Re: LF: TX > RX isolation test

[N1BUG <paul@n1bug.com>]

Hi Stefan,

On 07/19/2018 03:55 AM, DK7FC wrote:
> Well i don't feel like a real expert too.

>From my perspective you are a real expert! I have learned much from
you. I am grateful for your kindness and patience explaining things
to me.

> 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.

OK. Understood.

>> 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.

Oh, sorry, I created a misunderstanding. I meant 100:1 impedance. It
is 80 turns on the antenna side, 8 turns on the feedline side.

> 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.

Well, this is why I used the 9m long whip instead of a mini-whip or
similar. I have many trees to 25m height. There is only one way to
get above them, that is to put the mini-whip at the top of either
tower, eg. at 37m height. But both towers are connected to the TX
antenna ground system so there would no ground isolation.

Speaking of trees, I am a bit surprised my TX antenna is working
like it is. There is a small open space around it, but then many
trees around 3 sides of this small clear area. Along one side the
forest is not more than 6 or 7m from the vertical wire. But my half
power WSPR-2 signal was copied in G, PA, and DL. I guess this is not
too bad. There will be no more half power transmitting for me. When
I return the signal will be a little better. :-)

> 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 like the isolation idea of course. I am very well aware of common
mode issues! I have been at war with them a few times. Assuming I
find some type of antenna which is happy near the trees, then I have
some challenge to locate it where a solar panel can get sun.

Certainly solar power is possible. Whether it is possible this year
depends on the size of the panel / battery, which I have to think
about. I took the decision to delay replacing the roof on my house
for one more year so I could get the LF TX to full power and make
one new RX antenna in 2018. But even so, the budget has hard limits.

> 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).

Hmm, I don't think we are meaning the same thing when we say loop. I
was thinking something like this:

http://qsl.net/ve7sl/loop.html

but I think you have a simpler design in mind. Can you point me to a
link for more information? I probably have everything needed to make
this!

73,
Paul N1BUG