OPERATING INSTRUCTIONS
TYPE 874-GAL
ADJUSTABLE
ATTENUATOR
DESCRIPTION
The Type 874-GAL Adjustable Attenuator is of the wave-guide-
below-cutoff
type operating
in the TE1 ~ mode (inductive coupling).
The waveguide is excited by a coaxial trankmission line, as shown in Fig-
ure 1, and the output voltage is induced in a loop mounted on a sliding
plunger.
One end of the loop is terminated in a 50-ohm resistor and the
other is connected to the output cable. This arrangement makes the output
impedance of the attenuator approximately 50 ohms.
The attenuation is
varied by rotating the barrel which moves the plunger and loop along the
tube.
The barrel is calibrated in relative attenuation in dB. The attenu-
ators can be adjusted from the stop at the minimum attenuation position,
which occurs at about -9dB on the calibrated scale, to the stop at the max-
imum attenuation end of the range, which occurs at about 140 dB on the
scale.
Each revolution of the barrel changes the attenuation 20 dB. In
most applications the maximum usable attenuation is limited by leakage
from various parts of the driving circuit and not by the range of the atten-
uator. In this type of variable attenuator, as the attenuation is varied the
phase at the output connection is essentially constant with respect to the
input signal phase.
The method of coupling to the coaxial line permits the attenuator
to be used as a standard calibrated attenuator or as a device to couple a
small amount of energy from a high-level circuit for monitoring or meas-
uring purposes without introducing a large discontinuity in the main line.
The Type 874-GAL Coaxial Attenuator employs the new Type 874—BL
Locking Coaxial Connectors.
These connectors can be used interchange-
ably with the standard GR874~ Connectors.
When mated with another
locking
connector,
a firm mechanical coupling of the two is achieved
when the coupling shell is engaged.
The shielding is also improved sig-
nificantly, and, in general, the leakage is reduced by at least 50 dB.
The “quick-disconnect” feature of the standard GR874 Coaxial Con-
nectors is retained in the locking
type
if the coupling shell is not en-
gaged. However, in this case the shielding is not as effective.
