GPS comm interference problem

[Garage Guy]

Not sure whether to post this under GPS or COMM; it is the unholy marriage of the two...

Some background is here on this thread http://www.vansairforce.com/communit...ad.php?t=36776, but basically, where I am so far:

Our KX-155 comm interferes with GPS in our RV-6. When transmitting on frequencies near 121.175 MHz, which has its 13th harmonic pretty much on the main GPS satellite carrier frequency, it takes out satellite reception both in the panel mount GX60 and the GPS 496 handheld. The effect is not subtle.

I have put a GPS notch filter in the comm antenna line from the KX-155; no change. I have replaced the 12" coax pigtail on the KX-155 with a female BNC connector, to put the notch filter directly on the back of the KX-155; no change. I have put two notch filters in series (I have a collection of these now); no change. Someone said a ELT in the airplane can be a problem; I disconnected the ELT antenna, no change. I have swapped comm antennas (the GX60 comm doesn't have this problem); no change.

I don't know, but I think at this point that it is something other than the comm antenna that is radiating the interfering energy from the KX-155. Some GPS install manual (maybe the GNC300XL... I have looked through all of these I can find) says if a notch filter doesn't help, then shield the whole cable bundle coming from the comm tray connectors.

So,my questions at this point: What is the best way to shield an existing cable bundle? Or, any other ideas?!

Thanks,

--Paul

[kentb]

So I will recount my own problem.
I was getting interference from one of my comm radios on my engine monitor. Show up when I transmitted on only one of the radios and I would see my fuel level drop fast. Swapping antenna with the radio and the problem followed the antenna. The one giving me problem was a Bob Archer wing tip comm antenna. I also noticed that my FM radio (when I was in the hanger) would get a lot of static on it when I transmitted over the wing tip.
I have replace that antenna with another bent whip on the belly and have had not problem sense.

Hope this helps.

Kent

[Garage Guy]

Kent --

Hmmm, I linked to the wrong thread in the "some background" part of my post. Fixed now! Anyway, that thread explains my antenna setup, which is: Two belly-mount Comant CI-122 bent whip antennas for the comms. The GX60 GPS antenna is on top of the turtledeck, the 496 antenna is on the glareshield.

The thing is, the KX-155, with notch filter mounted directly to the tray connector, has the problem on either comm antenna. The GX60 comm works fine, no problem, on either comm antenna. So, I don't think it is an antenna problem, or an antenna bonding problem, or a coax-to-the-antenna problem.

At least, that's my best guess at the moment. There is a problem somewhere, though.

--Paul

[Garage Guy]

The latest on our KX-155 interfering-with-GPS problem here...

I borrowed another, recently-yellow-tagged 155 and swapped it in; same problem. So, I don't think it's our radio being badly aligned or the like.

I tried wrapping the 155's cable harness in copper tape, grounded at the tray, to shield it... hard to do this very well with the existing routing of wires, and my attempt didn't help.

So... don't know what to do next.

One option, looking better all the time, is to just placard the radio to avoid the problem frequencies when GPS is in use. The frequencies that interfere are in the ranges 121.1-121.25 and 131.2-131.35. That second range is, as far as I can tell, just for ARINC, so not an issue for us. ATC does use frequencies in the first range... but this is our #2 comm, not used that much for transmit anyway... so it probably won't be an issue often, if at all.

I'd feel better if the problem were really fixed, though.

--Paul

[az_gila]

...way you can improve the ground connection from the KX-155 to the airframe. A braid possibly?

[Garage Guy]

Originally Posted by az_gila

...way you can improve the ground connection from the KX-155 to the airframe. A braid possibly?

Gil --

Well... here's one thing I'm thinking about grounds. I put a notch filter on the comm antenna output right on the back of the radio. The radio's case is a Faraday cage, whether or not it is at the same potential as the airframe, and that notch filter should look like a short circuit at the 1575.42 MHz GPS carrier frequency. Sooo... if it's radiating at that frequency, it's coming from other conductors that pierce the Faraday cage, of which there are quite a few; shielding them would help; but that is na-ga-happen, at this point...

Another thing I'm thinking is that the avionics stack grounding is good already... the avionics trays are screwed into brackets riveted both to the front panel and the subpanel... the subpanel is riveted into the airframe.

But I could be wrong about all that. Some of those bits might have been primed before riveting, etc. And running a braid is easy enough to try. If I do that, should I run it to the firewall, or somewhere else?

--Paul

[az_gila]

Originally Posted by Garage Guy

Gil --

Well... here's one thing I'm thinking about grounds. I put a notch filter on the comm antenna output right on the back of the radio. The radio's case is a Faraday cage, whether or not it is at the same potential as the airframe, and that notch filter should look like a short circuit at the 1575.42 MHz GPS carrier frequency. Sooo... if it's radiating at that frequency, it's coming from other conductors that pierce the Faraday cage, of which there are quite a few; shielding them would help; but that is na-ga-happen, at this point...

Another thing I'm thinking is that the avionics stack grounding is good already... the avionics trays are screwed into brackets riveted both to the front panel and the subpanel... the subpanel is riveted into the airframe.

But I could be wrong about all that. Some of those bits might have been primed before riveting, etc. And running a braid is easy enough to try. If I do that, should I run it to the firewall, or somewhere else?

--Paul

...about a small braid to replace the ground wire that goes to pins S and 15 (the DC returns).
I presume that goes back to a single point ground. How long is that wire?

The other thing to check is that the covers of the KX-155 are screwed down evenly with no gaps.

[Garage Guy]

Originally Posted by az_gila

...about a small braid to replace the ground wire that goes to pins S and 15 (the DC returns).
I presume that goes back to a single point ground. How long is that wire?

Yes, single point ground terminal block on the firewall, and I haven't traced them out but those DC return wires would certainly be pretty long. But those pins are connected internally to the KX-155 case, aren't they? So I don't get why the length of those wires would be an issue as much as all the other wires coming out of the tray connectors... Anyway, a strap from the case to the firewall I can try. (Getting to particular pins on the connectors is another story!)

The other thing to check is that the covers of the KX-155 are screwed down evenly with no gaps.

OK I can check that too. Though a wavelength at 1.5 GHz is what, 8 inches? Any gaps there are a pretty small fraction of that...

--Paul

[az_gila]

Originally Posted by Garage Guy

Yes, single point ground terminal block on the firewall, and I haven't traced them out but those DC return wires would certainly be pretty long. But those pins are connected internally to the KX-155 case, aren't they? So I don't get why the length of those wires would be an issue as much as all the other wires coming out of the tray connectors... Anyway, a strap from the case to the firewall I can try. (Getting to particular pins on the connectors is another story!)

OK I can check that too. Though a wavelength at 1.5 GHz is what, 8 inches? Any gaps there are a pretty small fraction of that...

--Paul

...VHF getting out, not GPS frequencies getting in....

Unfortuneately, your problem is not unique.

I found this as a part of a RCTA update document that gives the harmonics involved.... and a caution...

2. The FCC requirement in CFR 47, of -40 dBc for aircraft spurious emissions, may not protect GNSS (including GPS) navigation receivers nor Aeronautical Mobile Satellite Service (AMSS) communication receivers from harmful interference because additional attenuation is required.
Of particular concern to GNSS are VHF 12th and 13th harmonics (especially at 121.150, 121.175, 121.2, 131.2, 131.25, and 131.3 MHz). Of particular concern to AMS(R)S are the VHF 12th and 13th harmonics (for AMS(R)S systems operating in the band 1525 - 1559 MHz, especially at 118.0 - 119.9 MHz and at 127.1 - 129.9 MHz; and for AMS(R)S systems operating in the band 1610 - 1626.5 MHz, especially at 121.1 - 123.85 MHz and 134.15 - 135.55 MHz).

3. Spurious emissions from the VHF antenna or a VHF box can be a problem. Additional filtering and shielding may be required to protect navigation receivers on an aircraft. Intersystem EMC issues are beyond the scope of this document. However, if simultaneous operation is required, the methodology stated in the following equation, if applicable, may be used to determine the harmonic levels that would be acceptable for a given set of conditions. The conditions that correlate with a particular manufacturer?s transmitter design should be specified in the installation information for the transmitter if the manufacturer intends for the transmitter to be used on aircraft that would include such receiving systems.

Another item to check is that the GPS reciever and it's antenna are grounded as per the manufacturer's instructions, and the grounding at the VHF comm antennae.

This quote might put the power levels involved into perspective. Your transmitter is probably around 8-10 watts.

One limitation of GPS is its susceptibility to interference. GPS signals are extremely weak, with satellites transmitting at power levels which measure only -160 dBW at the receiving antenna. This compares to the amount of light that can be seen from a 25-watt bulb at a distance of 10,000 miles. As a result, the GPS signals can be affected by both intentional and unintentional sources .

[Garage Guy]

Originally Posted by az_gila

...VHF getting out, not GPS frequencies getting in....

But, Gil, the VHF energy itself is not a problem. I know I have tons of VHF radiating (hopefully, mostly from the comm antenna ). It's those dang 12th and 13th harmonics that I need to get rid of.

Thinking it through... the KX-155 amplifies whatever VHF fundamental I want to transmit on, and because no amplifier is perfectly linear, generates harmonics too. If they stay inside the KX-155 case, there's no problem. But of course the whole signal, harmonics and all, is sent out over the comm antenna jack on the back of the case. So, I put a notch filter there to filter out the 1.5GHz ones. But I still get them .

So... they are either (a) getting out of the KX-155 case via some other route or (b) being generated outside the KX-155 somewhere.

For (a), I suspect the unshielded wiring harness, but as a practical matter that's not going to change, short of a complete panel upgrade, at which point the 155 probably gets replaced anyway...

For (b)... Well, there's no active amplification of the VHF comm signal after it leaves the 155. You need some nonlinearity to generate harmonics... where could that be happening?

Another item to check is that the GPS reciever and it's antenna are grounded as per the manufacturer's instructions, and the grounding at the VHF comm antennae.

The A-33 antenna installation manual doesn't say anything in particular about grounding the antenna (lots about using silicone to weatherproof it though), and anyway, the problem happens with a handheld GPS too... so I don't think it's a problem with the panel-mount GPS installation.

But comm antenna bonding... OK, yes, could be... Our aluminum Comant antenna bases are grounded through stainless screws, to steel nutplates, riveted to aluminum doubler plates. (This is one of the factory recommended methods...) I've checked the DC resistance there and the ground is good but those dissimilar metal interfaces could be acting like a rectifier at VHF, and so generating harmonics. But what makes me think that is not the problem, is that our 2 comm antennas are both mounted the same way, and switching the antennas between comms shows the problem goes with the comm, not the antenna.

I guess one way to distinguish between (a) and (b) is to put a shielded 50 ohm dummy load on the KX-155. If it still wipes out the GPS, the problem is (a), else (b). Maybe I'll do that and see what happens. Or, just on principle, go ahead and remove some paint, apply some alumiprep and alodine, and get direct bonding on the comm antennas; couldn't hurt anything, except the paint ...

--Paul