[TowerTalk] Which better: TOP of hill or SIDE of hill?

[Guy Olinger, K2AV]

I'm really not sure why you are having this problem. I would concede 
that for frequencies where 3 element yagis are common, gain effects 
will be foreground. Front to back is another matter if it's an issue 
and you really want to know.

It's easy enough to verify this. Just do a free space run on the 
antenna at issue and if it has limited radiation down and behind (as 
in a decent 3 element yagi), the gain effect of terrain down and 
behind is likewise limited.

In HFTA can you define a "zero" terrain point behind the hill and move 
the antenna away from the terrain "zero" point to the front of the 
hill or is it hard coded to the zero point used for the map data?

I just rechecked that I can model down a slope with TA using a dipole 
as an antenna. It does both sides of the dipole pattern, and I know 
that I can create a reverse pattern input to TA and work it backwards. 
If you place the antenna at the zero point in terrain TA appears to 
assume "flat" behind it.

In order to do the offsetting to get a correct bi-directional dipole 
result you have to have a terrain file where the "zero" horizontal 
point is sufficiently behind you. As far backward as forward, so that 
effectively you have two foregrounds defined.

Then (in TA), just drag the antenna "dot" to the middle of the range 
and set it at the various points you wish to test. It's very easy to 
check it out.

HOWEVER, neither program will recalculate a pattern for effects of 
near ground for wire antennas, two element (or "short" boom) yagis 
caused by sloping ground or interaction in a stack. Both programs 
presume enough height and sufficient horizontal gain to make that 
minimal, and really have no means to calculate otherwise.

This has consistently been a problem with stacks of two element beams 
on 40m at ordinary heights (e.g 50' and 120'), requiring both the 
antennas AND the neighborhood ground details (in 1 foot height 
increments) IN THE ANTENNA MODEL to get real world results. Those 
expecting a 3 db stack gain are in for a deflating surprise.

By 20 meters and three elements this appears to be a non-issue. No one 
puts a pair of two element 20m yagis on a tower at 25' and 60'.

Keep working on the problem. You can make the slopes really play for 
you.

If it were me, I'd have antennas on both sides of the slope, and 
optimize the whole thing for my favorite contests, including 
front-to-back issues.

73, Guy.

----- Original Message ----- 
From: "John WA2GO" <xnewyorka at hotmail.com>
To: <olinger at bellsouth.net>; <towertalk at contesting.com>
Sent: Wednesday, December 29, 2004 1:43 AM
Subject: Re: [TowerTalk] Which better: TOP of hill or SIDE of hill?


> Hello Guy (and all),
>
> I have spent the majority of the past week playing with HFTA. .....

> In my particular situation, however, I was wanting the software to 
> do something it cannot do. It is unable to take into account the 
> terrain that is BEHIND the beam. I wanted to find out whether I 
> would do better to Europe by placing a beam as much as 70' below the 
> top of a northeast facing slope,