[time-nuts] GPS antenna installation problem

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Mar 1 16:23:40 EST 2009


Bill

The coefficient of thermal expansion of solid PE is around 12x that of
copper (17x that of steel), so its seems unlikely that the the inner
conductor would contract faster than the surrounding solid polytethylene
dielectric.

However this only applies when the polyethylene and the inner conductor
are at the same temperature.

Perhaps thermal hysteresis of the dielectric during temperature cycling
fractured the inner conductor?

Foam dielectric cables should have lower stress due to temperature
changes, but may need better support.

Bruce

Bill Hawkins wrote:
> Group,
>
> My GPS time system consists of two Z3801A receivers with two HP cone
> antennas.
> I built a mast from plastic pipe (6" base to 2" arms) that is about 16
> feet tall.
> The antennas are 4 feet apart, each 2' from the center of the mast. The
> mast
> rises from a deck and is fastened 8' up at the roof line. The mast sits
> on a
> 3/4" sheet of high density marine plastic fastened to the deck with
> stainless
> hinges so that the antenna can be folded away from the house and brought
> to the
> deck railing. The mast was put up in 2003.
>
> The antenna cables are each half of a 100' coil of RG-8U. Each cable is
> about
> half in the house and half outdoors. N connectors are soldered to both
> ends, so
> no adapters are used. The cables leave the house through a waterproof
> boat deck
> fitting and travel about 5' under the deck to the mast. There's a
> service loop
> to allow the mast to be lowered, then the cables rise unsupported
> through the
> mast pipe and branch out to the antennas, which support the weight of
> the cables.
>
> Oh, and the location is Minneapolis, MN, USA.
>
> The problem is loss of signal during cold weather. Last winter (07-08) I
> lost
> the signal from one antenna during a cold snap, but it came back a week
> or so
> later when it warmed up outside. This winter, I lost The North antenna
> on Nov
> 22 when the low was 15, and it didn't come back. Then the South antenna
> went
> away on Dec 24 with a -17 low and didn't come back. I'm still running on
> holdover.
>
> I suspect that it's not a good idea to hang 20 feet of RG-8 from an N
> connector
> without some kind of strain relief, but I don't know why that would be a
> cold
> weather effect. Perhaps the center conductor shrinks more than cable and
> its
> braid, and pulls the center pin out. There's definitely an open circuit,
> looking
> at the receiver end of the cable. There's no alarm from the Z3801.
>
> Any thoughts, comments or ideas?
>
> Bill Hawkins
>
>
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