Practical_Antenna_Handbook_0071639586
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294 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 36'' 72'' 100'' 120'' 148'' 36'' 36'' 28'' 20'' 28'' 68 5 /8'' Boom to element bracket 4''w x 6''l x 1/4'' Thick aluminum plate 6'x1 1 /8'' OD 6'x1'' OD 5'8''x 7 /8'' OD 4'x 3 /4'' OD 3'x 5 /8'' OD 6'x 1 /2'' OD Centerline of Boom & Bracket Note: Length is to scale. Diameters & tapering are exaggerated for clarity. Figure 12.4A Stepped-diameter 20-m Yagi reflector half-element. Note: All tubing is 0.058'' wall thickness 100 mph wind survival (no ice) 6' or shorter tubing sections (Director & driven element are same except: • both have shorter tip sections • DE is split at boom & must be insulated from boom & bracket.) Dimple or pin nested sections where shown approach, the boom tubing should be reinforced with properly nested reinforcing inserts at each point where U-bolts encircle the boom. For 2-in OD boom material with 0.120-in wall thickness, 0.175-in OD tubing makes excellent inserts. All inserts should be pinned to the outer tubes with two stainless steel bolts at right angles to each other. Even after specifying a boom length there are countless ways to design a threeelement Yagi. The designs of Table 12.2 are based on the following “wish list”: • Free-space forward gain of 8 dBi or greater across the design bandwidth • Front-to-back ratios of 25 dB or more across the design bandwidth • Driven element offset slightly from the boom-to-mast bracket location but still well within reach from the tower top or mast • No need for boom or element truss supports • Input impedance “well behaved” and compatible with simple matching schemes • All element tubing less than 6 ft long for easy transportability and reduced scrap • Lightweight low-profile elements with 100 mph wind survival (60 mph with ½-in ice) Aluminum tubing of various diameters—all with 0.058-in wall thickness—is typically used in the construction of HF Yagis so that tubing in increments of 1 ⁄8-in OD can be nested (telescoped). Determining the length of each section is not a simple matter of “eyeballing” the element and selecting segment lengths that look about right. Instead, an understanding of mechanical design considerations and knowledge of materials strengths are required. Although not an easy read the first time through, Leeson’s Physical Design of Yagi Antennas does a superb job of relating those matters to the design of

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 295 Element bracket Mast U-bolts Aluminum boom-to-mast bracket Insert (pinned) 6'' Aluminum tubing insert under element bracket 0.175'' O.D. 0.12'' wall U-bolt Boom U-bolts 5' Aluminum tubing 6061-T8(drawn) 0.175'' O.D., 0.12'' wall centered & pinned at junction of boom sections Mast (2'' heavy duty galvanized steel) 10' Aluminum tubing 6061-T8(drawn) 0.12'' wall 2'' O.D. 6' 6' 2' 6' Insert 6'' long Insert 12'' long Insert 48'' long Insert 6'' long Alternative boom assembly for 6' with reinforced center. Figure 12.4B 20-ft Yagi boom with reinforced center section. HF Yagi antennas and provides not just equations (for those who want to “roll their own”) but also specific design examples (for those who simply want to build and install a known good design.) Although it would have been possible to design this Yagi with fewer different diameters of tubing for elements—i.e., with more pronounced steps between nested tubes— the resulting design would have required special techniques for implementing a double CW Mid Phone Reflector–DE spacing 100 7 ⁄8 100 7 ⁄8 100 7 ⁄8 DE–director spacing 134 3 ⁄8 134 3 ⁄8 134 3 ⁄8 Reflector tip length 216 5 ⁄8 214 3 ⁄8 213 1 ⁄2 Driven element tip length 210 207 5 ⁄8 206 3 ⁄4 Director tip length 199 5 ⁄8 197 3 ⁄4 197 Table 12.2 Element Dimensions and Spacings (in inches) for Three-Element 20-Meter Yagi, Optimized for Three Different Frequency Ranges

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 295<br />

Element<br />

bracket<br />

Mast<br />

U-bolts<br />

Aluminum boom-to-mast<br />

bracket<br />

Insert<br />

(pinned)<br />

6'' Aluminum<br />

tubing<br />

insert<br />

under element<br />

bracket<br />

0.175'' O.D.<br />

0.12'' wall<br />

U-bolt<br />

Boom<br />

U-bolts<br />

5' Aluminum<br />

tubing<br />

6061-T8(drawn)<br />

0.175'' O.D., 0.12'' wall<br />

centered & pinned<br />

at junction of boom sections<br />

Mast<br />

(2'' heavy duty<br />

galvanized steel)<br />

10' Aluminum tubing<br />

6061-T8(drawn)<br />

0.12'' wall 2'' O.D.<br />

6' 6' 2' 6'<br />

Insert<br />

6'' long<br />

Insert<br />

12'' long<br />

Insert<br />

48'' long<br />

Insert<br />

6'' long<br />

Alternative boom assembly for<br />

6' with reinforced center.<br />

Figure 12.4B 20-ft Yagi boom with reinforced center section.<br />

HF Yagi antennas and provides not just equations (for those who want to “roll their<br />

own”) but also specific design examples (for those who simply want to build and install<br />

a known good design.)<br />

Although it would have been possible to design this Yagi with fewer different diameters<br />

of tubing for elements—i.e., with more pronounced steps between nested tubes—<br />

the resulting design would have required special techniques for implementing a double<br />

CW Mid Phone<br />

Reflector–DE spacing 100 7 ⁄8 100 7 ⁄8 100 7 ⁄8<br />

DE–director spacing 134 3 ⁄8 134 3 ⁄8 134 3 ⁄8<br />

Reflector tip length 216 5 ⁄8 214 3 ⁄8 213 1 ⁄2<br />

Driven element tip length 210 207 5 ⁄8 206 3 ⁄4<br />

Director tip length 199 5 ⁄8 197 3 ⁄4 197<br />

Table 12.2 Element Dimensions and Spacings (in inches)<br />

for Three-Element 20-Meter Yagi, Optimized for Three Different<br />

Frequency Ranges

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