ANTENTOP- 01- 2014 # 018

Buried Antennas for Emergency Communications

The purpose of this article is to review some of the types of buried antennas which might be useful for amateur emergency communications and to present some of the results the author obtained with a buried 40- meter antenna.

Buried antenna properties

Because a buried antenna is immersed in a very lossy medium and because of the sudden difference in medium which, a radiated wave encounters at the interface of earth and air, a number of factors are drastically different for buried antennas as compared to an antenna in air.

Because of the antenna being in a different medium, the length/impedance versus frequency characteristics are different. For instance, a simple dipole buried 1 inch in soil of moderate conductivity would be about 17 and half feet long for 20 meters and have a center impedance of about 450 ohms. As may be imagined, these figures are very dependent upon the exact conductivity of the soil.

Because of the interface between earth and air, the radiation from a horizontal, buried antenna when it reaches the surface sets up a vertically- polarized ground wave. This factor, of course, is ideal for emergency communications work with vertically- polarized mobile stations.

Antenna forms

Many different forms have been tried for buried antennas and even complex directive arrays have been constructed. However, for amateur purposes, the dipole and 100 feet long wire are probably most useful forms. (Figure 1)

The formula for the length of a dipole depends upon ground conductivity as well as other factors and would not be of much use to the average amateur.

The best procedure for constructing a dipole is simply to cut it to 80% of the free- space length and then trim the ends equally until the lowest SWR is achieved. If buried in an area where ground conditions remain stable, the length does not have to be changed again.

In areas where ground conditions and surface conditions (snow, extreme changes of vegetation) are not stable, the 100 feetlong wire should be used.

Although an antenna coupler, such as a trans-match, is required to allow compensating for impedance changes with varying ground conditions, the antenna can then also be used for multiband operation. In typical soil the input resistance of such an antenna will vary from 50 to 600 ohms and the reactance from +- j400 ohms over the 2- 20 MHz range. The first resonance will be between 750 and 1800 kHz, which makes it effective from 80 meters on down. This type of antenna has been used by the Army in Viet Nam with good result over short tactical distances.

Construction

Whether a dipole or 100 feet long wire is used, the wire used for construction must be insulated along its length from the soil and care must be taken that moisture does not penetrate the tips of the wire or the connection to the feed line. Teflon insulated wire, numbers 22 to 26, is particularly suitable. Perhaps a less expensive method is to run plain rubber insulated wire inside plastic hosing. The ground connection for the 100 feet long wire can be a standard 4 or 5 feet TV type ground rod.

Efficiency

Many methods have been used for measuring the efficiency of buried antennas. Perhaps the most realistic for amateur purposes is to compare the field strength from a buried antenna to a good, surface quarter- wave vertical. Experiments made on this basis have showed buried antennas of the dipole and 100 feet long wire variety, when compared to a surface antenna resonant at the same frequency, to be about 40 db down for a burial depth of 1 inch. Roughly, this is about twice the order of magnitude reduction in signal strength as would take place between a 8 inch loaded 80- meter whip and a full size quarter- wave 80- meter vertical.

Experimental results

The author constructed a 100 feet long wire buried about 8 inches and operated on 40- meters. No impedance measurement were made but proper loading could be easily achieved with the use of a transmatch- type coupler, although some retuning was necessary periodically depending on whether the soil surface was moist or dry.

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