Balanced Load Measurements with VHF Bridge
THE -hp- Model 803A VHF Bridge, described in the April, 1950 issue of the Hewlett-Packard Journal, has made possible the direct measurement of impedance up into the I-HI' region. This bridge measures the impedance magnitude and phase angle of most types of high-frequency devices over a range from below 50 to above 500 mc.
One type of impedance measurement sometimes desired is the meas urement of balanced impedances such as balanced antennas fed by balanced transmission lines. Since the connection for the unknown impedance on the -hp- 803A Bridge is single-ended, some type of balancing device is required when measuring balanced unknowns,
A suitable balancing device for many such measurements is the balun shown in the drawing of Figure I, The balun is cut to be exactly one-half wavelength long at the operating frequency so that -the voltage at the input to the balun will be reproduced (out of phase) at the load end of the balun. Thus, the balun is effectively driving one half of the load while the bridge drives the other half, making the voltage across the load as a whole equal to twice the voltage delivered by the bridge. This balancing system is therefore roughly equivalent to a 1:2 voltage
© Copr. 1949-1998 Hewlett-Packard Co.
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