I’m not sure the RSPduo actually allows synchronized samples. You can use the
same sampling clock, but internally, the ADCs are fed from separate RF front
ends, so there’s the phase ambiguity in the PLL synthesizer. And, I’m not sure
if their firmware does sample for sample paired acquisitions. Certainly not if
you have multiple RSPs (I have a stack of the SDRPlay receivers on my desk at
work) - you can tie them all to a common clock, but the actual acquisition
isn’t synced.
Similarly for the plethora of “multichannel RTL-SDR” units out there (there’s a
5 input version) - they basically use 5 of the RTL-SDR chip sets, and a USB
hub, so there’s no synchronized acquisition (it’s not something the backend
sampler of the RTL-SDR does).
The USRP *does* have a way to do this (since they’re supplying devices to the
MIMO crowd which need 4x4 Tx/Rx) - but it’s pretty pricey, in ham terms. (I
also have USRPs on my desk at work - this is something I’m very interested in
at work).
The N210 is about $3k, and it does 2 in and 2 out. You *might* be able to put 4
channels in with a pair of 2 Rx direct receiver cards. I’ve not looked at the
details yet. The B210 has the AD9361, and only goes down to 70MHz. I’ve looked
through the user guide for the 9361, and it doesn’t look like there’s a mixer
bypass (I did think about deliberately misprogramming the LO PLL, so there’s no
LO, but I’m not sure that would work, or that there aren’t other high pass
blocks that cause problems.
One might also be able to gang up a set of Lime SDRs or ADALM Plutos - but
those typically don’t go down to HF, although they do expose enough digital
lines to be able to sync across multiple card (with some FPGA coding). The
Lime mini is $400 (per channel) and the front end only goes down to 10 MHz,
although there might be a “bypass” to allow lower frequencies. The Pluto uses
the AD9361.
And really if you’ve got a tuner on the front end, you’d like all the channels
to be fed by the same LO (so that the LO phase is the same for all converters).
On Sun, 17 Aug 2025 11:46:56 -0700, David Gilbert <ab7echo@gmail.com> wrote:
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 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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