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stevengrasley

Member
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?
 
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
 
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?
 
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.
 
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Substantial discoveries

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. :eek:
 
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.
 
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?
 
In signal processing, white noise is a random signal with a constant power spectral density. Harmonics are specific resonances at specific frequencies. So there is a big difference if you want to get real technical.

The thread Paige mentions is a good one. In that thread is the following:

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 .
Click to expand...

I was a radar product line manager at one of the big avionics companies for many years and have BSEE & MSEE so I wanted to get more deeply into this. After all, with my electrical engineer's attention to my panel, wiring & installation detail why did I run into this issue?

I spoke to the head technical guy at ACK. He told me flatly this has not been a problem with newer ELT's. He has not gotten any reports of this for a long time and as such it has not been a real concern for him. He felt the general guideline of keeping antennae 2' apart in all cases should be sufficient. My ELT and GPS antenna are about 2' apart but I'm having the problem.

He said the solution in the old days was to install a low pass filter with an upper cut off frequency around 1 GHz so that the 406 would still pass. I think he said I could find one from Micro Circuits. Before I do this I will make sure that the ELT antenna ring is the real problem. So I'm going to be doing some tests today or tomorrow (if I don't have too long of a Honey Do list...)

I spoke to the deep technical support guys at Garmin that handle the 430/530. (Try to get to these guys!) Anyway, they agreed this could be a real problem. I asked them why my SkyView GPS seemed to be immune even though it is located even closer to the ELT antenna than the Garmin? They of course do not know how the SkyView system works. But the harmonic from the 121.5 ELT is a known problem. Their primary recommendation is to move the ELT as far as possible from the GPS antenna. There options for me in this regard but I really hope I can find another solution like the low pass filter previously mentioned.

I also spoke to the great and very technically capable support guys at Dynon. They are aware of this issue but feel their GPS is essentially immune because of the way the system processes. I would like to understand more about what they are doing because in band noise is a problem no matter what. But there are some processing techniques that can help.

I'll post what I discover with my testing.

Oh...one last thing...all the companies were keen to highlight that there are no requirements in any TSO's to test for this interference problem. And they all agree that the spurious signal requirements that are tested for do not eliminate the problem. Interesting eh? In this case all the items that I am having problems with are certified. Go figure.
 
GPS interference testing

I disconnected the whip from the ELT and tested transmitting with each radio transmitting in the suspect band of 121.25-121.75. The challenge I had is that it was very easy to disconnect the coax from the whip, but the coax run is zip tied along the bulkhead just below the whip so the coax was dangling just below the whip. I knew the coax and the ELT where therefore still likely to "ring" a little. But my coax installation is really hard to get at in the aft of the fuse. I can reach the base of the whip by simply removing the empenage faring and reaching through the lightening hole.

The GPS signals were just slightly impacted throughout the subject band. But the GPS was still functional except when right on 121.5. At the center freq the GPS signals appear like they were shut off on the GPS. The signal impact was more pronounced on the GPS signal page the closer the freq was to 121.5. But the GPS did not lose lock unless the freq was 121.5. So, the "ring" and associated harmonics appear to be the problem. I reconnected the antenna and all freqs in the band 121.25-121.75 basically shut the GPS down.

A little research on minicircuits.com turned up a BNC in-line lowpass filter BPL-550+ with a passband DC-520 MHz and >40 dB stop band 920 MHz+. This should really knock down the problem harmonics but keep the ELT freqs clear. The BNC configuration should make the installation a snap. Mini-circuits also sells similar configuration bandpass filters that are an option. But VSWR and other considerations will likely make a lowpass filter the better choice.

I'll drop the filter in and post what the results are after I test the installation. Hoping this solves the problem. In the meantime I'm placarding the GPS for the 121.0-122.0 VHF band so I don't forget about this possible issue if I'm in the soup.
 
Solution

Seems like everyone went cold on this. In case some of you are interested I installed the filter noted in an earlier post. The band of problems was 121.25-121.75. After installing the filter the GPS signal strength is still impacted in this band, but the signal impact is dramatically reduced. Even at the center freq of 121.5 the GPS will maintain lock and stay in 3D DIFF. So this solution is a viable one for those that care to try it. Of course, use only a straight 406 MHz ELT and you have no problems at all.
 
stevengrasley said:
Seems like everyone went cold on this. In case some of you are interested I installed the filter noted in an earlier post. The band of problems was 121.25-121.75. After installing the filter the GPS signal strength is still impacted in this band, but the signal impact is dramatically reduced. Even at the center freq of 121.5 the GPS will maintain lock and stay in 3D DIFF. So this solution is a viable one for those that care to try it. Of course, use only a straight 406 MHz ELT and you have no problems at all.
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Most (all?) of the 406 ELTs also broadcast on the old 121.5 frequency.

From one of the manufacurers web pages -

"...TSO C-126-approved ELTs will also transmit on 121.5 MHz frequency for homing purposes only as the satellites alerting of the 121.5 MHz distress signal have been discontinued and replaced by the 406 MHz-detectable satellites..."
 
stevengrasley said:
Seems like everyone went cold on this. In case some of you are interested I installed the filter noted in an earlier post. The band of problems was 121.25-121.75. After installing the filter the GPS signal strength is still impacted in this band, but the signal impact is dramatically reduced. Even at the center freq of 121.5 the GPS will maintain lock and stay in 3D DIFF. So this solution is a viable one for those that care to try it. Of course, use only a straight 406 MHz ELT and you have no problems at all.
Click to expand...

I kinda thought it was a harmonics problem so glad to see you got it. Sounds like you just need a few more db's to completely eliminate the problem. Try two filters, one at the antenna and one at the equipment, and you should get enough additional attenuation to kill the problem.
:cool:
 
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