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Port Strobe
29th Dec 2004, 15:36
I'm having trouble understanding quite what is going on to generate the signals transmitted by the conventional VOR before Doppler VORs came to exist. I understand there are the reference phase and the variable phase signals. The reference phase signal is carried on a 9960Hz subcarrier which is frequency modulated at 30Hz with a frequency deviation of 480Hz. This signal is transmitted omnidirectionally such that at a given instant in time the phase of the signal is that same in all directions.

Now comes the tough bit, the variable phase signal. This is amplitude modulated at 30Hz but my understanding of how this is achieved is not great. I've read a couple of explainations but something isn't quite making sense with it. Some refer to rotation of the variable phase signal to achieve amplitude modulation but did the antennae actually rotate? If someone could please explain how this is actually achieved please I'd be very grateful. The points I'm having most trouble with are the mechanism for amplitude modulation of this signal and how all this relates to the phase of the received signal which is after all how the a/c determines its position. If you could highlight these points please I'm sure the penny will drop.

I have searched the forums and read some replies to questions along the same line but its not sinking in, sorry. For any mods viewing this I've also posted this in the Engineers forum to try and maximise my chance of an answer, I don't discriminate who gives me the correct answer :ok:

Thanks very much indeed

Tinstaafl
29th Dec 2004, 16:16
Yes, the antenna rotated. 1800 rpm I recall (or was it 3600?). Similar concept, structurally, as a radar antenna. The building generally looked like a cube with a rounded top 'dunces hat' mounted on the roof to shelter the bits and pieces.

Alex Whittingham
29th Dec 2004, 18:30
It was 1800 RPM, 30 cycles a second. The antenna was made up of a loop and sense type aerial that produced a combined polar diagram shaped like an artists palette with the centre at the 'thumb hole', the whole thing called a limacon. Rotating the loop aerial caused the asymmetric transmission to rotate, this cause the amplitude of the signal to go up and down. I'll try and post some diagrams tomorrow.

Port Strobe
29th Dec 2004, 22:41
Thanks for the quick replies so far guys. So from what you've told me we have a signal being broadcast in such a manner that it is amplitude modulated at 30Hz, which I understand now so that's cleared that up. Next question I've got is how this relates to the phase of this signal. What I understand is that if we look at the signals received by two different a/c at an instant in time then the amplitude of this signal depends upon the magnetic bearing from the station but I know the receiver equipment compares the phase difference to work out QDR so how does this varying amplitude relate to the phase of the signal and its spatial dependence?

Thanks very much for your time.

Alex Whittingham
30th Dec 2004, 08:58
Imagine a moment froxen in time, as below. At this instant the FM reference signal is at a phase of 0º, and going out in all directions. The AM signal has a maximum amplitude at a bearing of 270ºM and a minimum at 090ºM

The point of maximum amplitude on an AM transmission corresponds to a phase of 90º, the point of minimum amplitude is a phase of 270º. This means that, at a magnetic bearing of 270 from the beacon we have the FM at 0º phase and the AM at 90º. The phase difference could be read as 90º or 270º but, by convention, it is read as 270º.

flighttime2.0
30th Dec 2004, 10:15
test :

sorry guy's just having a little prop replying to posts ..

Port Strobe
30th Dec 2004, 11:19
Thanks very much for that, think I understand it enough to answer a question on it now. The diagrams help a lot as well.

Thanks again for your time, much appreciated.

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