On Jan 22, 9:02*pm, Channel Jumper Channel.Jumper.
wrote:
ARRL Antenna Book has all the answers.

Shortening the antenna will reduce the effective radiated power and will
reduce the amount of gathering ability of the antenna.

Uhh.. not really. for small scale shortening, the directivity will
remain roughly the same. Losses are fairly small, in general, so you
don't get a big change between loaded and unloaded (unless you pick a
particularly lossy way to load it)

What WILL change is that the dimensional tolerances and bandwidth will
be narrower. That is, you won't get the same gain over as wide a
band.

You will also not get as good F/B or side/back lobe performance in
general.


Every time you reduce the size by 25% - you throw away - probably 50% of
it's ability.

I don't think so. Can you demonstrate this with a simple model?

The physical size of a Yagi isn't all that closely related to the
effective aperture, at least for 10-20% sorts of size changes. Sure,
go to half or quarter size and losses will start to increase because
of the increased stored energy.



Antenna's are always a compromise when you get into the lower ranges.
It is easier to BUY a Mosley tribander - about $1000.00, then to try to
make your own.

As the elements are shortened or lengthened, all of the variables has to
be recalculated because the spacing will also change and the boom
length.

Yes, but today, you've got a LOT of help from very good and validated
modeling codes. No more are you looking at tedious cut and try.

The real thing that's going to affect performance for a small antenna
is mechanical details.. how are elements attached, what kind of
dimensional tolerances can you hold, how big is the tubing, stuff like
that.