The Model OIB-1 and Model OIB-3 Operating Impedance Bridges measure the operating impedance of antennas, networks, transmission line sections, and the common point of directional antenna systems while they are functioning at operating power levels. Both bridges operate with a through power of up to 5,000 Watts, and thus, provide an impedance measurement of the load while operating in its normal environment with RF power applied. This "operating impedance" cannot be measured by classical impedance bridge methods because the system characteristics are disrupted when the bridge is inserted in the circuit. Since classical design bridges are incapable of handling large amounts of power, they measure the low level impedance which may be different at operating power levels due to coupled impedances or impedance variations with power. The Operating Impedance Bridge applies almost all of the source power to the load thereby minimizing interfering effects from adjacent antennas or from strong signals on nearby frequencies. The OIB-1 and OIB-3 Operating Impedance Bridges thus satisfy a critical test equipment requirement of consulting and broadcast station engineers. The Operating Impedance Bridge connects directly in series with the transmission line, network, or antenna. The transmitter power is applied and a bridge balance is obtained by manipulating the R and X dials on the front of the bridge. A null reading on the front panel meter indicates bridge balance. Operating resistance and reactance are then read directly from the bridge dials. The bridge can also be used with the Delta RG-4 or other receiver/generator equipment to make impedance measurements. The bridge has been accepted by the FCC for measurement of license common point impedance. Application Bulletins 1 and 3, available on request, describe additional uses of the Operating Impedance Bridge. The OIB-3 Operating Impedance Bridge is an advanced version of the industry standard OIB-1. It provides all of the OIB-1 features plus extended resistance and reactance ranges, and an improved meter amplifier. The OIB-3 is furnished in a rugged drawn aluminum enclosure and does not require an additional carrying case. The Model CPB-1, CPB-1A and CPB-1B Common Point Impedance Bridges are similar to the portable Model OIB-1 and OIB-3 bridges, but are designed for permanent installation in the phasing equipment at the antenna common point.