Using the operating impedance bridge
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.
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 +1000Ω 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. However, with practice it becomes automatic. The actual correction is engraved on the panel of the instrument: X (corrected) = X/F. X = reactance (dial reading) and F = operating frequency in megahertz.
The OIB-3 is available in two versions. The first version measures up to 2MHz, and version 2 has an extended range to 5MHz. The operating ranges are identified by the bridge's serial number. Serial numbers 001 through 1224 cover the broadcast band, and serial numbers 1225 and above tune higher. Two-range bridges have meter DIR/TUNE switches marked Tune HI and Tune LO. The high position extends the tuning range to 5MHz, which greatly increases its usefulness.
This instrument offers a choice of 12" or 18" connecting cables and comes with a coaxial adapter. I have always found the 18" cables very useful when measurements have to be made in difficult areas using the transmitter as the power source (but not more than 5kW). Care should be taken to avoid contaminated measurements caused by random lead movements. Coaxial connectors are also available in the form of a large male UHF male-to-female PNC adapter. It is very important to ensure that both clip leads are properly grounded at the instrument input and output connections when used with an actual transmitter.
The OIB-3 requires only an RF power source to put it into action and has a built in detector and indicator so that additional equipment is not normally required when used with a transmitter RF power source. The built-in detector and tuning circuit usually provides adequate sensitivity when using a transmitter as an RF source.
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