[time-nuts] GPS shielding by power lines?

Scott Mace smace at intt.net
Sun Aug 10 14:05:22 EDT 2008


Around here they place cell antennas on a monopole down the center
of power transmission line towers.  The GPS antennas are invariably
directly below the 100-200kv lines.   The GPS signal must be reliable
enough or the cell sites wouldn't be there.

	Scott

Alan Melia wrote:
> Hi all,  in the process of setting up a GPS time standard for a Radio
> Astronomy facility (amateur) we installed a GPS receiver in a small cabin
> with a translucent roof, thinking that would not impede the GPS signal.
> After a lot of head scratching as to why we were not getting the performane
> we got at another site, we realised that the "convenient position" for the
> cabin was directly below a three phase 11kV power distribution line ( common
> UK rural electricity distribution system). We extended the cable and moved
> the antenna about 20 - 30 feet to the side of the line run, which was
> mounted on wooden poles at about 25 feet. In this position we immediately
> got a reliable fix. The fix and number of usable satellites degrades as we
> move nearer the lines.
> 
> The thought was that there as interference arcing or corona noise from the
> line insulators, and a receiver (AM) was deployed to listen for what was
> expected to be a substantial wide band noise signal....we didnt hear one! We
> are now confused about what the effect is. The signal could not be
> "screened" by the wires which are about 3 feet apart, but they definite
> provide a cone of interference directly under the run. The experiment was
> later repeated with two further different GPS receivers and produced the
> same result.
> 
> Has anyone seen this before? have you any idea of what level noise we should
> be looking for? I believe this is a wide signal so maybe an AM receiver is
> not the best choice The area is a rural, horticultural area (called market
> garden in the UK) We are obviouslt concerned to trace any noise sources in
> the vicinity of the Hydrogen line frequency at 1420MHz.
> 
> Alan G3NYK
> 
> 
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