On 12/5/2013 1:49 PM, John S wrote:
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.

Yes, the trend line is very close. But, please perform a calculation on
the 498R value using the equation and let me know if it agrees. If yours
does, then something is wrong here.

Thanks,
John


Helmut -

Please see this. I think Excel is screwed up.

https://imageshack.com/i/mrg919p