Practical_Antenna_Handbook_0071639586

578 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
The reflection loss is the amount of RF power reflected back down the transmission
line from the load. The difference between incident power supplied by the generator
(1.5 W in this example) and the reflected power is the absorbed power:
PABS = PFWD − PREF
(26.17)
If the load is an antenna, P ABS is split between radiated power and losses, such as
ground losses. The reflection loss is found graphically by continuing the righthand perpendicular
below the TLC point (or by laying out the impedance radius on the “REFL.
LOSS, dB” scale), and in this example (Fig. 26.7) it is –1.05 dB. Let’s check the calculations.
The return loss is –7 dB, so
10
⎛ P
− 7dB = 10log⎜
⎝ P
⎛ PREF
⎞
− 7 = 10log⎜
⎟
⎝ 1.5 W ⎠
− 7 ⎛ PREF
⎞
= log⎜
⎟
10 ⎝ 1.5 W ⎠
P
=
1.5 W
( −7/10) REF
PREF
0.2 =
1.5 W
(0.2) (1.5 W) = P
0.3 W = P
REF
REF
REF
FWD
⎞
⎟
⎠
The power absorbed by the load (P L ) is the difference between incident power P FWD
and the reflected power P REF . If 0.3 W is reflected, then that means the absorbed power
is (1.5 – 0.3), or 1.2 W.
The reflection loss is –1.05 dB and can be checked from:
⎛ P ⎞
L
− 1.05 dB = 10log
⎝
⎜
P ⎠
⎟
10
−
1.05
10
⎛ PL
⎞
= log
⎝
⎜ 1.5 W ⎠
⎟
P
=
1.5 W
( −1.05/10)
L
PL
0.785 =
1.5 W
(1.5W) × (0.785) = P
L
1.2 W = P
L
FWD