Delta Electronics, Inc.

Model OIB-2 High Frequency
Operating Impedance Bridge


  • Measures Operating Impedance
  • Resistance Range of -500 to +500 Ohms
  • Reactance Range of -800 to +800 Ohms at 10 MHz
  • Output for External Detector
  • Internal RF Amplifier
  • Rugged Aluminum Case
  • Portable
  • Lightweight


The OIB-2 is an impedance bridge designed for the measurement of operating impedances in HF antenna networks. The OIB-2 can be inserted in any part of an antenna system to measure the operating impedance of individual radiators, networks, and transmission line sections. It greatly simplifies these measurements and, due to its patented design principle, provides a degree of accuracy not obtainable with conventional bridges.

The OIB-2 permits the measurement of impedance under power with a minimum of insertion effects on the circuit being measured. The bridge handles a through power of up to 1,000 Watts at moderate standing wave ratios. Resistance and reactance values are read from two drum dials located on the front panel. An internal detector is provided so that when the bridge is operating in a high power circuit, no other instrument is required. An external detector connector is also provided so that the Model OIB-2 may be used with a well shielded generator and receiver such as the Delta Electronics Model RG-4 Receiver/Generator for low power impedance measurements.

Operating Impedance

The term "operating impedance" is defined as the complex ratio of the voltage applied to a load to the current flowing in the load when it is operating under normal power and in its normal environment. In many cases, the operating impedance differs substantially from the "self-impedance" or "cold impedance" of the load. In antenna systems, for example, a separate radiator has a self-impedance when operating in free space. When it is combined in an antenna array, its operating impedance differs from its self-impedance by the coupled impedances from adjacent radiator elements or its image.

Many loads have an operating impedance which differs with applied power level. In dielectric heating applications, for example, the operating impedance of the dielectric varies substantially with the applied power. A plasma load is another example of an impedance which varies with the applied RF power. In both cases, meaningful impedance measurements must be made at the operating power level.

Differences Between Bridges

The Model OIB-2 differs from bridges based on classical design. The OIB-2 can handle a substantial power level and causes a minimum of insertion effects. This permits the direct measurement of operating impedance. The Model OIB-2 is inserted directly in the circuit and the operating impedance of the load is measured under normal power. Classical design bridges are ordinarily incapable of handling large amounts of power and measure the cold impedance of the load. When matching circuits are adjusted from these measurements, it is found that an unsatisfactory match is obtained when power is applied.

In measuring the operating impedance of various elements of a directional antenna, the installation of a normal bridge within the antenna circuit disturbs the relative magnitude and phase of the currents in the various radiators. The element under measurement does not have the normal coupled impedance, and therefore, the results are incorrect impedance values.

The Model OIB-2 can be installed directly in the circuit of each element, each transmission line, or each matching network, and the operating impedance level throughout the system can be determined. The data obtained can be used to match the entire antenna system and determine the power level throughout the system. Another distinct advantage of the OIB-2 is that a signal generator of substantial power can be used with the bridge. Unlike bridges of classical design, the OIB-2 allows most of the signal generator output to be applied to the load. The interference effects from adjacent antennas in operation, or from strong signals on nearby frequencies, can thus be minimized. This permits measurements in an antenna array that would otherwise require closing down the entire station.


The OIB-2 contains a complete impedance measuring circuit and a sensitive null detector. When used with a transmitter or high power signal generator, no external detector is required. However, an external detector jack is provided so that the bridge can also be used with a low power signal generator and sensitive communications receiver as a normal impedance bridge for antenna or laboratory RF measurements. The bridge is housed in a deep-drawn aluminum instrument case and is supplied with a cover and carrying handle. It is designed for use under rugged field conditions.
Frequency Range:2 MHz to 30 MHz
Through Power Rating:1 kW with VSWR 3:1
Insertion Effect:Equal to 5" of 150 Ohm Line
Resistance Range:-500 to +500 Ohms
Reactance Range:-800 to +800 Ohms at 10 MHz
Accuracy:R and X, ±5% ±1 Ohm. Dials individually calibrated and engraved.
RF Source:Transmitter or signal generator
Detector:Internal for high power source. BNC connector for external detector when used with a signal generator.
Connectors:Input and output are Type N receptacles. 6" clip leads supplied. External detector connector is BNC.
Dimensions and Weight:9" high x 7" wide x 6-1/4" deep, 8 lbs
(228.6 mm high x 177.8 mm wide x 158.8 mm deep, 3.6 kg)
Order Number:920-0002-001

For additional information on any of our products, please contact Sales to discuss your particular requirements:
Delta Electronics, Inc.
5730 General Washington Drive
P. O. Box 11268
Alexandria, VA 22312
Phone: (703) 354-3350
U.S. Toll Free: 1-800-8-DELTA-8 (1-800-833-5828)
Fax: (703) 354-0216

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