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

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.

This effectively converts a 100-watt radio at the higher frequency to a 10-watt radio at the lower one, and relegates the performance to something more equivalent to the commonly used vehicular whip antennas than it does to a full-size dipole. This does not however eliminate the loop from one of its most important military applications, that of a small vehicular on-the-move antenna. It does require that care be taken in trading off antenna size, radiation efficiency, and transmitter power.

Loops can be arranged with the plane of the loop vertical or horizontal. Both give satisfactory performance for modern land HF combat communications. The horizontal configuration produces more lower-angle radiation useful for long distance (low angle) communication and surface wave (ground) LOS systems, at the expense of the near-vertical radiation required for NVIS (near vertical-incidence skywave) operation. NVIS is the most useful mode for operation in theater/corps-size areas so many of the world's armies (ie. Russia, China and Norway) have opted for this orientation.

The gain and pattern of a vertical small loop is shown in Fig. 4, and a horizontal small loop in Fig. 5.

[FIGURES 4-5 OMITTED]

Understanding efficiency is the key to understanding and effectively using small loops. Assuming the loop-tuning mechanism balances the inductive reactance of the loop itself with the capacitive reactance of the tuning capacitor, then the feedpoint impedance of the loop is the radiation resistance plus the loss resistance. In all radiating structures, radiation resistance increases with length, so we would expect the radiation resistance to be pretty small.

A common relationship for the radiation resistance of a small loop is:

Rr = 197 [Circumstance/ Operating wavelength ]4 (eqn 1)

If the loop circumference is 1/4 wavelength the radiation resistance is about 0.77 ohms--about a hundredth that of a full-size dipole--but then that's necessary to get the very large loop currents we're after.

If radiation resistance is the "good" resistance (that representing the conversion of applied radio frequency energy into radiation field) then the "bad" resistance is the "loss resistance." It includes the skin-effect resistance of the loop conductor plus the resistance of all joints and connections. If the connections are kept to a minimum and well-made the main loss is in the tuning capacitor and in the skin-effect resistance of the loop material itself. It is crucial the loop be made of a highly conductive material, and that it be large in cross-section. Assuming the loop is copper, the relationship for skin-effect loss resistance is:

Rs = 9.96 x 10-4 * "f * S / d

Where R is in ohms

f is the frequency in MHz

S is the circumference in feet

d is the conductor diameter in inches

Note that loss resistance changes as the square root of frequency, while radiation resistance changes as the fourth power of frequency. As frequency decreases from the loop's upper design frequency the radiation resistance decreases as the fourth power of frequency while the loss resistance decreases much more slowly. Efficiency is calculated as:

Efficiency = Rr / (Rr + Rloss)

Which decreases as the 3.5th power of frequency. This is why efficiency falls off so badly near the bottom of the frequency range. This is much easier to visualize on a graph. These data were computed for a loop designed for operation up to 30 MHz and the results are plotted in Fig. 6.

[FIGURES 6 OMITTED]

There are two other issues. The first is bandwidth. A radiating structure involving very low resistance and very high reactance is the definition of a high Q circuit, and such circuits have very narrow bandwidth. This means the tuning capacitance will have to be adjusted with even the smallest change in operating frequency.

The second issue is the tuning capacitor itself. It must be adjustable over the required range of values for the specific loop design, and must withstand the substantial voltages (easily several thousands of volts) that appear across it. In the case of "simple" air dielectric variable capacitors (see Fig. 7) this amounts to large spacings between the plates, which, to achieve the required capacitance, involves very big plates.

[FIGURE 7 OMITTED]

There are alternatives to air variable capacitors, the two common ones being vacuum variable capacitors (although the large glass enclosure makes them somewhat fragile for military purposes) and discrete component capacitors that are switched in and out of the circuit as needed. The switches have to withstand the very substantial r.f. current flows. One such switch is made by Kilovac Corporation and amounts to a vacuum relay. Even though fairly small (2-inch diameter) these relays are rated for 25,000 volts and 30 amps making them appropriate for most loop applications.

Whether the tuning capacitor is a rotary (air or vacuum variable) or made of individually switched components operation is much easier if adjusting the loop tuning capacitance for loop resonance is done automatically. This requires some kind of specially designed automatic loop tuner. Such equipment exists and is available in one form or another from loop antenna manufacturers. It is critical to mention, however, that the antenna tuner in a commercial radio probably isn't going to do loop tuning more than once. The high current and especially the high voltage dramatically exceeds the design parameters of these commercial tuners and the odds one would survive loop operation are extremely small. Military tuners and radios will have a somewhat better chance, but the real answer is a purpose-designed loop tuning system.

While there are some challenges with the successful design and application of loops it has been done quite successfully since well before WWII. The following photographs illustrate some more contemporary applications.

[FIGURES 8-10 OMITTED]

ACRONYM QUICKSCAN

CNR--combat net radio

GHz--gigahertz

HF--high frequency

JTRS--Joint Tactical Radio System

LOS--line-of-sight

MHz--megahertz

NVIS--Near Vertical Incidence Sky-wave

Ohm--unit of electrical resistance

R&D--Research and development

Mr. Farmer is a Vietnam-era Signal soldier and former lieutenant colonel in California's State Military Reserve, where he ran intrastate emergency communications. He's a graduate of USMC Command and Staff college. He's a professional engineer, has an extra-class Amateur Radio license and is president of EFA Technologies, Inc., in Sacramento, Calif. He has a bachelor's degree in electrical engineering and a masters in physics, both from California State University. He has published three books and more than 40 articles, holds four U.S. Patents and is a frequent guest speaker at communications and antenna-oriented conferences.

COPYRIGHT 2004 U.S. Army Signal Center
COPYRIGHT 2004 Gale Group