Practical_Antenna_Handbook_0071639586
428 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 Seal end with RTV or caulk Radiator element Center conductor with insulation left on /4 /4 Seal with RTV or caulk /2 Coax braid folded back /4 Insulator PL-259 End cap Seal with solder Pipe To XMTR Figure 19.2A Coaxial vertical based on coaxial cable. /4 0111057 FIG 17-05A Figure 19.2B Tubing coaxial vertical. 0111057 FIG 17-05B
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 429 2952 L (inches) = (19.1) F (MHz) The antenna can be suspended from above with a short piece of string, twine, or fishing line. From a practical point of view, the major problem with this implementation is that the coaxial cable begins to deteriorate after a few rainstorms because the center of the dipole, where the braid and the center conductor go in opposite directions, is exposed to the elements. This effect can be slowed (but not prevented) by sealing the end and the break between the sleeve and the radiator with either silicone RTV or bathtub caulk. A more weather-resistant implementation is shown in Fig. 19.2B. The sleeve is a piece of copper or brass tubing (pipe) about 1 in in diameter. An end cap is fitted over the end and sweat-soldered into place to prevent the weather from destroying the electrical contact between the two pieces. An SO-239 L coaxial connector is mounted on the end cap. The coaxial cable is connected to the SO-239 inside the pipe, which requires making the connection before mounting the end cap. The radiator element is a small piece of tubing (or brazing rod) soldered to the center conductor of a PL-259 coaxial connector. An insulator is used to prevent the rod from shorting to the outer shell of the PL-259. (Note: An insulator salvaged from the smaller variety of banana plug can be shaved a small amount with a fine file and made to fit inside the PL-259. It allows enough center clearance for 1 ⁄8-in or 3 ⁄16-in brass tubing.) Alternatively, the radiator element can be soldered to a banana plug. A standard banana plug nicely fits into the female center conductor of an SO-239. #12 Wire 2 to 3 turns 0.5 in. dia. 0.5 in. long Figure 19.3 5 For ; L 615 8 FT F MHz L IN 5 ⁄ 8-wavelength 2-m antenna. 7380 F MHz 5 ⁄ 8 -Wavelength 2-Meter Antenna The 5 ⁄8-wavelength antenna (Fig. 19.3) is popular on 2 m for mobile operation because it is easy to construct, and at low elevation angles it provides a small amount of gain relative to the standard l/4 ground plane. The radiator element is 5 ⁄8-wavelength, so its physical length is found from:
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428 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<br />
Seal end with<br />
RTV or caulk<br />
Radiator<br />
element<br />
Center conductor with<br />
insulation left on<br />
/4<br />
/4<br />
Seal with RTV<br />
or caulk<br />
/2<br />
Coax braid<br />
folded back<br />
/4<br />
Insulator<br />
PL-259<br />
End cap<br />
Seal with<br />
solder<br />
Pipe<br />
To<br />
XMTR<br />
Figure 19.2A Coaxial vertical based on coaxial<br />
cable.<br />
/4<br />
0111057 FIG 17-05A<br />
Figure 19.2B Tubing coaxial vertical.<br />
0111057 FIG 17-05B
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