In the April 2016 issue of QRP Quarterly (QQ) I described a magnetic loop antenna for portable QRP use. Based on that, I was asked to present on QRP loops for Four Days in May (FDIM) in 2017. This in turn inspired me to continue experimenting and testing loops. The following is an extension of the FDIM presentation.
My overall goal has been to build practical, inexpensive, portable, easy-to-build QRP loop antennas that can be used indoors or outdoors. To meet these criteria, I acknowledge that there are compromises. A butterfly or split-stator capacitor, for example, would have less loss but would be a major expense. Large copper tubing would be better than the coax I am using but wouldn’t collapse for carrying. In spite of such compromises, I have made many contacts with these loop configurations and they are definitely better than sticking a small whip on a QRP transceiver!
Some characteristics of magnetic loops
Magnetic loop antennas (MLAs) have a number of advantages beyond their relatively small size. They radiate both vertically and horizontally and are almost omni-directional except for a sharp null perpendicular to the center of the loop. This can be very useful for nulling out interference. In general they are known as quiet antennas, less susceptible to atmospheric noise such as thunderstorms. Consequently, it is sometimes possible to copy signals that cannot be heard on other antennas. They are also relatively unaffected by height above ground.
MLAs consist of an L-C circuit with a very high Q. As a result, tuning is very sharp and the bandwidth is very narrow. This can make for finicky tuning and frequent re-tuning when shifting frequency or turning the loop – the impedance can be affected by the surroundings, especially by anything ferrous. (The high Q also means excellent rejection of signals outside the bandwidth.) Therefore a practical concern for loop-building is finding a way to provide a gearing mechanism for the capacitor so that it can be fine-tuned.
Another challenge with these loops is that they have extremely low feed point resistance. This means that any other resistance, even minor, in the circuit can cause significant loss. Connections must be as low loss as possible. That’s why variable capacitors that don’t use wipers to connect to the rotor are ideal, though I am not assuming their use in these projects.
My experiments
In this short article I won’t describe the basics of magnetic loops. Instead, I will describe some of my experiments, including a very simple, low-cost but useable “breadboard” loop tuner. I will also include some of the improvements I have made in the original loop described in the QQ article.
Although I have experimented with copper tubing, unless otherwise noted the experiments below were done with a loop made of LMR-400 coax terminated by SO-259 connectors. I have made contacts with loops of 9’ circumference, but my normal loop is 12’ around. (15’ or even 20’ would be better, but gets unwieldy indoors.) The support for the coax loop is constructed of PVC pipe that can be taken apart for packing.
Coupling systems
Most of my loop experiments have used a small loop for inductive coupling, as in the diagram below (although this diagram, taken from AA5TB’s article, is upside down from the way I have built them).[i]
However, capacitive coupling can also be used. Often this involves a dual capacitor in place of the single capacitor in the figure above. The antenna is fed between the two halves of this capacitor with a second single capacitor in series with the feed line. To try that, I bought MFJ’s little QRP tuner MFJ-9232 that uses this kind of capacitive coupling. As I feared, it works but isn’t very practical. The knobs are too small, there is no vernier, and everything is so close together that hand capacitance interferes with tuning. I took it apart and rebuilt it into a larger box with a vernier dial and that helped. It is useable, especially if one “calibrates” it so that you have approximate settings to start with, but with two controls instead of one, it is still rather finicky for general use. I decided to stick with inductive coupling. [ii]
Double loops
In Magnetic Loop Antenna, (available from ARRL), Oldrich Burger and Marek Dvorsky describe a variety of multi-turn loop antennas. The authors, from the Czech Republic, have built and marketed such an antenna, with capacitive coupling, that claims to cover 80-10M. Inspired by them, I experimented with double loops with my original tuner, using both 9’ of 3/8” copper tubing and my 12’ LMR-400 loop, but with inductive coupling.
I found that the LMR-400 would nicely configure into two turns instead of one, and I was able to tune and make contacts with it. This adds versatility to the original design – depending on space, the coax loop can be configured into either a single or a double loop. However, to be able to use a double loop, it is helpful to position the loop connections on the tuner so that they are not directly opposite each other, but rather are offset. Also a smaller coupling loop will be needed – 1/5 or ¼ the diameter of the main loop. Tuning is sharper than with the single loop.
I then decided that a better option was to build a double loop of larger copper tubing with its own tuner. Since it is a double loop, it is much smaller overall than a single loop and thus relatively portable, even without dismantling it. Because it can be transported in one piece, very little setup is required. It is lower and less unwieldy than a larger single loop and thus can be used on a table without any further support system. (My original tuner includes a camera quick release that attaches either to a tripod or a photographic table clamp.)
The loop is made from about 12 ½ feet of ½” copper tubing. The tuner itself is built into a plastic electrical weather-proof box.
I began with a 395pf capacitor modified to reduce the resistance caused by stator wipers in an ordinary variable capacitor. To do this, I chose a two-gang capacitor and removed the plates from one section. I then connected the shaft to the frame with braided cable that is clamped tightly to the shaft and soldered with silver solder to a lug that happened to be welded onto the frame. The flexible cable is long enough to allow rotation. This capacitor had built-in 2:1 gearing, but I added a planetary gear to further increase the tuning ratio.
This turned out to not have enough capacitance for some bands, so I added another variable capacitor in parallel. I use that second capacitor to get into range for the band I am using, then do the actual tuning with the geared capacitor.
Inductive coupling works much as it does on single loop antennas, though I had to experiment a bit with the size, shape and position. On 40M, I was able to obtain a low SWR by squeezing the coupling loop a bit so that more of it was alongside the main loop. On 30M, the loop had to be more round. And on 20, it worked best when positioned across the main loops.
It is a bit harder to tune than my original tuner with a single loop and more gearing, but it is useable over a wider frequency range without retuning than I expected. Unfortunately, band conditions were very poor when I was testing this version so, though I did make contacts, I don’t yet have a good idea of its effectiveness. I doubt it is as efficient a radiator as a single loop though Burger and Dvorsky claim good results with these.
Construction details are beyond the scope of this article though the photos are fairly self-explanatory.
Additions to and lessons from my original tuner
“Improvements” to my original tuner, described in QRP Quarterly, are as follows. Some have relevance to the simple tuner project described later.
· I’m glad now that I had added the two terminals on top of the tuner in addition to the coax connectors that are used to attach the coax. With the 12’ loop, it will tune 80M by adding a capacitor (500pf) across them, and these terminals provide an easy way to install and remove this extra capacitance when changing bands.
· I tried making a carriage that would allow the feed loop to be positioned somewhat above the main loop, but that didn’t help. However, I did find that squeezing the feed loop into more of a football shape helped to lower the SWR, and made an adjustable sliding holder to allow that.· Adjusting the position and shape of the loop is one way to fine-tune the match.
· I also found that a slightly larger feed loop (about 25% rather than 20%) helped lower the SWR.
· For QRP, the capacitor would not have needed to be so hefty. In fact, it appears that almost any broadcast-band variable capacitor will work.
A simple bread-board tuner
Finally, I set out to make a simple “bread-board” style tuner that would cost little, use easily-available parts, and provide an easy way for someone to experiment with magnetic loops for QRP.
Unlike the tuner described in my QQ article, I separated the support system and the tuner itself. For the support, I simply used two pieces of wood and mounted the PVC directly to that. That in turn can be mounted on a clamp or tripod. I chose to mount a camera quick release plate on it. That allows me to use a clamp or a tripod with a quick release adapter for easy mounting and dismounting. I can set the tuner on this support plate, and it allows me to use a variety of tuners with the same support and loop.
The tuner itself is based on a small, single-gang broadcast band variable capacitor of 365-400pf. To ensure that it was easily available, I used a 365pf one from Antique Electronic Supply ($13.95 plus shipping).[iii]
Vernier dials and planetary drives add expense so I made my own using a DVD. I glued a knob to the center of the DVD so that I would have a way to attach it to the variable capacitor. I then used a grommet on a shaft to drive the DVD, with a large knob on the shaft.
Coax connectors (SO-239) were directly attached, one to the frame of the capacitor, the other to the lugs of the stationary plates. All that was mounted on a wood base. Hopefully the photos will be pretty much self-explanatory.
Although I have tested these loops on 20M, my main interest is 40M, and because of the small size, this is a greater challenge for these loops. Thus most of my experimenting has been on 40M cw, with the antenna indoors. However, as I noted earlier, I did find that by adding a capacitor (500pf) across the variable capacitor, I was also able to put it on 80M and make contacts. To make this possible, you may want to add terminals to the variable capacitor so that this supplementary capacitor can be added or removed fairly easily (see photo).
Tuning the loop
To reduce losses, the SWR at the feed point should be as close to 1:1 as possible, or at least 1.5 or under. Using my K2 and KX1 with internal auto-tuners, I have found I can hit this point very closely by ear, then tweak the setting based on how the auto-tuner tunes. However, it is helpful, at least while you are getting used to the antenna, to have an SWR meter or antenna analyzer to find the best position and shape for the coupling loop. In fact, I like to leave a small SWR meter in line so that when I want to move about the band, I can key and quickly tune the loop for a dip. Another option is to install a simple LED SWR indicator such as the one designed by N7VE, available as a kit from qrpkits.com.[iv]
Not only is the “loopty loop” ditty stuck in my head, but I can’t seem to stop making loop tuners – a case, apparently, of OCTD (Obsessive Compulsive Tuner Disorder). I hope that writing this article and making my presentation will get both of these obsessions out of my system – and that I haven’t transferred them to you!
Post-publication notes:
· If I were doing the “original” over, I would use a two-or three-gang variable capacitor with sections in parallel. (I know now that the spacing of a typical BC cap will handle QRP power).
· I’ve tried Superflex hardline instead of the LMR400 but haven’t been able to tell a difference so far.
· I’ve tried a rigid coupling loop from small copper tubing and it works but doesn’t allow fine-tuning SWR as well as a flexible one.
· The surroundings definitely make a difference in tuning.
Useful sources on loop antennas include:
· “Building a Magnetic Loop Antenna Series for Beginners [Video]:” http://forums.qrz.com/index.php?threads/building-a-magnetic-loop-antenna-series-for-beginners-video.548744/
o See also http://www.survivaltechnology.net/magnetic-loop-antenna-builders-buying-or-building-it-yourself/ (uses Chameleon kit)
· Burger & Dvorsky, Magnetic Loop Antenna, book available from ARRL.
· Facebook group on loop antennas: https://www.facebook.com/groups/DIYLOOP/
· “Magnetic Loop Antenna Calculator:” http://www.66pacific.com/calculators/small_tx_loop_calc.aspx
· “Small “magnetic” transmitting loop for 80-20M:”https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
· “The G4ILO Wonder Loop:” http://www.g4ilo.com/wonder-loop.html
· “The Underestimated Magnetic HF Loop Antenna:” https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
· Yatkes, Steve AA5TB, “Small Transmitting Loop Antennas” http://www.aa5tb.com/loop.html
· Zehr, Howard K4LXY, “The Magic Magnetic Vertical Loop Antenna.” QRP Quarterly, April 2016.
Commercially-available loops include:
· Chameleon: http://chameleonantenna.com/PORTABLE%20ANTENNA/PORTABLE%20ANTENNA.html (Also offers a starter kit for $99 – see info below)
· Alexloop: http://www.alexloop.com
· Alpha: https://www.alphaantenna.com
[i] Yatkes, Steve AA5TB, “Small Transmitting Loop Antennas” http://www.aa5tb.com/loop.html
[ii] For a thorough discussion of coupling methods, and loop design in general, see https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
[iii] https://www.tubesandmore.com/products/capacitor-365pf-variable. Note that Chameleon now sells a “loop starter kit” with geared capacitor, coax and connectors for $99. http://chameleonantenna.com/ACCESSORIES/ACCESSORIES.html
[iv] http://www.qrpkits.com/swrind_case.html