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

[Andy Talbot]

The first generation GPS were, IIRC, 50 Watts, +17dBW  EIRP.   The latest
one are higher power at up to 200W, or 23dBW EIRP
If you go through the link budget calculations, and assume a receiver with
a good noise figure, you can show that with a ground station antenna of
more than about 36dBi gain you can see the GPS signal above receiver
noise.    That gain corresponds to a dish of about 4.5 metres diameter at
1575MHz.   Bigger than one most radio amateurs are likely to fit in their
back garden, although there are a few moonbouncers around the World with
dishes that big.    Quite a few have 3m dishes.

Andy
www.g4jnt.com



On Fri, 20 Aug 2021 at 22:33, Gerhard Hoffmann <ghf at hoffmann-hochfrequenz.de>
wrote:

>
> 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-174  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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