The discussion got me to thinking about how we used to filter out nearby
interference on amateur TV - namely by using interdigital filters. This
led to a search for GPS interdigital filters which i did indeed find.
See Alison Microwave website at
<http://www.amlant.co.uk/DetailsAD430.htm> for one example of an
integrated antenna/filter/preamp. (I'm sure these aren't cheap, but I
haven't asked.) As for retrofitting, you could add a filter after the
antenna/amplifier assembly but I might be concerned that the amplified
GPS antenna is pretty wide and may have trouble with a Lightsquared
transmitter nearby. There are passive antennas though, and there are
in-line amps - you'd need to add the filter in between. We made these
ourselves for 439 and 1296 MHz - GPS L1 isn't much above that so with
some care it should be doable. The tuning can be finicky though :\
Ziggy
On 03/04/2011 03:31 PM, John Ackermann N8UR wrote:
Here's a measurement we did a few years ago on the HP 58535A:
http://www.febo.com/pages/hp_gps_splitter/port_1_hp_58535a_two_port_amp.png
John
----
On 3/4/2011 1:31 PM, Bob Camp wrote:
Hi
Ok, now it's pretty obvious the RF world near your GPS will be
changing a *lot* in the near future. Lightsquared and a bunch of
similar outfits will be camping out right next door with very high
power gear. They will be running 1.5KW from somewhere in town. GPS is
running 30 watts from off planet.
Has anybody tossed the various HP / Symmetricom GPS splitters on a
network analyzer? If so, what do the filters in them look like?
I probably should corner the market on these things before asking a
question like that.
The new neighbors will be at 1525 to 1559 MHz. GPS L1 is at 1575.42
MHz. That's what we are using for timing. L2 is down at 1227.5,
right now it's mainly military use. Obviously these guys are a bigger
deal for civilians than the military.
So the question is - do the built in splitter filters have any real
rejection 15 to 50 MHz off of center?
Probably worth checking. It would be a pleasant surprise if they
turned out to be useful.
Bob
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