[time-nuts] TBolt interference

John Green wpxs472 at gmail.com
Fri Apr 24 12:27:49 EDT 2015

In reading the posts regarding a TBolt being interfered with by a close by
GPS antenna/preamp/receiver, I don't have experience with Tbolts and
interference but I do have some with the Z3801. I had my Z3801 running for
a couple of years here at work. The antenna was a marine antenna mounted
about a foot off a metal roof maybe 30 feet away and 20 feet up. Lousy
location, I know. I got interested in how most cellphones now have GPS
built in, even if they aren't user accessable. I was interested in building
a micro power GPS jammer to attach to my own phone to keep it from sending
GPS coordinates. Don't ask me why. I have no interest in interfering with
anyone else's GPS, just mine. Anyway, while looking at the complexity of
doing it, I noticed there was a GPS jammer available from a certain Chinese
based company. I ordered one. Seems like it was less than $20. When It came
in, I examined the innards. A small VCO and some form of sawtooth generator
feeding a single stage MMIC amp to a small rubber ducky antenna. I was
standing in the lab intending to look at the spectrum on a spectrum
analyzer. I pushed the "on" button and the Z3801 immediately went into
holdover. Wow, I thought. Doesn't take much for that. I had a Garmin GPS in
my car, so I went out, turned it on, let it find satellites, hit that on
button, and nothing. Huh? I held the rubber ducky from the jammer up
against the Garmin and it kept operating perfectly. Go figure. I tried to
see if I had any results with my cellphone, but since I had no direct
access to the GPS, I could never tell. The jammer is in a box in my storage
building and has never been turned on since. So, some GPS receivers are
very prone to interference and others, not so much. I do know the military
PLUGR is virtually immune.

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