By Aubra E. Tilley, WM6T
Our interests in antennas include knowledge of their radiation patterns, which affect propagation, and their electrical properties, which affect impedance matching to transmitting and receiving apparatus and associated transmission lines. The antenna analyzer described here directly measures the electrical properties. However, it has adequate power output to make it a useful instrument for radiation pattern measurements.
The analyzer incorporates an analog signal generator with a digital counter readout, a precision impedance bridge and an SWR meter. Omitted from my initial ambitious goal is a built-in detector for the impedance bridge. Presently, an external communication receiver is used for this purpose.
The analyzer comprises three principal parts: a signal generator (oscillator and amplifier); bridges (impede ance bridge and SWR bridge); and the power supply (+5-V regulator, -5-V
Notes appear on page 8.
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inverter, 12-V battery and battery charger).
Housed in an aluminum enclosure, the oscillator provides frequencies from 1.6 to 30 MHz. The oscillator circuit uses a Motorola MC 1648 IC. This is an ideal component for a low-distortion oscillator with buffered output. As noted on the schematic of Fig 1, selected resistors are inserted between pin 5 and ground to improve the wave forms. Balun transformers B1 and B2 isolate the unit from power supply and ground thus reducing feedback effects from circulating currents in the chassis and power leads.
A power amplifier is included to improve SWR measurement accuracy and to provide useful power for radiation pattern measurements. An MC1350 IC is used as a driver for push-pull output transistors. This integrated circuit was designed for use as a video amplifier. Its broadband characteristics make it an ideal driver. Also, its excellent AGC characteristics are used to maintain a constant drive voltage to the bridge circuits. This feature permits making accurate SWR measurements without adjusting bridge amplitude levels.
The impedance bridge, shown in Fig 2, is of the series differential type. The advantages of the series differential R-C bridge accrue from symmetrical and nearly ideal dial scale factors. An unlimited variety of possibilities are available through the selection of the value of the differential capacitors in combination with fixed-value shunts. Fig 3 shows the computed reactance values versus the rotational angle of the differential capacitor at 10 MHz for the instrument being described. Figs 4, 5, 6 and 7 illustrate other possibilities using different capacitor combinations. Comparisons with other R-C bridge types are covered in a previous QEX article.1
Schematically, the series differential bridge is very similar to the common series R-C bridge. A simplified schematic is shown in Fig 8, where it can be noted that a variable capacitor is substituted for the usual reference capacitor. This variable is differentially ganged to the calibration capacitor. The two capacitor sections must be insulated from each other. In another configuration a conventional differen-
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