Hi Minto,
Am 10.08.2011 13:10, schrieb Minto Witteveen:
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.
I think the mechanism is that the unwanted signal on the screen causes a
potential difference between gate and source of the first (J)FET. So
this causes a current flow in the output stage and so a signal at the RX
input.
A common mode choke between RX and the antenna ground should form a low
pass filter for unwanted signals coming from the shack. Using a common
mode choke without a local ground should have little effect, except the
coax is some 100m long (between choke and probe) ;-)
Ah BTW regarding the discussion "the cable to the E field probe is the
actual antenna": One could just try what happens if one disconnects the
power supply. If the signal is still present then the cable is the
antenna, if the signal is gone: The probe must be the antenna. Isn't
it?! :-)
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.
Yes yes, totally agreed.
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).
It would almost have the same effect (when ignoring the C between cable
and ground along to the choke near the antenna ground) as placing the
choke near the antenna ground, both are in series and increase the
current reducing impedance, yes...
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.
Ah yes, that's what i meant above (should have read your mail completely
before answering ;-) ). This is the proof that Mal cannot be right when
saying "the coax is the actual antenna".
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).
Hm, i rather expect a galvanic coupling i.e. stray currenty on the
supply cable of the RX. What happens if you run the RX on batteries? The
same dependency?
There could be several reasons apply here...
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…
Today it's nice WX here! And in NL?
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…
Yes.
73, Stefan /DK7FC
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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