Before the days of plastic antenna analyzers, you either used a GR bridge or a slotted line (for VHF), or just knew the SWR from a cheap wattmeter or Bird, or if you were lucky, a Delta OIB-2. This is the cousin of the famous OIB-1 and OIB-3 used by by broadcast engineers. Freq range is 2-30 MHz, 1 kW thru-power. Reads +/- 500 ohms resistive and up to +/- 800 ohms of reactance, and includes the meter amplifier for precise null. Dials were hand cut and calibrated at the factory in Alexandria, for 5% accurate results. Measures the Complex Impedance of antennas and feedlines while being driven under power. In multiple antenna arrays, this is helpful to see effects of mutual coupling from parasitic elements. Also to measure the input impedance of a big grounded grid amplifier with cathode current at operating level. I've used one for a local 1.49 KHz kW AM station for ATU tuning with reduced accuracy. I'll try and post a photo soon. Here are some links to reports, manual attached.
$650, like new.
http://www.luetzow.us/OIB1.htmlhttp://radiomagonline.com/transmission/antennas/radio_using_operating_impedance_2/
