"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 and 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?

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