RF ground is a vague term: People claim they know what it is when they see it, but can't define it. Many radio amateurs are easily misled by untruths about RF grounds. What follows is by no means the last word on RF grounding, but should help fill a void in the radio amateur's literature.

In my opinion, an RF ground is something that presents a low impedance at all frequencies of interest on the desired ground surface. All frequencies of interest usually means just the transmission frequency and all spurii. Spurii usually include harmonics, but may include mixing products. The ground surface is the tricky part. What do you want to be at ground potential? Surely you don't expect your microphone to be at ground potential on all frequencies, do you? Think about it: A coiled microphone cord is a bigger radiator than many "rubber duck" antennas! With proper matching, you could probably make a better antenna out of the mic cord shield! In this case, low impedance means "small reactance and resistance." Some radio amateurs just look at the resistive part of the impedance and forget about the reactance, which often is huge!

The size and shape of conductors are very important in evaluating a ground surface. Take, for example, a tall, thin aluminum structure that is grounded at one end. No matter how well you ground that one end, the structure still radiates and receives RF energy--that's why it's called an antenna. The shape that offers the lowest impedance is a sphere. (The Earth is a pretty good approximation of a sphere.) Size is also important--you wouldn't expect a metal-covered tennis ball to present a low impedance on 160 meters.

Now that we have some idea what a ground should be, the question remains as to how to achieve it. It's pretty ridiculous to have to buy a large metal sphere the size of the Earth just to get a good ground (although it would work well!). The closest practical approximation--a short, thick wire connected to the Earth--works pretty well as long as the wire is much shorter than a wavelength. Notice that I said wavelength. If you expect your "good RF ground" to eliminate the second harmonic of a 10-meter signal (56 MHz), you need a wire much shorter than 8 feet long. (No, four feet is not much shorter, and would not work. In fact, a 4-foot "grounding wire" hooked up to the Earth might be a good radiator at 56 MHz. Much shorter means 2 feet or less in length-at 56 MHz, that is.)

Following this line of reasoning, it may seem impossible to get a good ground at UHF. Actually, you can get a good ground at UHF by using a large metal plate, which is often called a ground plane. A large, flat surface also presents a low impedance at UHF. (Yes, a "flat Earth" would work just as well as a spherical one-at least in terms of grounding-if you were far enough from its edge.)

But if you live in a second-floor apartment (as I do), how do you get a good HF ground? First, you should determine whether or not a ground is needed. (An ac ground should be considered a necessity for those operating equipment from commercial power lines.) Many antennas, however, such as dipoles and loops, operate just fine without an RF ground.

As for TVI, many amateurs mistake TV fundamental overload as a grounding problem. (It is highly unlikely that TV sets will ever be designed to operate in a strong RF field.) First, install a high-pass filter on the TV set. If the transmitter is indeed radiating energy on a TV channel, try to improve the transmitter shielding, and try an absorptive harmonic filter (See the 1988 ARRL Handbook, p 40-9). Grounding will help if the chassis or mic cord of the rig is hot with RF. In this case, a quarter-wave, tuned counterpoise will provide grounding at one frequency (and its harmonics) on a small surface area. Grounding may or may not help situations involving RF feedback. It may be easier to dissipate and isolate the RF from various cords and wires using ferrite beads and toroids.

Some people have asked me about measurement techniques for evaluating RF grounds. Well, if you connect a meter between two points that are at the same RF voltage, you may get a nonzero reading anyway! The meter leads can act as a loop antenna that can be expected to pick up RF fields. If you find an easy way around this problem, let me know about it.-Zachary Lau, KH6CP, ARRL Lab Engineer

The US Army Signal Corps recently reported on the results of improved grounding techniques for mobile field tactical radio stations.1These mobile stations are usually installed in trucks, and use vertical antennas. The ground system consists of a cable connected to a copper rod driven several feet into the earth. The typical ground resistance and RF impedance obtained, therefore, are often not optimal.

In order to improve the effective ground, field tests were made employing a number of ground rods mechanically connected in parallel around the mobile radio station. The ground rods were driven into the earth at various depths, and the resultant ground resistance and RF impedance measurements were recorded. Then the tests were repeated with ground rods mechanically connected in series and driven to different depths in the earth.

Analysis of the recorded data and the field radio transmission tests indicated that the use of four series-connected ground rods, driven only a foot or two into the earth, provided the most efficient ground system. These improved field grounding techniques should be applicable to Amateur Radio stations, particularly during Field Day operations.-Lt Col A US (ret) David Talley, W2PF, Suite 1533-S, 10275 Collins Ave, Bal Harbour, FL 33154

1 Signal, Mar 1988, pp 79-80