[TowerTalk] Circularly Polarized Receive Antenna

[David Gilbert]

Before he retired, my oldest son used to write software for this sort of 
thing.  He told me that with eight synchronous antennas he could beam 
form in any direction ... up/down and left/right.  And of course for 
receive they wouldn't have to be full size antennas, and depending upon 
the resolution they wouldn't have to be spaced a large percentage of a 
wavelength apart.

Units like the RSP Duo from SDRPlay (about $300) are synchronized 
sufficiently to allow beam forming with software like SDR Uno, although 
with just two antennas you don't get much gain.  You can get a lot of 
rejection in a very sharp notch that way, though. There are some pretty 
impressive YouTube videos out there demonstrating the effect.

Dave


On 8/17/2025 11:11 AM, Jim Lux wrote:
> with N antennas, one can theoretically null N-1 sources, so if the noise isn’t sort of “generic atmospheric noise distributed in the direction of your desired signal” some sort of multichannel coherent receiver could be a good thing.
>
> And, one doesn’t need those antennas to be particularly “special” or spaced in any particular way - that is, if you’re doing adaptive cancelling, it’s not like beamforming with a 4 square where some arrangements are better than others.  Obviously, “farther apart” is probably better than close together, but realistically, some smallish loops oriented in different directions would probably work.
>
> The trick is that there’s not a lot of inexpensive hardware out there that provides synchronized acquisition of the signals - you can find RTL-SDRs (at the low price end) and various other SDRs that provide synchronized clock, but they tend to have separate USB interfaces and the sample streams are not synchronized.
>
> For this application you don’t need huge dynamic range, assuming you have some front end filtering to knock down things like AM BC stations.
>
> Somewhere along the line, someone is going to finally build a N input RF front end for HF at a “consumer” price point (e.g. not the $10k for a USRP with 4 inputs).  Then, it’s a matter of implementing the combiner algorithm, and then you can feed the “fixed up” RF into whatever receiver you are comfortable with (including whatever AGC and filtering you like).
>
> That’s going to change HF receive antenna concepts a lot - because, after all, we typically don’t need “gain” on receive, what we look for is directivity. (if gain was what we need, then people would spend some time building lower noise front ends for HF receivers, and they don’t)
>
>   
>
>
> On Wed, 13 Aug 2025 12:30:19 -0700, Jim Brown<jim at audiosystemsgroup.com> wrote:
>
> On 8/13/2025 4:46 AM, Brian Beezley wrote:
>> Fading can be a problem on 160. When it is due to polarization rotation
>> of the incoming signal, a circularly polarized receive antenna can
>> eliminate it.
> Very interesting ideas, Brian! However -- the overwhelming issue for
> many of us is local noise. I have two reversible half-wave Beverages,
> half-wave on 160M, that are effective as high as 20M, one to EU/VK, the
> other to SA/JA. I also have a phased pair of VE3DO loops spaced 5/8-wave
> on 160 that are also effective on 80M. All of these antennas are
> vertically polarized. There's a noisy home with a solar system in the
> direction of EU, another in the direction of SA, and a retreat center
> with a large solar system in the direction of JA.
>
> My point is that for most hams, receive noise is the dominant factor in
> what we can hear. With the same Beverages and TX antenna, I could work
> EU on 160 CW a few nights a year when I moved here in 2006. I haven't
> heard EU on CW for five years. I do serious weak signal work on 6M, and
> noise from most directions is limiting me by 12 dB or more.
>
> BTW -- beginning with their K3, introduced in 2007, Elecraft has had the
> option of a second synced RX, and I've been using it since 2008. I'm
> phasing the two VE3DO loops with a DX Eng NCC-1 noise canceller, which
> is a very nicely engineered unit. Measured responses are in this pdf.
>
> http://k9yc.com/VE3DO.pdf
>
> 73, Jim K9YC
>
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