[time-nuts] Re: Testing a GPS mag mount antenna

[Gerhard Hoffmann]

Am 20.08.21 um 22:14 schrieb skipp isaham via time-nuts:
>
> Hello to the Group,
>
> I picked a box of used (removed from commercial radio APRS type 
> service) mobile/vehicle GPS Antennas. They are mostly the classic 
> square molded, black plastic magnetic mount type, about the size of a 
> bar of soap when cut to square (2/3 the size of a large bar of soap). 
> The coax length terminates to an SMA connector.
>
> I'd like to use some of these unmarked (obviously also unbranded) 
> antennas for a few projects. The initial goal is to first set up a 
> system to test (good/bad) the antennas, then determine their operation 
> voltage, I suspect them to be 3.x to 5.x Volts. They are probably not 
> "new enough" to be the type to operate of 3 or 5 Volts DC.
>
> For testing... I purchased a nice NOS Mini-Circuits bias-Tee.
>
> The intent is to now operate the antenna through the bias-tee, in to 
> an analyzer. I would initially start the bias supply off at 3 Vdc, 
> while also monitoring current.  If I don't receive an adequate/valid 
> GPS signal off air, I could increase the bias up to 5 Vdc (rinse/repeat).
>
> Should I be able to "see something" on or around the GPS frequency 
> other than what I suspect will be something visual looking like a 
> noise/pulse source/signal?

You won't see anything interesting on the spectrum analyzer. The 
signal(s) look like noise, and they are buried in the real noise.

Deeply!  In a real receiver, there are probably just 1 bit ADCs, aka 
comparators, and the receiver  needs to know the pseudo random

polynomial that was used to blow up the bandwidth of the 50 baud message 
to some MHz in order to reverse that effect.

And you have to know which part of the polynomial is currently used. 
This is done by search & correlation tries. Sloppy wording, I know.

Only when that reversal is done you have a positive signal/noise ratio. 
GPS receivers are 95% math, the rest is electronics

and packaging.

You may see a noise molehill at the nominal frequencies, but that means 
only that the pre-amplifier and maybe some filtering works.

And there is a hefty preamplifier to make up for 5 meters of El Cheapo 
RG-174Â  coax. The GPS pseudo-noise is only

1 promille of that what you see.

Disclaimer: Last time I was involved in this was with the Plessey 
1010/2010 chip set in a previous life, for GPS/Glonass combined,

which was new then.


Gerhard, DK4XP





> I don't yet have a GPS receiver with a signal strength indicator, else 
> I could probably not have to send this post.  But, I do have access to 
> an analyzer, I bought the bias-tee (was reasonable in price) and I'd 
> like to test these 30 antennas to see if they work and determine if 
> 3.x volts is enough... or 5 volts is required.
>
> Thank you in advance for any replies and comments.
>
> cheers,
>
> skipp
>
> skipp025 at yahoo.com
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