[time-nuts] GPS shielding by power lines?

Alan Melia alan.melia at btinternet.com
Sat Aug 9 17:15:55 EDT 2008

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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