[time-nuts] GPS Antenna Feed Line Decision
kb8tq at n1k.org
Sat Sep 2 16:26:53 EDT 2017
A lot depends on what comes after the feed cable. The “disto amp” will determine
a lot. You likely need 10 db of net gain in front of it to keep things running ok. For an
antenna that is *really* 26 db (as opposed to 26 db +/- 6 db), that would come out to
16 db of feed line loss. This isn’t a terribly surprising outcome. The antennas are designed
for installations that run 150’ or more of coax ….
> On Sep 2, 2017, at 2:57 PM, Clay Autery <cautery at montac.com> wrote:
> Having decision-making problems for the materials for my GPS main
> feedline. Going to use a TM LMR stock, just can't decide how big to go
> with it...
> 26 dB 5vdc antenna on top of a 38 foot mast. Feed will come down the
> inside/center of mast and exit near the bottom, thence routed through a
> window and to the GPS distro amp. Antenna will feed GPSDO, NTP Server,
> Blitzortung System Blue station, and one other device TBD.
> Just cannot decide how big to go with the antenna to distro amp feed...
> Assuming 50 feet total (38' mast + 12 feet to amp in shack) @ 1800 MHz
> (closest to 1725 MHz), here are the losses from just this piece
> (ignoring the amp to device jumpers):
> -240 = 5.45 dB XXX - too much loss?
> -400 = 2.85 dB
> -500 = 2.30 dB XXX - too hard to find
> -600 = 1.85 dB
> -900 = 1.25 dB
> Money not necessarily a consideration as this is a short run for a
> permanent installation. Don't anticipate ever moving the GPS antenna to
> the tower.
> For 900 and likely 600, likely would not be able to do it in one piece
> as routing it out of the mast and into the shack would get complicated.
> Would likely bring it out of the mast at the bottom with a right angle
> connector, and then use a smaller diameter jumper for the last 12 feet.
> 500 is pretty uncommon stock wise and it and connectors are harder to find.
> I already have the tooling for both 240 and 400... but I definitely
> don't want to challenge ANY of the devices for signal gain.
> So it mostly boils down to easy vs. more effort ($$ aside).... Is it
> worth the additional trouble to move from -400 to -600 or -900? To NOT
> lose the 1-1.6 dB additional?
> I'd appreciate your recommendation and reasoning. Thanks in advance!
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
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