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Using the operating impedance bridge
When greater sensitivity is required, the built-in detector is tuned to the frequency of interest by using the detector tuning control for the appropriate frequency range. If still greater sensitivity is required the internal battery operated meter amplifier can be switched into use to provide increased sensitivity. In this case it is advisable to turn the sensitivity control toward minimum prior to switching on the meter amplifier. When an upscale reading is obtained, the appropriate tuning control should be adjusted for maximum deflection and a sharper null.
When measuring conditions result in very low RF power levels, an external detector can be connected via a double-shielded cable to the BNC connector on the OIB-3 panel. When a suitable receiver/generator, such as the Delta Electronics RG-4B (next page) is used for this purpose, it provides a convenient RF power source for the operation of the OIB-3 impedance bridge in situations lacking a transmitter RF source.
Because it is an operating impedance bridge, the OIB-3 can be inserted anywhere in the transmitter to antenna path with a maximum of 5kW RF power. Therefore the actual operating impedances can be measured under operating conditions, and individual tuned circuits can be measured and adjusted under operating power as necessary using the receiver generator as a source of RF signal.
When measuring impedances under transmitter power the line is opened and the cable end coming from the transmitter is connected to the IN connector on the side of the case. The OUT terminal of the OIB is connected to the interrupted line going to the load. When using transmitter power and a coaxial line is interrupted it is essential that the clip lead cables are properly grounded to the system. It is also worth mentioning that when the bridge is inserted into a coaxial line high accuracy is obtained by using coaxial adapters instead of clip leads. The instruction manual that comes with the bridge provides very clear and straightforward instructions for its general operation. It also provides an interesting explanation of the manner in which the OIB operates.
I have found the OIB-3 to be invaluable when working with antennas involving negative towers that are returning power to the system. Negative towers are very easily measured by merely reversing the connections to the OIB. In other words the power source (transmitter) is connected to the OUT terminal on the bridge and the RF load to the IN terminal. Bridge operation is the same as for positive towers and the measured impedance is read from the bridge dials. The operating load impedance will be read as a negative of the impedance measured. It should be noted that the resistance dial is calibrated below zero to about 5Ω on the minus side of zero. This eases the measurement of anticipated operating resistances near zero and eliminates the need to reverse the bridge connections when measuring negative towers.
E-mail Battison at firstname.lastname@example.org.
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