Please keep any replies simple as I am a microlight (Ultralight) pilot not an electrical engineer - thanks!

When using a Trig Transponder in my microlight aircraft there is a horrible click / crackle in my headset associated with the pulses from the transponder. The noise is present even when the radio is not on (i.e. just the intercom is connected between pilot and passenger)

My intercom system is Flycom and I know others with Flycom intercoms have experienced the same problem.

Have contacted the manufacturers of the intercom, Flycom. They are aware of the problem but don't have a solution.

Have tried a big aluminium "ground plane" at the point the Trig Aerial is connected to the aircraft. Have tried the ground plane with and without an earthing wire.

Have wrapped the wires to the transponder aerial in metal tape and re-routed them away from other wires.

Have tried wrapping the transponder and the intercom boxes in aluminium foil, with and without an earth wire

I also get interference to a lesser extent from flashing strobe lights. (Collision avoidance flashing lights on each wing)

I can reduce the noise by enclosing the Trig aerial with my hand - but it is very difficult to fly with one hand under the aircraft! Maybe if I take a passenger.........?

Any thoughts? Please keep them simple!

Thanks

Paul

 

Welcome to AAC!
I'm no expert on avionics, but it seems to me that if the various RF screening methods you have tried were unsuccesful, perhaps the interference breakthrough is (at least partly) via the power supply to the transponder and intercom units.
Have you tried adding extra capacitive decoupling components to the supply, or ferrite beads on the supply wires?
Do the units have a star ground arrangement?

 

Welcome to AAC!
I'm no expert on avionics, but it seems to me that if the various RF screening methods you have tried were unsuccesful, perhaps the interference breakthrough is (at least partly) via the power supply to the transponder and intercom units.
Have you tried adding extra capacitive decoupling components to the supply, or ferrite beads on the supply wires?
Do the units have a star ground arrangement?

Thanks for response.
Haven't tried ferrite beads.
Sorry, not sure what a star ground arrangement is.

 

I agree that the intercom seems to picking up the noise through the power input (since the strobe lights also cause a problem), which is why your various shielding experiments were not successful.
Where are the intercom power and ground leads connected as compared to those for the transponder?

 

I agree that the intercom seems to picking up the noise through the power input (since the strobe lights also cause a problem), which is why your various shielding experiments were not successful.
Where are the intercom power and ground leads connected as compared to those for the transponder?

Thanks for your thoughts.
The Intercom is under the pilots' seat (see attached picture) and the transponder unit is under the base tube (main tube providing the frame to the microlight) - see picture, just above the front wheel, about where the "a" is of NatWest
The aerial for the transponder is small, about 3 inches long and protrudes underneath the a/c - just below and slightly to the rear of the passengers green trainer. (Doesn't show in this picture as it is not my a/c)
They are about 1m apart. I could try moving them slightly further apart but there is not much room in a microlight aircraft.
I will need to check, but I think the power leads probably both go via the ignition switch (?) as they both become live when the ignition is switched on?
I don't recall seeing a ground lead for the intercom as it is a "plug in" box, perhaps it has integral ground? Apologies for not being too clear on this. Can check when next at the airfield.
Appreciate your input.
Paul

 

Sorry, not sure what a star ground arrangement is

In the left-hand circuit the wire sections AC and BD are shared by both the intercom and transponder devices. Heavy current use by one device can thus result in a supply voltage change to the other device, by virtue of the voltage drop across the wire's resistance.
In the right-hand circuit point B is a 'star ground' from which all ground wires radiate. Thus wire section BD (likewise AC) is not shared.

 

View attachment 182567
In the left-hand circuit the wire sections AC and BD are shared by both the intercom and transponder devices. Heavy current use by one device can thus result in a supply voltage change to the other device, by virtue of the voltage drop across the wire's resistance.
In the right-hand circuit point B is a 'star ground' from which all ground wires radiate. Thus wire section BD (likewise AC) is not shared.

Many thanks
Will need to check out our wiring.
On hols next week for 3w, but a little task for my return!
Regards
Paul

 

You may need to power the intercom directly from the battery.
You can do that with a relay energized by the ignition switch with power directly from the battery to the relay contacts, and ground directly to the battery.
A large capacitor (say 470μf) directly across the power to the intercom may also help.

 

In addition to the other good info here, sometimes a Common Mode Choke helps on the power lines when sharp impulse current causes noise.
https://en.wikipedia.org/wiki/Choke_(electronics)#Common-mode_choke

 

In addition to the other good info here, sometimes a Common Mode Choke helps on the power lines when sharp impulse current causes noise.
https://en.wikipedia.org/wiki/Choke_(electronics)#Common-mode_choke

I'm learning lots!
Thanks
Paul

 

My intercom system is Flycom and I know others with Flycom intercoms have experienced the same problem.

Do this system and the transponder both comply with the EMC Directive?

 

Do this system and the transponder both comply with the EMC Directive?

Factory fitted by aircraft manufacturer so I would hope so.

 

Hmm. For compliance, a device in normal use is supposed to be not significantly affected by conducted or radiated interference . In my book your intercom appears 'significantly affected' in normal use; but I don't know what the official limits and test conditions are.