Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 309
limited range. In an even simpler configuration, one or more lower beams can be fixed
in position by being mounted directly to sides of the tower at the appropriate height(s).
A DXer or contester in the northeastern United States might, for instance, attach beams
aimed at 45 degrees (to maximize coverage into Europe) and 135 degrees (for South
America and the southern half of Africa) directly to the tower at points roughly a wavelength
below the top beam.
In practice, stack spacings on a guyed tower will be determined as much by guy
wire attachment heights and element clearance distances as by any theoretical projections
of stacking gain versus spacing. Luckily, exact stack spacing (stacking distance) is
not critical.
Because the optimum stack spacing is expressed in terms of the number of wavelengths
between the top and bottom beams in the stack, one might conclude that only
monoband beams can be stacked with good results. That isn’t true. Because the stacking
gain curve exhibits such a broad peak as a function of spacing, multiband beams covering
a 2:1 frequency span can be spaced as much as 3l/4 apart at the lowest frequency of
operation, corresponding to a spacing of 3l/2 at the highest frequency. Thus, amateur
HF tribanders or five-banders (when the 12- and 17-m WARC bands are included) covering
a 2:1 frequency span can be stacked with very competitive results.
The normal assumption with stacking is that the feedline configuration to each beam
in the stack is identical in length and impedance—a “T” connection, in other words, as
shown in Fig. 12.9—thus applying roughly equal transmitter power in phase to the two
Yagis. Enterprising HF DXers and contesters have found, however, that a useful adjunct
to the two-beam stack is a coaxial cable relay control box with a four-position switch and
associated transmission line phasing sections that allow the user to select beams in phase
(BIP), beams (180 degrees) out of phase (BOP), upper beam only, or lower beam only. All of
these choices are useful at one time or
another not only because we are usually
interested in communicating over a
wide range of distances but also because
Upper
feedline
Relay
switching
Lower
feedline
To station
/2 to 3
2
/2
Upper & lower feedlines should be exactly
the same length, measured from antenna feedpoint
to switch box.
h
the optimum elevation angle for HF ionospheric
communications is always
varying.
One advantage to using one or more
fixed lower beams or having the upper
and lower beams independently rotatable
is that transmitter power (and receiving
capability) can be split between
two different directions at the same
time. Thus, if looking to establish communications
with two different parts of
the world, the upper beam might be rotated
in the direction of one continent
while the lower beam is aimed at another.
Figures 12.10 and 12.11, provided by
W. L. Myers, K1GQ, are plots of antenna
Figure 12.9 Feeding stacked Yagi antennas.