Note
Ham Radio, Jun 1977, p. 84.
The Millen 90673 Antenna Bridge is based on the "Macromatcher" described in QST, January, 1972.
The 90673 is a radio frequency impedance measuring device which is intended for use in the frequency range from 1.8 to 30 MHz.
An "extra" provided by this bridge is the measurement of RF resistance outside the normal high-frequency limit. Using additional coils, specially made for the VHF range, it is possible to obtain approximate resistance readings up to 60 MHz, but without any accompanying reactance calibration.
A null detector, with diode rectifier and DC amplifier, is built in and indicates on a 1 mA meter. The bridge requires an external oscillator, such as a grid-dip oscillator.
Resistance measurements are made by varying a differential air dielectric variable capacitor. The resistance range covered is approximately 5 to 150 ohms. At frequencies below 15 MHz, values up to 200 ohms are measurable. The accuracy of resistance measurement is better than ±10% over most of the range.
Reactance is measured by tuning a coil-and-capacitor circuit, first to an initial null with a resistive reference and then to a new null with the unknown impedance. This requires simultaneous balancing of resistance and reactance controls. The reactance range is calibrated in ohms of reactance at 1 MHz and indicated inductive and capacative values.The reactance value at the operating frequency is obtained by dividing the idicated reading by the frequency in use.