Height effect on antennas.

  • 1
  • Question
  • Updated 3 years ago
  • Answered
Taking advantage of so many bright OMs on this forum I wanted to get your opinion.
I understand the positive effect height has on gain for DX on beam antennas. I have this question though. What's better? A beam antenna 30 to 40ft from the ground vs a beam on top of a 100ft building on a 10ft tower.
If the ground plane is at the base of the tower then the 30ft tower on the ground will work better. But if the plane ground is the earth then the one on top of a building is way higher and therefore better.

For the questions sake both are the same identical antennas with identical setups and on the same location, same ground material.

I made a very simple graphic:

Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

  • 1752 Posts
  • 534 Reply Likes

Posted 3 years ago

  • 1
Photo of Walt - KZ1F

Walt - KZ1F

  • 3040 Posts
  • 643 Reply Likes
That is true Sal, but the building roff gives you great HAAT. The roof may or may not not present as a ground.
(Edited)
Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

  • 1752 Posts
  • 534 Reply Likes
Reading that take off angles of 5 to 10 degrees are ideal for DX, an antenna at 10ft will hit the base of the tower plane about 60 ft away from the antenna. That is why I think that HAAT more helpful for DX than Height from tower base...

Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

  • 1752 Posts
  • 534 Reply Likes
Hi Phil,

Have you tested the antenna at different heights with the same DX station?
Photo of Phil m0vse

Phil m0vse

  • 205 Posts
  • 38 Reply Likes
No I haven't, but as soon as Storm Barney has passed over I will definitely give it a go.
Photo of Phil m0vse

Phil m0vse

  • 205 Posts
  • 38 Reply Likes
I do find that the hex is a lot less directional when fully down (at about 10ft) it is practically omni-directional.
Photo of Mike KD2CJJ

Mike KD2CJJ

  • 122 Posts
  • 33 Reply Likes
Phil, yes.. That is because of the take off angle increasing.. All horizontally polarized antennas act this way..   The lower the take off angle the more directional it will become..   The lower the less directional it will become.. at some point you will have virtually an omni directional patttern (very little nullls)..  

Another benefit of height!  However!  I find this to be a disadvantage some times.. Many of my buds have verticals running on one slice and their directional antennas on another... 
Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

  • 1752 Posts
  • 534 Reply Likes
This is very interesting information. 
Phil, you are welcome to test it with my station. Given the distance between your location (I'm guessing UK because of the Clark SCAM) and Barbados we should be able to see the difference in performance at different heights.
Photo of KF4HR

KF4HR

  • 507 Posts
  • 140 Reply Likes
More height is normally better (to a point) but in this case with the antenna only being 10 feet (or less) off the top of the building, the radiation pattern could be negatively affected by the metal in the building or perhaps surrounding objects on the top of the building (other antenna's, HVAC, roof flashing, etc).  The longer cable loss would be a minor issue at HF frequencies but if that was a concern the issue could be compensated for by larger diameter coax.  Another concern might be, in case of servicing, how difficult would it be to gain access to either of these antenna locations?  My .02.
Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

  • 1752 Posts
  • 534 Reply Likes
The servicing and access is a great point. A shorter tower can be serviced easily. Anything above 25 ft starts to get a bit trickier.
Photo of Walt - KZ1F

Walt - KZ1F

  • 3040 Posts
  • 643 Reply Likes
And that is one of the reasons I got a US Towers TX series. I had one motor put on it for raising/lowering and should I need to get to the antenna I have a son to do the cranking from vertical to horizontal. I did that when I first installed the tower and it tore both shoulders. When I had the rotor serviced I had tower workers do it and they climbed it after I lowered it to 22'.
Photo of Peter Driessen

Peter Driessen

  • 11 Posts
  • 0 Reply Likes
An excellent review of the effect of antenna height on DX performance is at
http://www.nzart.org.nz/assets/pdf/20...
Photo of EA4GLI - 8P9EH - Salvador

EA4GLI - 8P9EH - Salvador

  • 1752 Posts
  • 534 Reply Likes
Great read. Need to go over it a couple of times.... with this forum I have information overload! Thanks Peter.
Photo of KY6LA - Howard

KY6LA - Howard, Elmer

  • 3541 Posts
  • 1396 Reply Likes
Perhaps one of the best sources of information is the ARRL Antenna Handbook

I can particularly recommend the computer program High Frequency Terrain Analysis (HFTA) which is included on a CD in the book which allows you to design and test the effectiveness of different antenna heights and configurations based on the exact terrain location of the antenna.

I have used HFTA very effectively in contests to determine optimum heights for different bands to different areas around the world... There are many cases where a higher antenna may actually be too high for a particular band so that you get secondary power lobes at high angles if you are too high.  In many cases we got much better results to say EU when we cranked down the 100' beam to 70' on say 20M...

The beauty of HFTA is that it takes into account the terrain of the surrounding area to determine the communications effectiveness..

Finally .. you have to be very concerned about the composition of the roof.. if it is metal, and the antenna height is in the near field of the frequency, (10' on 20M)  it will definitely have a major effect on the antenna performance so in such cases the higher the better... if it is a less conductive material and/or you our outside of the near field ... say 30' on a 10M antenna then there will be much less effect of the roof....

Again.. strongly suggest you use HFTA when designing antenna locations.. it works.