×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

transmission power required for VHF/UHF at higher altitudes.

transmission power required for VHF/UHF at higher altitudes.

transmission power required for VHF/UHF at higher altitudes.

(OP)
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!  

RE: transmission power required for VHF/UHF at higher altitudes.

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.

RE: transmission power required for VHF/UHF at higher altitudes.

(OP)
Thanks for the info!

RE: transmission power required for VHF/UHF at higher altitudes.

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.

RE: transmission power required for VHF/UHF at higher altitudes.

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.

RE: transmission power required for VHF/UHF at higher altitudes.

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: Eng-Tips.com Forum Policies

RE: transmission power required for VHF/UHF at higher altitudes.

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close