GPS / Ham radio interference

[Stephanie Koles]

Last year, I got a different car (old: 95 Pontiac Bonneville; new: 99 Pontiac Bonneville)... About that time, I noticed that my DeLorme Earthmate was not holding onto satellites very well at all -- I'd completely lose GPS every now and then (not even 2D coverage)... At first, I thought it might have been a windshield issue -- I know some windshields in some cars actually have tiny specks of metal in them, which some have said interfere with GPS signals (makes sense to me). This issue SEEMED to get better when I moved the Earthmate near the side window (vs. the windshield). The intermediate nature of the problem, however, made me question whether it really was a windshield issue. However, when chasing on 3/20, I found another link. Whenever I key up on my ham radio (I was talking on 146.55 and .525), the GPS signal would degrade and I'd eventually completely lose the signal. As long as I kept the transmission less than 4-5 seconds in length, the GPS would just go down to 2D, before slowly getting back to 3D. Unfortunately, I even moved down to low power (5W from the radio -- Yaeusu FT-8900R, though with a mag-mount, trunk-mounted Comet SB-15 antenna, which is 5/8 wave on 2m w/ 4.15db gain) and still have the problem. I didn't use this radio very much before last year, so that might explain why I didn't notice the GPS interruptions before last year.

At any rate, has anyone else ever had this problem? I'm not really sure how to go about fixing this either... I can't roof-mount the antenna, since it's already on the big size for any mag-mount, so I really need to keep it on a short leash. I suppose I may be able to make a little metal box to put the GPS device in (not so much a box, obviously, but like a box with the top off, so the GPS has a clear view of the sky, but there would be metal wall between it and the ham antenna on the trunk). Ideas? Experiences?

IIRC, I think GPS runs about 1.5gHz, so I'm a little surprised that this is an issue.
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One thing you have to remember is the GPS receiver has a RF chain that converts 1.5 GHz down to a usable IF so the CPU in the unit can demodulate the GPS signal. In the process of downconverting the GPS signal, these IF(Intermediate Frequency) sections of the receiver can pick up strong signals from other sources(Your ham radio). Not all GPS receivers are completely shielded against this and it may be unavoidable. Ideas to minimize this may be to not use a cigarette lighter adapter on the GPS receiver as the power wire acts like a pickup antenna for stray signals from your 2 meter rig. If your only using battery power on the GPS receiver, the issue may be unavoidable. You could try mounting the 2 meter antenna on the roof as a permanent installation instead of a magnet mount antenna and this might help as the coax from a mag mount may radiate a certain degree. Mounting the 2 meter antenna in the roof provides a good ground plane and will help reduce the amount of RF in the car itself. Just some observations and ideas for you

 

[Tom Burgess]

Ok I have the same gps. I have it mounted on the driver side dash door post. I use a yeasu 8100 dual bander mounted below my laptop at my knee as seen below.

Here is the low tech way to test for this problem. The first test is to try a different radio in place of yours.If it does not cause problems then your radio is most likely not the problem. If you can not find a radio to test it with try this, get some foil and place it between your gps and radio. under the gps works best or around the radio. Make sure you do not touch the ant connector while trans. :unsure: You can place the foil between the gps and antenna on the back glass . shinny side toward the antenna. My guess is rf leak from the main radio as ham equip. is not sheilded like comm. equip. I hope this helps. Only other way is to find a rf meter to check things out.

 

[Pat Palmer]

First, find out if the interference is coming through the air or the wiring. Is the computer plugged in to the vehicle's power system? If so, try unplugging the computer and run just on batteries as a test. I had that problem with my laptop using HF. Disconnecting the 'puter from the vehicle's power system did the trick, but caused some inconvienience.

If that doesn't yield results, your radio is most likely overloading the GPS reciever. Try to find a spot further from the antenna. Remember, doubling the distance from the antenna reduces the field strength of the signal to 1/4 the value (Inverse Square Law).

Lastly, (and possibly most horrifically) consider a permanent hard mount for the ham antenna. This may be an RF ground issue. Mag mounts have very poor ground, both DC and RF.