Practical_Antenna_Handbook_0071639586

C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 195
L
300-
parallel
line or
twin lead
To
XMTR
Figure 6.11A Folded dipole fed with 300-Ω line.
Assuming we can match the 300-Ω line to the transmitter, we have the same power
going into the folded dipole as we did into the single-wire dipole. For equal power, if
one feedpoint current is half the other, its corresponding feedpoint voltage must be
twice the other (because P = VI = constant). Thus, in free space the transmission line
“sees” a feedpoint impedance of
Z
FOLDED
2V
=
I
2
V
= 4
I
= 4Z
DIPOLE
DIPOLE
DIPOLE
DIPOLE
DIPOLE
≈ 290 Ω
(6.9)
The impedance step-up ratio for a two-wire folded dipole is 4:1 if the two conductors
are of equal diameter. When they’re not, the relationship is more complicated but the
impedance step-up is generally proportional to the ratio of the unfed wire diameter to
the fed wire diameter. Thus, a folded dipole can be designed to provide a specific feedpoint
impedance to the transmitter and transmission line, within limits, by making one
of the two wires larger than the other.
In many installations, the best feedline for the folded dipole will be 300-Ω twin-lead
or, better yet, open-wire line connected to a balanced wire ATU at the transmitter end.