As a sidebar, we found while testing 432 beams at Central States, that
our older beams, only a couple years, seemed low in gain. We
ScothBrighted the elements, Al welding rod, as I remember, with a hobby
brass driven element and T match, and got 3 or 4 10th's more on a 17
foot beam I designed for EME than I had on the 1st range test.

Sorry about the looong sentence.

tom
K0TAR

Ian White, G3SEK wrote:


I agree that the maximum *theoretical* gain - ignoring losses - is
achieved when the element currents are as high as possible, and the
feedpoint resistance is as low as possible. This also requires that the
element spacing is as close as possible... which leads to the
interesting conclusion that a compact beam should have more gain than a
full-sized one!

In practice, of course, this doesn't happen. The reason is that losses
can *never* be ignored in this particular problem. As the element
currents rise and the feedpoint impedance drops, the I^2*R losses in the
elements and the matching losses to 50R rapidly overtake any theoretical
increase in gain.

This means that high-gain beams with deliberately high element currents
are only a theoretical curiosity. The underlying theory has a valid
place in academic textbooks such as Kraus, but it isn't relevant to
practical antenna engineering. (Even superconducting elements and
matching circuits wouldn't make such antennas practical.)

Also, it isn't correct to apply generalizations about 2- and 3-element
yagis to a long, multi-element yagi. In particular, the first 2 or 3
elements of a long yagi cannot be considered in isolation from all the
other elements.

It is true that gain optimization in multi-element yagis tends to reduce
the feedpoint impedance towards 15-20R, but this is a remote side-effect
of all the other design parameters. A low feedpoint impedance certainly
isn't a desirable design aim in itself, because it leads to significant
matching losses and a reduction in the SWR bandwidth.

Numerous designers have found that when they are getting close to a
gain-optimized design, it is usually possible to raise the feed
impedance back towards 50R by inserting an additional first director
with a very close spacing ahead of the driven element. (This technique
may have been developed after you ceased to take a close interest in
yagi design, Dave.)

The close-spaced first director is mostly an impedance-changing device,
and it has relatively few side-effects on the overall gain and pattern.
With a multi-element yagi, it is usually possible to take out most of
these side-effects in the next round of optimization. The result is a
yagi that can be fed directly from 50R coax (through a balun) which
eliminates matching losses and greatly improves the SWR bandwidth. If
the re-optimization is done well, any decrease in gain is almost
undetectable in simulation, and completely undetectable on the air.