R: Re: LF: TX > RX isolation test

["marcocadeddu@tin.it" <marcocadeddu@tin.it>]

Hi Paul,

a "magic T" at the RX end will allow you to power a remote E-probe ;-) don't be worried of your 100m coax run: I live at first floor of my building and although the building is not a sky skraper, but a common italian condominium the closest antenna is 50m of coax away! On the other hand losses are quite small on LF / MF so for a proper matched antenna are negligible I guess your RG59 and RG6 are more than useful!

Keep on and do not worry if you feel to not understand something: you are not alone! :-) and the reflector should also be a room were more experts can explain to common (or almost common) people esotic tales ;-)

73, Marco IK1HSS  

----Messaggio originale----
Da: paul@n1bug.com
Data: 18-lug-2018 23.34
A: <rsgb_lf_group@blacksheep.org>
Ogg: Re: LF: TX &gt; RX isolation test

Hi Stefan,

> Be happy to find some new challenges to expanse your knowledge spectrum.

I am, but I feel like a fool asking so many questions and sometimes
having difficulty understanding how things work at this part of the
electromagnetic spectrum.

> Here i have the same experience with Linux, a big struggle to me! :-)

I understand! I use linux every day but for routine things like
email, web browsing, etc. For this I do not need (and do not have)
much 'under the hood' expertise with linux.

>> 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.

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?

> 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.

Yes, that's correct.

>> 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.

> 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.

I would like to continue using this antenna on LF and MF, at least
until I have more RX antennas installed. The preamp I use now is
(perhaps foolishly) band pass filter, preamp, and two way splitter
all in one box. This has been good because with the filter I can TX
will full power on 160m, HF, VHF without interference to my LF and
MF receivers.

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.
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.

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!

> 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.

A loop is also in my plans. I have many plans, not enough time or
money. :-)

73,
Paul N1BUG