ELT Interference with 430W

[stevengrasley]

I recently had a call from the FAA because they had noticed some anomalies in my ADS-B signals. After I put my heart back in my chest from being in my throat it turns out the FAA guys were great. I guess I'm part of the learning curve on ADS-B system implementation. After a little work suffice it to say that there are a lot of software versions you need to make sure are correct to insure everything is going to work properly between EFIS and GPS and transponder. I have a full Dynon SkyView system with the 430W as the certified GPS source. After a bit of work I think the issues for 2020 compliant ADS-B performance are just about resolved.

However, since I was digging into things I thought I should do some checks on the issues I have read about for 121.5 ELT interference. I have an ACK ELT-04 which is both 121.5/406. The ELT antenna is in front of the vertical stab and the GPS antenna is just aft of the slider when it is all the way open. There is close to 3' of separation max. Suffice it to say that when the mic is keyed on any freq 121.25-121.75 its like the GPS signals are just shut off! Bam! Below 121 and above 122 and there are no problems. So, although I was told all the new modern combi ELTs don't create this problem, that is not true. So, what to do about it? Any suggestions?

[RVDan]

Where is the com antenna located?

[sailvi767]

I am a little confused on this. The ELT should not normally be transmitting. When you key the mike you're transmitting on comm. Does the ELT share the antenna with the comm radio?

George

[PaigeHoffart]

First make sure it's the ELT causing your problem (remove it, and see if your comm takes out the GPS).

If it is the problem, you could try a notch filter, or re-locate the antenna(s) (ELT / GPS).

See this thread
http://www.vansairforce.com/communit...ad.php?t=37374

Paige

[krw5927]

This is very interesting. I also have the E04 ELT with the antenna near the VS and a com antenna just behind the slider. I'll have to try transmitting on 121.25-121.75 as you say and note the results.

One factor might be where your GPS antenna for the 430 is located. Is it on the same stretch of skin behind the canopy near the transmitting com antenna? Or did you do like a lot of builders and put it under the cowl?

[airguy]

The ELT antenna is a natural resonator at 121.5 frequency - so if a comm antenna anywhere else on the aircraft is transmitting near that frequency, the ELT antenna will "ring" like a bell (electronically speaking, it will resonate at its resonant frequency of 121.5 after being excited by the other transmitter) even though the ELT itself is not actively transmitting. That would normally not be a problem, except for the proximity of the ELT antenna and the GPS antenna in this case. The ELT antenna "rings" loudly enough to blank out (RF white noise) the very weak GPS signals from the satellites. The GPS signal is still there, it's just way too weak to pick up through all the other RF noise, like trying to have a quiet conversation in a loud bar.

The only permanent solution would be relocate one of the antennae to increase the separation.

[stevengrasley]

Glad to see the chatter here. I spent a good chunk of yesterday getting much deeper into this (spoken to Garmin, Dynon, ACK, a couple of avionics shops, etc) and have a lot to share. But work schedule has me slammed until later...I'll be back with a post on what I have learned so far, some comments on previous posts above, and also let you know about some further testing I am doing. Clearly I am not alone.

[stevengrasley]

Airguy, you are perfect in your explanation of the "ring". But it's not really as much of a white noise problem as it is a harmonic. The L1 frequency for GPS is nearly a perfect multiple of 121.5. As such, the "ring" has a harmonic just above 1 GHz at or near the L1 where it wreaks havoc with GPS receiver limiter circuits because the ring is many, many times "louder" than the GPS signals. My com antennae are on the belly between the gear. The GPS is up top just behind the slider full aft position. The GPS antenna is near the ELT whip. Getting further away is not the best option. Notch or low pass filters on the ELT antenna are an option. But other causes need to be eliminated and I'm going to run some tests this weekend to learn more to share.

[stevengrasley]

I also will share what I learned from all the vendors I spoke to. Very interesting...
But work is killing me today and all this airplane stuff is much more important, right?

[flightlogic]

Greg is correct. Not important whether you call it white, brown or harmonic.