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Author Topic: Calibrating an SWR meter?  (Read 2744 times)
K4RLK
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« on: June 05, 2011, 07:16:43 PM »

I have an MFJ-815B swr/power meter, and an MFJ-949E tuner. The tuner reads about .5 higher than the swr meter at the exact same frequencies. I need to verify which one is correct, and don't have an analyzer. Any suggestions?
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W8JX
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« Reply #1 on: June 05, 2011, 07:18:58 PM »

Are they in series by chance?
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K4RLK
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« Reply #2 on: June 05, 2011, 07:22:07 PM »

Negative. I swap the coax between the 2 to compare the readings. And of course, the tuner is set to bypass when comparing it to the swr/power meter.

Are they in series by chance?

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AC5UP
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« Reply #3 on: June 05, 2011, 07:24:33 PM »

A 50 Ohm dummy load should read 1:1, a 100 Ohm dummy load should read 2:1, a 25 Ohm dummy load should read 2:1...............................
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W8JX
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« Reply #4 on: June 05, 2011, 08:03:12 PM »

A 50 Ohm dummy load should read 1:1, a 100 Ohm dummy load should read 2:1, a 25 Ohm dummy load should read 2:1...............................

Good point!
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WB6BYU
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« Reply #5 on: June 05, 2011, 08:43:41 PM »

So one question is whether both meters show a dip at the same tuner settings, or whether you would
end up with different settings to get a good match.


Getting them to read zero reflected power with a 50 ohm dummy load is the easy part.  That just shows
that the reflected power is zero.  Then you also need to make sure that the reflected power reading
corresponds to the forward power reading to get an accurate SWR.  The best way to do that is to check
it with a load that has a known SWR.

Probably the easiest way is to put two 50 ohm dummy loads in parallel with a coax T connector.  That
gives you 25 ohms at the T for an SWR of 2 : 1.  By the time you connect that to the SWR meter with
a coax jumper you can't be guaranteed that the meter sees 25 ohms, but at least the SWR should
be 2 : 1 and you can check the calibration.

Note also that the calibration varies with the power level:  if you are running low power to where you
need maximum sensitivity to reach full scale, the reflected power will read low due to the non-linearity
in the response of the detector diode.

I have a MFJ-949C, and each of the power scales (30 and 300 watts, forward and reflected) has its
own power calibration setting.  If you have access to a known good watt meter you can calibrate
all 4 scales, which will improve the accuracy of the readings.  But don't expect great precision from
it - most meters aren't accurate enough to tell you whether your SWR is 2.1 : 1 or 1.8 : 1.  The
important thing is to know whether a particular adjustment makes the SWR higher or lower.
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STAYVERTICAL
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« Reply #6 on: June 06, 2011, 05:15:58 AM »

SWR depends not only on the load impedance but also on the coax length, loss, and several other factors.
So you should eliminate any feedline from the calibration.
If you put a resistive dummy load at the output (antenna) terminals of both the SWR meter and the tuner, you will soon see how they stack up.
Most SWR meters work by reading the forward and reverse voltages/currents, and are made to operate at a specific impedance, normally 50 ohms. So as previously stated, if you put a resistive load at the antenna terminals you should get a 1:1 match.
Other swr ratios are just the ratio's of 50 ohms to the load impedance.
Dont know what sort of SWR meter you have, but they are mostly just voltmeters who's dials are calibrated in SWR units.
To be accurate they need to be set to maximum scale on forward power, and when set to read reverse, will indicate the reverse voltage due to the reflected energy. The scale is calibrated so it acts like a crude but effective analog computer, doing the calculation,
Forward voltage + Reverse Voltage  divided by Forward Voltage - Reverse voltage.
So as an example if the forward voltage is 1 (full scale) and the reverse voltage is 0.5 (half scale)
then the SWR is = (1 + 0.5 )  /  (1 - 0.5 ) = 1.5 / 0.5   which equals 3.
So the SWR at half scale is 3:1.
Simplicity itself.
Some more sophisticated SWR meters read these voltages and do the calculation by microcomputer, but its all the same process.
If you find the SWR meter is off, many have a potentiometer inside to adjust them for calibration.
Hope this helps , 73s.
« Last Edit: June 06, 2011, 05:18:40 AM by STAYVERTICAL » Logged
VK1OD
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« Reply #7 on: June 06, 2011, 01:02:12 PM »

I have an MFJ-815B swr/power meter, and an MFJ-949E tuner. The tuner reads about .5 higher than the swr meter at the exact same frequencies. I need to verify which one is correct, and don't have an analyzer. Any suggestions?

If you place the 815B between the tx and 949E using good 50 ohm cables, and switch the 949E to the internal dummy load, you should get VSWR=1:1 on the lower HF bands on both meters at high power (for best accuracy).

If you don't, the relevant VSWR meter is out of cal (unless something else is faulty). This is not a lab standard dummy load, but it should be good enough for the purpose of checking the VSWR meters, and give you a better cal than the one you describe.

(I would not depend on the explanations of how a VSWR meter works above, they are seriously flawed. Read more widely.)

Owen
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