[time-nuts] GPS Antenna Feed Line Decision

Clay Autery cautery at montac.com
Sat Sep 2 16:47:42 EDT 2017


Can't say what the gain tolerances are, but it appears to be a pretty
decent quality part.

The distribution amp is an HP 58516A GPS L1 Distribution Amp with the
external power supply tap which will be fed with an ultras stable/clean
adjustable linear power supply.  As I understand it, this is a unity
gain amp, so there should be little to no insertion loss or distro losses.

The distribution amp will be mounted near the devices such that amp to
device jumpers will all be < 1 meter/3.28 feet.  Likely a lot less.  ALL
Times N-connectors until reaching a connector TM doesn't make.

Clay Autery, KY5G

On 9/2/2017 3:26 PM, Bob kb8tq wrote:
> Hi
> 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 ….
> Bob
>> 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!
>> 73,
>> -- 
>> ______________________
>> Clay Autery, KY5G
>> MONTAC Enterprises
>> (318) 518-1389

More information about the time-nuts mailing list