Practical_Antenna_Handbook_0071639586

196 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
4:1
BALUN
Figure 6.11B Folded dipole fed with coaxial cable.
75- coax
to XMTR
Although coaxial cable can be connected to the antenna feedpoint directly, it will experience
a roughly 6:1 VSWR over its entire length.
Another popular alternative is to use a 4:1 balun transformer at the feedpoint (Fig.
6.11B). This arrangement provides a reasonable match between the folded dipole and
either 52- or 75-Ω coaxial transmission line.
Adding a third wire, also unfed, on the opposite side of the fed wire from the first
unfed wire, creates the three-wire dipole. A three-wire center-fed dipole offers twice the 2:1
SWR bandwidth of a conventional dipole. The impedance step-up ratio for conductors
of equal diameter is 9:1, so this antenna makes an excellent mate to 600-Ω open-wire line.
As before, all three wires are tied together electrically at both ends of the antenna.
For both the folded dipole and the three-wire dipole, the fundamental tradeoffs
when compared to a single-wire dipole are increased bandwidth on the design band
versus increased weight and loss of multiband capability.
Bow-Tie Dipole
Another method for broadbanding a half-wave dipole is to use two identical dipoles
fed from the same transmission line and arranged to form a bow-tie shape, as shown in
Fig. 6.12. The use of two identical dipole elements on each side of the transmission line
increases the apparent conductor cross-sectional area so that the antenna has a slightly
improved length/diameter ratio.
The bow-tie dipole was popular in the 1930s and 1940s, and became the basis for the
earliest television receiver antennas. (Each analog TV channel is 6 MHz wide, so a
broadband antenna is required to cover each group of adjacent channels: low VHF, high
VHF, and UHF.) The bow-tie also attained some measure of popularity in amateur cir-
I
R
I
I
R
75- coax
to XMTR
I = Insulator
R = Rope
Figure 6.12 Bowtie dipole.