• AT HIGH AND ULTRA-HIGH FREQUENCIES the desirability of constraining electro-magnetic fields within very definite confines has led largely to the adoption of coaxial transmission lines for the transference of power from one point to another. Ideal transmission lines of this type, having inner and outer conductors of zero resistance and an intervening medium of zero power factor, are theoretically capable of transferring power with zero energy loss and with zero external field, and the properties of actual lines approach those of the ideal very closely.

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A New Audio-Frequency Microvolter page 6

For measuring circuits, in particular, coaxial lines have been found almost indispensable. Because of the uniform distribution of inductance and capacitance along their lengths and the absence of appreciable losses from radiation, they generally follow the conventional "engineering solution" of the long line with great accuracy at frequencies extending into the hundreds of megacycles. To the properties of low losses and of low external field must therefore be added the further virtue of accurate predictability.

To obtain maximum benefit from coaxial lines, however, it is of great importance to use proper terminal equipment. For convenience, a plug and jack system is often highly desirable for use in setting up measuring systems. The plugs and jacks used should have two very definite properties, namely, (1) they should be as short as possible and have a characteristic impedance differing from that of the line as little as possible, in order to minimize reflections resulting from impedance mismatch, and (2) they should have as continuous an external shield as possible, in order to minimize external fields.

The Type 774 Coaxial Terminals, shown in Figure 1, have been designed with these two properties in mind. In order to reduce impedance mismatch in lines having different characteristic impedances they have been made with as short internal conductors as possible and with as low capacitance as possible. In order to provide as continuous an external shield as possible, lugs have been provided for four connections to the outer shell from the cable sheath at points uniformly distributed around the circumference.

The solid dielectric is polystyrene, which has both a low dielectric constant and a low power factor. These properties make possible the low capacitance and low losses of Type 774 Coaxial Terminals.

A plug unit and a jack unit are available for mounting on panels, and a similar pair of units for terminating coaxial cables. The plug connector and the jack connector make it possible to join two cables having identical terminations, that is, two plugs or two jacks.

Figure 1. A group of Type 774 Coaxial Terminals.

Figure 1. A group of Type 774 Coaxial Terminals.

For many applications the capacitance of these units is the factor to be considered in determining their suitability. The capacitance for each Type 774 Unit is listed in Table I. In addition to the total capacitance listed in the first column, there is given, for many units, a figure called "insertion capacitance," which is the capacitance added to a circuit when that particular unit is plugged in. This is lower than the total capacitance because of the overlapping when a plug unit is plugged into a jack.

In addition to the connectors listed in

Table I, another unit, for use with Type 684-A Modulated Oscillator, is available. This adapter can be installed in place of the output binding posts provided on the oscillator. This type of output terminal is necessary when the oscillator is used as a power source for impedance measurements at frequencies above a few megacycles, as for instance with the Type 821-A Twin-T.

The importance of maintaining the continuity of the external conductor in


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