Re: LF: TX > RX isolation test

[Andy Talbot <andy.g4jnt@gmail.com>]

Could the coupling also account for your higher noise level at the original closer spacing?  Your main, tuned Tx antenna is picking up large levels then coupling this, 21dB down, into the vertical and swamping its own noise pickup.

I use a short Procomm active antenna (an earlier version of this  http://procom.dk/products/520-aac-1  ) located directly under my 7m high Tee , and looking at the output from that on a spectrum analyser there is a definite peak at 137kHz when the big antenna is connected.   Removing the Tx coax connection, and thus de-resonating, kills the noise peak.  

Incidentally, On Tx with 700 watts (about 10kV on the antenna 7 m above it)  the Procomm is so badly overloaded it supplies a horrible almost square wave signal of 6 volts pk-pk. Around +28dBm.   Several times  I've forgotten to de-power it on transmit and left that input to blast the SDR-IQ which complains loudly by flashing its red overload LED and sending a message to its driver software.   But it survives that degree of overlaod

I have blown one of those Procomm antennas over the years - not sure how, I just found it was dead one day.  It could have been by overloading on Tx;  it could have been another reason.

Andy

www.g4jnt.com

On 16 July 2018 at 02:20, N1BUG <paul@n1bug.com> wrote:

First, the rest of the story on my RX antenna...

Last winter I used a 9m tall "low noise vertical" for receiving:

http://blog.n1bug.com/2017/11/21/

It was only about 12m away from the TX antenna. It seemed to work OK
for a omnidirectional antenna.

This Spring, when I moved it away from the TX antenna, it stopped
working. The signal level from the antenna was too low to amplify. I
ended up with the noise floor being largely noise of the preamp itself.

Today I put the RX antenna back in the original location. Output
level from the antenna increased more than 20 dB to the same level
it had been last winter. My LF transmitter is not ready so I made
some isolation tests at low power with a signal generator.

First I connected the signal generator directly to the receiver and
verified calibration is linear, eg. for every 10 dB change in
generator output level I see exactly 10 dB change at the receiver. I
calibrated the receiver so that it would read the same dBm level as
the signal generator.

Next I connected the signal generator to the TX antenna and measured
signal level from the RX antenna into the receiver. Isolation is
only 21 dB!

This means that during last winter when I was transmitting with 200
watts I probably had about 1.5 watts coming into the receiver.

Paul N1BUG