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Most radio engineers today will recall uncomfortable evenings and cold nights spent in dog houses making antenna impedance measurements, or worse, still trying to balance a laboratory model GR bridge on a freestanding steel ATU cabinet, adjust a signal generator or oscillator, tune a detector and probably try to take notes. The development of the operating impedance bridge by Delta Electronics about 40 years ago changed all that, and together with the receiver generator made antenna impedance measurements not only much easier to perform, but more accurate because antenna measurements can now be made under actual operating conditions. Almost inevitably measurements made with actual operating power, or at least sufficient power to produce operating conditions, are more accurate than cold measurements made using flea power.
OIB
The first version of the OIB was known as the OIB-1, and as it became widely used a few minor changes were made. Today the OIB-3 is very similar in appearance to the OIB-1 and is shown at left. Its impedance measuring range now covers from -1000 to +1000 ohm resistance and -900 to +900 reactance to 1MHz. When the OIB was first introduced it became possible for the first time to measure circuit impedances under operating conditions at any point in the antenna system.
Remember that the bridge is calibrated at 1MHz and it is necessary to apply a correction to the reactance read directly on the reactance dial. Many years ago I found it was quite easy to forget to make this correction....