R: Re: LF: TX > RX isolation test

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

Hello Paul, hello Roelof,

just a quick feedback with this concern.
I have a MiniWhip like antenna hanging from the same mast of my MF/LF antenna it is located about 9 meter above a metal roof (giving me a blessing low impedance on my TX antenna ;-) ). The MiniWhip is about 1.5 meter from the vertical wire and about 1 m from 1 of the top loading wires.

The first time I tried it, I was really surprised from the lower noise and similar signal I had receiving with the TX antenna.. indeed they are coupling as reported in the previous messages, but I mean this is not really important: it is true that absolute noise levels and signal levels are higher in comparison with a remote E field antenna but reception is enjoyable and you have not a sensitive device connected to a big aerial.

In my case a have 2 vakuum relays switching in and out the LF base coil and of course if I switch to MF while listening to LF the detuning of the close TX antenna makes signals different (lower) but not performance is not looking to improve, so I leave it as it is ;-)

Last but not least: I was also worried about the survivability of active devices in my MiniWhipLike antenna.
Roelof was so kind to offer me free replacement of J310 and they are still alive althoug in some case I feeded the TX antenna up to 700W!

73, Marco IK1HSS  

----Messaggio originale----
Da: roelof@ndb.demon.nl
Data: 16-lug-2018 16.16
A: "N1BUG"<paul@n1bug.com>, <rsgb_lf_group@blacksheep.org>
Ogg: Re: LF: TX &gt; RX isolation test

Hello Paul,

Noise received on your TX antenna is re-transmitted and this is responsible for the increase 
in noise level on your RX antenna. One way to get rid of it is to detune your TX antenna on 
receive.

I have been using an active E-probe antenna since 2004 with very rewarding results on LF / MF, 
albeit mounted just 6 meter from my house. The band noise from the antenna at 477 kHz is 14 dB 
over the noise floor of my PERSEUS, so sensitivity is more than needed. 

There is no need for a good radio earth with this type of antenna, due to the small 
capacitances involved, typical 3 pF. 

This antenna is prone to common mode problems. I have solved this by connecting the shield of 
the coax to a ground stake at the bottom of the mast and near the house. Requirements for this 
ground are modest. On poor soil one square metre of chicken wire will work already. A suitable 
common mode choke, e.g. 20 turns RG174 on a FT-140-77 toroid can help as well. 

To achieve best results some experimentation is on order, which is time well spent!

73,
Roelof Bakker, pa0rdt