RPM and VOR interference

[Capt. Manuvar]

Is it true that VOR reception cannot be trusted At RPMs that are multiples of 60 e.g 1800, 2400? i Think is got something to do with 60Hz
Thanks

[2Donkeys]

I don't think that things are quite as clear cut in real life as your post implies.

It is true that certain combinations of VOR, receiver and Propeller will interfere with one another at certain RPMs. The symptoms normally take the form of needle oscillation rather than an apparently sound, but false indication.

I've never heard anybody advance a theory about the precise RPMs at which this occurs, but I would be surprised if the relationship was quite as straightforward as your post implies. The most likely cause, I would think, is RF harmonics from the engine and/or associated ancilliary gear which would not presumably occur at evenly spaced prop RPM intervals.

2D

[david viewing]

2D

I'm not convinced that an engine or accessories could generate significant harmonics in the VHF range, excluding of course spark noise from faulty installation which would affect the entire VHF spectrum.

In theory electronic equipment like FADEC or electric RPM generators might produce VHF interference, but that's all a bit specialised.

It is true that the alternator can generate hash and you can demonstrate this by tuning your car radio to an unused FM channel. You are likely to hear an engine speed dependant whine.

However, I think it is reasonable to assume that aviation alternators are designed for RF compatibility and do not radiate significantly. The whine on the car radio is likely to be coupled by the power wires, rather than by RF, and may enter the audio section of the radio rather than the front end.

So an alternative explanation might be that it would be fairly easy for the alternator to 'modulate' the aircraft battery DC at some multiple of engine speed. This DC ripple might be able to find it's way through the Navaid power supply and on to the VOR needle. A simple test would be to trip the 'Alt' switch.

David

[Capt. Manuvar]

Thanks for the replies. I read about it in some flying magazine a while ago. Someone wrote in asking why his VOR behaves badly at 1800rpm.
Capt.M

[2Donkeys]

I'm not convinced that an engine or accessories could generate significant harmonics in the VHF range

I am about as far from being an expert on things radio as it is possible to be...

That said, I would be pretty sure that the suppression of RT emissions from aero-engines and accessories is a significant issue, and failure of suppression equipment frequently causes disruption to VHF voice reception and transmission.

Have you never had the experience of a passing motorbike causing the television or radio in your house to flicker or crackle?

[UV]

Yes it is true!

I recall from the CPL and ATPL theory (and this is very theoretical!)that the interference indeed happens at multiples of 360.

A bearing from a VOR is calculated from the difference in phrase signals being received from from the VOR reference signal, and a bearing signal.

This difference, received in your VOR, is then translated into a Bearing. Obviously to a maximum of 360 degrees.

Anything more and you will have to look it up the theory books!

Hence you seeing this interference at 1800 rpm (360 times 5).

[dmjw01]

failure of suppression equipment frequently causes disruption to VHF voice reception and transmission
... probably the most commonly heard example of which is the "whoop whoop" noise induced by some people's strobes.

[IO540]

UV

This may be in the exams but I am 99.9% sure it isn't true. The signal emitted from a VOR is continuous in both time and the radial information and does nothing special at the cardinal points.

All sorts of interference scenarios are possible; one cannot rule out anything outright really. Most planes flying are very old and their wiring is similarly old. Where cables are shielded, the shield connection can be corroded or not even made in the first place - most avionics shops are no more than followers of wiring instructions; they usually aren't electronics experts. Look at how many planes have their radio transmissions badly affected by their strobe inverters, and that's just a really simple case.

A multiple of RPM is perfectly possible if the magneto/ignition leads are radiating.