|To:||"John WA2GO" <email@example.com>, <firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] Which better: TOP of hill or SIDE of hill?|
|From:||"Guy Olinger, K2AV" <email@example.com>|
|Date:||Wed, 29 Dec 2004 12:30:39 -0500|
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.
----- Original Message ----- From: "John WA2GO" <firstname.lastname@example.org>
To: <email@example.com>; <firstname.lastname@example.org>
Sent: Wednesday, December 29, 2004 1:43 AM
Subject: Re: [TowerTalk] Which better: TOP of hill or SIDE of hill?
Hello Guy (and all),
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,
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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