Re: Airspy+ or SDRPlay 2 or both
Hi Mark,toggle quoted messageShow quoted text
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
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...