Michael,
The ARRL Antenna Book latest edition has a program called "High
Frequency Terrain Analysis" (HTFA). You download topographical
information from the *exact* location where the dipole will be, import
that into HFTA, tell it you have a dipole and how high. Pick a compass
direction and it will do ray tracing based on the topo info and the
antena and give you a very good idea what type of lobes you'll have.
HFTA only works for horizontal polarization.
http://www.arrl.org/catalog/9043/HFTA.pdf
-Scott, WU2X
On Tue, Jun 17, 2008 at 10:51 PM, Michael Baker <k7ddmjb@qwest.net> wrote:
> Hi Gang,
>
> Let me pick the brain of the great collective and see what pops out.
>
> I take EZNEC and put up a dipole for 20 meters at about 1/2 wl in
> the air and I have a set of patterns that look like what I would expect to
> see.
> What happens if the dipole is not working against a normally flat
> surface like when it is on top of a mountain?
> My field day site is at about 6800' and drops off from that altitude
> to about 1100' in a radius of about 270 degrees and about 250' from the
> proposed antenna site. If I mount the antenna on this plateau on a short
> mast (20') do I still have the low angle of attack that I would have at that
> 5700' drop off or is it still primarily effected by the 20' height above the
> near field ground? Is there anyway to model this?
> Don't get me wrong, I know the site works well I am just trying to
> understand what is going on.
> Anyone care to give me a bit of a readers digest version of an
> answer as I am not an engineer, just a long time ham with questions. ;>)
>
> My own best guess is that I have some residual effect from the near
> field because of the proximity to ground so I get a pretty vertical angle of
> attack for at least one lobe. In the far field ground which is sooooo far
> away I an thinking I get the good clear field of attack on the horizon and a
> lot of lobes from the various reflected signals/waves (Frenzal Effect?)so it
> performs well. How close am I?
>
> Michael Baker K7DD
> k7ddmjb@qwest.net
>
>
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