Originally Posted by
scifi
I think RoadRabbit has it 100% correct, the VOR radials are set in stone, or more precisely their aerials are set in concrete on the roof of the building.
You would be mistaken.
VOR's actually emit two radio signals, one is an omnidirectional master signal and the second is a directional, rotating azimuth signal. The phase of the azimuth signal changes as it rotates at 30Hz, such that the phase of the azimuth signal only matches the phase of the master signal when the two are aligned, i.e. at Station North. Hence the difference in phase between the signals is used by the receiving avionics to determine the radial from the station.
The master signal can and is changed periodically to bring Station North to broadly match Magnetic North over a coming period. That exact period changes according to national requirements; some VOR's in the US go for a long time without being adjusted, those in some European states are adjusted much more frequently.
It is important though to note that during that period, Station North will only exactly match Magnetic North for just part of that period. The difference between Station North and Magnetic North is the Station Declination and it is listed in the AIP. Please note that declination and variation are
not the same thing.
Since a VOR receiver is only measuring the difference in phase betweem two signals, there is nothing about the system that is set in concrete and unchangeable. Engineers periodically 'adjust' station north by changing the phase outputs.