Use existing redundant ELT antenna for PA 1090mhz reception?

[peteD]

I have a redundant external ELT(121.5 mhz) antenna hidden in the fin forward fairing and am trying to tidy up my PA installation.

Could I use this for 1090 mhz reception or would the losses be too great?  .....in which case I could cut it to the correct length, but would prefer not to if possible.

Thanks for any advice!

[exfirepro]

Hi Pete,

You don’t say what type of aircraft, not that that affects the principle.

Bearing in mind the relative strength of most 1090MHz signals, you can usually pick them up on the proverbial piece of wet string. That said, however, the trigger levels for the Bearingless (Pure Mode-C and Mode-S) Alerts depend on receiving signal at predetermined strength levels from a tuned (or at least near tuned) antenna.

Your 121.5MHz antenna is going to be off frequency by a factor of almost 9 times over-length and there is no way to predict what this is likely to do to received signal strength. Better IMHO to go for a correctly tuned internal or external antenna (depending on aircraft construction) rather than risk not receiving the designed warnings at the correct times.

Regards

Peter R

[peteD]

Thanks Peter, it’s a certified a/c, hence my reluctance to alter the length of the antenna. As it is only used in Rx mode, I was hoping it might be acceptable, but point taken about bearingless targets...although it is external as opposed to the present stock internal.

Presumably I could do a rough comparison by swapping antennas while displaying the signals and strength page of PA?

Thanks

[exfirepro]

Hi again Pete,

I understand your concerns with a certified aircraft. You can certainly try comparing the signal levels being reported for Mode S aircraft in the PAW Traffic Screen. There is no harm in testing it. I would be particularly interested in your results as I and a colleague did most of the flight testing for Lee to set up the trigger levels in the first place. In this respect screenshots of the Traffic Screen would be most useful if you can manage them safely (usually safer on the ground or with a second pilot). Testing is also easier with a well briefed colleague with a Mode-S transponder in a separate plane as a ‘known target’. If stuck, when my mate wasn’t available, I used to go hunting FlyBees as they descended towards the Edinburgh TMA, though Mode-S only CAT is becoming a rarer beast these days.

If your tests result in higher than normal strength signals (which I would think unlikely) you can adjust the trigger points by selecting a shorter setting in ‘Mode C/S Horizontal Sensitivity’ - as we would do if operating close to concentrations of high power CAT transponders. The opposite (obviously) applies in the case of weaker signals. My concern would be if the signals are too weak to reach the minimum trigger level (on the Long Range Setting) to trigger alerts before the other aircraft gets too close. Please let me know how you get on. You can contact me by PM if you need any specific details or don’t want to comment on anything on the open forum.

Best Regards

Peter

[Paul_Sengupta]

Personally I think it'll work very well, but how long will your coax run be? Coax attenuation at 1GHz is quite high!

[peteD]

Was planning to use existing installation to rear seat area, so it’s probably 1 1/2 to 2 meters
Thanks

[Paul_Sengupta]

Should be fine then.

[peteD]

Thanks all, will post results

[peteD]

Was going to post screenshots but  says I can't load  bigger than 512kbs, file is 400ish, tried 200 ish still not playing???
Anyway, as would be expected, signals not as strong, but I think usable with a tweak to sensitivity!

update:seems to have loaded, this is using ELT ant.

[peteD]

Stock PA ant(tuned), Apologies for the differing size screen shots, it was done in a hurry! and uploading seems to be a problem as above
43BF7A is a local ground station(ADSB?) and shows comparative signal strengths..i think

[exfirepro]

Hi again Pete,

Thanks for posting your results.

Yes that does seem to show a reduction in received signal strength, which shouldn’t of course have any serious effect on ‘Known Position’ targets - they should just appear on screen as normal. I would advise in the interests of safety running a longer Mode CS Range (i.e. Medium or even Long Range) until you can determine by flight testing how close Mode C or S Aircraft are getting before the visual warnings and audio alerts are triggered. CAT Mode-S (with it’s significantly higher power output) is usually a safe way to do some initial testing as it provides a reliable signal while following a predicted path and is easier to see at longer range, or try to fly a prearranged route with a colleague equipped with Mode-S (but not PAW/ADS-B). That will give you a better idea of what is a safe and practical Range setting for your setup.

Please let us know how you get on.

Best Regards

Peter

[Vic]

The only (slight) concern I would have with using the ELT antenna is, of course, that it is perfectly tuned to Airband frequencies. This means that possibly the SDR could get swamped and suffer a loss of sensitivity at times that you are transmitting on the aircraft's radio due to its proximity and strength of signal that won't be rejected by the ELT antenna.

This signal shouldn't be sufficient to damage the front end of the SDR but it might be worth repeating your tests whilst doing a long radio transmission on a permissable frequency to see if ADSB reception is degraded /corrupted in any way.

[peteD]

Good point, will check that!
Thanks for everyone's advice.

[tnowak]

PeteD,

Can your ELT antenna be removed from the ELT or from the attaching coax?
If so, why not just remove the ELT antenna and replace it with a homemade or purchased transponder antenna?

You can always restore everything back to normal at a later time.
Tony

[peteD]

Hi, the ELT has been removed(as was required a while ago) and I have the remaining antenna concealed in the fin fairing(as originally fitted) the associated coax terminating in the rear seat area where the ELT was located. It’s a certified a/c so reluctant/difficult to change things. I have considered “shortening” the existing...