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 299
headings of most interest or use to them. One of the most frequent uses of wire beams
is on the lower bands (e.g., 40, 75/80, and 160 m), where rotatable beams are more expensive
and often far more difficult (and expensive) to install at useful heights. Bear in
mind, however, that the small element diameter of wire beams leads to a narrower
bandwidth and a slight reduction in the maximum obtainable forward gain.
Dimensions for some three-element wire Yagis are provided in Table 12.3. In all
cases, #12 uninsulated wire is used, and the Yagis are assumed to be about 70 ft above
average ground. These beams give up about 0.5 dB of forward gain relative to ones made
of aluminum tubing in order to get reasonable F/B and F/S ratios and a good match to
either 50-Ω or 75-Ω feedlines through a 4:1 balun. Since the height chosen is a smaller
percentage of a wavelength as the frequency decreases, there is greater ground loss at the
lower frequencies and the elevation angle of peak forward gain increases. Using a 4:1
balun to bring the feedpoint impedance up to something compatible with commonly
available coaxial lines, the 2:1 SWR bandwidth for a 75-Ω system is about 1.5 percent of
the center frequency. The dimensions in this table are not correct for Yagi elements made
of aluminum tubing and will be slightly off for different heights above ground.
Multiband Yagis
Largely because of the costs and complexities associated with erecting multiple beams
high in the air, amateurs and professionals have developed a variety of approaches to
covering more than one frequency from a single boom:
• Interlaced elements
• Trapped elements
• The log-periodic beam
• Adjustable (motorized) elements
In principle, beams with interlaced elements are easily understood; in practice, the
electrical interactions and mechanical issues that are created by putting all these elements
on a shared boom add to the design complexity and cost, and almost always lead
to some compromises in performance. Years ago, most beams with interleaved elements
were designed with manual calculations, supplemented by experimental results
at test ranges. Today, a lot of the drudge work is handled by antenna modeling programs
and the design of an effective multiband beam can be optimized far quicker.
One advantage of interlaced beams is that the user can opt to have a separate feedline
for each band. In some applications (such as amateur multitransmitter stations),
this is highly desirable. One disadvantage of interlaced beams is that they can be mechanical
nightmares to construct, erect, and keep up in the air. Another disadvantage
(common to all multiband beams, it should be noted) is that often the performance on
one or more of the covered frequency ranges is compromised. Nonetheless, for some
users they represent the best solution to a specific need.
Arguably the most popular of the multiband antennas is the trap tribander (sometimes
a five-bander if provision has been made for the 12-m and 17-m WARC bands).
Although most tribanders are sold into the amateur market, trapped tribanders designed
for key shortwave broadcast bands or long-haul military HF comm links are also
available.