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SWR meter as power meter
On Nov 30, 9:23*pm, Richard Clark wrote:
On Tue, 30 Nov 2010 13:50:35 -0800 (PST), JIMMIE wrote: 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 Hi Jimmie, On reading this, several thoughts came to mind. *You write about having used a linear scale to guesstimate SWR. *Sounds good and it immediately leads us to an existential question: * * * * "What value is there in knowing the value of SWR?" The first motivation following a glance at the SWR meter is to LOWER the SWR, for whatever value it may reveal. *In this sense, the value is a trivial consideration - relative indications are enough to achieve the goal. Using the feed from a Bruene style detector pair into a Log-Amp will give you a power response in a linear scale. *If you want to know the SWR with accuracy, you can throw the reverse switch for the second power reading and compute, or you can throw the reverse switch and start twisting nobs to make the power indication go away. consider:http://rfdesign.com/mag/503rfd33.pdf Says it all in one page - with pictures, schematics, and math too. I don't quite catch the drift of the "capacitance meter" paired up with NE555 osc. *I presume you mean to construct a DC-Freq converter? 73's Richard Clark, KB7QHC Heathkit made a linear scale capacitance meter years ago dont remember the model #. CM? I think. Circuit use a tube in a multivibrator configuration the NE555 is just a more modern approach to th same circuit. Cap to be tested is in series with the osc and meter. Current through the circuit is a linear function of capacitance or for that matter frequency. I never got around and may never still build the circuit because I can always bread board a little circuit to use with my function generator when I want to measure a capacitor. I cal the circuit using some precision caps before use. I agree with your view of usefulness of knowing SWR. I use it mostly as a relative health indicator of my antenna system. . A high SWR on my 40M dipole may indicate something is not well and I should look out the window while an unrealistically low reading on 2M probably means I have water in my coax. BTW the reason I bought the little home brew SWR meter was to source the meter and box for this project. My lack of real need for it has kept the little hamfest treasure an SWR bridge. Jimmie |
#2
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SWR meter as power meter
JIMMIE Inscribed thus:
On Nov 30, 9:23Â*pm, Richard Clark wrote: On Tue, 30 Nov 2010 13:50:35 -0800 (PST), JIMMIE wrote: 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 Â*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 Â*vacuum tube osc should I do this . Jimmie Hi Jimmie, On reading this, several thoughts came to mind. Â*You write about having used a linear scale to guesstimate SWR. Â*Sounds good and it immediately leads us to an existential question: "What value is there in knowing the value of SWR?" The first motivation following a glance at the SWR meter is to LOWER the SWR, for whatever value it may reveal. Â*In this sense, the value is a trivial consideration - relative indications are enough to achieve the goal. Using the feed from a Bruene style detector pair into a Log-Amp will give you a power response in a linear scale. Â*If you want to know the SWR with accuracy, you can throw the reverse switch for the second power reading and compute, or you can throw the reverse switch and start twisting nobs to make the power indication go away. consider:http://rfdesign.com/mag/503rfd33.pdf Says it all in one page - with pictures, schematics, and math too. I don't quite catch the drift of the "capacitance meter" paired up with NE555 osc. Â*I presume you mean to construct a DC-Freq converter? 73's Richard Clark, KB7QHC Heathkit made a linear scale capacitance meter years ago dont remember the model #. CM? I think. Circuit use a tube in a multivibrator configuration the NE555 is just a more modern approach to th same circuit. Cap to be tested is in series with the osc and meter. Current through the circuit is a linear function of capacitance or for that matter frequency. I never got around and may never still build the circuit because I can always bread board a little circuit to use with my function generator when I want to measure a capacitor. I cal the circuit using some precision caps before use. I agree with your view of usefulness of knowing SWR. I use it mostly as a relative health indicator of my antenna system. . A high SWR on my 40M dipole may indicate something is not well and I should look out the window while an unrealistically low reading on 2M probably means I have water in my coax. BTW the reason I bought the little home brew SWR meter was to source the meter and box for this project. My lack of real need for it has kept the little hamfest treasure an SWR bridge. Jimmie Jimmie. Have you seen this ? http://ironbark.bendigo.latrobe.edu.au/~rice/lc/ I've just built this version. http://panteltje.com/panteltje/pic/lc_pic -- Best Regards: Baron. |
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