Practical_Antenna_Handbook_0071639586

C h a p t e r 4 : T r a n s m i s s i o n L i n e s a n d I m p e d a n c e M a t c h i n g 145
I
I max
0
/ 4
/ 2
Z
Length
Figure 4.13 Quarter-wavelength shorted stub.
Series Matching Section
The series matching section—also called the series section transformer—is a generalization
of the Q-section that permits us to build an impedance transformer that overcomes the
Q-section’s limitations. According to The ARRL Antenna Book, with appropriate choices
of transmission lines this form of transformer is capable of matching any load resistance
between about 5 and 1200 Ω. In addition, the transformer section does not have to be
located at the antenna feedpoint.
Figure 4.14 shows the basic layout of the series matching section. Three lengths of
coaxial cable, L 1 , L 2 , and L 3 , serve to form the entire feedline connecting the antenna to
the transmitter. Lengths L 2 and the line to the transmitter (L 3 , which is any convenient
length and doesn’t appear in the equations) have the same characteristic impedance,
usually 75 Ω, while section L 1 has a different impedance. Note that only standard, easily
obtainable values of impedance are used here.
The design procedure for this transformer consists of finding the correct lengths for
L 1 and L 2 . You must know the characteristic impedance of the two lines (75 Ω and 50 Ω,
respectively, in this example), along with the complex feedpoint impedance of the antenna,
Z L = R L + jX L .