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Date: Thu, 19 Jul 2018 09:55:19 +0200
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: TX > RX isolation test
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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