Many thanks to SWLing Post contributor extraordinaire, 13dka, who brings us a three part series about the new SULA homebrew antenna project. This first article describes this affordable antenna and demonstrates its unique reception properties. The second article will focus on construction notes. The third and final article will essentially be a Q&A about the SULA antenna. All articles will eventually link to each other once published.
This wideband unidirectional antenna is an outstanding and innovative development for the portable DXer. I love the fact that it came to fruition via a collaboration between Grayhat and 13dka: two amazing gents and radio ambassadors on our SWLing.net discussion board and here on the SWLing Post. So many thanks to both of them!
Please enjoy and share SULA Part 1:
Introducing the Small Unidirectional Loop Antenna (SULA) 1-30MHz
A small and simple, unidirectional and DX-capable loop “beam” for SWLs!
In early June, Andrew (grayhat), SWLing Post‘s resident antenna wizard suggested a variation of the “cardioid loop” on the SWLing Post message board: The original “cardioid loop” is a small loop receiving antenna deriving its name from a cardioid shaped (unidirectional) radiation footprint. The design is strikingly simple but it has a few downsides: It relies on a custom preamp, it needs a ground rod to work and it is unidirectional only up to 8 MHz.
Andrew’s version had the components all shuffled around and it did not only lose the ground rod, it also promised a nice cardioid pattern over the entire shortwave, from a small, diamond shaped loop. Wait…what? It can be made using parts available on Amazon and your DIY store:
You need some 3m wire and PVC tubes to create a support structure to hold the wire, a 530 Ohm resistor and a 9:1 balun like the popular “NooElec One Nine”. Since it’s a “lossy” design, adding a generic LNA like the NooElec “LANA HF” would help getting most out of it. When you put that all together you have what sounds like an old shortwave listener’s dream: a small, portable, tangible, and completely practical allband shortwave reception beam antenna with some more convenient properties on top, for example, it is a bit afraid of heights.
That sounded both interesting and plain crazy, but the .nec files Andrew posted were clearly saying that this antenna is a thing now. Unfortunately Andrew suffered a little injury that kept him from making one of those right away, I on the other hand had almost all the needed parts in a drawer so I ended up making a prototype and putting it through some of its paces, with Andrew changing the design and me changing the actual antenna accordingly, then mounting it upside down. Let me show you around:
- Small, diamond shaped wire loop (with 76cm/29.92″ sides), needing as little space as most other small loops.
- Unidirectional with a ~160° wide “beam” and one pronounced minimum with a front/back-ratio of typically 20dB over the entire reception range 1-30MHz.
- Moderate height requirements: It works best up to 3m/10′ above ground, where it gives you…
- …a main lobe with a convenient flat takeoff angle for DX
- Antenna is comparatively insensitive to ground quality/conductivity.
- Wideband design, works best on shortwave and is pretty good up to 70cm.
A functional small beam antenna for shortwave reception that’s just as small and possibly even more lightweight (prototype:~250g/9oz) than your regular SML, that can be easily made out of easy to obtain parts and easily carried around for mobile/portable DXing and due to its cardioid shaped directional pattern also for direction finding, a “tactical” antenna that’s also doing DX? Unlike conventional, Yagi-Uda or wire beams it can achieve a low takeoff angle at only 3m/10ft height or less, the front/back ratio is typically better than that of a 3-element Yagi, with a particularly useful horizontal pattern shape. That it’s rather indifferent to soil quality could mean that more people get to reproduce the good results and being a real wideband antenna is making the SULA an interesting companion for multiband radios and SDRs. Really? A miracle antenna? Is it that time of year again? If I had a dollar for every….
The proof is in less than half of the pudding
Full disclosure: I’m not an optimistic person.
Of all the things I expected to happen, that it actually works was not the top one. But let me just share with you how it greeted me out in the field, the first thing ever recorded from such an antenna at the dike (sorry for the video orientation):
This was the first time ever I heard a small pile-up of specifically Japanese stations out at the dike, calling for lovely sounding TF/LA5MUA in Iceland. That I had the new antenna turned a bit towards Japan may actually have contributed to that specificity. These signals came in with a comfortable SNR, and this was only the passive version of the SULA (I forgot to bring the power bank for the preamp that night). Most importantly this didn’t sound “meh” at all, which was what I expected more than anything else – that it kinda works but it’s too “meh” to bother with. Rarely been more happy to be wrong! After changing the feedpoint to “outside the loop” for improved lobe shape and stability, the real SULA confirms the 4NEC2 simulation in most if not all of its predictions, it…dare I say that…it works!?
Expectations need to be reasonable of course, beam antennas are not like flashlights, the highest difference in “brightness” between front and back is 1-3 S-units, even though the SULA has the potential to be better than that. Let’s also not forget that this is a quite lossy antenna, that’s why it may not work well for many radios without adding a preamp to the antenna, and even with that help it will not have super hot levels. What we hope for is that it can yield a better SNR from its smaller reception lobe compared to regular loops of that size, and the evidence collected so far indicates that this is very often the case, sometimes more, sometimes less…
The key information you’re all probably waiting for is where on the big roster of antennas this one could be sorted in and after only 4 weeks in chronically disturbed conditions here’s some mostly speculative performance ballpark guesswork – the overall performance of the preamplified version is comparable with ideal results from regular, high quality active loop antennas of comparable size, but the SULA offers some potential perks over them.
Whereas “Ideal results” for an SML are what the ML-200 is picking up at the beach, because it’s close enough to the water to have good low angle coverage. The more you move away from the water, the more the results are supposed to drift apart in favor of the SULA. But even with a preamp it’s pretty much deaf on LW and still quite insensitive on the lower part of MW…and that still brings us right back to the good things:
Even down on MW it has a pretty deep minimum on the backside that can be used to separate pretty strong AM stations on the same frequency and it generally works pretty OK at night on MW. Beyond 1 MHz it opens up and starts doing what it says on the imaginary tin. The pronounced backside minimum (to avoid the term “null”) is getting flatter with increasing frequency but it has still excellent (>13dB) directionality up to 10m and beyond. This is pretty educational, too: All of a sudden I can tell if a signal comes short path or long path and an azimuth map is my new friend. That’s fun!
As a pseudo-practical example, here’s how it sounded when I accidentally turned the back of the antenna through the loudest station in a pile-up (IU5PTB). Note how a station from Belgium suddenly comes through. There’s no QSB on the Italian station, I was just obsessed with fading him in and out with the antenna, nothing personal either. 🙂
Up in the FM band the pattern changes due to the loop becoming a “big” (1 Lambda) loop for the wavelength and the unidirectionality makes place for a bidirectional pattern shifted 90°, which is then perpendicular to the plane of the loop. The loop stops having losses and if you have nearby FM transmitters you may need to add an FM bandstop filter in order to avoid overloading the LNA, that the SULA is a surprisingly competent antenna on FM, VHF and beyond for its odd 3m restriction is probably not helping with that.
Elevation/takeoff angle and height
Again, this antenna works up to 3m/10ft of height, more will quickly deteriorate the lobe pattern. At 3m/10ft the takeoff angle is typically centered at 25° on 20m, lowering the antenna to 1.5m/5′ makes that 30°, 5m/15′ would yield 20° but at the cost of directionality. Mind you, 25° is only the gain peak of the lobe, the 10° takeoff angle is there too and only a few dB quieter over average ground.
The takeoff angle of the SULA still depends a bit on frequency as in “higher=flatter” but the slightly steeper angles on the 160-40m bands at constant 3m height are rather welcome variations catering for the different usage of the 80- and 40m-band . The best is maybe that this behavior is all “wideband”, with similar properties on all bands and all frequencies on shortwave.
This video is an unlucky pairing of an amplified YouLoop with the SULA, trying for RNZ after sign-on on 15720. It was made to illustrate the newly discovered, surprisingly steep angles at the dike for the (amplified but unmodified) YouLoop, disqualifying it as an antenna to compare with the SULA. What remains is the recording of what is basically a “steep angle/flat angle” demonstration with one of the world’s favorite DX stations, admittedly making the SULA sound inappropriately better.
BTW, this is Auckland VOLMET on yet another morning with thoroughly messed up conditions, USA skip severed, no signal from WWV but Auckland comes in long path.
The low height requirements are making this great for portable operation on short, portable fiberglass poles or for permanent installation in small backyards. Obviously you can turn its main lobe away from your house for some attenuation of QRM from the house. For the same reason it could be good on a balcony, depending on the noise situation of course. It probably only kind of works indoors or in the attic, this may not be the ideal indoor antenna in very noisy environments, or when the relative height of the antenna is too high, or just because the patterns may be all pretty off indoors. OTOH, mounting this 3m high over a big metal roof on a high-rise could be pretty awesome…
Another nice trait of the SULA is its relative independency from ground properties – the difference in takeoff angle between “good ground” and “poor ground” is only +/- 5° (10° flatter near salt water), so in theory it could bring in some DX where other antennas would have great difficulties to do that. Whether that’s true or not…that’s for you to find out!
So far we only verified some basic properties of the SULA on open area test sites, there’s only one of these antennas in existence right now…and building your own would be a chance to explore something (I think) new and if you’re inclined to, share your results!
Everyone would be super interested in practical demonstrations, particularly comparisons with other antennas and performance in different environments!
Stay tuned for Part 2: SULA Construction notes!
We will post Part 2 of the SULA antenna series within the next few days. Once posted, we will link to it here. You can also follow all three parts of the SULA series by bookmarking the tag SULA Primer.