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Re: [Paparazzi-devel] Interference w/ GPS

From: address@hidden
Subject: Re: [Paparazzi-devel] Interference w/ GPS
Date: Wed, 4 Jun 2008 18:58:19 GMT

Those transmitters work well, and I believe they are made by Lawmate. I have 
used them in many applications but they do continually revise the hardware so 
you may likely have a different version.

I also have one of those antennas and I tried it a couple of years ago.  I 
don't recall exactly, but I may have concluded it to cause interference.  Or 
maybe it was just too heavy.  I don't remember.

I have used many of the 500mW and 1W transmitters within 30cm of the GPS, using 
the Tiny as the power source, and typically see around 3dB loss in satellite 
signal strength.  But I have found that the antenna is critical.  I have never 
been successful in making my own antenna (they work great but kill 
GPS/IR/RC/etc.) and I have bought many bad antennas over the years.  The cheap 
included whips usually work pretty well.
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These are the ones we currently use ... Now is that the lawmate video transmitter you speak of. I am not sure since it does not list a manufacture

On Tue, Jun 3, 2008 at 5:38 PM, address@hidden <address@hidden> wrote:
The lo pro style antenna should be ideal for preventing GPS interference, unless of course it is circularly polarized in the opposite direction.

If your TX is radiating, much of that radiation will use the wiring as antennas so you would need to use pass-thru EMI caps on each wire tied to your shielding at exactly the point of entry.

Many of us have had very good results with the Lawmate video transmitters sold by Blackwidow, Rangevideo, etc.
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We use patch antennas for the video on the bottom of the plane similar to the lo pro antennas. We did some testing and did conclude that it was the tx itself and not the antenna. Also we use a separate battery for the video.

On Tue, Jun 3, 2008 at 3:54 PM, address@hidden <address@hidden> wrote:
I've never noticed any EMI emissions from a video TX but certainly the RF output has a tremendous effect on the GPS and everything else.  You could temporarily connect a high gain antenna to your TX and point it away from the GPS to verify that the interference is indeed from the TX itself and not the RF output.

If the problem is indeed RF, make sure that your TX antennas are as far below the GPS ground plane as possible and try using a ground plane antenna on your TX (to reduce upper hemisphere transmissions) and/or increase the size of your GPS ground plane to reduce sensitivity on the lower hemisphere.

You may be getting ground reference interference as well that may require a separate, isolated power source for your video system.

Lastly, the analog video wire can emit HF noise - try coax, being careful of ground loops.  Other wires can be looped through ferrite rings to suppress emissions.

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So we are using anywhere between 1 and 3 2.4 GHz video transmitters on our UAV's and have been getting a lot of interference. The problem is that the transmitter itself is leaking RF and interfering with the gps not the antenna (or at least i think so). Does anyone have any suggestions for how to stop this interference. Also does anyone know any high quality small transmitters for any frequency's that don't have as much signal leakage?

We have tried wrapping the transmitters in grounded aluminum foil with little to no success as well

Also does anyone know if you can buy a single transmitter that can broadcast on multiple channels simultaneously?

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