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Using the operating impedance bridge
Many test instruments have been developed in the history of broadcast engineering. Some have had brief lives; others simply filled the needs that many engineers recognized, but little more.
In terms of the most-used all-around test tool, the oscilloscope is the clear winner. But the operating impedance bridge (OIB, sometimes also called an operating inline bridge) is probably the ultimate answer to the RF engineer's prayers. Prior to the development of the OIB more than 35 years ago, the AM engineer had to lug a heavy cold measurement RF bridge, some type of RF signal generator and a detector/indicator to a dog house and hope that AC power was available.
Changing history It seems rather strange that no one thought of the impedance bridge earlier in the history of radio. I've often wondered if the impact of TV and FM, with their associated coaxial applications, was what gave Charlie Wright the idea for the OIB-1, now manufactured by Delta Electronics.
The latest version is the OIB-3, which incorporates minor modifications from the original OIB-1. The OIB-2 handles up to 1kW and covers the 100kHz to 30MHz range. It is popular in the HF market.
The introduction of the receiver/generator completed the measuring package. Delta's RG-4 and Potomac's SD31 and SX-31 combination operate on AC power or internal rechargeable batteries and cover the 100kHz to 30MHz band. Both consist of a precision oscillator plus a receiver covering the same band. The receiver becomes the detector for the RF bridge, and high variable sensitivity is obtained.
A station transmitter operating at suitably reduced power (less than 5kW) can provide the signal necessary for an OIB to adjust networks and measure antenna impedance.
It is the term operating that separates the hot bridge from the cold bridge. Before the OIB, engineers frequently found that a cold impedance measurement made with flea power from a signal generator differed appreciably from the impedance existing when the circuit was hot and in normal operation.
Using the OIB The OIB is often used to measure the impedance of a new AM antenna. It is connected in series with the antenna connection to the ATU. The input connection is fed from the network output, and the output connector goes to the RF lead to the tower, as shown in Figure 1. The FCC requires that the base operating impedance be measured as closely as possible to the point where the antenna current is metered.
Many ATUs are designed with a current jack located immediately before the tower connection. The jack allows a meter to be plugged in when making readings, and a shorting plug is used when not taking antenna current readings. This is an ideal place to insert the OIB.
The most discernible difference between the OIB-1 and OIB-3 is the addition of resistance and reactance switches in the latter, which extend both ranges to 900V resistance and j900reactance, respectively.
The OIB-3 panel layout can be seen on the next page. Most labels are self-explanatory. However, the L-C switch deserves mention. If the sign of the reactance is known beforehand, this switch should be set appropriately to L or C. Otherwise, the meter movement should be observed. If increasing the reactance dial increases the meter reading, the load is capacitive, and C should be selected. The resistance dial has a 5V negative calibration to aid in checking low negative resistance.
Measuring SWR To measure SWR, feed the bridge input from the transmitter, and connect the bridge output into the transmission line. Set the Resistance dial to the known line resistance and the Reactance dial to "0" (Rj0). Set the Forward/Reverse switch to Forward and increase the sensitivity control until a full-scale reading is obtained. Now set the switch to the Reverse position. The SWR can be read directly on the bridge meter.
The bridge is designed and calibrated for use at 1,000kHz. It is important to apply a small frequency correction for reactances measured at something other than 1,000kHz. This correction is shown as X/Fmc, where X is the measured reactance (the sum of adder switches and reactance dial reading), and F is the frequency measured in MHz. If the frequency is above 1,000kHz, the corrected value will be more than the dial reading. If the frequency is less than 1,000kHz, the corrected reactance will be less than the dial reading. If the adder switches were needed to obtain a null, it is important to remember to add their values when recording the total measured figures.
When inserting the OIB into a coax line, standard coax connectors can be inserted in the input and output sockets. This is advisable if power of more than a few watts is used. Leads with heavy clips on one end and coax connectors on the other (measuring 12 or 18 inches) can be supplied, but ground-clip connections sometimes become loose, and an ungrounded OIB can be damaged - not to mention harmful to people and equipment in the area.
It is probable that DA engineers find the greatest use for the OIB. It is essential for setting up networks in phasors and ATUs. It can be inserted at the common point in a phasor, and adjustments in distant ATUs that affect the common point impedance can be observed and the necessary corrections made without shutting down the transmitter.
An unstable tower will often fluctuate between negative and positive while tuning the array. This is something that a cold bridge can't measure. The OIB's reverse measurement feature is invaluable.
To measure the negative tower, the input and output connections are reversed (with the input connected to the antenna under measurement and the output connected to the transmitter). The bridge is operated in the usual way. Remember that a negative tower is taking power from the antenna. The null is obtained and the R and X values are recorded with the values reversed in sign.
Very precise and highly accurate measurements can be made with an OIB. Proper use in accordance with the unit's instruction manual and safe engineering practices makes antenna adjustments much easier.
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