[aprssig] Center Frequency of GPS Signal

Rich Garcia k4gps at arrl.net
Tue Nov 9 18:21:04 EST 2004

Could but I do not believe in this case. It comes and goes, if it were
overload it would stay all the time since the radio station is 24/7 with no
power changes. Last 2 problems I have had were Channel 67 3rd harmonic
taking out the GPS unit and then 92.7 FM doing the same.

In the case of 92.7 the engineer that they have on retainer (long gone days
of actually having one on staff) was a ham and I knew him. Late one night we
went to the site 100 yards away from my GPS to power down the amps, upon
arriving to the transmitter shack my GPS III lost lock, dragging a Spectrum
Analyzer in (all 55 lbs of it!) I found the box/door that holds the final
tube was to blame, the copper finger stock was not making good contact and
there went the shielding, clearing up each time I pushed on the door.

Channel 67 was another story PAX TV, took a call from very high up to shut
down their transmitters even at 00:00hrs. 3rd harmonic spike went away but
level readings were still inside of FCC guidelines so that was a fight.
Eventually moving the GPS antenna behind a 16ft ground mounted dish about
200ft away from the main tower and tilting the antenna to the opposite
horizon away from the broadcast antenna cleared the problem right up. Lots
of work but we found a fix that did not involve lawyers or the FCC!

RF Overload...would be easier to explain instead of the 15th harmonic but
with having replaced the GPS 4 times and having plenty of GPS units at other
locations with larger broadcast transmitters than this one I doubt it. Now
that I have the center freq I can bring my Spectrum Analyzer over there and
take a look at the signals. I am sure boaters at the Stuart Inlet will also
appreciate it!


-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org]On Behalf Of John
Sent: Tuesday, November 09, 2004 5:19 PM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] Center Frequency of GPS Signal

Rich, could it just be RF overload on the GPS rx

John K4AG

Rich Garcia wrote:

>Been a long time since I have had to look at the GPS signal on a SA, does
>anyone know off hand what the Center Freq is ? I kind of remember it to be
>somewhere in the 1,450 Mhz range...
>I think a local radio station has a harmonic that is a bit stronger than
>allowed and is wiping out the GPS band, I need to track that down and be
>sure before I make any phone calls. Interesting thing is that the radio
>station is literally right on the beach at a very busy local inlet next to
>marina's and such, whenever I am in the parking lot or driving past the
>station my GPS III will either lose lock or the signal strength will drop
>almost nothing on all birds that I can see.
>aprssig mailing list
>aprssig at lists.tapr.org

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