Mike & Becca Krzystyniak wrote:
> Did they decomission the VOR system too? Seems most major airports have
> installed and are using large VOR arrays like these...
>
> http://www.panoramio.com/photo/4839614
>
>
> Mike K9MK
>
>
>
That's not an unusual antenna system. It's the standard Pseudo-doppler
way to generate the VOR signal, although if you have enough flat land,
you just put it on the ground.
But versions up on a pedestal are also fairly common (it reduces
multipath, and frees up ground space). The first one I saw was off the
end of the runway at Santa Monica when I was learning to fly. Hanging
on the edge of the embankment at the west end.
It's kind of a clever idea (and it's even antenna related...)
The VOR signal (on VHF, just above the FM broadcast band) is a
combination of an AM and a FM modulation at 30 Hz. The FM is actually a
subcarrier at about 9kHz. Imagine a rotating dipole radiating the FM
signal.. It's sort of like a beam sweeping around 30 times a second.
You make a simple receiver that can detect the FM and the AM separately,
and you compare the phases of the two to figure out where you are (angle
wise) from the transmitter.
The rising zero crossing of the FM signal is (I think) when the "beam"
is pointing north, so if the 30 Hz AM signal goes through its zero
crossing positive at the same time, you know you're north.
The transmitter used to be done with a crossed coil goniometer scheme
spinning at 1800 rpm (and may still be, although these days, I bet
they'd do it with DSP), but an even cleverer way is to have a central
transmitter, and then just transmit a replica of the signal from a
smaller antenna that appears to move in a circle around the central
antenna. The moving antenna has a doppler component (aha.. the FM
part), and the "phased array" effect from the combination of the signals
gives you the AM.
BTW, the receiver was also real simple... the two signals from AM and FM
drive the coils of a two phase synchro type receiver.
http://en.wikipedia.org/wiki/VHF_omnidirectional_range has some details
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