Practical_Antenna_Handbook_0071639586

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