Hi Paul,
Am 17.07.2018 23:03, schrieb N1BUG:
I find myself in a strange, alien world on LF. All my experience at
HF, VHF, UHF seems to be no help down here. ;-)
Be happy to find some new challenges to expanse your knowledge spectrum.
Here i have the same experience with Linux, a big struggle to me! :-)
In hindsight, the TX> RX antenna isolation measurements I made the
other day make no sense at all to me. I now question whether the
results mean anything.
Your measurements appeared meaningful to me.
But as far as i understand your main interest is to reach the best
sensitivity of the RX antenna when it is standing far away from the TX
antenna.
If the preamp is designed for 50 ohm input, why is a 50 ohm resistor
not a suitable 'dummy load' for receiver testing?
It is. But do you think that the antenna and the 100:1 transformer
represents a Z=50+j0 Ohm load? Then it should be possible to transmit on
that antenna as well, with limited power of course. If you have a signal
generator, eventually with a 50 Ohm output impedance, just connect it to
the antenna (wire+transformer) and measure how the voltage of the signal
generator drops. It should drop to 50% if the antenna load is actually
50 Ohm. I bet you already know all that stuff.
All in all i would favour the JFET preamp solution but if you like, you
can also treat the RX antenna like a TX antenna and resonante it (using
a coil on a ferrite rod for example) to 137 kHz and then transform to 50
Ohm and connect your 50 Ohm preamp. Your transformer will have a winding
ratio of just 4:1 maybe, e.g. 20:5 turns.
The 9m vertical is in a small open area but it has trees on 3 sides.
Trees are 10m away on one side, 15m on another side, and finally
about 25m the other side. It is 12m from the TX vertical and about
22 or 23m from each of the two grounded towers (33m and 35m tall)
which support the TX vertical.
I think that should be fine. 9m vertical wire is quite good on LF.
I think it's not so easy to connect the 9m vertical directly to the
preamp. It could be done by moving the preamp outside and finding a
12v battery to power it, then running coax back to the receiver 60m
away.
Yes. Then, try as i said, build a small coil to compensate the imaginary
part and then transform to 50 Ohm and then use a normal RG58 cable. The
transformer helps you to decouple the grounds from each other, which can
be very helpful!!
The reason for my interest in trying a directional antenna (K9AY,
EWE, ...) is most of my interference comes from the southwest.
During winter there are often storms with lightning to my southwest
but not in any other direction. I also have some local man made
noise (power line problems and some industrial motor speed controls)
to the southwest, but no known noise makers to the north or east.
Yes yes it is a good idea to use a directional antenna.
But, a normal loop will also improve the situation and is easier to
build. the K9AY stuff is more complex/ advanced. But certainly a good
choice.
73, and have fun!
Stefan
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