But Hilton, that doesn't explain the effect observed. What DOES explain it
is the antenna pattern that the FAA chose for the VORs. In their infinite
wisdom they never considered that aircraft would fly much above FL250.
Remember, this was in the late 40s and early 50s that the system was
designed.

Therefore, they "squished" the antenna pattern to squirt more signal at a
lower radiation angle than an isotropic ("all angles") radiator. Think of
it as a ball of dough (isotropic) that has been squished to become a pancake
(low angle radiation). If you are ABOVE the pancake, you receive less
signal strength than if you are in the dough, so to speak.

There are two effects here. One is "radio horizon" which limits low
altitude reception to what the antenna can "see". The equation for this is
that radio horizon (in miles) equals the square root of the aircraft
altitude above the VOR (in feet). Thus, an aircraft near San Diego
receiving SAN VORTAC (which is on an island near Pacific Beach, damn near as
close to sea level as you can get) at an altitude of FL180 will have a radio
horizon of 134 miles, almost exactly what the fellow said, and will be
almost in the dead center of the antenna "beam" pattern. However, take that
same aircraft in the same geographic spot and honk it up to FL500, the radio
horizon moves to 224 miles, but you have climbed yourself way above the beam
and the signal strength has dropped below usable..

Howzat?

(Signal strength, BTW, falls off as the SQUARE of the distance.)

Jim




"Hilton" wrote in message
.net...
However, the FAA has depicted cylinders of various diameters stacked
upon each other. Given that the VOR is line-of-sight, I did not
understand why, for example, a VOR would be received 130nm out at FL180
yet only be received 100nm at FL500. Doesn't it logically follow that at
the higher altitude the VOR would be able to be received further out?

No, then it wouldn't fit in the semi-sphere. Signal strength drops off
(non-linearly I believe) as you move away from its source, so the further
you go, the weaker it becomes, hence the semi-sphere. Since the sphere
tapers off at the top, so too do the cylinders.