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HEWLETT-PACKARD
JOURNAL
TECHNICAL INFORMATION FROM THE -hp- LABORATORIES Vol. 6 No. 5
BLISHED BY THE HEWLETT-PACKARD COMPANY, 275 PAGE MILL ROAD, PALO ALTO, CALIFORNIA JANUARY, 1955
A Precision Wave Guide Attenuator Which
Obeys a Mathematical Law
FOR about a year and a half -hp- has been which collectively cover the range from 3.95
manufacturing a new type of direct-reading to 18 kmc.
precision wave guide attenuator known as a Basically, the attenuator consists of three sec
rotary attenuator*. This device is distinguished tions of wave guide in tandem. In each section a
by the fact that its attenuation follows a predict resistive film is placed across the guide as shown
able, mathematical law not re in Fig. 2. The middle section is a short length of
lated to frequency. Other than round guide which is free to rotate axially with
the cutoff attenuator which has respect to the two fixed end sections. The end
several disadvantages in wave sections are rectangular-to-round wave guide
guide use, this rotary attenuator is generally con transitions in which the resistive films are nor
sidered to be the most accurate attenuator avail mal to the E field of the applied wave. The con
able for wide band microwave applications. struction is symmetrical and the device is bi
The attenuator has a calibrated range of 0 to directional.
50 db which is accurate within 2% of the db read When all films are aligned, the E field of the
ing at any frequency in a wave guide band. This applied wave is normal to all films. No current
accuracy is obtained directly from the calibrated then flows in the films, and no attenuation oc
dial; no calibration charts are required. The curs. If the center film is now rotated to some
VS WR of the attenuator is less than 1.15 and the angle 0, the E field can be considered to be split
insertion loss is less than 1 db. into two components: E sin 9 in the plane of the
Until recently, the rotary attenuator has been film, and E cos 9 at right angles to it. The E sin 9
produced only in 8.2-12.4 kmc size wave guide. component will be absorbed by the film, while
It is now being produced in five wave guide sizes the E cos 9 component, oriented at an angle 6
with respect to the original wave, will be passed
unattenuated to the third section. When it en
counters the third film, the E cos 9 component
will be split into two components. The E cos 9
sin 9 component will be absorbed, and the E cos2