[time-nuts] TBolt interference

Bob Camp kb8tq at n1k.org
Fri Apr 24 18:03:15 EDT 2015


I believe that you will find that all GPS devices are pretty easy to jam when you
are right on top of the receiver. . The signals are so far down in the mud that it 
does not take a whole lot to mess up tracking them.  Your little gizmo is likely 
putting out a signal that is massively stronger than the GPS at the ranges you tried. 

Yes, some devices have front end filtering and others do not. That helps with 
off channel (like cell phone) “jammers”. Some GPSDO’s have receivers in them with
added filtering to allow better performance in a cell tower. 


> On Apr 24, 2015, at 12:27 PM, John Green <wpxs472 at gmail.com> wrote:
> 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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