Hello Roelof, Stefan, all,

Thank you for your comments.

> ... If the whip is grounded at the top of the pole and the pole is
connected to a clean RF earth 
> and the coax is connected to a noisy shack earth, then the common mode
chokes will probably help.

> When you ground the shield of the coax e.g. when it enters the house or at
the bottom of the mast 
> the local noise flows to earth and does not reach the antenna.

This is what I thought, and I made a pi shape filter : Gnd - CMC - Gnd. 
Pse see attached.

My CMC (common-mode-choke) was made of a thin coax (1.8D-QEV 1.8mm outer
conductor)
31 turn wound on a FT240-77 toroid, was about 2 kilo-ohm on 136kHz.

One Gnd used is the tower (about 16 ohm earthing-resistance at AF i.e.
ground resistance meter),
the other is a buried copper mesh 15 meter long, 17 ohm, leftover of my
earthing experiments.

In pi configuration, I am expecting an attenuation 40dB or more,
but I don't know how much it is in reality.
I am not sure how it will behave on buildings, either.


> And the coupling capacity between the windings of an isolation transformer
can easily reach the order of 
> magnitude of the probe, i.e. a few pF....
Yes indeed.
My isolation transformer has 27pF between the windings, 43 kilo-ohm on
136kHz.
Much more than the 3.1pF of my Mini-DP.
It was made of bifilar twist of wrapping-wire, 20 turns on FT-82-77 toroid,
about 700uH.
I use it in pi configuration also : Gnd - transformer - Gnd(RX).
The RX is connected to an 1meter square earthing plate beneath my shack.

I have attached some pictures, including my Mini-DP, 
Mounted at the end of a FRP pipe, 2m away from the tower.

73'S

Hideho YAMAMURA, JF1DMQ

JF1DMQ_136k_CMC_017.JPG
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JF1DMQ_136k_CMC_usage.PNG
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JF1DMQ_136k_IsoTr_014.JPG
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JF1DMQ_136k_MiniDP_040.JPG
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