[TowerTalk] Mystery Hy-Gain tribander

[Bill VanAlstyne]

K4SB wrote:
> ...they all have an impedance of 50 ohms, but, this was a
> design I developed myself (by mistake ) and the first director
> is so close to the driven element you wouldn't believe it.
> About 18 inches on the 15. None of these yagis have a F/B
> of less than 50 db at resonance. The greatest determining
> factor in the impedance, besides that 1st director, is the
> spacing and length of the 3rd director.
>
> And believe me, the spacing between elements is critical. I have a
> 1000 mm long rule obtained from McMaster Carr, and those placings are
> set to within .5 mm.

The problem with super-critical yagi designs is that they're, um,
super-critical. What he said.  :-)  If you have the time and patience to fiddle
with them until they are JUST RIGHT, and also have the toolset and skillset to
create mechanical structures that can hold close tolerances through normal
environmental stresses... more power to you! (Up to 1500 watts. ;-)  The reason
you never see yagi designs like this commercially, of course, is that it's
impossible to use such a design on a kit antenna that will be assembled by end
users with sometimes very minimal facilities and skills. Commercial antennas
have to be idiot-proof -- though some manufacturers (Cushcraft comes immediately
to mind) take this concept way overboard, IMO.

Having said that, it's been my experience that you can model, optimize, and
build an excellent yagi (I use YO by K6STI) that can boast significantly better
performance than your typical commercial yagi kit, WITHOUT making the design
super-critical. Of course, you have to pay the piper somewhere, and that's
usually in the area of input impedance. A highly efficient yagi with a
reasonably stable, tolerance-forgiving design typically has a fairly low input
impedance, and that requires a matching device of some sort. But there are lots
of good ones. I particularly like the T-match, basically a double gamma match.
It's balanced, so it doesn't introduce common-mode currents in the antenna boom,
coax, etc. the way a gamma can. (Not that that's necessarily a serious problem.
But it can be.)

Bill / W5WVO

>
> Have purchased 1 commercial antenna in almost 50 years, a
> TH11-DX...and don't ask.
> That thing is an absolute killer on 17, but I have severe doubts it
> gets much further than a couple hundred yards away on all the others.
>
> 73
> Ed
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