Tuning Electrically Short Antennas for Field Operation

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Tuning Electrically Short Antennas for Field Operation

Bob McGraw - K4TAX
 From another source.   F Y I --


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.”


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