Practical_Antenna_Handbook_0071639586

334 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
When deployed over normal (i.e., lossy) ground, the Beverage antenna responds to
vertically polarized waves arriving at low angles of incidence. These conditions are
representative of the AM broadcast band, where nearly all transmitting antennas are
vertically polarized, and of incoming long-haul DX signals on the amateur bands. In
addition, the ground- and sky-wave propagation modes found in these bands (VLF,
BCB, MF, and low HF) exhibit relatively little variation with time.
The Beverage works best in the low-frequency bands (VLF through 40 m or so).
Beverages are not often used at higher frequencies because high-gain rotatable directional
arrays—notably, Yagis and cubical quads—have a much smaller footprint than a
set of six or so Beverages would require, are highly efficient, have excellent gain in their
main lobe, are relatively easy to erect to a useful height at a modest cost, and can also be
used for transmitting. In addition, the likelihood of “corrupting” the Beverage’s unidirectional
pattern with horizontally polarized signals increases because the usual height
of the Beverage above ground begins to be a sizeable fraction of a wavelength at the
higher frequencies.
The Beverage performs best when erected over moderately or poorly conductive
soil, even though the terminating resistor needs a good ground. One wag suggests that
sand beaches adjacent to salty marshes make the best Beverage sites (a humorous oversimplification).
Figure 14.2 shows why poorly conductive soil is needed. The incoming
ground wave (broadcast band) or low-angle (long-haul DX) E-field vectors arrive essentially
perpendicular to the earth’s surface. Over perfectly conducting soil, the vertical
waves would remain vertical. But over imperfectly conducting soil, the field lines
tend to bend near the point of contact with the ground. (To say it another way, a perfectly
conducting ground cannot support a voltage or E-field between any two points
on the ground plane. Hence, the only E-field that can exist immediately above a perfect
ground is vertically polarized.)
As shown in the inset of Fig. 14.2, the incoming wave delivers a horizontal component
of the E-field vector along the length of the wire, thus generating an RF current
in the conductor. Signals arriving from the unterminated end of the wire build
up a voltage on the wire as they travel, or “accumulate”, down the wire, but that voltage
is absorbed, or dissipated, in the termination resistor. Signals arriving from the
termination end, however, travel to the feedline end, where they are collected by the
step-down transformer or the feedline itself. If the Beverage is properly terminated
by the transformer in conjunction with the feedline impedance, those signals simply
keep moving through the feedline to the receiver, never to reappear. If there is a mismatch
at the junction of the Beverage wire and the feedline or at the receiver itself, a
portion of the signal headed for the receiver will be reflected back onto the Beverage
wire, but some signal still continues to the receiver. The reflected portion continues to
travel along the Beverage until it arrives at the termination resistor, where it is absorbed.
Are there optimum lengths for a Beverage? Some Beverage experts suggest specific
lengths that provide “natural” cancellation of signals (called a cone of silence) off the
back of the antenna pattern for a given band of frequencies. Some sources state that the
length can be anything greater than or equal to 0.5l, yet others say greater than or equal
to 1l is the minimum size. One camp says that the length should be as long as possible,
while others say it should be close to a factor called the maximum effective length (MEL),
which is