The QRPer Board

I am getting ready to build a QRP 9:1 unun as the basis for a random wire antenna. If anybody can answer a couple of questions, it would be greatly appreciated.

1. Conventional wisdom has the toroid wrapped with three conductors wrapped in parallel, then connected to give the correct final turns ratio. I understand that parallel wrapping helps to distinguish each of the conductors. Is there any reason that wrapping with three wires twisted together would not work (assuming that I can identify the ends of each of the wires before making final connections)?

2. I believe a capacitor connected between the coax center conductor and shield (ground) is not used for the 9:1 (as distinct from a 49:1 for an EFHW). Is it correct to wire the 9:1 without the capacitor?

Thanks and 73 de Brian - K3ES

So, I've only built a couple 9:1's, so I'm certainly not an expert. But, the ones I've built have worked, so FWIW ...

1. The wire I used to wrap the toroid (a 240-43) was a bit thick (maybe 14-gauge?) to twist all three strands together and then wrap the toroid. However, I have seen bifilar 9:1's made with a twisted pair and they seem to work.

2. Correct. I haven't used a capacitor and I haven't seen others use capacitors in 9:1's. The capacitor across the primary side of a 49:1 transformer supposedly reduces the SWR somewhat on the higher bands. I assume you're using your 9:1 on a random wire antenna and since that antenna is deliberately not resonant on anything (i.e., it's resonant on nothing and all of nothing's harmonics ), and will be using a tuner anyway, a slight tweak to the SWR on some select bands wouldn't seem to be necessary.

Good luck with the build!
73 Skip K4EAK

Thanks, Skip.

73 de Brian - K3ES

I found this one being quite good

https://vk6ysf.com/unun_9-1_v3.htm

my only additional recommendation is to also wind this

https://vk6ysf.com/balun_choke_balun_hf_reisert.htm

and insert it in the same box, connecting the choke at the output of the 9:1, this will effectively stop the feedline from becoming part of the antenna (CMC) then, by the way, you'll need to use a counterpoise, but that shouldn't be a problem; a run of insulated wire laying down on ground will do

oh and as for the random, you may want to check this

http://qrper.net/viewtopic.php?p=267#p267

Andrew (grayhat) wrote: ↑Wed Dec 07, 2022 5:53 pm I found this one being quite good

https://vk6ysf.com/unun_9-1_v3.htm

my only additional recommendation is to also wind this

https://vk6ysf.com/balun_choke_balun_hf_reisert.htm

and insert it in the same box, connecting the choke at the output of the 9:1, this will effectively stop the feedline from becoming part of the antenna (CMC) then, by the way, you'll need to use a counterpoise, but that shouldn't be a problem; a run of insulated wire laying down on ground will do

alternatively, you may wind the 9:1 and the 1:1 choke in two separate boxes, that way, you may place the 9:1 at the feedpoint and the choke further down the coax, so using the coax braid (between the 9:1 and the choke) as a counterpoise (that is a part of your antenna in both tx and rx), this may be fine for SOTA/POTA where the coax won't meet a "noise cloud" but a "no go" for a permanent installation, where the braid would pick up all kinds of "domestic noise"

HTH

Thanks all! I am going to build a QRP 9:1 using tri-filar winding, and no capacitor. I'm not yet sure about radiator length. All I'm lacking now is a bit of shop time. When I get things together, I will let you know how it works.

Best 73 de Brian - K3ES

Brian - K3ES wrote: ↑Mon Dec 12, 2022 6:44 pm Thanks all! I am going to build a QRP 9:1 using tri-filar winding, and no capacitor. I'm not yet sure about radiator length. All I'm lacking now is a bit of shop time. When I get things together, I will let you know how it works.

Best 73 de Brian - K3ES

I'll refrain it


http://qrper.net/viewtopic.php?p=267#p267

forgot, as for the radiator length, check the link I posted, then decide based on the lowest band of interest and on the type of installation; I mean, if it's for a permanent installation (e.g. at home), then you may consider a longer antenna, if instead the antenna will be used for POTA or the like, a shorter one will be easier to deploy, but the lenght still is a factor; if you take the time to read these

http://qrper.net/viewtopic.php?p=267#p267

viewtopic.php?t=60

and to check the included links, you'll find this useful reference

https://udel.edu/~mm/ham/randomWire/

which shows the "good" lengths for a "random" wire antenna, now the author correctly states that, to be efficient on a given band, the antenna should be at least (around) 1/4 wave, this means that, for example, willing a random which will efficiently cover the 40m band, the length should be above 10m (33ft); this does not mean that one can't use a shorter antenna, just that such an antenna won't be as efficient; the "classic" size, that is 84ft for the random and 17ft for the counterpoise is a pretty good one, covering all the bands from 80 to 10 meters, but won't be so easy to deploy, operating in the field, in such a case, one may give up some bands and pick a shorter antenna, for example, a 35ft random will allow to operate on 40, 30, 20, 17, 15, 12, 10 meters and won't be difficult to deploy, add a decent counterpoise (which may still be 17ft if you want, or for example, two or three 10ft wires) and you'll be in business

Brian - K3ES wrote: ↑Mon Dec 12, 2022 6:44 pm Thanks all! I am going to build a QRP 9:1 using tri-filar winding, and no capacitor. I'm not yet sure about radiator length. All I'm lacking now is a bit of shop time. When I get things together, I will let you know how it works.

Best 73 de Brian - K3ES

I finished my build this month, and used the antenna for Winter Field Day... Er... used Winter Field Day to test the antenna. Particulars are:

- 9:1 unun auto-transformer was built on a FT50-43 core, wrapped with 9 turns of tri-filar (3 twisted strands) 24 AWG magnet wire.
- I connected a 144 ft radiating wire, with the objective to enable tuning for CW on the 10m through 160m bands.
- I used a three 17 ft wires for a counterpoise.
- LDG Z-11Pro II tuner was successful matching 10m, 15m, 20m, 40m, 80m, and 160m bands with 1.5:1 or better SWR.

I am pleased with the results, and am working on a more complete field report, so more to follow. Thanks to all who helped with tips and advice.

73 de Brian - K3ES