Practical_Antenna_Handbook_0071639586

284 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
unacceptable: they may be mechanically too fragile or too heavy, but most likely they’re
just plain unobtainable. If all six of the listed dimensions are not scaled by the same factor,
performance on the scaled band(s) may well be less impressive than expected. A
practical approach is to use simple scaling to get into the ballpark with dimensions for
the new band based on the original band, then use antenna modeling software (Chap.
25) or experimentation to fine-tune all the dimensions so as to optimize performance
around standard aluminum tubing diameters or copper wire gauges.
Many different Yagi designs exist. In fact, the potential number is infinite, limited
only by users’ willingness to experiment or to use modeling software to examine various
dimensional configurations. One reason for having many different designs is that
usually one or more characteristics of the antenna must be compromised. Thus, one
design may be developed for a three-element Yagi with wide SWR bandwidth, while
another may favor a high F/B ratio over a narrow bandwidth centered on the design
frequency.
Some designs are based on mechanical tradeoffs rather than electrical performance
specifications. One such commonly encountered constraint is total boom length. (A
manufacturer might choose a shorter boom length to keep material costs and antenna
prices down. An amateur backyard experimenter might choose a shorter boom length
because of limitations of his rotator or proximity of a nearby tree.)
For obvious reasons, boom lengths of 20 ft or slightly less are very popular. In particular,
3-in irrigation pipe makes a very strong boom. (But there are many commercial
Yagis for those bands that butt two suitably thick-walled 10-ft aluminum tubes together
to form a 20-ft boom—usually with interior and/or exterior reinforcing sections for
some distance on either side of the junction.)
In the 85 years since the publication of the original Yagi-Uda paper, practicing scientists
and engineers, along with amateur experimenters, have continued to improve
the design. While much of Uda’s later work was specifically aimed at the burgeoning
VHF consumer television market, shortwave broadcasters and the U.S. military joined
hams in bringing the Yagi to the HF bands. Here is a summary of some of the most important
things to keep in mind when working with Yagi antennas:
• Exact interelement spacings are not critical on a Yagi; they are typically 0.1 to 0.3
wavelengths for popular proven designs. Once established for a design,
however, changes to spacings interact with element lengths and do affect Yagi
performance.
• Wider element spacings (for a given number of elements) provide greater SWR
bandwidth and more constant forward gain and F/B ratio across the entire
band. Of course, wider element spacings translate to longer booms!
• Maximum forward gain and maximum F/B ratio seldom occur at the same
frequency; in general, longer boom lengths (for a given number of elements)
make it easier to optimize multiple parameters over a wider common range of
frequencies.
• As a glittering generality, Yagi gain is proportional to boom length. Stuffing
additional elements onto a fixed boom length usually worsens performance.
• A reflector 3 percent longer than a half-wavelength in the conductor is a good
starting point. (Said another way, start with a reflector length that is no longer
than l/2 in free space or air.)