[time-nuts] Outdoor GPS Antenna Selection

Martin VE3OAT ve3oat at storm.ca
Mon Jan 8 16:54:12 EST 2018

Hi, Duane,

I am actually using a small commercial GPS antenna with the small 
(lossy!!) coax and getting good results.

It is either a Garmin or Gilsson "puck" style antenna with 15 feet of 
that terrible coax, mounted on a 3 foot wooden pole at the peak of the 
roof.  The antenna sits on a cast-off 10 inch aluminum pizza plate 
(turned upside down for drainage) for ground-plane, covered by a large 
plastic funnel to keep off some of the snow.  The coax is just long 
enough to reach my Thunderbolt receiver in the room below the peak. 
Lady Heather shows that there are almost always at least 7 birds in 
view at any time and the setup seems to work fine.

You can engineer your solution for optimum results but I would guess 
that, as long as you are above a certain minimum level of signal for 
the receiver, you will get reasonably good performance.

... Martin    VE3OAT

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Duane Wheaton wrote :

I'm selecting an outdoors antenna for a Jupiter Pico T Timing GPS 
(TU36-D400-020) but don't clearly understand what "dB" rating of 
amplified antenna to use. From web references, it looks like the 
degree of amplification required is dependent on 1.) The length of run 
of coax lead-in, combined with the characteristic loss of the type of 
coax used 2.) The amount of amplification needed to increase the very 
weak GPS signal to the range the receiver requires it. If I'm running 
20 feet of RG-58, would a 26dB antenna be sufficient?

The datasheet for the receiver family states, "1575.42 MHz at a level 
between –115 dBm and –133 dBm into a 50 Ω impedance." So the receiver 
needs at max a -115 dBm signal. The typical received signal power from 
a GPS satellite is −127.5 dBm. So it looks like I need to amplify the 
RF signal for the GPS receiver requirements + compensate for the high 
loss of the GHz signal traveling through ordinary RG-58 coax. Am I on 
track, here? Recommendations?

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