[time-nuts] GPS Antenna Feed Line Decision

Wes wes at triconet.org
Sat Sep 2 19:02:26 EDT 2017

This is just a cascaded noise figure situation.

The first stage is the antenna (preamp) which has 26 dB gain (assumed) and an 
unknown noise figure. Assume it's a dB or so. Let the second stage gain be a 
negative value equal to the cable loss and the second stage noise figure be 
equal to the cable loss in dB.  Assume a lousy 10 dB NF and 10 dB gain for the 
distribution amp.

If the cable loss is 5 dB then the cascaded noise figure and gain are: 1.26 dB, 
31 dB gain.

If the cable loss is 2 dB then the cascaded noise figure and gain are: 1.13 dB, 
33 dB gain.

I would use RG-6.

See: https://www.pasternack.com/t-calculator-noise-figure.aspx

The secret here is the 26 dB gain.

Wes  N7WS

On 9/2/2017 11:57 AM, Clay Autery 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,

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