Hi Paul,
Am 18.07.2018 23:34, schrieb N1BUG:
Perhaps so but now I do not understand how the coupling can be only
-21 dB. The efficiency of the TX antenna is< 0.1% and surely the
little RX antenna is very inefficient also. How can I have only 21
dB loss of signal between them? What is the method of coupling or
signal transfer for antennas in such close proximity?
Well i don't feel like a real expert too. But here i would say that the
equivalent circuit is a constant voltage source (the TX antenna while
transmitting) coupled to the RX system. The coupling element is the
capacity between the wires and it will be in the sub-pF range. We are in
the near field you know... There may be some more coupling impedances,
like stray currents in the ground but for understanding the basic
dependency i have this circuit in mind. Depending on the impedance of
the RX system there will be a resulting voltage on the RX terminals
(your 50 Ohm input of the preamp for example) given by the injected
current over this small coupling capacity. And this capacity will rise
when you put the RX antenna closer to the TX antenna of course.
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?
Ah. I see. In fact I have no idea what complex impedance it presents.
I would expect is is less than 1 Ohm due to the extreme winding ratio of
your transformer, if you actually have 100 turns primary and 1 turn on
the preamp side.
If I understand correctly, one suggestion is to convert this antenna
into an e-probe by eliminating the transformer and putting a JFET
preamp at the antenna. It would be reasonably simple (maybe) to feed
DC power over coax. I don't have any RG58 but I have a lot of RG-59
and RG-6 direct burial cable. I use it for my 160m RX antennas.
One concern for me is that I don't make my RX situation any worse
than it was last winter. To address that, maybe I can leave the
existing RX antenna just as it is and make a new one with JFET
amplifier and power over coax feed at the more distant location
where this one failed to work. I don't know if that is a good idea.
That is a good idea. Instead of a 9m long wire you can try a much
shorter wire but at a higher height above the ground. This can lead to
better results and it will lower the coupling capacity to the TX antenna.
Just search in the web for the PA0RDT mini whip to get some ideas for
the schematic design. A 0.3m long wire is quite fine for LF and MF, as
long as it is at the highest point, above all trees. The effective
height is essential.
Any coax cable is fine of course.
Until now I did not see anyone use long coax runs with e-probe type
antennas. Loss is obviously not a concern, but common mode problems
could be. The coax to reach that location would be about 100m.
No problem!
But can't you use a big battery and a solar module instead of power over
coax? Then you can use a (or 2) transformer to decouple the grounds
between the house and the RX site. Would be a pity if you bring the
noise from the house over to the RX site via the coax! Just experiment
by using a battery in the first tests. I would use 2 1:1 transformers to
decouple the grounds of the sites.
I see most people put e-probe antennas very close to their home with
short coax runs. I can try that and see what happens. I also read
the suggestions to find a quiet location for the antenna. My
experience at 160m is probably useless at LF, but I have always
found that any RX antenna in close proximity to my house (and
neighbor's house) was a disaster on 160m. Very noisy!
Same on LF!
A loop is also in my plans. I have many plans, not enough time or
money. :-)
A loop is even easier i find. And it is very quickly done. And the trees
will give useful supports, they don't lower the RX signal much (at
LF/MF). You just need some wire, a capacitor, a small ferrite core.
That's all! With that you can build a resonated loop with a 50 Ohm
output to feed your existing preamp. I would even recommend to start
with this project first! You will be surprised! Done in 1 day. And you
can transmit on that antenna with say 5W (when using good capacitors
such as WIMA FKP-1).
73, GL, Stefan
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