Reg Edwards wrote:

"Tom Ring" wrote
Here's the one that works best for long booms -

G = 10 log (5.4075 B + 4.25) for B GT 1

Where G is gain in dBd and B is boomlength in wavelengths.

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Thanks Tom,

If we have 5 or 6 elements spaced approx 1/4-wave apart, that makes a
boom length of 1 wavelength

Make that: "If we have 5 or 6 elements optimally spaced along a boom
length of 1 wavelength, we can hope for something in the region of"

a gain of 10 dB relative to a dipole.

Can experienced users of Yagi design programs confirm that 10 dB is in
the right ballpark?

That's about the best gain we can hope for, given a one-wavelength boom
to play with. On the other hand, poor choices of element lengths and
positioning will produce many dBs less forward gain... or even gain in
the opposite direction.


Yes please, I would like to see the equation for short booms and fewer
elements on your return from the salt mine.

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.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek