Larry - K7SV wrote:
> Hi all,
>
> We're in the midst of building a fairly long boom yagi for 20M. When
> completed it's going to be at 190ft. We expect to initially tune it (T
> Match) at 15 ft and then tram it to 80 ft or so for further tuning before
> bringing it up to full height.
>
> Initial thought is to tune it 200khz low at 15 ft, but that's just a guess.
> Are any of you aware of data available that might make this a little easier
> than simple trial and error. This will no doubt display a curve that sharply
> rises as one moves an antenna from ground level to some height above ground
> and from there on up to our working height the percentage of change becomes
> fairly small. I'd guess from 30 feet on up the rate of change would be
> greatly reduced from what one would see between ground level and that
> height.
>
> As I think about the question, I find I'm asking myself if there is anything
> in modeling that might provide some answers, but if that were possible I'd
> expect that Steve NR4M who developed the design based on YO would not be
> looking for your help!
>
>
modeling will certainly tell you the general magnitude of change you
might expect, but you'd need to include all the other stuff around the
antenna when you make the measurements (e.g. if you're tramming up to 80
ft, you'd need the guy wires, the tower, all the other stuff)
When you say "long boom", I assume that means you have a fair number of
elements and a lot of gain? More gain means more stored energy in the
system, which means the tuning of the various elements is more critical
to get the expected performance (particularly for front/back and
sidelobe performance). Again, modeling might tell you how sensitive the
design is.
And, of course, just because the feedpoint impedance happens to be what
you expect doesn't mean that all the other elements are correct.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|