Many thanks to SWLing Post contributor extraordinaire, 13dka, who brings us Part Two of a three part series about the new SULA homebrew antenna project. This first article describes this affordable antenna and demonstrates its unique reception properties. This second article focuses 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 Part 2:
Part 2: SULA Construction notes
The drawing [above] has all you need to know. You basically need to put up a symmetrical wire diamond starting with a balun at the one end and terminating in a resistor at the other end of the horizontal boom, the sides are supposed to be 76cm/29.92″ long so you need to make yourself some…
I used 0.63″/1.6cm square plastic square tubing/cable duct profiles from the home improvement market to make the support structure. You can use anything non-conductive for that of course, broom sticks, lathes… The plastic profiles I used had the advantage of being in the house and easy to work on with a Dremel-style tool and everything can be assembled using the same self-tapping screws without even drilling. The profiles are held together with 2 screws, for transport I unscrew one of them and put that into an extra “parking” screw hole on the side, then I can collapse the cross for easy fit into the trunk, a rucksack etc.
These profiles are available in different diameters that fit into each other like a telescoping whip. This is useful to make the support structure variable for experiments and to control the loop shape and tension on the wire. The booms end up at 1.075m each, the profiles come in 1m length, so that’s 4 short pieces of the smaller size tube to extend the main booms by 37mm on each side
On the resistor end of the loop that smaller tube isn’t mounted in the “boom” tube but to the side of it in order to keep the wire running straight from the balun box on the other side.
You can use anything non-conductive to bring it up to height. On second thought that is indeed bad news if you were planning on putting that up on your metal mast…and we have no data on what happens when you do it anyway. I don’t know if the smallest (4m) telescoping fiberglass poles would suffice for portable operation, but I’m a fan of just using the big lower segments of my 10m “HD” mast for the stiffness they give me (3 segments for the height, the 4th collapsed into in the base segment for easy rotation). Telescoping masts also give you easy control over…
The published patterns are for 3m/10′ feedpoint height over “average” ground. Increasing height further has no expectable advantage, instead it will deteriorate the favorable directional pattern of the loop. Flying it lower, or even a lot lower in windy weather on the other hand is causing a surprisingly moderate hit on performance.
The wire should be 2.5mm diameter or (I think) AWG10, again, you can use whatever you see fit, I used flexible copper litz wire hoping this is a good choice for a mobile version of the antenna. Thinner wire may work too, but 2.5mm provides a better match across the entire SW range. Speaking of which, here are the SWR/Impedance curves for this version:
The Nooelec “One Nine”-balun works fine, but it needs the R1 center tap connection cut to achieve galvanic isolation (which is a recommendable modification that can be easily undone). Please also see here: https://swling.com/blog/2019/10/the-nooelec-balun-19-v2/
You can also wind your own: Andrew writes “otherwise it can be wound on a binocular core just like the “NCPL” transformer but winding 2 turns to the antenna and 6 turns to the coax (winding ratio 3:1 which results in a 9:1 impedance transformation – the winding ratio is the square root of the desired impedance transformation ratio), the core may be a #61, #73 or #43 – but the transformer is important, since a bad one will ruin the antenna performance.”.
For the terminating resistor I used 4x 2.2kOhm and one 22 kOhm 1% tolerance metal film resistors in parallel to get A) as close as possible to 530 Ohms and B) to get a nice “pack” of resistors. The wire ends of the resistors can be twisted together, then generously tinned to form a pretty solid pack of resistors that has some mechanical robustness and removing the need for a box etc.. For easy testing of different resistors on the prototype I used luster terminals to connect the resistor pack, you should simply solder the loop wire to the resistor pack and apply shrinking tube to make it even more rugged.
Of course you can use any kind or size resistor except wire resistors but I recommend staying as close to 530Ohm as possible. Less resistance will make the lobes steeper while more resistance tends to form extra backside lobes or at least decrease the F/B ratio.
The optional “current balun” or “common mode choke” consisting of clip-on ferrites on the coax at the bottom of the mast is optional but highly recommended. It will help with both preventing the coax becoming part of the antenna and keeping noise away from the antenna. I could see once again how important that is when I compared the SULA with other antennas at the dike: While charging my Icom from a powerbank, the other antennas (without chokes) picked up some converter hash from that power bank, propagating over the coax up to the antennas, while the choked feedline to the SULA kept that away. Amazon has packs of 10 that should do fine, a row of at least 6 of them should go onto the coax.
Our attempts to have the feedline going to the balun via a choke on the horizontal boom resulted in reduced directionality and other bad results on some frequencies. Also, it made the weight-distribution worse, so don’t do that.
If you make your own balun and preamp and wait for the right scrap parts to show up you can save a lot of money but if you have none of the things needed and just want to source the most convenient options off Amazon etc. we’re talking substantial $150 or even more for the full system including coax and mast etc.. What you may get in exchange is a quite capable little beam antenna disguised as SML, with a performance profile promising some fun with the upcoming high frequency band openings between 15 and 6m. I’m looking forward to have this antenna up when the conditions are – for a change – slightly elevated again at the dike. At the same time I hope nothing goes wrong and everyone brave enough to make such an antenna (based on weird stories of some sleep-deprived OM) can find as much benefit in Andrew’s awesome design as I do.
The donut modification
The SULA derives its directionality from termination in the middle of the loop. But what happens if you replace the resistor with… thin air?
As you can see, you get a nice donut-shaped and flat omnidirectional pattern like from a vertical with a good groundplane! In other words, the antenna still has its pretty low takeoff angle but now with good reception from all around. At the same time, the losses decrease and here’s maybe the most interesting part, the most gain increase is happening on LW and MW , while it retains its low angle profile! That doesn’t turn it into a MW DX antenna unfortunately, since directionality is an indispensable requirement for MW DX antennas but it’s certainly a great option to have when you’re not on the hunt for specific stations or countries, or when the DX station has a frequency all for itself.
You may have noticed the 3rd variant – the wires connected directly to form a regular loop. This might give you excellent NVIS performance and less of that pesky low-angle DX. Unfortunately, both variations will greatly mess up the formerly good impedance match and how exactly that is bad is yet another aspect of the antenna nobody has explored yet.