at higher frequencies for small values of attenuation, the method outlined
in the following paragraph should be used.
To eliminate the correction and to make the indicated relative atten-
uation agree very closely with the actual attenuation between —9 and 140
dB on the scale, the voltage across the line, at the actual coupling point,
should approach zero.
To accomplish this, connect an adjustable short-
circuiting stub (Type 874-D2OL or -D5OL) to the end of the main line op-
posite the source.
Adjust the stub until a voltage minimum appears at the
coupling point. To find this stub adjustment, substitute a Type 874-VRL
Voltmeter Rectifier and a Type 874-VI Voltmeter Indicator, or a Type 874-
TL Tee and a detector, for the attenuator.
Adjust the stub until the indi-
cated voltage is minimized; then replace the attenuator.
(If the Type 874-
VRL is used, connect the end containing the series resistor, engraved R,
to the generator.) If the frequency is changed the stub must be reset, but a
frequency calibration can be marked on the stub, for convenience when a
number of measurements are to be made. At frequencies below 300 MHz, a
Type 874-WNL Short Circuit can be substituted for the stub, without fur-
ther adjustments.
With the Type 874-WNL Short Circuit, accurate results
can be obtained at scale readings greater than zero. At frequencies below
1500 MHz, the indicated attenuation is accurate at scale readings as low
as —9 dB, and, at frequencies between 1500 and 4000 MHz, readings above
0 dB on the calibrated scale are accurate if the stub is set as previously
outlined.
An alternate method of setting a voltage minimum at the coupling
point,f or frequencies above 1500MHz, utilizes the fact that the attenuation
constant for the desired mode is smaller than that of the spurious modes.
If the attenuator is set to a relatively large value of attenuation, the stub
can be properly set by adjusting
it
until maximum output is obtained from
the attenuator. However, a voltage minimum does not occur at the coupling
point unless the effective source impedance, seen looking back toward the
generator from the coupling point, is a pure resistance.
When the genera-
tor and the detector are not matched, the above condition can be obtained
by the use of suitable pads (Type 874-G1OL or -G2OL) at the generator
and the detector.
At the higher frequencies the attenuation is reduced as the wave-
guide approaches its cutoff frequency.
To correct for this effect, multiply
the indicated attenuation by the factor found in the frequency correction
graph, Figure 3, corresponding to the operating frequency.
The impedance of the adjustable arm of the attenuator is nominally
50 ohms.
However, the impedance varies appreciably with frequency, as
shown by the plot in Figure 4 of the VSWR seen looking back into the out-
put connector for a typical unit. The tolerance on the dc resistance of the
50-ohm resistor is ±
10%.
TYPE 874-GAL ADJUSTABLE ATTENUATOR
3
