[TowerTalk] Tower placement

[K4SAV]

Congratulations on having a QTH with good DX potential.  You need HFTA, 
terrain analysis program!  It was included on the CD ROM with the ARRL 
Antenna Book 20th edition.  With this software you can model your 
terrain and move your towers around to see the effect. 

It is very hard to give you an answer as to which is best.  There are 
going to be trade-offs.  A couple of general conclusions can be made.  
For best low angle performance, a terrain which slopes away from the 
antenna is best, as opposed to one which drops sharply (as over a 
cliff), or flat terrain. For example, if you have a cliff immediately in 
front of the antenna, and a flat floor below, the first ground bounce is 
the valley floor, so the antenna functions as if it is very high, which 
isn't bad, but it's not as good as for a sloping ground in front of the 
antenna.  In the case of the sloping ground, the reflection angle of the 
first bounce is at a lower angle because of the sloping ground.  
However, if this reflected wave happens to hit an  upward slope next, 
then a lot of the low angle wave generated by the first reflection is 
going to be sent upward (not good). So if the ground continues sloping 
down, or there are no obstructions ahead, then low angle stuff is 
maintained.

In a situation where you have ground rising in front of the antenna, 
your low angle performance is going to be restricted, maybe severely, 
depending on the height of the obstacle in front. 

So, already, you are seeing some trade-offs.  These are general 
conclusions, but the real answer is far more complex, because with 
rugged terrain, the bounces can get really weird, and terrain a half 
mile of more away may significantly effect propagation.  It will be well 
worth your effort to model this using HFTA.  It's not perfect, but it's 
a lot better than a guess, and a lot easier than moving towers.

Jerry, K4SAV

Bill Axelrod wrote:

>I have a new mountain-side QTH and am looking for some advice regarding 
>tower placement since my knowledge of the effects of topography as it 
>affects HF propogation is sorely lacking.
>
>First, let me describe the property.  I have 37 acres on the side of a 
>mountain in WV.  I have an approximate 270 degree clear view covering West 
>to North to East behind the house.  Then there is a heavily wooded steep 
>rise of 100 feet or so from the front of my home my home facing South.
>
>There is a ~150 ft wide by ~180 ft deep cleared "shelf" area behind the 
>house that drops approximately 30 ft over the 180 ft.  At the end of the 
>shelf the ground drops approximately 100 ft quickly (maybe a 50 to 60 degree 
>slope), then another more gradual slope to the valley floor.  It appears to 
>be a couple of miles before the next ridge of equal height to where I am 
>located.  Maybe more.
>
>I plan to put up a 70 ft tower with a HF beam this summer.  My question is: 
>Where on the property should I put the tower for the best DX propagation?
>
>Would I be better off putting the Tower as far up the mountain as possible? 
>Or would I be better off putting it as close to the end of the shelf (and 
>sharp drop-off) as possible?  Which would be best to get the optimum take 
>off angle?
>
>Conversely, I also plan to put up a NVIS wire antenna for close-in stateside 
>contesting.  What location would be best to get the optimum high take off 
>angle?
>
>My interests are DXing and contesting.  There are no power lines to get in 
>the way and no neighbors who can see my property from their homes.  The XYL 
>will be happy as long as I don't put the tower directly in her view from the 
>deck.
>
>Thanks and 73...   Bill K3WA 
>
>_______________________________________________
>
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>
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