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[TowerTalk] Noise puzzle (longish)

Subject: [TowerTalk] Noise puzzle (longish)
From: Pete F5VNB <>
Date: Wed, 08 Dec 2010 16:05:29 +0100
List-post: <">>
I would appreciate any observations on the following:
My antenna is a 3 el SteppIR mounted on a 22.5m telescopic tower.
The rx noise floor is direction dependent: beaming ~120 deg from JN04BK 
it can be 8dB higher than the reverse bearing (A/B checks are quick 
thanks to the SteppIR). The differential appears to be highest on 
18/21MHz, and not there at all on 50MHz. Rig is a K3, no premap except 
50MHz, measurement uses a 200kHz wide spectrum display with the 
averaging set to 10secs.

OK, I think, this must be a noise source out in that direction and since 
it is on a line from the tower and through the QTH (~100ft separation) 
it must be something in the house. So, transceiver hooked up to battery, 
the power switch is pulled - and the noise is the same.

Hmm, inverse square law says it should be something pretty obvious and 
not too far away - except that I live in the French countryside and 
there is nothing in that direction except for woods, fields, a vineyard 
or two and then a house every mile. But there is a power pole ~150ft 
away that sends the underground feed to the house. All other power lines 
are between 0 and 120 degrees bearing and ~750ft away.
So out I go armed with a HT on 21MHz am, wandering around looking for an 
increase in noise. Down the drive, across the lane, into an empty field 
and there is no obvious increase in noise.

Bear with me, this gets a bit more interesting now: there are gusts up 
to 100kph forecast for today so I lower the antenna to about ~30ft.
Back at the rx, the noise floor differential has disappeared; on 21MHz 
it may have even slightly reversed.

I am puzzled; is the noise now lower because the main lobe is now more 
elevated and the source is close to the horizon, ie it *is* a long way 
off. Or could it be something common mode that appears when there is 
more feeder/tower in the air?

What does the team think?

Pete, F5VNB


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