I decided to build another antenna and chose a 49:1 UnUn (or Transformer depending on which side of the fence you’re looking at it from).
I had about 70 feet of 18 gauge “stealth” black wire that I used in my previous project – the 9:1 UnUn – to use for this project.
After reading about antennas I decided I wanted one that didn’t require a tuner (my 7100 doesn’t have one built-in) because that’s less I had to lug downstairs in the kitchen which is my favorite operating position due to my view.
I also wanted one that was multi band since I don’t fancy going out and readjusting my Super Antenna MP1-C at night for 40 meters. I like my 9:1 in the attic for tuning around but it definitely needs a tuner.
I got some parts from Home Depot but mostly on Amazon. This build wasn’t as cheap because the toroids are a little more pricey. All in all, I think it was less than $50 to build 1 but if I had more toroids then I could probably build 10 with all the extra parts and pieces I had left laying around.
After I built the transformer, I had a lot of trouble tuning it to resonance. I tried to use the 70 feet of wire and put it on a spool and unspool it while I watched my NanoVNA which was connected directly to the SO239. No bueno.
I also tried to hang the wire in a tree with 50 feet of coax and methodically tried to make the wire shorter and shorter (by folding it like people seem to have good luck with on YouTube). No bueno.
So then I decided to bench test my transformer to see why I couldn’t get it to show any signs of me assembling it correctly. I have a few bags of 1000 and 330 ohm resistors in my Arduino kit so I grabbed 4 x 1000 resistors and put them in parallel to make up the 2500ish ohms of resistance that a 49:1 transformer expects. Then I attached the resistors to the antenna and the ground/counterpoise lug (which is attached to the SO239 ground anyway). Then placed my cheap multimeters leads across the SO239s contacts. Lo and behold its reading about 50 ohms. Great.
After hemming and hawing more I did some math and came up with 64.88 feet at 7.2125MHz (1/2 way into the phone band of 40 meters) and 66.8 feet at 7.00 MHz. Then I ran the wire all round my house from the 1st floor up the stairs and up to the 2nd floor bathroom at 66.8 feet. I used a 50 foot coax connected to the transformer with the NanVNA on the other end.
Wow! 40 meters was resonant across the whole band! But 20 wasn’t yes less than 2:1 SWR so I made about 15 trips up and down the stairs shortening the wire by folding it and twisting the end upon itself until I got 14.350 to about 1:1.8. I knew I could do better so I unwrapped it and rolled it around a shampoo bottle to confirm that folding and twisting it was equivalent to spooling it (something that didn’t work outside the other day). It did. My “final” length is 63 feet and 2 inches. I know that will change with actually how its installed and when I actually cut the excess wire off.
Then I ran another experiment to see what went wrong the other day. I connected my NanoVNA directly to the transformer. Whoa it was all over the place. Then I “ground” it by touching the outside of the PL259. Stabilizing. Hmm. Then I used a 3 foot coax. Much better but it got real good when I touched the connector with my bare finger.
Could the outer jacket(s) of the 50 foot coax be contributing the stability of the measurements? You bet!
That’s exactly where I went wrong outside earlier that day. I tried to unspool the wire while the NanoVNA was connected directly to the transformer. Had I used a suitable coax like 25-50 feet OR touched the connector to ground it (or maybe used a counterpoise on the lug), then maybe I would have gotten some reasonable readings.
So today’s experiment is going to be putting the wire in the same position outside except taking measurements at the end of the 25 or 50 foot coax. Maybe also grounding it and seeing if that makes a difference. And then actually cutting the excess off and re-tuning once its in its final resting spot at my QTH after doing Rx and Tx tests and comparing with a 9:1 and SuperAntenna MP1-C.
Later that day…
So I setup my 49:1 on my pergola and did a head-to-head test with my 9:1 (30 feet wire) as well as my Super Antenna MP1-C on 40 and 20MHz.
I could not tell much of a difference between reception with the Super Antenna and the 49:1. They were both excellent.
My 9:1 paled in comparison. While it is pretty good, both other antennas pulled in stations that I would have rolled right past with the 9:1.
I got 2 SOTA QSO’s almost immediately with the new antenna! AB0MV (my first YL) and AE0Q were both operating nearby and I had a short QSO with them as they were also using a 49:1 at 66 feet (didn’t get which radio they were using).
So I took down the 9:1 and put the 49:1 in its place, cut the wire at 63 feet 2 inches and put it up a little higher in the trees for more HOA stealth and setup the MP1 to 40 meters to compare the rest of the day. I really can’t tell much of a difference between either antennas. I’m definitely happy with the build.
On both 40 and 20 meters my 49:1 worked as well or maybe a small smidge better some times in terms of clarity/noise.
I think I might ditch the 9:1 or something. Theres no reason to use a 9:1 especially since it requires a tuner and either of the 2 antennas – 49:1 and MP1-C I can tune to less than 2:1 SWR on 40 and 20.
While the 49:1 shows promise on higher frequencies, since the sunspot cycle sucks, I’ll worry about those bands later on.
73!
