transmission power required for VHF/UHF at higher altitudes.

[1dontknowmuch]

The 2002 SkyTower project in Japan, where they used a UAV at 65,000' to transmit broadband and HD video link, reported that it took only 1/10,000th of the power required to transmit the same signal from a ground based transmission tower.
Would anyone know of a fairly accurate formula to work out the different transmission power requirements for various altitudes?
Any info greatly appreciated!

 

[Compositepro]

This is not an aeronautics issue but an electronic one. A geosynchronous satellite can cover the entire U.S. using a solar array for power while a ground based radio station will cover a few hundred miles with 50 kilowatts. It's the inverse square law. A satellite uses a focused beam whereas a ground based transmitter cannot be focused.

 

[1dontknowmuch]

Thanks for the info!

 

[Higgler]

Sounds plausible.
Compare 22,000 miles to 12 miles. Every time you shorten your distance by a factor of two, your power required decreases by a factor of 4. Hence the drop in required power is actually alot less than this 10,000 factor based on that.
Picture your flashlight spot shining on the wall. At 12 inches distance compared to 22,000 inches, it's alot brighter. Same thing for 12 miles vs. 22,000 miles.

 

[AviatorJim]

This is more of a line of sight issue. The higher the altitude, the larger area you can see "line of sight" as the curvature of the earth does not hide areas at a greater distance. You also don't have to transmit through trees, building, etc. . .
..
Broadcast satellites and UAV's don't use a focused signal, They generally radiate a hemisperic pattern. The fixed ground station uplink comes out of a directional antenna.
..
An aircraft at 35,000 feet can transmit a VHF voice signal to all horizons with as little as 25 watts. On the ground, my 5 watt handheld radio is lucky to talk to other people on the ground within 2 miles. I can talk to aircraft in flight at 10 to 15 miles (general aviation aircraft at 2000 ft above the ground). If I am also in flight, 50 to 100 miles is common.
..
To cover the same geographic area using ground stations requires multipls transmitters, at very high power. From 65,000 feet the UAV can see hundreds of miles in every direction.

 

[IRstuff]

The Earth is a giant, resistive, ground plane; that makes propagation parallel to the ground behave like an extremely lossy transmission line

TTFN

FAQ731-376

 

[rb1957]

there's also the signal-to-noise ratio of the receiving antenna to consider