blueflyer

blueflyer

Well Known Member
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."
 
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.
 
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....
 
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.
 
Filter

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

TED 4-70 GPS NOTCH FILTER

notch_filter.JPG


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/catalog/avpages/tednotch.php
 
educated

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

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.
 
Simmilar experience

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?
 
Im using a Sportys SP-200. I just bought a choke, so I hope to get out to the airport and try it in the next few days.

I called AvMap again. Although they didnt have an answer, the service was good.
 
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How far away from the GPS does the radio have to be before the interference stops? Does removing the antenna from the handheld (if possible) cause this distance to decrease? Are you running the handheld on ship's power? If so, try running it on batteries to see if the interference is coming from the power connection.

Do other handhelds cause the same problem? Using a different brand of handheld might be an easy cure, if not exactly the cheapest.

If you find that you don't need much distance to stop this interference it might just be a matter of relocating the GPS antenna (or feeding a belly antenna with the handheld). Sometimes there's enough leakage from the local oscillator, CPU or other innards in a handheld that, while the radiated energy is small, can cause interference to other receivers because of close proximity. Moving them even slightly apart can resolve the problem. Otherwise you might have to find someone that can do a spectral analysis to see what frequencies the handheld is radiating.

Mark Olson RV-7A F1-EVO AA0MH
 
radio interference

When I first flew my 6A everytime you would key the radio (apollo sl60) at 122.8 it would set off my ELT. Rather annoying. They put some of those ceramic magnets on the cable and it cleared the problem up. Don't know about the GPS, never had that problem.
 
Allan Stern said:
When I first flew my 6A everytime you would key the radio (apollo sl60) at 122.8 it would set off my ELT. ...
Click to expand...

Wow, what brand ELT was that? I'd like to see what DO-160 radiated susceptibility category they tested it to. Completely unacceptable.

Blueflyer: Put the choke on the GPS power cable. PM if you have questions.
 
Choke is on

I have the choke in place on the GPS power cable now. Now, I just need to get out there to test it. I'll report back with the results. Thanks for all the help so far.

fl-mike said:
Wow, what brand ELT was that? I'd like to see what DO-160 radiated susceptibility category they tested it to. Completely unacceptable.

Blueflyer: Put the choke on the GPS power cable. PM if you have questions.
Click to expand...
 
Update: I flew today with the choke on the power to the GPS and the notch filter on the SP-200. I had to same results....the GPS lost its fix as soon as I went to 119.90. What a bummer. I may concede and just buy another brand handheld.
 
Does it interfere with the GPS even when you are not transmitting??
If it interferes while listening, it might be the IF in your handheld. Different brands of radios use different IF's. If this is the case try disconnecting the antenna (but don't press the PTT while is it disconnected) and see if this clears the problem. If this fixes it, you will need to get the comm antenna away from the GPS unit.

Are you using a external antenna or the rubber duck type antenna??
If you are not using an external antenna, this may be your problem. Running an properly installed external antenna can help considerably with RFI issues.
 
Yep, it interferes with the GPS as soon as I switch to 119.90. No matter whether transmitting or just listening on the frequency. I have the handheld comm running on batteries, with the comm tied to an external belly antenna.

You have given me another idea to try. I do have a second comm antenna on the top of the rear fuselage. I will connect the comm radio into the rear fuselage antenna to see if that makes a difference.
 
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