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SWR meter as power meter
On Nov 29, 2:51*pm, Owen Duffy wrote:
If it is the type where calibration varied with frequency, you could copy what was done with some commercial instruments of the seventies, eg the Oskerblok. 1. Calibrate the sensitivity knob with a logging scale. 2. Add a power scale to the meter face. 3. Measure a set of points of sensitivity setting vs frequency where the meter scale is correctly calibrated for power. 4. Create an interpolated smooth graph as a lookup, and attach it to the instrument. Be prepared for significant linearity error at the low frequency end if you calibrated the meter for low fsd power. If it is a Bruene type sensor, the response should be reasonably flat within a frequency range which you must establish. The interesting thing with either type of sensor (and both are used in Bird elements, though the first type is frequency compensated), is that although the Pfwd and Pref power readings are of each of no value by themselves, the power in the line is Pfwd-Pref irrespective of the load impedance or line Zo. Owen Owen, the meter is just a 1 to 100 linear scale meter on on 100 uA movement. Ive used it for years just to "guesstamate" VSWR. Not really as inaccurately as one may think. My thought was to cal a scale for the "full scale set" knob so that when the meter is adjusted for full scale the knob position will indicate power. Part of the reason for doing it this way is that it is a nice large good quality meter and I dont want to risk damaging it by taking it apart. I may have other uses for it later on. Im thinking "linear scale capacitance meter similar to the heathkit model. I intend to use an NE555 osc instead of vacuum tube osc should I do this . Jimmie |
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