Copyright I Hewlett Pack Aro Co

Figure 2, Seur set of u at eguide slotted line equipment including HO'JB carriage assembly with four 810B slotted sections, and two broadband probes*

The new slotted sections and probes that have been developed will be described ¡n the following. Remaining instruments will be described in future issues.

slotted sections

The focal point of the whole program is the set of slotted line sections shown in Figure 2. The set consists of four slotted waveguide sections, a carriage assembly, and two types of broad band probes. Each component of the set is available separately so that only those components necessary for a given measurements project need be used. The complete set of slotted sections with either probe covers a range of 3950 to 12,400 mc.

To achieve a moderate selling price, the set is designed so that all four slotted sections are usable with one carriage assembly and one probe. In effect, this arrangement eliminates three carriage assemblies, thereby saving much costly machining from the set as a whole. To put into use a slotted line for any of the four waveguide sizes from 3950 to 12.400 mc. it is only necessary to set in place the desired slotted section and tighten four thumbscrews.

The carriage assembly has been carefully designed to obtain mechanical accuracy and freedom from wear. The probe carriage rides on two solid one-half inch diameter stainless steel bars that are precision ground and honed to within 0.0002", The necessary longitudinal bearing is obta ined on the front supporting bar through use of two linear ball bushings, special bearings that provide the advantages of ball bearings in linear motion applications. Such an arrangement allows the front bearings to be dust sealed and gives a positive support to the carriage without need for spring-loading. The rear support for the carriage consists of two opposed ball bearings that ride linearly on the rear support bar.

The linear type bearings and freedom from sliding parts make the probe carriage adaptable to an unusual technique in standing-wave measurements. In Figure 3 the probe carriage is shown coupled to a slow-speed motor in such a way as to give a reciprocating motion to the carriage. By coupling the output of the probe to a low-frequency or dc oscilloscope through a suitable amplifier, a visual indication of the standing wave pattern is obtained. The oh-vious feature of this technique is that adjustments in loading or tuning can be made much quicker than with the conventional point-by-point technique. Sweep voltage for the oscilloscope can be obtained from a contact sliding on a resistor as shown in the illustration. The motor drive arrangement is not provided but can easily be made up in breadboard form in the laboratory.

The travel of the probe carriage is 10 cm. or more than three-quarters of a guide wavelength at the lowest operating frequency of 3950 mc. The probe position relative to the right-hand or output face of the slotted section is indicated on a metric scale with a vernier reading to one-tenth of a millimeter. In ad-

Figurt- Breadboard set-up lor motor drive for use with Model HO'JB carriage assembly.

d it ion, the probe carriage is equipped to mount a dial indicator (Figure 4) so that null patterns can be measured with very high mechanical accuracy. The dial indicator is not provided with the carriage, although any standard dial indicator can be used. The dial indicator shown in Figure -t reads up to 2.5 cm in divisions of 0.01 millimeter.

The Model 8WB slotted sections and the Model 809B carriage assembly are machined to very close mechanical tolerances. With reasonable care in interchanging sections and aligning joints, VSWR measurements of 1,02 or better can be made,


Two probes are available for use with the skated sections. Both cover the complete 3950 to 12,400 mc range. The Mojel 444A probe is unconventional in that it provides rectified output but is an untuned device. Being untuned, this probe is unusually convenient to operate and allows a series of measurements to be

Figure 4. -hp- Model 8(J9B carriage assembly u itb dial indicator.




Figure 5. Iy pica! response and efficiency characteristics of Model 444A untuned probe compared u itb com entional probes having tuned detectors, made rapidly. In addition, the probe design is such that good efficiency and response characteristics are provided. The efficiency and response of the Model 444A are compared in Figure 5 with those of rwo conventional tuned type probes.

The high efficiency and flat response of the 444A probe have been achieved by locating the detector element quite close to the r-f pick-up antenna, thereby minimizing residual reactances. The detector element itself is a modified 1N26 silicon crystal, and the probe is thus suitable for general - purpose applications where a crystal type detector can be used.

The second probe. Model 44 2A,

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