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Date: Wed, 18 Jul 2018 15:16:50 +0200
From: DK7FC <selberdenken@posteo.de>
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In-Reply-To: <ebf9700f-a8f7-fb34-7566-0d9501e5ee82@n1bug.com>
Subject: Re: LF: TX > RX isolation test
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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