Practical_Antenna_Handbook_0071639586

C h a p t e r 8 : M u l t i b a n d a n d T u n a b l e W i r e A n t e n n a s 219
Multiple Dipoles
Another approach to multiband operation of an antenna consists of two or more l/2
dipoles fed from a single transmission line, as shown in Fig. 8.1B. There is no theoretical
limit to how many dipoles can be accommodated, although there is certainly a practical
limit based on the total weight of multiple dipoles and their spacers. One trick is to remember
that bands related to each other by a 3:1 ratio of frequencies can probably be
covered by a single dipole cut for the lower-frequency band. Such is the case, for instance,
with 40 and 15 m and possibly even with 80 and 30 m.
Assuming the dipoles are at least l/2 above ground at the lowest frequency, a reasonably
good match to the feedpoint impedance is provided by either 50-Ω or 75-Ω
coaxial cable. The two sides of the coax (center conductor and shield) can be connected
to the center of the multi-dipole directly or through a 1:1 balun transformer, as shown in
Fig. 8.1B. Each antenna (A-A, B-B, or C-C) is cut to l/2 at its design frequency, so the
approximate overall length of each dipole can be found from the standard expression
for dipole length.
Overall length (A + A, B + B, or C + C):
L (feet)
or, for each side of the dipole (A, B, or C):
L (feet)
=
F
=
F
468
(MHz)
234
(MHz)
(8.1)
(8.2)
As always, close to the earth’s surface these equations are approximations and are
not to be taken too literally. Some experimentation will probably be necessary to optimize
resonance on each band. Also, be aware that the drooping dipoles (B and C in this
case) may act more like an inverted-vee antenna (see Chap. 6) than a straight dipole, so
the equation length will be just a few percent too short. In any event, a little “spritzing”
with this antenna will yield acceptable results.
A
1 : 1 BALUN
A
B
B
C
Coax to
XMTR
C
Figure 8.1B Multiband dipole consists of several dipoles fed from a common feedline.