Practical_Antenna_Handbook_0071639586

204 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
Multiband Fan Dipole
The basic half-wavelength dipole antenna is a very good performer, especially when
cost is a factor. The dipole yields consistently good performance for practically no investment.
A bonus is that a standard half-wavelength dipole (but not any form of folded
dipole) exhibits a current maximum at its feedpoint not only on its fundamental frequency
(where it is l/2 long) but also on its third harmonic (where it is 3/2 l in length).
Thus, a 40-m half-wavelength dipole fed with 52-Ω coaxial cable on 7 MHz can also be
used on 21 MHz with reasonably comparable VSWR. However, its pattern changes
from the classic bidirectional figure eight pattern on its fundamental frequency to a
four-lobe cloverleaf pattern at its third harmonic.
Dipoles for multiple bands can be hung in parallel and fed from the same feedline
and center insulator. Because a dipole operated far from its resonant frequency exhibits
a much higher impedance, most of the transmitter power will go into the dipole that is
resonant on the band being used. Figure 6.18 shows three dipoles (A1–A2, B1–B2, and
C1–C2) cut for different bands, operating from a common feedline and balun transformer.
Each of these antennas is a half-wavelength on a different band.
When building this antenna, try to keep the dipoles separated from each other a bit.
Note that it will not be necessary to include a separate dipole for any band having a
wavelength one-third that of a lower-frequency band that already has a dipole. For example,
if you cut A1–A2 for 7 MHz, don’t bother including a separate dipole for 21
MHz. If you do, the feedpoint impedance will not be what was expected.
Rope
INS
Rope INS
A1
B1
C1
INS
C1
or
1:1
BALUN
INS
C2
B2
A2
Rope
INS
Rope
Rope
INS
Rope
Coax to
receiver
Figure 6.18 Multiband dipole antenna.