Sorry, there's no theoretical basis for declaring what the "best
balance" of those parameters is. So there's no theoretical basis for
deciding what the feedpoint impedance will be for the "best balance".

But. . .

If the Yagi impedance is very low, it indicates very strong coupling
between elements and high element currents. This indicates a sharply
tuned antenna which might have high gain if the losses are minimized,
but also narrow bandwidth. This is a common result of trying to squeeze
too much gain from too small an antenna. To understand why, try googling
"super gain" or "supergain" antennas or look this topic up in an antenna
text.

If the Yagi impedance is high -- close to that of a dipole -- it means
that there's very little coupling from the driven element to the
parasitic elements. Consequently, the parasitic elements won't have much
current with which to produce fields, and they won't do much. The
antenna won't have much gain relative to a dipole, and its pattern won't
be much different from a dipole.

So while a Yagi having an impedance outside very roughly the 25 - 35 ohm
range can still perform well in one or more respects, you should look
carefully at it to see what tradeoffs have been made.

Of course, this applies only to the resonant feedpoint resistance at the
center of the driven element, which can be transformed over quite a wide
range by various structures and networks.

Finally, the above comments are pretty broad generalizations, so they're
subject to numerous exceptions. But they're a good starting point for
understanding some basic properties of Yagis, and hold often enough to
be reasonable rules of thumb.

Roy Lewallen, W7EL

ve2pid wrote:
Hi to all,

I read somewhere that, in the case of Yagis, ''in the range of 25-35
Ohm you get the
best balance between gain, pattern, bandwidth and element currents.'''

Is that true? And if so, I would like to have the theoretical
explanation behind this.

Thanks and 73 de Pierre