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 441
and above, other methods are often easier to implement. For example, we can replace
the LC tuner with stripline tank circuits.
The balun transformer provides an impedance transformation between balanced
and unbalanced circuit configurations. Although both 1:1 and 4:1 impedance ratios are
possible, the 4:1 ratio is most commonly used for VHF/UHF antenna work. At lower
frequencies it is easy to build broadband transformer baluns, but these become more of
a problem at VHF and above.
Figure 19.6 is a 4:1 impedance ratio coaxial balun often used on VHF/UHF frequencies.
Two sections of identical coaxial cable are needed. One section (A) can be any
convenient length needed to reach from the antenna to the transmitter. Its characteristic
impedance is Z 0 . The other section (B) is cut to be a half-wavelength long at the center
of the frequency range of interest. The physical length is found from
vF
L(inches) = 5904 (19.9)
F(MHz)
where L = cable length, in inches
F = operating frequency, in megahertz
v F = velocity factor of coaxial cable
The velocity factors of common coaxial cables are shown in Table 19.1.
Regular polyethylene 0.66
Polyethylene foam 0.80
Teflon 0.72
Table 19.1 Coaxial Cable Velocity
Factors
Hard line velocity factors typically lie in the 0.8 to 0.9 range, but it is wise to check
the manufacturer’s specifications for the exact number. Alternatively, a better approach
for either coaxial cable or hard line is to measure the velocity factor of the specific piece
of cable you intend to use. See Chap. 27 (“Instruments for Testing and Troubleshooting”)
for more information.
Example 19.1 Calculate the physical length required of a 146-MHz 4:1 balun made of
polyethylene foam coaxial cable.
Solution
L = 5904
F
v FF
= (5904)(0.08)
146
= 4723.2
=32.4in
146