From another source. F Y I --
73
Bob, K4TAX
An article by two well-known radio amateurs, “Tuning
Electrically Short Antennas for Field Operation,” appeared in
Microwave Journal. Authored by QEX Editor Kai Siwiak, KE4PT,
and award-winning researcher Ulrich Rohde, N1UL, the article
points out that both Amateur Radio and military applications
exist for 20-W battery-powered radios equipped with whip
antennas. “In general, the whip antenna which makes the radio
portable is not optimized for signal propagation: A whip
antenna has no ground return or proper counterpoise,” the
article notes. “While some users drag a wire of up to 8 meters
behind, this is not an ideal solution.” As the article
explains, electrically short antennas — typically 0.1 λ or
shorter — look like a capacitor, with a typical capacitance of
25 pF per meter of length. “At 2 MHz, where the wavelength is
150 meters, an inductor of 84 μH is required for resonance,”
the article says. But just getting a good VSWR is not all
there is to it. Rohde told ARRL that loading coil placement in
a short vertical antenna is critical, and “the greater the
elevation of the coil, the better the radiation. He said that
“center loading” — he considers the “best compromise” to be
more on the order of two-thirds’ loading — can dramatically
affect both the antenna’s transmitting and receiving
performance, as opposed to base loading, as found with popular
so-called screwdriver antennas. Radials of some sort also are
essential. As the article points out. “With center loading,
both the radiation resistance and integrated surface are
larger, which are better for radiation.” Inductors are the
lossy components of an antenna tuner, while capacitors “are
infinitely better.” The authors conclude that, for optimal
operation, antenna radials should be 0.25 λ, with one
sufficient for tuning, and up to four producing a symmetrical
azimuth. “Connecting the HF radio ground to a large metallic
object is a good choice,” the article said. Ulrich told ARRL
that optimizing an antenna in the manner the article describes
will produce “significantly better” signal reception, although
a short antenna will also have a narrower bandwidth. The
objective should not be to get a good VSWR but to keep in mind
that there’s a difference between resonance and radiation.
“These requirements for optimum antenna performance make HF
manpack radios somewhat complicated and unattractive,” the
authors concede.. “Nonetheless, the well-matched and radiating
antenna provides the most success, and some of these highly
portable radios provide vital communications in disaster areas
— recently in Puerto Rico and South Florida.”
http://www.arrl.org/news/tuning-electrically-short-antennas-for-field-operation-is-research-topic
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