Bird Technologies 4331 - Manual (Page 29)

tenuation require allowance for normal instrument er-
rors. The correction may be determined by direct rigid
connection of the Thruline Wattmeter units, or of the
Thruline Wattmeter - Termaline Load combination, in cas-
cade. Slight juggling of the zero settings is permissible
for convenience in eliminating computation, provided read-
ings are being taken fairly well up on the scale.
3. Attenuation By Open Or Short Circuit Method. More
clever by far than method (2) is one depending on the
high directivity (null balance) to which the Thruline Watt-
meter elements are held. They should, and do, exhibit
good equality between forward and reflected readings
when the load connector is open or short circuited. In
this condition, φ = 100 percent, the forward and reflected
waves being equal in magnitude, and p = infinity. Say
that this is checked on open circuit, and then a length of
line of unknown attenuation, also open circuited, is con-
nected to the load connector. The ratio φ then shown is
the attenuation in two passes along the line (down and
back).
Expressed in dB,
the dB figure may be compared with published data for line type and length
by remembering to halve N
dB
or to double the line length, because twice
the line length is actually being measured.
This measurement should be supplemented by one of
φ (Insertion), or at least
by dc continuity and leakage checks, since the attenuation measurement
alone can be in error from faults such as open or short circuits part of the
way down the line.
using the equation: N ,
dB
=
F
HG
I
KJ
10 log
W
W
r
f
Theory of Operation
15