Bridge Measurements In The College Laboratory

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The efficient operation of a college electronics laboratory for student use requires the permanent installation of certain major items of equipment. Such apparatus as vacuum-tube bridges, Q meters, impedance bridges, and the like, which are so frequently required as accessories to experiments, should be available without the necessity of setup.

This does not mean that a student should be deprived of the opportunity to make his own setups. It does mean that a worthwhile experiment, whose subject is a vacuum-tube bridge, may require assembling the necessary components but, when a certain amplifier experiment requires a knowledge of tube parameters, a bridge should be available

Figure 1. View of audio-frequency capacitance and inductance bridges with generator and detector in the Electronics and Communication Laboratory at The Cooper Union.

already set up with power supplies, tone source, and detector.

The accompanying photographs show some of the permanent installations which have been found useful in the Electronics and Communication Laboratory of the Electrical Engineering Department at The Cooper Union. The severe dust and dirt conditions of a metropolitan location have led to the use of the hinged, counterbalanced wooden covers which also provide mechanical protection of the enclosed equipment.

The apparatus in Figure 1 consists of the General Radio Type 716-C Capacitance Bridge, Type 667-A Inductance Bridge, Type 1302-A Oscillator, and Type 1231-BRFA Amplifier and Null Detector with Filter. In Figure 2 are the Type 821-A Twin-T Impedance-Measuring Circuit, Type 1330-A Bridge

Figure 2. The Type 821-A Twin-T Impedance Measuring Circuit at The Cooper Union. A metal shield covers the table top, and equipment is protected from dust and other damage by the hinged cover.

Figure 2. The Type 821-A Twin-T Impedance Measuring Circuit at The Cooper Union. A metal shield covers the table top, and equipment is protected from dust and other damage by the hinged cover.

Figure 3. View of equipment for the measurement of cable characteristics at 12 Mc with the Type 916-A Radio-Frequency Bridge.

Shown in Figure 3 are the General Radio Type 916 Radio-Frequency Bridge, RCA WR-67A Oscillator, and National NC-57 Receiver. This inexpensive oscillator has, with occasional calibration, been satisfactory for this bridge. One experiment performed with this equipment makes use of the cable circuit attached to the wall. This consists of a long line of RG-8/U which branches at a tee junction to two short lengths of RG-ll/U, one terminated by a capacitive-resistive and the other by an inductive impedance. The circuit is matched at the tee at 12 Mc. The send-ing-end impedance is measured over a substantial range centered about this frequency. The cable characteristics are measured on samples, and verifying computations performed from this information and the termination constants.

Figure 4 shows an installation built

Oscillator, and National NC-57 Receiver used as null detector. Among the uses to which the 716-C and 821-A are put is the measurement of dielectric constant and dissipation factor by means of the Type 1690-A Dielectric Sample Holder which appears in Figure 2.

Figure 4. The setup for antenna measurements at very-high and ultra-high frequencies with the Type 1602-A U-H-F Admittance Meter. The cone antenna under measurement is shown at the extreme upper left of the photograph. The special adjustable line can be seen at the front of the test bench.

around the Type 1602-A U-H-F Admittance Meter. The usual unit oscillators, mixer, and the like are provided, with a Hallicrafters receiver as 30-Mc detector. It was desired particularly to use this equipment for antenna impedance measurement. For this purpose, a ground plane of moderate size was mounted above an instrument cabinet at a sufficient height so that antennas under test are little affected by persons moving about the laboratory. A special connector constructed from a Type 874-EL Ell is inserted at the center of the ground plane. This connector includes a solenoid-operated switch for remotely short-circuiting the transmission line at the base of the antenna. The line is made of brass tube and rod with the central conductor supported by polystyrene screws.

A useful accessory to this equipment is the special constant-impedance adjustable line which can be seen to the right of the admittance meter. This is built up of two Type 874-LK Units. The principal purpose of this device is to increase the adjustable length for use between 100 and 500 Mc. A rack-and-pinion drive makes its operation particularly convenient, and the dangling patch cord of the single unit is eliminated.

Various antenna models equipped with banana plugs can be conveniently connected. Typical student data taken on a cone antenna are reproduced in Figure 5.

In the conduct of this experiment as

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