automatic based on VSWR sensing and a self-tuning servo system to control the drive 

motor. Peaking the loop tuning capacitor for strongest band noise on Rx will get the loop 

antenna tuning in the right ballpark for Tx with a low VSWR. 

Failure to pay very careful strict attention to construction details in relation to eliminating 

all sources of stray losses and making bad siting choices such as close proximity to ferrous 

materials are the two main reasons why small magnetic loop antennas sometimes fail to 

live up to their performance potential; instead behaving as a proverbial “wet noodle” with 

associated poor signal reports. Conversely a well built / sited loop is an absolute delight. 

Transmitting loop antennas intended for optimal coverage of the most popular portion of 

the HF spectrum from 3.5 MHz to 30 MHz are best segregated into at least 2 distinct loop 

sizes. A nominal 0.9m diameter loop for covering all the upper HF bands from 20m 

through to 10m (and perhaps also tunable down to 30m depending on capacitor min/max 

ratio), and a 2m diameter loop for covering the lower bands 80m through to 30m. For best 

operation down at 160m and improved 80m performance increased loop diameters of 3.4m 

to 4m should be considered. 

An important thing to note about vacuum capacitors is they don’t have a uniform RF 

amperage current rating over their entire capacitance range, but it is less at small plate 

mesh / low capacitance end. So one needs to factor this characteristic into design 

calculations and make sure you operate the capacitor within its ratings over the desired 

loop tuning range. Manufacturers like ITT Jennings provide Nomographs of this. This is 

another good reason for restricting the loop tuning / operating range over a nominal 2 to 

3:1 range so the Vac cap always works in its optimal VAR / current “sweet-spot” region. 

The saving grace with the current ratings of vacuum capacitors is they are continuous RMS 

Amps, i.e. key-down CW operation; and can be considerably safely exceeded when 

8

unning relatively low duty cycle SSB voice modes / PEP transmissions. All is OK with 

vacuum capacitors as long as the rated glass/ metal seal temperature is not exceeded. This 

is unlikely to occur in practice as the silver plated copper mounting clamps efficiently 

heatsink and remove any heat into the copper loop conductor. 

Mono-band loop operation yields the best result as the optimum loop inductance to 

capacitance ratio can be chosen and the majority of the tuning capacitance can be provided 

with a fixed vacuum capacitor. A much smaller vacuum variable capacitor can then be 

deployed in parallel to achieve fine vernier “bandspread” tuning across the whole band of 

interest, e.g. 40m or 80m, etc. 

Top-band operation at 1.8 MHz is always the hardest challenge for any antenna type, small 

loops (typical dimensions of 0.02λ) included; but their on-air performance can nevertheless 

be authoritative with a commanding signal presence. There are no "free lunches" (and few 

cheap ones) when shrinking the size of antennas as the free space wavelength has not yet 

been miniaturized by nature redefining the laws of physics! Consequently antennas of 

such diminutive size must always be placed into proper perspective when compared with 

the performance attainable from a full-sized λ/2 horizontal dipole for 160m. However, 

most amateurs haven’t got sufficient residential block size and/or mast height in a fraction 

of wavelength to accommodate a 160m dipole that works properly with a decent radiation 

efficiency and ability to put its radiated power in a useful direction. Similarly, reasonably 

efficient and efficacious Verticals for 160m operation unfortunately exceed the allowed 

height by a great margin that’s permitted by local council and residential building code 

regulations. Then a huge amount of real estate is required to accommodate the extensive 

radial system. 

The practical on-air performance of a loop on the 160/80m bands will be highly dependent 

on what antenna you use as a reference comparison, e.g. a centre-loaded mobile whip or 

full size resonant dipole/monopole, etc. and what path is used, NVIS, ground wave, sky 

wave, etc. The loop conductor diameter is determined by the desired loss resistance due to 

skin-effect, and choices can range from modest 6mm copper tubing to large bore 100mm 

copper or aluminium tube. Commonly used conductor diameters used to construct a 

magnetic loop are 20mm and 32mm soft copper tube. Heavy wall thickness tubing is not 

required as the RF current flow is confined to the conductor surface due to the skin-effect. 

Note that the radiation efficiency is not related to the loop size. Loop antenna efficiency is 

determined by the conductor tube diameter and its conductivity. This conceptual notion is 

counterintuitive for many folks. A small loop will also be efficient and radiate power very 

effectively on 80m and 160m but the resultant L–C ratio and stored energy will often be 

such that the loop’s Q factor will be so high as to yield an impractically small 

instantaneous bandwidth that’s not useful for SSB communication purposes. Achievable 

bandwidth is roughly proportional to loop size / diameter and Q is inversely proportional to 

the loop diameter. Depending on its construction a small loop of nominal 1m diameter can 

exhibit an intrinsic radiation efficiency of 90% over the 1.8 to 30 MHz frequency range. 

Copper tubing is the preferred material to fabricate the loop as it has a higher conductivity 

than aluminium. 

Larger size semi-rigid Heliax coax such as LDF550 / LDF650 / LDF750 will conveniently 

make excellent loop construction material for the smaller diameter 20m to 10m HF band 

loops when run at the 100 to 400 Watt power level. 

9

The larger bore 2-inch LDF750 can be used on the lower bands to beyond 1 kW. In 

relation to resistance and conductivity, small loop antennas inherently exhibit very low 

radiation resistances, which compete with the ohmic resistances of the loop conductor and 

the resistances from connections and welds, including the tuning capacitor connection. 

Magnetic loop antennas will typically have a radiation resistance in the order of 100 to 200 

milliohms. This means that every additional milliohm caused by a poor contact will cost 

you one percent efficiency. That is why professional magnetic loop antennas for 

transmitting purposes will never have mechanical contacts and everything including the 

capacitor plates should be welded or silver soldered. It is not uncommon to experience 60 

Amperes or more of RF circulating current in the loop and capacitor when fed with several 

hundred Watts of power. 

In the practical deployment and siting of a loop antenna there are extrinsic factors of both a 

beneficial and deleterious kind affecting the radiation and loss resistances when the loop is 

not strictly deployed in a free space scenario. When the loop is mounted over a perfectly 

conducting ground plane reflector or copper radial wire mat an electrical image is created 

that increases the effective loop area. This increase in turn beneficially increases the loop’s 

radiation resistance by a substantial factor. Such a favourable situation is easy to facilitate. 

Conversely if the loop is placed over average ground (a reasonable reflector) the radiation 

resistance increases but a reflected loss resistance is also introduced due to transformer 

effect coupling near-field energy into the lossy ground. Similarly when ferrous / iron 

material is too close, the magnetic near-field of the loop will induce by transformer action 

a voltage across the RF resistance of the material causing a current flow and associated I 2 R 

power loss. This situation might for example arise when the loop is mounted on an 

apartment balcony with nearby iron railing or concrete rebar etc; the deleterious influence 

can be minimised by simply orienting the loop to sit at right angles to the offending iron or 

steel material. Another loss contributing component is due to current flowing in the soil via 

capacitance between the loop and the soil surface. This capacitive coupling effect is again 

minimised by keeping the loop at least half a loop diameter or more above the ground. 

The transformer analogy for the loop antenna is a good one. The HF communication link 

may be visualised as a reciprocal “space transformer” with the loop acting as a secondary 

“winding” loosely coupled to the distant transmitting antenna. The magnetic field 

component of the incident electromagnetic wave induces a small RF current to flow in the 

loop conductor by means of induction that in turn gets magnified by the loop resonator’s 

high Q that’s appropriately impedance matched to the coax transmission line. 

A freestanding transmitting loop is best supported a metre or two in height on a short nonmetallic 

mast section of 100mm diameter PVC drainpipe and pedestal foot fashioned from 

plastic plumbing fittings. The loop can also be placed on a rotator drive plate and turned 

for best signal strength or it can be oriented in angle to null-out particularly bad QRM. 

Care must be taken not to touch the loop when transmitting and to keep a safe distance 

away from the loop’s magnetic near-field to ensure conservative compliance with 

electromagnetic radiation / EMR standards for human exposure to EM fields. A distance 

equal to or greater than one or two loop diameters away is generally a safe field strength 

region. RF burns to the skin from touching the loop while transmitting are very unpleasant 

and take a long time to heal. 

10

Concluding remarks: 

The proof of the pudding is always in the eating so experimentally inclined amateurs are 

encouraged to gain some first hand experience by getting into the shack workshop and 

constructing some homebrew loops. Such empirical validation of efficacy is always very 

gratifying, particularly when a VK station can have a solid 5 and 9+ QSO on 20m with a 

USA or Canadian station from an elegant looking Lilliputian indoor loop sitting on a table 

fed with a modest 50 Watts! What we ultimately seek from any antenna is reliable HF 

communication at all times when a band is open for DX and, simply put, that means 

radiating most of the RF that’s applied to the antenna in a useable direction and take-off 

angle. The underestimated magnetic loop antenna satisfies that basic criteria very well. 

A well designed and constructed small magnetic loop antenna is perhaps one of the rare 

few instances were a proverbial gallon of performance can be extracted from a pint bottle! 

© Leigh Turner VK5KLT 

7 July 2009 

11

Small Magnetic Loop Antenna Project - QRPBuilder.com

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