Radio oddity

[drauk]

Here's the scenario:

Box 1, a GNS530, tuned to Elstree, 122.400.
Box 2, a GNS430, tuned to D&D, 121.500.
Audio panel (an oldish King model) set to Auto for box 1 (i.e. listen and talk on box 1), set to "Phones" for box 2.

Press the PTT and there is loads of fairly loud background noise. Switching off "phone" for box 2 cures it. On other frequencies on box 1, eg. Brize Zone, 119.000 it doesn't happen. 122.400 is the only one I found it on.

Any thoughts as to why this is happening?

[pumper_bob]

Is it possible the AM transmitters on finals for 10 are interfering?

[IO540]

Switch off anything that might be transmitting e.g. DME or transponder.

Switch off other avionics/electrics in turn.

How far do you need to be off 122.40 to make it disappear?

[Blinkz]

When I was flying a twin Garmin 430 setup I had a similar problem, when freqs are selected that are quite close to each other you get loads of interference. Never found out why tho sorry.

[drauk]

Being any distance from Elstree didn't seem to make any difference - it happens when I am a couple of miles away and happens when I am 100 miles away. I'll try switching off other bits of equipment tomorrow. As I say, it's only when the audio panel has box 2 set to 'phones'. This suggests that the background noise is coming FROM box 2, but it only comes when you key the mic and are tuned to certain frequencies on box 1. I will also try different frequencies on box 1 to see if it still does it, or if it is some magic associated with 122.40.

Next time someone with similar kit is able it would be great if they could try it and see if it happens to them as well.

[Keygrip]

I get the same hassle - Garmin 430 box 1, various stuff as box 2 (so it's not the aircraft, it's the radios).

119.75 on 2, 121.5 on the Garmin is the worst combination.

[IO540]

Many years ago, radios had a rotating turret full of crystals of various frequencies, and a big knob with gears behind it with which this was rotated between a set of contacts.

All modern radios use digital frequency synthesis, where you (crudely speaking) take a crystal (just one crystal), and multiply the frequency using a phase locked loop within which is a programmable counter. So you have various other frequencies floating about inside the equipment, and their harmonics.

Plus you have all kinds of other muck e.g. a switching power supply running at some tens of kHz and loads of harmonics coming off that.

I can't believe Garmin have done this badly because they sell thousands of these things, but maybe there is a specific installation procedure (earthing specific things) which has not been followed? My experience of UK avionics shops is that universally they do not understand the workings of the equipment and merely follow wiring diagrams.

I would start by setting everything up to show the interference and then, behind the panel obviously, clip split ferrite beads onto one wire at a time (close to the respective equipment enclosure) and see if the problem changes. If you see any change then you've got it - it's conducted/radiated.

[LateFinals]

It might just be there was a certain well-known individual on duty in ATC at Elstree who your avionics took exception to !!:)

[OpenCirrus619]

This might be caused by some sort of heterodyne / mixing effect and is related to the difference in frequencies - as opposed to absolute numbers.

Next time you tune to Brize on '1' try tuning '2' to 118.1 (0.9MHz less).

Just my pennies worth....

OC619

P.S. Does anyone know what the IF is for these radios?

[BigAl's]

...455kHz...?? Can't imagine it's anything special.

Sounds like breakthrough to me... bear in mind this is AM we are dealing with. Only happens on one freq, lots of loud background noise etc.

Ferrites can be useful, just bear in mind you have to pinpoint what to stick your ferrite on (don't whack them on everything) and also to get the best results you will need to take at least one coil around the core. Also bear in that this won't work with coax cable (ie the aerial cable)... which is where it may just be coming from!.. either that or on the audio lines...:E

[S-Works]

same problem, my avionics guy tells me it is a result of bleed over from the antenna being so close to each and the frequencies being so close.

[IO540]

don't whack them on everything

Actually, you do stick them everywhere :) - one wire at a time. They don't work on coax cables but I don't think coax cables are the problem here, unless poorly earthed and even then I can't see what that frequency relationship would mean.

my avionics guy tells me it is a result of bleed over from the antenna being so close to each and the frequencies being so close.

That's a total lack of understanding of radio, if given as an explanation for the specific example given by drauk. If that was the case, having 117.0 and 117.1 on the two radios wouldn't work at all. Neither would 127.1 and 127.2 etc etc etc.

[drauk]

Some interesting discussions, but keep in mind that the problem isn't about transmitting and receiving as such, it's when I push the PTT for the radio on box 1, if and only if box 1 is (a) tuned to certain frequencies and (b) the audio panel is set such that I can listen to the output of box 2, I get a load of noise in my ear. Isn't the existence of any substantial sound a bit odd - don't audio panels in aircraft arrange for a small amount of what the mic picks up to be played through the ear piece to make it all sound and feel more natural?

[denhamflyer]

two things:-

1. Yes you do get audio feedback to the ear this is called sidetone and is important that we can hear ourselves when talking, but this is normal audio frequencies.

2. If it only occurs when PTT is active then it is possible (but I am suprised on modern sets) that a Heterodyne effect is occuring i.e. the resultant frequency is the difference between the two (or there intermediate counterparts) and that this is not being filtered out (by being in the audible spectrum). This is not normal in FM sets but with a poweful enough signal bypassing/resonating with the IF (intermediate) stages could cause a problem, early boat VHF had similar problems. The other side would be that is poor EMC immunity and you have a found a "sweetspot" in their circuits.

[Three Yellows]

IO540
That's a bit harsh on Bose, and I believe that he is correct. The two freqs do not need to be next door to each other... its more a question of harmonics and spacing of the aerials. On most light a/c their isn't enough real estate to get the aerials far enough apart. Even on my state of the art DA42 the G1000 is capable of splitting the comms, i.e allowing pilot to talk on one freq and co-pilot to talk on another, but this isn't recommended by Garmin as the aerials are not far enough apart. (Yes I know that it wasn't quite the original point here, but its the same sort of thing)

3Y

[IO540]

I was referring to the avionics bloke, not bose-x.

I agree it is likely a harmonic issue.

[Zulu Alpha]

Box 1, a GNS530, tuned to Elstree, 122.400.
Box 2, a GNS430, tuned to D&D, 121.500.
These frequencies are 900KHz apart.
Do you get the same effect if you keep the same difference but use another frequency ie try 121.600 and 122.500.
If so it is something to do with the difference frequency.
If not then I don't know

[Three Yellows]

I was referring to the avionics bloke, not bose-x.


oh, ok, sorry. I misunderstood... I thought you were beating up Bose... and I need him so that I can get an IR!

[S-Works]

:O:)

I'm honoured....