[time-nuts] Trimble Thunderbolt - Receive Sensistivity

Brian Kirby kirbybq at bellsouth.net
Fri Aug 29 01:37:12 UTC 2008


My problem is I am using a Motorola antenna with 24 db of gain, looking 
at Trimble they expect 35 db gain.

I am in an apartment and the management let me put up a GPS antenna on 
the edge of the roof.  I came down with 20 feet of RG6 and then jumped 
into RG59 coax to run inside the apartment.  I "guesstimate" there is 40 
to 60 feet in the RG59 run.

I added a cheap DSS amplifier at the GPS splitters and its improved.  I 
am going to look into moving the amp to where the coax comes into the 
building , the RG6/RG59 junction - but I will need to figure out a bias 
tee scheme to feed 12 volts to the DSS amp, and then I need to take it 
out and bias the antenna at +5 volts.

Brian - KD4FM

Mark Sims wrote:
> The Thunderbolt does not seem to be the most sensitive receiver around,  but it does not seem particularly bad.  Sensitivity in a time receiver can be a bad thing...  more sensitivity tends to make it more susceptible to multipath, etc.  These things were meant to be mounted on cell towers, etc where they have a pretty clear view of the sky.  Once you have that,  you don't need high sensitivity.  Also their proximity to high power RF transmitters makes overly sensitive front ends a problem.
> 
> On the subject of GPS amplifiers...  adding external amplification to a GPS may not improve its performance and can actually degrade it.  All amplifiers amplify signal and noise,  plus distort everything in the process. (fundamental laws of the universe: 1) You can't get something for nothing,  2) You can't break even,  3) You'll die trying)   Ideally,  you want the amplifier at  the antenna and you want the amplification to match the cable loss and no more.  Generally GPS receivers with poor sensitivity have poorly designed front ends or signal processing.  Yelling in their ears won't make them work any better.
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