Radio Interference

[G Miller]

I am based in the Seattle area. Ten miles or so east of KSEA is Tiger Mt, a 3000 foot peak with numerous very high power commercial transmitters. Since purchasing my 1970 C model some 15 years ago I have been experiencing significant radio interference when flying within 5 miles of Tiger Mt. I experimented with placing ferrite beads on the outside of the cosxial cables connecting the radios to the antennas and heard some reduction in interference. During the recent annual my A and P and I measured the resistance between the antennas and the fuselage skin and found them to be within spec. We decided to replace the 50 year old coaxial cables with new RG400 shielded coax. Wow! The interference is gone. For those owners out there who are having interference with high power transmitters, I suggest swapping out the old cables for new ones. You will not be sorry.

[carusoam]

Nice positive pirep, GM!

Thanks for sharing your experience...

Best regards,

-a-

[0TreeLemur]

Cool outcome @G Miller .  I'll hijack your thread with a corollary.  With the engine off, I hear a buzzing noise from the certified pulse-width modulation (PWM) panel light dimmer over the speaker in my aircraft when it is active- but not when the lights are turned full up or full down.  This noise is not loud enough to hear when the engine is running but I thought that was odd though.  When I had the headliner down last weekend to see about the dome light switch, I discovered that the wire running to the overhead speaker is a shielded single conductor wire.   They used the shield as a conductor to carry one of the speaker connections  .  Anyone else notice this?   How hard to replace that wire?   Seems like it would be hard.

[Yetti]

the shield on the speaker wire is ground.  It might be frame ground and is just there to make sure.  would have to look at the wiring diagram to be sure.  My overhead light has bare resistors and such.  I think I covered them with heat shrink.

[jaylw314]

6 hours ago, Fred₂O said:

Cool outcome @G Miller .  I'll hijack your thread with a corollary.  With the engine off, I hear a buzzing noise from the certified pulse-width modulation (PWM) panel light dimmer over the speaker in my aircraft when it is active- but not when the lights are turned full up or full down.  This noise is not loud enough to hear when the engine is running but I thought that was odd though.  When I had the headliner down last weekend to see about the dome light switch, I discovered that the wire running to the overhead speaker is a shielded single conductor wire.   They used the shield as a conductor to carry one of the speaker connections  .  Anyone else notice this?   How hard to replace that wire?   Seems like it would be hard.

In theory, using the external shield as a return path should be okay--in fact, it should prevent any RF interference, in the same way twisted wire pairs.  Aside from that, analog audio signals are usually too slow frequency (up to 20-50 kHz?) to produce RF that would interefere with radios anyway.   I don't know if it's standard practice, though

[Yetti]

talking with a carrier pilot.  talking about the incidence of cancer may be higher for that class of people.   He was mentioning when the radar would come around waiting to launch, the speakers in their headset would buzz.   I thought that was interesting because I thought fighters were hardened.  but it is just a plexiglass canopy.