[TowerTalk] Noise puzzle (longish)

[Bill]

This thread will go on for years I'll bet.
Bill



On 12/8/2010 10:05 AM, Pete F5VNB wrote:
> 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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