[Towertalk] 20M Yagi ?

[WD4K]

The terrain here is typical of the mid south US area. High hills with
gradual slopes. I have a topo but without getting too specific, the hill
here is 4 acres on an already high ridge with nothing in the way of
obstructive hills within miles. The approx 800 ground height is on a flat
top of probably 1/2 mile square, more or less. The slopes are down 2-300
feet within 1-3 miles away. Gradual is the best way to describe it. No
clifftops or severe angles although it is difficult to walk some of it. "you
can see forever, is a good way to describe the view" I am 20 miles outside
of Nashville and can look down on the city lights at night. The near field
for JA starts sloping after 100', Eu slope starts at near 300'out and carib
out 1/2 mile or so. The only near level ground is due south for a mile or
two. Understood that the modeling needs to be done with specifics so I offer
these very "unspecific" guesstimates as generic guides for some "what if"
fun. Tommy WD4K

-----Original Message-----
From: Maurizio Panicara [mailto:i4jmy@iol.it]
Sent: Tuesday, July 16, 2002 11:54 AM
To: towertalk@contesting.com; Bill Tippett
Cc: WD4K_1@Bellsouth.net
Subject: Re: [Towertalk] 20M Yagi ?


Bill,

I meant the situation as a 200/300 ft drop, a wall as it happens in
proximity of a cliff edge, then a smoother but consistent terrain slope to a
total amount of 800 ft.
Having tested something similar on 20 15 and 20m, I can promise that or You
move back the tower with antennas from the cliff edge some WL to set the
first reflection on the terrain where the tower is, or the reflection you
look for to fill the null would not occurr.
In other words and elements detuning problems apart, it's totally useless to
have a lower antenna (unless the antenna is too low) if the terrain doesn't
extend enough to insure the reflection you need.
In such case the tower height is almost neglectable compared to antennas
effective height.
This is an intuitive geometrical fact, also perfectly confirmed by any of
the software you mentioned: what counts is not the ground exactly below the
antenna tower but away enough, where the reflection you are looking for
occurs.

73,
Mauri I4JMY




> I4JMY wrote:
>
> >In case of a 200/300' real drop, and 800' hill top location, it's quite
> >possible that the first reflection is so far that the equivalent height
> >of the antenna/s from ground is 800'.
>
>         I interpreted this to mean 800' above sea level and 2-300'
> above surrounding terrain.  Assuming this, the effective height is
> 2-300' plus the tower height.  The immediate terrain near the
> antenna is very important and this is why there is no substitute
> for a model.  If for example you wanted a 10 degree ground reflection
> and had one antenna at 40', that would occur at 214' from the tower,
> for an antenna at 30' it would occur at 170', and for one at 20' it would
> be at 113' (assuming flat terrain).  It might be possible to locate the
> tower on a flat area of the hilltop to achieve this.
>
>         Both antennas out-of-phase might be another viable solution
> since I believe this would also give a high takeoff angle.  I just took
> a quick look at two of my KLM's in free space and the BOP TOA peaks at
> 24 degrees azimuth, so it might be possible to choose a different phase
> angle to steer the maximum lobe to whatever TOA is desired.  For example,
> if the top one leads the bottom by 90 degrees, the TOA peak drops to 12
> degrees.  As I said before this is a complex problem and there is no
> substitute for looking at it problem with some good models assuming the
> actual terrain in question.
>
>                                                 73,  Bill  W4ZV
>
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