Practical_Antenna_Handbook_0071639586
0111057 FIG 17-01B 442 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s One approach to neatly joining the coaxial cables is shown in Fig. 19.12. In this example, three SO-239 coaxial receptacles are mounted on a metal plate. This arrangement has the effect of shorting together the shields of the three ends of coaxial cable while the center conductors are connected in the manner shown. This method is sometimes provided as part of a commercial antenna; the author’s commercial 6-m Yagi has just such a bracket on it, the only difference being the use of BNC connectors. Because the balun of Figs. 19.6 and 19.12 is mounted at the antenna feedpoint and, hence, is almost always located outdoors, special care must be taken to weatherproof the ends of the transmission line—especially the long run back to the transmitter. The delta match gets its name from the fact that the physical layout of the matching network used when tapping out on the two sides of a driven element or dipole a distance corresponding to the transmission line impedance has the shape of the Greek letter delta. Figure 19.13A shows the basic delta match scheme. The delta match connections to the driven element are made symmetrically on both sides of center, and can be made from brass, copper, or aluminum tubing, or a bronze brazing rod bolted to the main radiator element. The width (A) of the delta match is given by A(inches) = 1416 F(MHz) (19.10) while the length (B) of the matching section is 1776 B(inches) = F(MHz) (19.11) The transmission line feeding the delta match is balanced line, such as parallel transmission line or twin-lead. The exact impedance is not terribly critical because the Cable ends or solder lugs Solder Solder Mounting plate SO-239 connectors Figure 19.12 Practical implementation of 4:1 balun using connectors.
C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 443 L A D D B Center line Delta match 468 L F(MHz) 600- line 118 A 2D F(MHz) 148 B F(MHz) Figure 19.13A Delta feed matching system. dimensions (especially A) can be adjusted for a good match. Although 450-W line is commonly used, 300-W or 600-W lines are equally good alternatives. Figure 19.13B shows a method for using coaxial cable or hard line with the delta match. The 50- or 75-W coax or hardline impedance is transformed up to the delta match impedance with a 4:1 balun transformer such as the one in Fig. 19.6. Center Driven element Metal rods BALUN Figure 19.13B Practical VHF delta match.
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C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 443<br />
L<br />
A<br />
D<br />
D<br />
B<br />
Center<br />
line<br />
Delta match<br />
468<br />
L F(MHz)<br />
600- line<br />
118<br />
A 2D<br />
F(MHz)<br />
148<br />
B F(MHz)<br />
Figure 19.13A Delta feed matching system.<br />
dimensions (especially A) can be adjusted for a good match. Although 450-W line is<br />
commonly used, 300-W or 600-W lines are equally good alternatives. Figure 19.13B<br />
shows a method for using coaxial cable or hard line with the delta match. The 50- or<br />
75-W coax or hardline impedance is transformed up to the delta match impedance with<br />
a 4:1 balun transformer such as the one in Fig. 19.6.<br />
Center<br />
Driven<br />
element<br />
Metal<br />
rods<br />
BALUN<br />
Figure 19.13B <strong>Practical</strong> VHF delta match.
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