Practical_Antenna_Handbook_0071639586

188 p a r t I I I : h i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
suspended from the same center support, eliminating any downward pull on the center
of the antenna. Another mechanical advantage of the inverted-vee is that there is no
need for substantial tension in the antenna wires themselves, since they are not being
called upon to support the weight of the feedline or the center insulator. Hence, there is
no reason an inverted-vee can’t be constructed from #18 or smaller gauge wire.
Figure 6.6 shows a typical inverted-vee installation. The operating frequency and
the height of the center support will determine the approximate height above ground of
the ends, but there is no reason the ends can’t come almost to the ground if provision
for preventing people and animals from touching the antenna is included. Angle a can
be almost anything convenient, provided that a > 90 degrees; typically, a is between 90
and 120 degrees. On 80 m, the minimum practical height for the center support is about
35 ft for a = 120 degrees and the overall length of the antenna’s footprint is about 110 ft.
Conversely, if backyard space is at a premium, reducing the included angle to 90 degrees
leads to a minimum center support height of nearly 50 ft, but the footprint is only
slightly more than 90 ft. An inverted-vee for 40 m could have minimum dimensions—
both height and footprint—of approximately 50 percent to 60 percent of these figures.
Sloping the antenna elements down from the horizontal to an angle (as shown in
Fig. 6.6) lowers the resonant frequency for a given length of wire. Thus, the wire lengths
may need to be cut slightly shorter for any given frequency than for a true horizontal
dipole cut for the same frequency. There is no precise equation for calculation of the
overall length of the antenna elements; the concept of “absolute” length holds even less
for the inverted-vee than it does for regular dipoles. There is, however, a rule of thumb
that can be followed for a starting point: Make the antenna about 6 percent shorter than
a dipole for the same frequency. The initial cut of the antenna element lengths (each
quarter-wavelength) is then
L
Coaxial
cable to
transmitter
L
S
R
I
Insulated
support
L 220
F MHz
Feet
I
R
S
I Insulator
S Support stake
R Rope
Figure 6.6 Inverted-vee dipole.