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@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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