Yes, I think I can say quite definitely coupling accounts for most
of the higher noise level when the Rx antenna is near the Tx
antenna. I see that noise peak and it moves around as I change
resonance on the Tx antenna by adjusting the variometer. It seems to
be a necessary evil with this particular Rx antenna in order to have
enough noise output to successfully use it.
I am now greatly concerned about my overall Rx system. I presumed
this 21 dB coupling meant that the noise output of the Rx antenna in
proximity to the Tx antenna is 21 dB lower than noise output from
the Tx antenna. I just tried connecting the Tx antenna to the
receiver and approximately confirmed this. It looked more like 18 dB
noise floor increase on the Tx antenna, but with fluctuation from
summer static this is a rough estimation.
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!
This reasoning leads me to wonder if something is drastically wrong
with my receiver/preamp system.
Paul
On 07/16/2018 02:24 AM, Andy Talbot wrote:
> 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 <http://www.g4jnt.com>
>
>
> On 16 July 2018 at 02:20, N1BUG <[email protected]
> <mailto:[email protected]>> 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/
> <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
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