Re: LF: TX > RX isolation test

[N1BUG <paul@n1bug.com>]

Hi Stefan,

>> I was planning to build a double size K9AY loop (like W1VD uses) but
>> its 'gain' at 137 kHz models to -55 dBi. It seems to me this puts it
>> at or below the noise output of the little vertical when moved away
>> from the Tx antenna. If that is so, then I will never be able to use it!
> You will need a preamp for this antenna.

Yes, I know. I have a preamp with the short vertical, but unless I
put it close to the Tx antenna to get strong coupling between them,
I do not have enough noise from the antenna to overcome preamp
noise. I believe the same will happen with the K9AY, maybe even worse.


> But when the TX and RX antennas are coupled together, your noise may 
> rise, lowering the RX performance. Thus it is better to keep the 
> antennas as decoupled as possible and use a preamp (with an internal low 
> pass or even band pass filter) instead.

I know! That is why I tried moving the Rx antenna farther from the
Tx antenna. But then, with one 20 dB gain preamp the noise increased
only 2 dB between 50 ohm termination and Rx antenna. Adding a second
preamp improved the situation somewhat. Then I had a high noise
floor but only about 8 dB increase in noise from the antenna over
the 50 ohm termination. S/N ratio of WSPR signals and DCF39 carrier
increased.

I cannot make instant measurements between the two Rx antenna
locations but I feel S/N ratio is better when the Rx and Tx antennas
are strongly coupled. Then I have about 20 dB noise increase on the
antenna with a single preamp. Daytime S/N of K3MF and K3RWR
definitely seems better with the strong Tx to Rx antenna coupling.
When I move the Rx antenna away from the Tx antenna I lose a few dB
S/N on them, even with the two preamp setup.

I don't know if this is correct but I read that for optimum S/N one
should have 10 dB noise increase on the antenna when compared to a
50 ohm termination. I also read that when using a SDR the A/D
converter has some influence, then the number should be 13 to 15 dB
to reach optimum S/N. This part I did not fully understand, and I do
not know if the source of this information was reliable.

Paul