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 in soil of
moderate conductivity would be about 17 ½
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.
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 long wire are probably
most useful forms. (Figure
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
long 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.
Whether a dipole or 100 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 long wire can be a standard 4 or
5 TV type ground rod.
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 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. Roughly, this is about twice the
order of magnitude reduction in signal strength as would take
place between a 8 loaded 80- meter whip and a full size quarter-
wave 80- meter vertical.
The author constructed a 100 long wire buried about
8 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.