Ian White GM3SEK wrote:


There isn't really "an" equation for gain versus boom length, because
boom length is only an indicator of the potentially achievable gain. But
nothing "makes" a yagi deliver any particular gain figure.

Quite the opposite: very often, yagi designers will deliberately
sacrifice some forward gain to obtain better performance in other
important respects. These can include a cleaner pattern, a more
convenient feed impedance and better tolerance to element loading by
rain and ice; and all maintained over a wider bandwidth.

Yagis that have been single-mindedly optimized for maximum gain tend to
be poor performers in other respects - especially tolerance to element
loading.

The equation that Tom quoted is just the best-fit line through the gain
figures for one particular family of reasonably good long yagi designs.
Other good designs will have similar gain, but different performance in
other respects. And for every good yagi design, there are many more that
come nowhere near this level of performance.


Ian is correct, and I should have stated that this equation is for well
designed yagis. Also there were 3 families of antennas involved, plus
some odds and ends. The K1FO models all are within about 2 tenths of a
dB of the line in question.

Also, if we move to pure gain number, as in the multiplication factor,
the line produced by well designed yagis is virtually ruler straight
above 1 wavelength, and up to a a point I can't remember. It's above
10. And then it starts to roll off, which makes sense. I'd have to go
look at the models again to find where the rolloff becomes noticable.

tom
K0TAR