Theory of Operation
7
Component Testing
The Bird 4304A is very helpful in component testing, and may be employed in
several ways:
Note:
Measuring small attenuations requires correction for nor-
mal instrument errors. For two wattmeters, do this by simply con-
necting the wattmeters directly, with no line between them, and
adjusting their zero settings. For one, make sure to note exact
readings, or their difference, on the initial equality check, and cor-
rect for this.
Insertion VSWR or
may be measured by placing the component
between the wattmeter and a good load resistor.
Attenuation (power lost by heat in a line) as well as insertion VSWR or
may be measured by inserting the unknown line between two
Thruline wattmeters, or between a Thruline wattmeter and a
Termaline absorption wattmeter.
Line loss using open circuit calibration: The high directivity of elements
can be exploited in line loss measurements by having the load
connector open or short circuited. The forward and reflected waves
then have equal power, so that
= 100% and
=
. Open circuit
testing is preferred, because a high quality open is easier to create than
a high quality short. To measure insertion loss, use an open standard to
check forward and reverse power equality, then connect an open,
unknown line to the wattmeter. The measured
is the attenuation for
two
passes along the line (down and back). This can then be compared
with published data for line type and length (remember to halve N
db
or
double the line length).
Note:
This measurement should be supplemented with either
time domain reflectometry or DC continuity and leakage checks,
since the attenuation measurement alone cannot detect faults
such as open or short circuits partway down the line.
