Richard wrote:


What has happenned is that I, who know little of the art, am relying on
whoever condescends to make a comment. And comments so far have tended to
highlight how unsatisfactory it is taking a model and bulding a yagi on the
dimensions.

Mine came out nearly perfect. 6m version...

There seems to have been a desire to point out to me how
impossible a task it is to build a decent antenna unless one factors in
alteration of boom size and element diameters. Technically these comments
are correct, you cannot expect to get the same antenna performance if these
things are ignored and I feel I've been concentrating on these details for
fear of not doing enough. But on the other hand, will the difference on
receive amount to much whether elements are insulted from a boom or not .
Probably not eh.

It does make a difference, but I don't think it's anything that would
ruin an antenna. Lets say "perfect" design shows 9.4 dbi. Lets say "not
quite perfect" design shows 8.4 dbi gain. Hardly noticable in the real
world. Most consider 6 db to equal an S unit in theory as an
example...When building my 6m yagi, "three element" I copied the eznec
NBS specs exactly as far as element lengths and dia. The dia was 1/2
inch , which was easy to copy using 1/2 inch copper tubing. I gave no
real thought about boom size or type, DE type, etc. I built the antenna
per specs using welded to the boom elements, and welded to the boom one
piece DE, matched using a simple gamma match. The antenna is all one
piece including the boom. If I had used a split DE, I would have still
used the same lengths...Anyway, I built it and tested it using local
stations as signal sources. The gain and f/b "seemed" to be almost
exactly as modeled, as well as I could measure. IE: the modeled f/b is
appx 10 db. Thats about what I got. The forward gain seemed to be about
right on also. "appx 9.4 dbi".
The easy way out for you, would be to get an existing 2m yagi, and
simply trim the elements. You could rescale in the model program to tell
you how much to take off. Sure, the spacings will change slightly, but
again I bet it won't be noticable. If anything, being the spacing will
actually slightly increase in wl, the gain "could" actually increase
slightly. I bet it doesn't drop much... The modeling program would give
you a good idea of what to expect from it. MK

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