Practical_Antenna_Handbook_0071639586

336 p a r t V : h i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
ages are in the 250- to 300-ft range; they have made it possible to pull weak stations out
of the noise on 160 and 80 m on so many occasions it is hard to believe there is much
improvement left to be gained by lengthening them. Of course, the longer a Beverage
is, the narrower the main lobe—so in principle the greater the number of them you need
to cover the compass rose.
Like the nonresonant longwire antenna, the Beverage needs a termination resistor
that is connected to a good ground. This requirement is somewhat in conflict with the
assumption that Beverage antennas work best over lossy ground, but the solution is to
make a good artificial RF ground with radial wires at the terminated end. As in the longwire
case, insulated or bare wires, at least 0.2l long on the lowest frequency of interest,
make the best radials. However, a substantial improvement in the ground is possible
using wires only 15 to 20 ft in length (which is much less than l/4 at the frequencies of
interest), atop or just below the surface of the soil. Some experienced Beverage users
recommend just one or two radials for each band, laid out at right angles to the Beverage
wire itself. Bear in mind that while a ground rod driven into the earth at the terminating
resistor makes a great mechanical tie point for joining the end of the Beverage to
the resistor and any radials, it makes a lousy RF ground. The author has had good results
(maybe not “perfect” results, but . . .) with two 0.1l radials for each amateur band
of interest, oriented at right angles to the Beverage wire itself. Wires in close proximity
to ground—even lossy ground—should be thought of as providing nonresonant capacitive
coupling rather than resonance, so their exact length is not important.
In addition to the radials and ground rod, Misek also recommends using a wire connection
between the ground connection at the termination resistor and the ground connection
at the receiver transformer. According to Misek, this wire helps to stabilize the
impedance variations at higher frequencies.
The basic single-wire Beverage antenna of Fig. 14.1 consists of a single conductor
(#18 to #8 wire gauges are most common) erected about 8 to 10 ft above ground. Because
the RF power in a receiving Beverage is minuscule, the size and metallic composition of
the wire are primarily set by mechanical (strength) and metallurgical (corrosion lifetime)
considerations, rather than electrical. In contrast to most antennas, the greatest threat to
Beverage longevity comes from dead trees and limbs falling on them or animals running
into them. #14 THHN copper wire from building supply outlets is probably the most
consistently economical source, although the antenna may have to be retensioned periodically.
Some have used copper-clad steel or electric fence wire, but the author has
found that the use of alternative metals or bimetallic wires often brings with it a new set
of problems greater than the slight stretchiness of softdrawn copper wire.
Some Beverages are unterminated (and, hence, bidirectional), but most are terminated
at one end in a resistance R equal to the antenna’s characteristic impedance, Z 0 .
For the heights discussed, Z 0 is likely to be between 300 and 800 Ω. The receiver end
should also be terminated in its characteristic impedance, but ideally requires an
impedance-matching transformer to reduce the antenna impedance to the 50-Ω or 75-Ω
standard impedance used by most modern HF transceivers and receivers.
Installation of the Beverage antenna is not overly critical if certain rules are followed:
• For safety of humans and animals, install the antenna at a height of 8 to 10 ft
above ground. Check its entire length frequently; because they are so low,
Beverages have an annoying habit of collecting fallen branches or entire trees!