I recently built a couple of identical WSPR transmitters to be used in direct comparisons between magnetic loops and other antennas, or even between various kinds of magnetic loops. They are now available on Github.
The transmitters are based upon the Si5351 clock generator and their RF section was inspired by the excellent Zachtec’s products. An onboard GPS receiver is used to automatically provide timing and location. In the absence of a GPS fix a button can be used to start transmitting. The same button can also be used to cycle between 5 predefined WSPR frequencies. There are no onboard filters therefore external filters must be used in order to maintain compliance with FCC rules. SMA connectors provide connectivity for both the HF and the GPS antennas.
The firmware is based upon existing libraries and was created around version 1.0 of the board. Version 1.1 introduces an important extra feature: auto-calibration.
I also built another magnetic loop, identical to the one I already have. Together, they will serve as a testbed for my comparison tests.
I really like going QRP from a not so popular location on East River, right between Manhattan and Queens. I normally don’t plan for it. I just go whenever I feel like it and, obviously, I have time to do that.
Yesterday was one of such occasions. It was a particularly interesting day because the WPX CW Contest was going to start at 8PM local time and I had come home relatively early from work.
I wasn’t planning to try to contact CW stations at the very beginning of the contest (too late for me to stay at the river, bands too crowded). Instead, the idea was trying to catch those operators warming up their amplifiers (and their fingers!) in the hours immediately preceding the start of the competition.
Sure enough, those stations were exactly there, as I expected them to be. In one hour I made 9 contacts, all Europeans (plus Russia). Another fun day at the river! Here is the video:
After describing my mini loops (here and here), this time I am going to write about the classic “full size” version that I have been using for a while.
At Dayton the Hamvention of 2018 I took my loop with me and I mounted it on a PVC support that fitted a military style backpack. It made quite a splash! Eventually, someone took a picture that ended up on the August 2018 QST and the 2019 ARRL Calendar front-covers. That’s when I proposed the QST editors an article which described how to make the antenna. QST accepted but took its time, finally publishing the article with the May 2019 edition.
The K1FM Loop consists of a LMR400 radiator (124″) fed by an LMR240 exciter (25″). The radiator loop is terminated by a geared, multi-stator variable capacitor made by Alps (14pF to 165pF, P/N 42-36251-1). The exciter, instead, is constructed to be terminated by a 3way BNC adapter. The geared capacitor and the loop configuration have been inspired by the AlexLoop while the exciter construction and the selfie stick by Alpha Loop.
The exciter loop takes a bit of patience in order to be manufactured this way. Quite a bit, actually. Of course more classic approaches can be used as long as the overall size remains the same. Both loops are mounted on the selfie stick using a custom 3D printed bracket that fits on standard camera mounts.
The other critical aspect of this project is obviously the capacitor. It is the capacitor which defines the bands you can use, the resolution you get (how “touchy” the loop will be…) and – last but not least – how much power the loop will be able to handle. The 15-165pF geared type is some kind of a sweet spot, a very good compromise for a loop this size when used QRP. Unfortunately Alps no longer makes this capacitor but from time to time they pop-up on Ebay for about $20 each. You can, of course, replace this capacitor with equivalent ones (similar, non geared capacitors are more common and much easier to find).
The capacitor enclosure is fundamentally just a box that holds the capacitor against the selfie stick, connecting it to the radiator loop. The box I used is made by uxcell and measures 3.9″ by 2.6″ by 2″. I mount it against the stick using a custom 3D printed ABS bracket.
The list of all necessary materials, from the main loop to the last nylon screw, is here. Overall the cost for a single antenna should be around $60 but the final cost will vary depending on how much you will spend for the capacitor and for the 3D prints.
Performance of magnetic loops this size have been extensively analyzed and reviewed. Mine is no different than the others. Please do not expect miracles! Ultra-Portable QRP is hard stuff and you will need to rely the 3 P’s: Propagation, Perseverance, Patience. In a nutshell this loop performs at around the same level of full sized half-wave dipoles or of a vertical antennas. Yet, you are running 5W watts into it therefore plan for the right time, the right band and the right location if you want to work DX on voice. All in all, I find it a bit lacking on 40 and I don’t use it on that band unless it is inevitable. 20 and 30 are very fine and – when conditions permit – so are 17, 15 and 10.
Other than the loop, I normally carry my FT-817, a WinCamp Headset adapter, a PC Headset, a Palm Radio CW Pico Paddle, a mini tripod and a military backpack with a PVC antenna mount. I recently added FT8 and wireless headsets, but that’s going to be detailed in other posts.
Finally, I’d like to spend two words on RF safety. As a licensed Amateur Radio operator you should already be aware of the following but I, for the sake of safety, I’ll repeat what you probably already know. Loops can be dangerous around people. Do not exceed power! By doing so you would irreparably scar your capacitor. If you really go heavy, you basically built a soldering machine: sparks will fly and fires might start. Moreover, even when QRP, this loop will exceed FCC exposure guidelines when used in close proximity. To be on the safe side, nobody should be closer than 2 meters from this loop, regardless of how much power you are using. Finally, do not touch the radiator while transmitting because you could be burnt, electrocuted or both (~800V at 5W, ~4000V at 100W!).
In the effort of improving my ultra-portable HF antenna setup, I first looked into a Capacitively Coupled Magnetic Loop, better known as the Army Loop (or Patterson Loop). The Army loop performed well. It was so portable that I enjoyed using it before, during and after numerous Tennis matches… Here is one example:
The Army loop, however, sometimes presented a little bit of a challenge because in order to tune you need to operate two capacitors, instead of one. Generally speaking, one capacitor affects resonance while the other affects the coupling impedance. The main problem I found is that even when the SWR is very low, such impedance isn’t always what you would expect it to be (around 50 ohm). You can still match it by operating the second capacitor but there really isn’t a way – while portable – to tell how close you are to the perfect match. That’s the main reason why I decided to look for alternatives.
The idea for my next portable loop came from the Wonder Loop by Wonder Wand (not to be confused with the ‘G4ILO Wonder Loop’, which is unrelated). Wonder Wand’s Wonder Loop looks like an Army loop, but only has one capacitor. After taking a look inside I was intrigued to see a Toroidal transformer. My first reaction was: WFT?!? Then I remembered seeing something similar on the excellent N4SPP page about loops (probably the best page about magnetic loops actually available). Here is what Frank N4SPP has to say about this coupling method:
The advantages of such design are two: no need for a coupler loop and only one capacitor. Precisely what I wanted!
At this point you might ask why don’t I just use the Wonder Loop. The thing is that the Wonder Loop isn’t simply ultra-portable, it is microscopical. It’s radiator is made of a 1 mm thin copper wire… a bit too much, I guess. All I wanted was the same concept as the Wonder Loop but with a radiator of my choice (and at a smaller price!).
Here is the schematic I started working on:
This time I used a smaller Hammond Enclosure, the 1594ASGY. The capacitor, instead, is a dual gang 266 pF by Mike’s Electronic Parts. Both gangs are engaged in parallel as well as all the trimmer capacitors (trimmers are used to raise the minimum capacitance and also raise the maximum a little bit). By doing so, the loop covers from 40 to 10 meters. Here is the is the first test item:
And the analyzer results:
As you can see, on 20 meters and above not only the SWR is low but also the impedance is constantly around 50 ohm. No need for a second knob! On 30 and 40 meters, instead, conditions deteriorate considerably but I never expected any good performance from such a small loop on those bands anyways. Below 14 MHz I generally use my loop to listen only. It might happen to make a contact once in a while, but it is rare.
The loop responded as I wanted it to… But was it performing? To find out, yesterday I did the first field comparison of four different foldable loops:
1) Toroidal Coupled Loop 6FT (the antenna I’m writing about today)
2) Army Loop 6FT (the antenna I described last time)
3) Inductively Coupled 6FT Loop (classic design, just smaller than usual)
4) Inductively Coupled 11FT Loop (the classic loop! Practically a photocopy of the Alexloop)
Testing was performed on 20 meters WSPR by sending 3 frames on each antenna starting at 7PM local time. By the time I finished with the last antenna it was already 8PM and it’s possible that during such time propagation conditions evolved. The ideal way to perform this test would be transmitting with all the antennas at the same time, but I am not equipped to do that… At least not yet!
Finally, the gathered data:
As expected, the test shows the 11FT loop constantly being on top of the game. Loop dimensions are critical and on 20 meters there is a considerable difference between 11 and 6 feet circumference. On the other hand, the 6 Feet loops (Toroidal, Capacitive and Inductive) seem to be delivering similar performances. There are no clear winners or losers, which is good news because that means I can keep using the latest version (toroidal) which is smaller and simpler to use.
Future plans include building the biggest toroidal loop possible, as well as making my 11FT inductive loop lighter and easier to transport.