Practical_Antenna_Handbook_0071639586

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: