Hi Stefan, (et al)
Well I beg to differ.. :-)
What I think happens is this: The outside of the coax picks up
electromagnetic radiation like any antenna (including QRM generated by
fluorescent lamps and Alinco switching power supplies). This signal travels
along the coax to the Miniwhip. (also in the direction of the receiver but
that is not important here as the signal is on the outside of the coax).
Upon arrival at the miniwhip this signal on the outside of the coax has
nowhere to go but to the _inside_ of the outer mantle of the coax – it
‘rounds the corner’ at the end of the coax so to speak.
This is exactly the reason why a common mode choke is advised when an
asymmetrical feedline like coax is connected to a symmetrical antenna like a
dipole – at the intersection (It works to-way of course).
The signal at the inside of the mantle (traveling back to the receiver) is
not balanced out by a same signal on the inner conductor, thus creating a
net signal in the receiver. (of course some of the signal will also be
picked up by the E-probe and other stray capacities, but not enough to
exactly compensate)
So how to avoid the QRM that is picked up by the coax to ‘travel back’ via
the inside: for the miniwhip it is indeed best (as Roelof mentioned) to
short these signals to earth _outside_ the house, preferably as close to the
miniwhip as possible. Grounding there would to the trick, aided by a (large
enough) common mode choke between the ground point and the house. The QRM
that is picked up in the house would be – after attenuation by the choke -
directed into the ground and not up into the pole and the miniwhip.
Whatever happens in the house would then be largely irrelevant. Adding a
common mode choke close to the rig will do little extra. (it would only
attenuate QRM getting from the shack’s earth system to the outside of the
coax).
Any signals picked up by the vertical coax between the earthing point and
the whip will add to the received signal, but at low frequencies it will not
be much.
So far for theory. Now the proof of the pudding: DCF39 is now > S9+40 dB. My
old trusty QRM generator (Alinco SMPS) generates S9+25 at 135.500. When I
switch off the miniwhip (cut the power) DCF39 drops down to just above the
noise floor. As expected. But the Alinco signal only drops down some 15 dB
and remains the only signal that is audible. This is exactly what I would
expect: the QRM travels along the outside of the coax to the miniwhip,
‘rounds the corner’ and comes back via the inside of the coax shield.
Further proof that it indeed takes this route: if I disconnect the coax in
the shack the Alinco smps signal disappears also (so it is not received via
any other path).
Last year I already bought 3 meters of copper pipe to drive into the ground
in the backyard. Bet never got around to finish the job…
The main reason the signal strength is much higher with the elevated
miniwhip is (I think) caused by the fact that I am surrounded by other
houses, gardens, trees etc. Not comparable with an open field…
Regards,
Minto pa3bca
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Ceterum censeo Carthaginem delendam esse
-----Original Message-----
From: Stefan Schäfer
Sent: Wednesday, August 10, 2011 00:03
To: [email protected]
Subject: Re: LF: Re: HB9ASB...
Hi Minto,
Am 09.08.2011 22:48, schrieb Minto Witteveen:
You are right w.r.t. the cable being (a significant) part of the working
of the Miniwhip antenna. [...]
I don't think so. There should be no difference between a 5m and 10m
long cable. I think about a capacitive divider. The probe has about 3
pF, that's one plate of the C. The other one is the cable and metal
connected. Once if this part of the C has say >10 * 3 pF, the difference
between longer cables become smaller and smaller.
I think it is just the S/N that rises due to lower becoming noise and
higher signal levels. On a flat field without trees and houses, you have
excellent reception even with a 2m pole :-)
73, Stefan
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