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Tuning Electrically Short Antennas for Field Operation:

from The ARRL Letter on June 13, 2019
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Tuning Electrically Short Antennas for Field Operation:

An article,"Tuning Electrically Short Antennas for Field Operation," by two well-known amateurs, 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 [that] 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."


The ARRL Letter

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