Practical_Antenna_Handbook_0071639586

138 P a r t I I : F u n d a m e n t a l s
⎛
2 2
n – Γ ⎞
Loss = 10 log ⎜
2
⎟
⎝ n – nΓ
⎠
⎡
= 10 log ⎢
⎣
⎡
= 10 log ⎢
⎣
⎛
= 10 log ⎜
⎝
⎛ ⎞
= 10 log ⎜
1.019 ⎟
⎝ 0.974 ⎠
= 10 log (1.046)
= (10) (0.02)
= 0.2 dB
2 2
(1.04) – (0.25)
1.04 – (1.04) (0.25)
1.082 – 0.063
1.04 – (1.04) (0.063)
1.019
1.04 – 0.066
⎞
⎟
⎠
2
⎤
⎥
⎦
⎤
⎥
⎦
Compare the matched line loss (A = 0.15 dB) with the total loss (Loss = 0.2 dB),
which includes mismatch loss and line loss. The difference (i.e., Loss – A) is only 0.05
dB. If the VSWR or the total line length were considerably larger, however, the loss
would rise.
Impedance Matching in Antenna Systems
Although antennas are reciprocal devices, a receiving antenna can do an excellent job
even in the absence of perfect matches between the antenna and the feedline or between
the feedline and the receiver. This is especially true in the MF and lower HF regions,
where reception is almost always limited by atmospheric noise.
In contrast, proper matching of the transmitter to the feedline and the feedline to
the antenna is essential for maximizing our radiated signal. That is because maximum
power transfer between a source and a load always occurs when the system impedances
are matched.
Of course, the trivial situation is when all three sections of our system—transmitter,
feedline, and antenna—have the same impedance. The most obvious example would be
an antenna (such as a half-wave dipole in free space) with a 75-Ω resistive feedpoint
impedance fed from garden variety 75-Ω coaxial cable or hardline that is connected to a
transmitter with a 75-Ω output impedance.
However, in real life the feedpoint impedance is rarely what the books say it should
be. Even our ubiquitous l/2 dipole seldom appears to be exactly 75 Ω or purely resistive
to its feedline. Then there are our wire arrays, our Yagis and our loops—all with