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HEWLETT'PAGKARD
m p ' f T E C H N I C A L
JOURNAL
I N F O R M A T I O N F R O M T H E - n p - L A B O R A T O R I E S
VOL. 3 No. 3
| PUBLISHED BY THE HEWLETT-PACKARD COMPANY, 395 PAGE MILL ROAD, PALO ALTO, CALIFORNIA NOVEMBER, 1951
Recent Developments in -hp-
Waveguide Type Measuring Equipment
February, 1951, issue of the Journal loading are interdependent. To obtain high
J_ announced the -hp- program for a com probe output, then, the designer must con
plete series of waveguide measuring equip struct a probe having high efficiency; that is,
ment to cover the frequency range from 2600 a probe having maximum output for a given
to 18,000 megacycles. At that time the Model loading effect on the line.
810 interchangeable slotted sections, the In tuned type probes the probe tuner in
Model 442 and 444 probes, and the Model creases the efficiency of the probe by adjust
440 detector mount were described. The suc ing the match between the probe antenna
ceeding issue described the Model 485 de and the detector element. From an opera
tector mounts, the Model 370 fixed attenu tional standpoint, however, a tuned probe is
ators, and the Model 912 high-power ter inconvenient to use. If the probe is a single
minations. adjustment type, the probe must be retuned
Since those issues were published, a num for each change of frequency. If the probe is
ber of other items have been put into pro a two-adjustment type, not only must the
duction. In addition, considerable interest probe be retuned when frequency is changed
has been shown in the Model 444 untuned but the tuning operation itself is often diffi
probe and Model 806B coaxial slotted sec cult. Consideration of these factors led to a
tion, and pertinent application data regard decision to design the Model 444A as an un
ing these devices will be presented here. tuned probe. To accomplish such a design, it
UNTUNED PROBE becomes necessary to make the probe as effi
A realistic measure of electrical quality cient as possible.
for an r-f probe designed for slotted line Since no tuner was to be incorporated in
work is the relation of probe rectified output the probe, it was extremely important that
to loading on the line caused by the probe. residual reactances between the probe an
However, since probe output is obtained at tenna and the probe detector element be held
the expense of extracting energy from the at a minimum. This was achieved by locating
line, the characteristics of probe output and the detector element unusually close to the
antenna and by using a type 1N26 silicon
crystal whose construction was more suitable
than other types for the purpose. As a result
of this design, the rectified output of the
Model 444A was high, relatively constant
over wide r-f frequency ranges, and did not
-hp-MOOEL444/i exhibit the large peaks and troughs common
KILOMEGACYCLES with conventional probes.
Fig. 1. Comparison of probe loading at ?:For a description of probe loading effects, see "Good Practice in
constant penetration for -hp- Model 444A Slotted Line Measurements," Hewlett-Packard Journal, Vol. 3,
untuned probe and for tii'o tuned probes. Nos. 1 and 2, September and October, 1951.
P R I N T E D I N U . S . A . C O P Y R I G H T 1 9 5 1 H E W L E T T - P A C K A R D C O .