[Towertalk] 20M Yagi ?

[Michael Tope]

Mauri,

How about the case where the ground immediately under the antenna
is very close, but there is also a  nearby cliff? Ray tracing to the point
of
distant reflection suggests this antenna will produce a very low angle
of radiation. On the other hand, the electrical field induced in the
ground immediately under the antenna will be out-of-phase with the
electrical field from the antenna suggesting that the antenna will
have a null towards the horizon.

A concrete example of this sort of antenna is the local club station.
They have a 160 meter inverted-vee mounted on a 40' telephone pole
close to the edge of a ~500' mesa. Looking in the direction of the 500'
dropoff, what is the effective height of this antenna? Is it 40' or 500'?

Anyone know how to properly simulate the pattern of a dipole in
this configuration?

73 de Mike, W4EF....................................

----- Original Message -----
From: "Maurizio Panicara" <i4jmy@iol.it>
To: <towertalk@contesting.com>; "Bill Tippett" <btippett@alum.mit.edu>
Cc: <WD4K_1@bellsouth.net>
Sent: Tuesday, July 16, 2002 10:54 AM
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
> >
> > _______________________________________________
> > Self Supporting Towers, Wireless Weather Stations, see web site:
> http://www.mscomputer.com
> > Call 888-333-9041 to place your order, mention you saw this ad and take
an
> additional 5 percent off
> > any weather station price.
> > _______________________________________________
> > Towertalk mailing list
> > Towertalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
> _______________________________________________
> Self Supporting Towers, Wireless Weather Stations, see web site:
http://www.mscomputer.com
> Call 888-333-9041 to place your order, mention you saw this ad and take an
additional 5 percent off
> any weather station price.
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk