Re: Airspy+ or SDRPlay 2 or both

Leif Asbrink
 

Hi Mark,

Ever been at a field-day?

Antenna noise maybe -146 dBm/Hz (28 dB above room temp.)

We then would want to place the noise of our receiver
at -152 dBm/Hz, 6 dB below the rx internal noise (NF=22 dB)

The noise floor in 500 Hz is then -125 dBm.

With 100 dB dynamic range, the strongest undesired
signal we can tolerate is a bit below -25 dBm. An
interferer at -25 dBm would degrade sensitivity by 3 dB
if the mechanism is reciprocal mixing.

If we would tolerate a 1.5 dB S/N loss, max signal would
be -28 dBm.

Assume someone else in the field-day operates the same band
with 100W (+50 dBm) the attenuation between antennas
has to be at least 78 dB.

The free-space path loss at 14 MHz over a 100m distance
is 31 dB between dipoles. 47 dB more is needed and I
think orthogonalization (one V and the other H or the
nulls of the dipoles point to each other) would easily
give 20 dB attenuation, but I think 47 dB over a whole
band would be very difficult.

Maybe the staions want 1.5 kW???

Admittedly the above is a special scenario, but it is real.

Another case is when two operators on e.g. 14 MHz use beam
antennas with a gain of perhaps 8 dB. When they point towards
each other they might have only 35 dB attenuation between
them if antennas are high above ground at a distance of 1 km.
Maybe even less.

Surely few people on HF need 100+ dB dynamic range, but some
actually do....

On VHF it is a very different story with much higher antenna
gains and much lower antenna noise.

Another thing is the possibillity to monitor the band while
transmitting. It is not hard to tune a directional couple for
30 dB directivity on a reasonable antenna (impedance not
changing with time.) With -20 dB coupling the loss of NF
would be 20 dB, but the loss of S/N about 15 dB.
The tx signal would be attenuated by 50 dB. We need 78
so 128 dB dynamic range which we can get today would
do the trick for a power of 100W.

One can also arrange antennas that do not couple much to
each other. Maybe two parallel dipoles in the X direction
separated by a few wavelengths that are shifted in the X
direction by the optimum amount. (0.125 wavelengths?)

There are so many interesting things we can do these days...

73

Leif

Rob,

As I've asked in other forums (I know, "fora"), what good is 100+ dB dynamic range when no one will ever need that on HF?

I've tried very hard to find any HF signal that's more than 80 dB over the atmospheric noise level, and have yet to observe such an animal.  That said, I did find some broadcast AM signals that were 80 dB (and even a little more) above the noise, but those are all 50 KW towers 7 miles from me:

https://drive.google.com/open?id=1Nr0j5gcYqjNlWJiQceW_T0xeHj1I0oKV

And my RSP1A is handling that like a champ.  Best money I ever spent!

Mark  KF5VQY

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