Mobility favors small antennas: small-loop high-frequency antennas

Army Communicator, Spring, 2004 by Edward J. Farmer

In our modern suite of communication options, high-frequency radio has the unique property of requiring no infrastructure. A complete voice and data radio station is easily man-portable and capable, with proper use, of communicating with any other spot on earth.

When the German army was developing the doctrine that became Blitzkrieg it was obvious from the outset that a paradigm shift in communications was essential. Heinz Guderian, the architect of Blitzkrieg" said, "I want to command over the radio from the front, not talk about it in the rear on a telephone." Since he was originally commissioned as a signal officer and spent much of his career with issues related to staff organization and communication, he had an unusual perspective on the essential roll of communications in maneuver warfare, and how it could be achieved.

A complete HF radio system is easily man-portable, but performance improves with the size of the antenna--and a full-size antenna can be over a hundred feet long. Mobility favors small antennas, and the "holy grail" of HF antenna research a physically small antenna capable of "full-size" performance. One of the notable efforts along the way, but certainly not the holy grail, is the small loop.

[FIGURE 1 OMITTED]

Small-loop antennas have been around for a very long time. While opinions vary as to whether the antennas were loops or top-loaded monopoles, the German army in WWII fielded a number of scout and command vehicles with loop-like antenna structures. Probably the most famous is Erwin Rommel's command vehicle, as seen in Fig. 2.

[FIGURE 2 OMITTED]

The idea of a loop antenna comes from the realization that radiation field is the space integral of antenna current over distance. Long antennas with low current produce the same field intensity as small antennas with high current. The problem becomes designing a radiating structure that promotes the flow of very large radio-frequency currents. The obvious "cut-to-the-chase" answer is, "make a closed loop." If the loop circumference is fairly small its radiation resistance will be small. Because such a structure will be inherently inductive there will be some inductive reactance opposing current flow, but it can be easily eliminated by adding some series capacitance to form a series-resonant circuit. In such a situation, the net reactance is zero and the resistance is the radiation resistance plus the loss resistance of the loop, both of which are very small--perhaps even less than an ohm. This "short circuit" promotes the flow of huge currents and therefore the possibility of large fields from physically small structures.

As the circumference of the structure increases, so does the radiation resistance. Also, the phase of the antenna current in one place is sufficiently different from the phase of the current in another that the radiation pattern becomes a strong function of the frequency of operation, and the expected performance only occurs near the design frequency. This causes such a loop to behave more like the linear antennas with which we are more familiar. A classical "full size" loop has a circumference of one wavelength at its intended operating frequency, and isn't especially useful for military purposes.

The "small loop" term is usually reserved for closed-loop antennas in which the current around the loop is more-or-less in-phase, so the loop antenna can be treated as a magnetic dipole. This criteria limits the antenna to a circumference of about 1/4-wavelength at the highest frequency at which it is to be used. Also, it becomes harder and harder to match a radio to a small loop as the frequency increases--the feedpoint impedance becomes quite large and extremely reactive. Matching a radio to a small loop is one of the very interesting engineering challenges of loop antenna engineering.

The components of a small loop are shown in Fig. 3.

[FIGURE 3 OMITTED]

The advantage of a small loop, at least at the high end of its frequency range is that it provides gain and patterns very similar to what one would expect from a full-size (1/2-wavelength) dipole at the same frequency. This is a huge advantage--a physically small, lightweight, easy-to-deploy antenna that provides about the same performance normally obtained only after three Soldiers do 15 to 30 minutes work erecting masts and stringing wire.

There are two significant limitations. First, loops are sensitive to objects moving in their vicinity (near field) so re-tuning can be a frequent requirement.

Second, as frequency decreases from the size-defining highest frequency so does efficiency. While a loop will theoretically operate at any lower frequency the efficiency decreases so significantly that practical issues restrict it to about an octave (2:1 frequency range), so the lowest frequency is generally assumed to be about half the highest frequency. While the antenna's pattern remains the same as frequency decreases, the loss in efficiency dramatically reduces the gain. At the lower frequency the loop's gain will be down by about 10 dB from what it was at its highest frequency.