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

[Bob kb8tq]

Hi

The GPS signals are spread spectrum transmissions. If you look for them with
a normal spectrum analyzer they are 10’s of db’s below the noise floor. Yes,
that’s a bit weird, but it does work. 

Your gizmos may or may not include a preamp. First thing I’d do is take a look
at current into the device vs supply voltage over at least the 3 to 5V range. They
should pull 10ma or more. 

If they have a preamp (and it has reasonable gain), you should see the noise
from the preamp come up as the power comes on. As an example, a 20 db
preamp should give you about a 20 db “noise hump” on an analyzer that can
see down to the noise floor. 

Have fun !!!

Bob

> On Aug 20, 2021, at 4:14 PM, skipp isaham via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 
> 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?
> 
> 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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