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MFJ259 conversion help
On 12/5/2013 11:31 AM, Helmut Wabnig wrote:
On Thu, 05 Dec 2013 10:26:33 -0600, amdx wrote: On 12/4/2013 9:14 AM, John S wrote: On 12/4/2013 8:48 AM, amdx wrote: On 12/4/2013 4:50 AM, John S wrote: On 12/3/2013 9:07 PM, amdx wrote: I may be asking for something that doesn't have an answer. I connected a voltmeter to the R meter of my MFJ259. I checked a bunch of resistors and recorded the voltages. Now I have all these voltage readings vs. Resistance and don't know how to relate them except for a conversion graph. Is there a math function that relates these voltages to Resistance? Load Resistance Voltage reading 7.5 ohms 0.0388 volts 15 0.0444 volts 25.5 0.0478 volts 39 0.0577 volts 50 0.0614 volts 100 0.0807 volts 140 0.0891 volts 174 0.0935 volts 221 0.0980 volts 249 0.1000 volts 365 0.1060 volts 498 0.1090 volts If you can figure this out, I'll reward you with the SWR chart :-) Thanks, Mikek V = 0.0191*ln(R) - 0.0077 You're welcome. Thanks, I'll learn about that and see if I can make it work for me. Mikek Good. I can tell you how I accomplished it if you are interested. Hi John S, Please don't let me work you if your not interested, this is now probably more of a curiosity than a way to make the MFJ259 more useful. I ran all the numbers with the formula, it is not accurate enough to me usable. I don't know if that is a calibration error, resistor error, (I used 1% resistors) a me error, or the wrong formula. I don't think it correlates very well with my measurements. Might need to use fixed font to read this. My Measurements. Real R Calculated R Measured Voltage 7.5 ohms 11.4 0.0388 volts 15 15.3 0.0444 volts 25.5 18.3 0.0478 volts 39 30.7 0.0577 volts 50 37.3 0.0614 volts 100 102 0.0807 volts 140 158.9 0.0891 volts 174 200 0.0935 volts 221 253 0.0980 volts 249 284 0.1000 volts 365 384.6 0.1060 volts 498 457.4 0.1090 volts This is really only usable at 100 ohms. Only 2% error. 52% error low end and 8.1% top end Is there a better formula. I'm trying to use a digital meter in place of basically a hand drawn dial that is very nonlinear. I'm wondering if I calibrated 50 ohms to read a bit lower or higher voltage it might help Anyone's helpful thoughts, Thanks, Mike Well I did it using an old dirty trick: adding one fake data point at the end to bend the curve. http://img560.imageshack.us/img560/1845/yi2.gif You must then not use the values which are above the last valid data point. You may program this formula into an Arduino .-) w. FWIW, I measured a 549 ohm resistor and the voltage was. 0.1105mv Mikek |
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