Practical_Antenna_Handbook_0071639586
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298 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 Insulators Rope 0.96L Insulators Rope Insulators Rope Driven element Insulators Rope L (Top view) Figure 12.5 Wire beam especially useful for HF bands. parallel to, and about 0.2 to 0.25 wavelengths apart from, each other. This particular set of dimensions is for a parasitic director, usually (but not always) made evident by the unfed wire being shorter than the driven element. Although the two-element wire beam is the most common, multielement wire beams are also possible. Some leading DXers and top-scoring contesters use wire Yagis of up to seven (!) elements fixed on compass Band 80 40 20 Design frequency 3.600 MHz 7.100 MHz 14.125 MHz Reflector—driven element spacing 38′3″ 19′5″ 9′9″ Driven element—director spacing 35′6″ 18′0″ 9′ 1 ⁄2″ Reflector length 139′4″ 69′3″ 34′7″ Driven element length 134′5″ 67′10″ 34′ 1 ⁄2″ Director length 129′9″ 65′9 1 ⁄2″ 33′1″ Forward gain at design frequency 9.6 dBi 11.8 dBi 12.7 dBi F/B ratio at design frequency 20 dB 25 dB 32 dB F/S ratio at design frequency 20 dB 25 dB 24 dB Z IN at design frequency 85 Ω 22 Ω 17 Ω Matching network l/4 Q-section 4:1 balun 4:1 balun 2:1 VSWR bandwidth 75 kHz (75 Ω) 100 kHz (75 Ω) 200 kHz (75 Ω) Table 12.3 Three-Element Copper Wire Yagi Dimensions

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.

298 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<br />

Insulators<br />

Rope<br />

0.96L<br />

Insulators<br />

Rope<br />

Insulators<br />

Rope<br />

Driven element<br />

Insulators<br />

Rope<br />

L<br />

(Top view)<br />

Figure 12.5 Wire beam especially useful for HF bands.<br />

parallel to, and about 0.2 to 0.25 wavelengths apart from, each other. This particular set<br />

of dimensions is for a parasitic director, usually (but not always) made evident by the<br />

unfed wire being shorter than the driven element. Although the two-element wire beam<br />

is the most common, multielement wire beams are also possible. Some leading DXers<br />

and top-scoring contesters use wire Yagis of up to seven (!) elements fixed on compass<br />

Band 80 40 20<br />

Design frequency 3.600 MHz 7.100 MHz 14.125 MHz<br />

Reflector—driven element spacing 38′3″ 19′5″ 9′9″<br />

Driven element—director spacing 35′6″ 18′0″ 9′ 1 ⁄2″<br />

Reflector length 139′4″ 69′3″ 34′7″<br />

Driven element length 134′5″ 67′10″ 34′ 1 ⁄2″<br />

Director length 129′9″ 65′9 1 ⁄2″ 33′1″<br />

Forward gain at design frequency 9.6 dBi 11.8 dBi 12.7 dBi<br />

F/B ratio at design frequency 20 dB 25 dB 32 dB<br />

F/S ratio at design frequency 20 dB 25 dB 24 dB<br />

Z IN at design frequency 85 Ω 22 Ω 17 Ω<br />

Matching network l/4 Q-section 4:1 balun 4:1 balun<br />

2:1 VSWR bandwidth 75 kHz (75 Ω) 100 kHz (75 Ω) 200 kHz (75 Ω)<br />

Table 12.3 Three-Element Copper Wire Yagi Dimensions

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