[time-nuts] Roof Penetrations for a GPS Antenna Lead
exray at bellsouth.net
Tue Apr 1 22:59:36 EDT 2008
Use short piece ( say 3 feet ) of 1 inch conduit with a weatherhead attached
to it, it has a plastic piece on the exit portion which you can drill out to
fit your coax and seal with RTV if you wish.
Leave a rain loop as it leaves the weatherhead. Use a roof cone and roof
mastic to waterproof the outside conduit/roof interface. Drill a small pilot
hole from BELOW near a rafter. Leave a small drill in the hole so you can
find it from the outside. Then go OUTSIDE and drill the hole in your roof to
fit the conduit. Use conduit clamps to secure the conduit to the rafter.
Mount the antenna to the conduit.
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Stan, W1LE
Sent: Tuesday, April 01, 2008 9:55 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Roof Penetrations for a GPS Antenna Lead
Hello The Net:
I am also pondering adding a commercial GPS antenna on the roof at a height
to locate the bottom of the antenna to just clear the ridge line.
Choices I have addressed for my sloped and shingled roof:
1. Secure the antenna to an existing roof penetration, like a drain pipe
vent for the shack bathroom. Snake the coaxial cable down the vent pipe,
cut the drain pipe inside the attic and add a PVC tee section, allowing the
coax to come out the horizontal "tee" outlet. Seal as needed to keep
vent gasses outside.
I do not suspect the vent gasses from the septic system to be corrosive
GPS antenna rated for outside installation.
The coaxial cable can be a "poke thru" the "tee" and sealed or a
connector could be applied and sealed.
2. Use a separate vent pipe penetration fitting exclusively for the GPS
antenna and stub mast of non corrosive piping. Put the penetration near
the ridge line,
but not on the ridge line, with a stub mast, maybe 2' long.
Any roofer could do the watertight installation. Penetration flanges are
available in a wide variety of sizes, 1" thru 4" and sized for PVC DWV
(drain, waste, vent) piping.
Inside the attic I would use wooden blocks secured to the rafters to support
the stub mast from moving and to keep it centered in the rubber
I would locate the roof penetration just above where I wanted to
penetrate the shack ceiling.
Could use a plastic electrical wall plate at the ceiling penetration,
and a bulkhead RF coaxial connector.
3. I have a continuous ridge vent of fibrous material with shingle
material, applied over it.
It would be possible to sneak (snake) a cable thru this material and
still maintain weatherproof integrity. The GPS antenna could be mounted
resistant sheet metal, bent to compensate for the roof pitch.
The cable would be mounted on a direct path to the shack.
4. Right now the GPS antenna is on a ground mounted tripod,
at about the height of the top of the gutter and roof drip edge.
I use a continuous vented aluminum drip edge, so I could also poke thru
that with a coaxial cable. The GPS antenna could then be mounted
on sheet metal. The existing performance on the tripod
is good for my Trimble Thunderbolt. The ridge line is about 10' higher,
Putting the GPS antenna on the roof would give me some lightning
being within the "zone of protection" of the towers near the house.
5. Could put the GPS antenna and larger coaxial cable on the nearby
tower, mounting it on a side arm.
Minimum of 70' to the house and another 15' after the bulkhead
penetration point, into the shack.
I do not expect any significant improvement in capture performance by
having the antenna at the ridge line height vs. the gutter height.
There are nearby deciduous trees to the east thru north and NW.
Stan, W1LE Cape Cod, FN41sr
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