Richard Fry wrote:
. . .
Statements on this newsgroup about the free space gain of a very short
doublet antenna being not much less than a full-size 1/2-wave dipole
might lead some people to conclusions different than shown in the FCC
chart.

Only if some erroneous statements were made here that I don't recall
seeing or if a person doesn't look at the chart carefully.

The chart shows that in the absence of ground loss (the "theoretical"
curve), the difference in gain between a very short vertical and 0.25
wavelength high vertical is 0.4 dB (300 vs 314 mV/m). This is
approximately the number I've quoted before on a number of occasions
(0.45, or less than 0.5 dB). I don't recall seeing anyone here ever
claim that this will be the gain difference of real antennas with loss
(the other curve) -- as the antenna gets shorter, the radiation
resistance drops, and so the fraction of power lost in the ground system
increases. The curve clearly shows how strong this effect is when using
the ground system adopted by AM broadcasters, and in fact the ground
system resistance can be inferred from the graph. This was first
described and shown in detail by Brown, Lewis, and Epstein in their
classic 1937 paper. Jerry Sevick, W2FMI, built and measured a number of
very short 40 meter verticals with elaborate ground systems to show that
the loss could be kept very modest in a real installation. His
experiments were published in QST in the '70s.

One shouldn't overlook another potentially large cause of loss when
using a very short antenna, either -- the matching network. Short
dipoles and verticals have a large amount of capacitive reactance which
requires an inductor to match, and loss in the inductor can become large
compared to the antenna's feedpoint resistance. But like ground system
loss, this isn't a loss inherent in the antenna itself.

The actual performance of a vertical can be made to approach the
"theoretical" curve arbitrarily closely with a sufficiently elaborate
ground system. The system the broadcasters have chosen isn't pefect --
it's simply one that's been deemed to be good enough for the job at
hand. Ground systems can be made which have lower loss.

What you have to always keep in mind is that if you put 100 watts into a
lossless antenna, 100 watts must be radiated. The lowest possible gain
of such an antenna is 0 dBi, which is what you get if that 100 watts is
spread equally in all directions. An infinitesimally short dipole has a
directional pattern which is similar to that of a quarter wave dipole,
just a little fatter. That's why its gain is a little less.

Even though we can't ever build a lossless antenna, it's useful to
understand them. It allows us to separate, and trade off as we wish, the
various factors affecting gain and loss.

Roy Lewallen, W7EL