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Date: Wed, 10 Aug 2011 14:47:55 +0200
From: =?UTF-8?B?U3RlZmFuIFNjaMOkZmVy?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Re: HB9ASB...
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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=20
> electromagnetic radiation like any antenna (including QRM generated by=20
> fluorescent lamps and Alinco switching power supplies). This signal=20
> travels along the coax to the Miniwhip. (also in the direction of the=20
> receiver but that is not important here as the signal is on the=20
> outside of the coax).
> Upon arrival at the miniwhip this signal on the outside of the coax=20
> has nowhere to go =C2=ADbut to the _inside_ of the outer mantle of the =
coax=20
> =E2=80=93 it =E2=80=98rounds the corner=E2=80=99 at the end of the coax=
 so to speak.
I think the mechanism is that the unwanted signal on the screen causes a=20
potential difference between gate and source of the first (J)FET. So=20
this causes a current flow in the output stage and so a signal at the RX=20
input.
A common mode choke between RX and the antenna ground should form a low=20
pass filter for unwanted signals coming from the shack. Using a common=20
mode choke without a local ground should have little effect, except the=20
coax is some 100m long (between choke and probe) ;-)

Ah BTW regarding the discussion "the cable to the E field probe is the=20
actual antenna": One could just try what happens if one disconnects the=20
power supply. If the signal is still present then the cable is the=20
antenna, if the signal is gone: The probe must be the antenna. Isn't=20
it?! :-)


> So how to avoid the QRM that is picked up by the coax to =E2=80=98trave=
l back=E2=80=99=20
> via the inside: for the miniwhip it is indeed best (as Roelof=20
> mentioned) to short these signals to earth _outside_ the house,=20
> preferably as close to the miniwhip as possible. Grounding there would=20
> to the trick, aided by a (large enough) common mode choke between the=20
> ground point and the house.  The QRM that is picked up in the house=20
> would be =E2=80=93 after attenuation by the choke - directed into the g=
round=20
> and not up into the pole and the miniwhip.

Yes yes, totally agreed.
> Whatever happens in the house would then be largely irrelevant. Adding=20
> a common mode choke close to the rig will do little extra. (it would=20
> only attenuate QRM getting from the shack=E2=80=99s earth system to the=
=20
> outside of the coax).
It would almost have the same effect (when ignoring the C between cable=20
and ground along to the choke near the antenna ground) as placing the=20
choke near the antenna ground, both are in series and increase the=20
current reducing impedance, yes...

> Any signals picked up by the vertical coax between the earthing point=20
> and the whip will add to the received signal, but at low frequencies=20
> it will not be much.
> So far for theory. Now the proof of the pudding: DCF39 is now > S9+40=20
> dB. My old trusty QRM generator (Alinco SMPS) generates S9+25 at=20
> 135.500. When I switch off the miniwhip (cut the power) DCF39 drops=20
> down to just above the noise floor. As expected.
Ah yes, that's what i meant above (should have read your mail completely=20
before answering ;-) ). This is the proof that Mal cannot be right when=20
saying "the coax is the actual antenna".
> But the Alinco signal only drops down some 15 dB and remains the only=20
> signal that is audible. This is exactly what I would expect: the QRM=20
> travels along the outside of the coax to the miniwhip, =E2=80=98rounds =
the=20
> corner=E2=80=99 and comes back via the inside of the coax shield. Furth=
er=20
> proof that it indeed takes this route: if I disconnect the coax in the=20
> shack the Alinco smps signal disappears also (so it is not received=20
> via any other path).
Hm, i rather expect a galvanic coupling i.e. stray currenty on the=20
supply cable of the RX. What happens if you run the RX on batteries? The=20
same dependency?
There could be several reasons apply here...

> Last year I already bought 3 meters of copper pipe to drive into the=20
> ground in the backyard. Bet never got around to finish the job=E2=80=A6
Today it's nice WX here! And in NL?
>
> The main reason the signal strength is much higher with the elevated=20
> miniwhip is (I think) caused by the fact that I am surrounded by other=20
> houses, gardens, trees etc. Not comparable with an open field=E2=80=A6
Yes.

73, Stefan /DK7FC


>
> Regards,
> Minto pa3bca
>
>
>
>
>
>
> -----------------------------------------------------------------------=
---=20
>
> Ceterum censeo Carthaginem delendam esse
> -----Original Message----- From: Stefan Sch=C3=A4fer
> Sent: Wednesday, August 10, 2011 00:03
> To: rsgb_lf_group@blacksheep.org
> 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=20
>> 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 differenc=
e
> 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 hav=
e
> excellent reception even with a 2m pole :-)
>
> 73, Stefan