This one is a real hum-dinger!

[blueflyer]

I have an AvMap GeoPilot II Plus mounted on the front windscreen of my airplane (on the vertical bar). I use a sportys SP-200 handheld radio for comm. When I switch my handheld comm to one certain frequency (119.900) ....my AvMap goes haywire and loses its GPS reception. As soon as I switch frequencies away from 119.900....the GPS magically regains its GPS fix and works wonderfully. It does this everytime, without exception.

How in the world can this be solved? I called AvMap who said "hmmm, we dont know."

[Jamie]

That's a weird issue. Did you try Google? Looks like a lot of people have reported the same frequency on different radios causing GPS interference issues.

See here.

[blueflyer]

Is there a way to "shield" the GPS unit? For example, build an air gizmo out of a material that deflects radio frequencies but still allows GPS signals to penetrate it? Im just thinking out loud on that....

[fl-mike]

most likely the 199.900 MHz is a harmonic of something in the GPS. Might be the processor clock or bus, might be a GPS IF.

Does it do it when the GPS is operating off batteries?

If it only does it when you are DC powering the GPS via the lighter socket, I'd try a clamp-on ferrite on the power cable near the GPS.

[Don Jones]

IIRC Most if not all panel mount GPS's have a notch filter installed to prevent interference from comm radio's.
I know my GNC300XL has one installed.

Edit: I just checked the GNS430 install manual and there is a section on harmonic interference with comm radios and gives a list of things to do to eliminate these issues including installing a 1.57542 Ghz notch filter in the comm antenna cable as close to the radio as possible.

[Walt]

This would likely fix your problem of spurious emiissions from the handheld...
(copied from spruce website):

TED 4-70 GPS NOTCH FILTER



Your global positioning system receives its positional information at 1575.42 megahertz. The communications radios on-board your aircraft typically broadcast in the 121.5 megahertz range. That is a wide separation in frequency bandwidth but it's not the complete picture. Your comm's are also producing harmonics which can produce significant noise at the GPS frequency. Now you can filter that signal and avoid potentially hazardous situations.

The TED 4-70 notch filter is a simple low cost way to clean up the receivers frequency environment. The TED 4-70 notch filter installs quickly and easily via standard BNC bayonet couplings, in-line between radios and antenna. The 4-70 rejects in excess of 50dB of 1575.42 signal at the source. Each filter is individually tuned and checked to assure maximum performance. Of course, the 4-70 also passes strenuous environmental criteria so important in aircraft applications. Major GPS manufacturers recognize this problem and recommend using the
4-70 on new and retrofit installation.

http://www.aircraftspruce.com/catalo...s/tednotch.php

[blueflyer]

I appreciate the speedy replies. I must admit, I was unaware there was such a thing as a notch filter and/or its purpose.

[blueflyer]

I went out and purchased the notch filter.....and it doesn't solve my problem. Same thing happens. As soon as I switch to 119.90 my GPS loses its GPS fix and will not reaquire until I switch frequencies.

I have an AvMap GeoPilot II Plus. I tried calling them about it but they told me they were too busy to deal with it because they were about to release a new product. I appreciate their honesty but they didn't do anything to keep me as a customer. I plan to call them again to give them a second chance at customer service.

[scsmith]

I have a GPS flight data recorder in my sailplane that used to loose fix when I transmitted on certain frequencies. The fix for me was to put ceramic choke ferules on all the power leads and the GPS data cable to the remote display.
Something like that might help?

[Doug]

Which brand hand held are you using?
Doug