Practical_Antenna_Handbook_0071639586

308 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
Some implementations of the Moxon utilize wires with spreaders (visualize the
driven element of a cubical quad pointing straight up in the air), some use aluminum
tubing for the element centers (i.e., the portions at right angles to the boom) and wires
for the element ends, and others use aluminum tubing for the entire radiating structure.
One recent implementation converted a Cushcraft 40-2CD two-element shortened Yagi
to a Moxon, thus trading loading coil losses in the original design for the possibly
smaller losses of full-length elements with bent ends. A properly designed Moxon also
boasts higher F/B ratio than the typical two-element Yagi.
The dimensions shown in Fig. 12.8 for a 20-m Moxon rectangle constructed with #12
wire yield a free-space forward gain of 6.14 dBi (or 4.0 dBd) in conjunction with F/B
and F/S ratios of nearly 24 dB at the design frequency of 14.1 MHz. Z IN = 51 + j0.4 Ω at
the design frequency, and the SWR stays below 1.6:1 across the entire band. If all six
segments of the 20-m design in the figure are scaled according to wavelength, the design
should translate easily to other bands while retaining the 50-Ω resistive input impedance.
Stacking Yagi Antennas
There are two distinctly separate reasons to stack antennas on a single mast or tower:
• Desire for increased gain and pattern flexibility from interconnecting two or
more antennas for the same frequency
• Need to put multiple bands on one support because of limitations (space, funds,
zoning, etc.) preventing the use of multiple supports
Stacking for Gain
As we saw in Chap. 5, splitting the available transmitter power between two identical
antennas can lead to increased signal strength in certain directions far from the antennas,
at the expense of signal in other directions. Extensive experimentation and computer
modeling have shown optimum HF or VHF stack spacings for a pair of identical
beams mounted one above the other to be in the one wavelength range. Under ideal
conditions, stacking two 3-element Yagis will provide the same peak forward gain as a
single 6- or 7-element Yagi on a much longer boom—often at much lower support cost.
However . . . the improvement from stacking will be impaired—or even
Ânonexistent—if the upper and lower beams are too close together or if the lower beam
is too close to ground. Generally, stacking will be a disappointment if the lower beam is
not l/2 above ground or more. Of course, the higher the frequency, the more flexibility
is possible on a support of a given height. A 70-ft tower is just about the shortest that
can host a two-Yagi, 20-m stack, while an experimenter can put a 10-m stack at many
different heights and spacings on the same tower.
To enjoy the benefits of stacked Yagis around the entire compass rose, the obvious
(and expensive!) approach is a rotating tower—or a hybrid tower where only the portion
from the lowest beam up rotates. But in many instances, coverage of a full 360 degrees
is not a necessity, and the lower beam in the stack can attain as much as 300
degrees rotation by mounting its rotator on a short outrigger mast supported a foot or
two from the nearest tower leg at both ends of the mast. Some modern rotator controllers,
such as the Green Heron units, can be interconnected in a master/slave configuration
so that the lower beam follows the commands for the upper through the more