RST Engineering wrote:
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

Very comprehensive and "engineer like" elaboration to Hilton's answer.
I think I finally get the picture. Thank you!

Antonio