Practical_Antenna_Handbook_0071639586

C h a p t e r 7 : L a r g e W i r e L o o p A n t e n n a s 213
To X 1 and X2
Figure 7.6 Quarter-wavelength coaxial
matching section.
The feedpoint impedance of a
1l loop is the vector combination
of three components:
4
matching section
(75 coax)
4
= 246V
F MHz
ft
• Baseline feedpoint impedance
of a simple l/2 “bent” dipole or
inverted-vee in free space
• Effect of radiated fields from
the second l/2 section on the fed
section
• Ground reflection effects, if too
close to ground to use free-space
assumptions
52 coax to
receiver (any length)
The resulting input impedance for
the array is around 110 Ω for any
reasonable height above ground,
so it provides a slight mismatch to
75-Ω coax and a 2:1 mismatch to
52-Ω coax. A very good matching
to 52-Ω coax can be produced
using the scheme of Fig. 7.6. Here,
a quarter-wavelength coaxial cable
matching section (see Chap. 4) is
made of 75-Ω coaxial cable. The
length of this cable should be
L = 246 F
(7.6)
Fv
where L = length, in feet (ft)
v F = velocity factor of coaxial cable
F = frequency, in megahertz (MHz)
The impedance Z 0 of the cable used for the matching section should be
Z0
= ZLZ
(7.7)
S
where Z 0 = characteristic impedance of coax used in matching section, in ohms
Z L = feedpoint impedance of antenna, in ohms
Z S = source impedance (i.e., 52-Ω characteristic impedance of line to receiver in
standard systems)