Hello Stefan,
why not use the "big" TX antenna also for RX. With a sufficient
atenuator of course.
On 137kHz I alway used my TX antenna for RX, but a 40dB attenuator was
required to avoid overload at the RX.
73, Rik ON7YD
Quoting Stefan Schäfer <[email protected]>:
Dear Lowfers,
Yesterday i saw the extreme influence of my 100m vertical tx antenna
to the small active antenna. Now I have some ideas how to avoid
that and would be thankful for any comments and hints.
During the QSOs there was much QSB in the RX signals, just as if
someone is playing with the AF-volume. First I thought it is caused
by a loose contact in the Converter or in the fiber optic cable but
later I found, that it was caused by the angle of the tx antenna to
ground (Wind was good but with some QSB ;-) ).
In my last QSO with IK1HSS I disconnected the TX wire from the
loading coil and than, the whole signal was almost gone (when
choosing “visual mode LOW” at the argo monitor). Switching to
“visual mode HIGH” and increasing the AF a little bit gave almost
the same QRM lines by DFC39 (vy strong yesterday). Now, the SNR was
much better than before!
In the following pic you can get a impression of the dimension of
the influence of the TX ant (The RX ant is isolated to the rest of
the rig by an optic fiber cable and battery supply, so “just”
coupled by the E field to the TX ant):
[cid:[email protected]]
In the left part the TX wire was disconnected from the coil and mode was HIGH
In the middle part I connected the wire to the loading coil
In the right part I switched to the LOW mode!
I am sure that the TX ant does not only increase the QRM but also
the wanted signal. So the benefit will not be so strong than between
left and right part in the pic. But the SNR increases
significantly, as I saw in the QSO with IK1HSS. So a disconnection
of the TX wire makes sense.
What are the alternatives?
Placing the RX ant apart from the TX ant to reduce the influence? I
think with a 100m vertical I can forget that unless I do not want to
spend 100s of meters of RX cable ;-)
Bringing the ant out of resonance? That is the one and only solution
I think. Any better ideas?
I could disconnect the loading coil at the lower end to ground. That
could be done with a relay (My RX/TX sequencer provides an output
“12V @ RX” that could directly be used for this). Probably there
will be a residual coupling capacity of the coil to ground and the
relay contacts and so on, say 30pF but since my ant C is 580pF that
would be quite enough for deadjusting, I hope so.
As I thought about protecting the contacts and the PA in the case
that there will be a mistake (contacts have to be closed without a
voltage at the relay) in the circuit, i thought some Varistors or
even better some antiparallel diodes could be a good choice. That
brought me to the idea that a power diode out of a SMPS would easyly
handle the antenna current with fast enough switching times.
So do you think just two antiparallel diodes (or perhaps 2x2, 2 in
series, and perhaps with an additional 10…100k Resistor parallel)
without a relay could be suitable to solve the RX TX ant influence
problem? Could the TX signal distortion be neglected? When the
voltage across the diodes (RX case) is less than the forward
voltage, do the diodes act as an open contact, just with a few pF
reverse capacity? I think so but I don’t know much about the RX
voltages that are given by such an antenna. There will be some
experiments… :)
Comments welcome ;-)
73, Stefan/DK7FC
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