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):
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… J
Comments
welcome ;-)
73,
Stefan/DK7FC
