Hi Gerhard,
I think one has to compare two different situations:
The pole that is on a noisy roof (and its noisy ground) and on the other
side the pole that is far away in the garden, with its own and clean
local earth. In the garden a metallic pole should make no difference, as
you say.
73, Stefan
Am 11.08.2011 05:33, schrieb Gerhard Hickl:
Roelof!
With interest I was following all the discussions about the "Mini
Whip"-like antennas.
In your description of the "PA0RDT Mini Whip" you suggest to use a non
metallic pole for mounting the probe and an (optional) earth-rod to
ground (sorry Andy!!) the shield.
I might be completely wrong but isn't it the same when one uses a
grounded metallic pole and connect the shield of the coax on top of this
pole?
BTW: I have an insulating pole (5m) on top of the roof and yet no
grounded shielding of the coax but just in the shack. I will connect the
shield to the lightning protection system at the bottom of the pole and
see about the difference.
Actually it isn't even a true coax cable but a singe shielded twisted
pair where the two inner wires are in parallel....Z=? Who knows????
So considering my "fluffy" set-up, I think my VLF grabber is performing
very well but for sure there is room for improvements as well.
73
OE3GHB
Gerhard
Am Mittwoch, den 10.08.2011, 21:26 +0200 schrieb Roelof Bakker:
Hello Minto, Stefan,
Some food for thoughts:
When I tested a mini-whip in the garden with battery feed and also with a
battery fed SLM, I found the noise lower than when used inside and
operated from the mains.
The coax cable from the house station was connected in parallel with the
mini-whip by means of a BNC T-piece.
The noise increased by 9 dB. My assumption is that this noise is received
on the shield of the coax inside the house.
Without connecting the shield to a separate radio earth, the noise
received on the shield inside the house will travel over the shield all
the way to the antenna. There the noise will be received by the sensitive
probe. This will be by means of capacitively coupling, I guess. Many
people have reported a large drop in local noise by connecting the shield
at the bottom of the mast.
This also applies to normal active whips of course.
73,
Roelof, pa0rdt
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