Does an SWR meter need a minimum of power

My MFJ 962D has meter ranges of 300W and 3000W, even with the meter set on 300W, my 10W hardly budges the SWR needle. The radio's meter can be off the scale and the tuner's SWR meter is around 1.3 reflected, I have calibrated the SWR on the tuner and still the same. Does an SWR meter require a minimum amount of power to show an accurate SWR reading? I was told by the seller it showed accurate results when he was using 100W output, at the moment I am getting readings in the ballpark then fine-tuning using my radio's meter. I can live with it as it is and was more intrigued than annoyed.

Hi Mark. An SWR meter will require a minimum amount of power, depending on how it has been constructed.

I don't know your 962D, but should read ok with your Foundation levels. I base that on a snippet of the calibration text I have just read in the manual (below). Maybe you could try calibrating?


With the 3000/300 Watts button pushed out for the 60 Watt reflected scale, Transmit 10 Watts
as indicated on the reference meter. Adjust the LO REF trim pot to set the reflected power scale
to 10 Watts

SangueG wrote:

Hi Mark. An SWR meter will require a minimum amount of power, depending on how it has been constructed.

I don't know your 962D, but should read ok with your Foundation levels. I base that on a snippet of the calibration text I have just read in the manual (below). Maybe you could try calibrating?


With the 3000/300 Watts button pushed out for the 60 Watt reflected scale, Transmit 10 Watts
as indicated on the reference meter. Adjust the LO REF trim pot to set the reflected power scale
to 10 Watts

I have calibrated as per the manual, still the same, 10W is quite a small percentage of 300W. Thinking about it and reading the scale, 1.3 is pretty close to 10W reflected and 1.3 is the maximum reading I get.

Hi Mark,

As you may be aware SWR (or VSWR, however you want to identify it) is a ratio-metric reading based on forward and reflected wattage readings.

The formula looks less elegant typed than it does written but is basically as follows :-

VSWR = (1+SQRT(Power reflected / Power forward)) / (1-SQRT(Power reflected / Power forward))

So if you had 100W forward and 4W reflected the Power reflected divided by the Power forward = 0.04 with the square root of that being 0.2 so the formula would look like :-

VSWR (1+0.2) / (1-0.2) = 1.2 / 0.8 = 1.5

Therefore 100W forward and 4W reflected gives a SWR reading of 1.5

The same would be true at 1000W forward with 40W reflected, or at the other end of the scale 10W forward with 0.4W reflected.

As you can see from the latter example QRP power levels read on a QRO or high power calibrated meter becomes a bit difficult! With your MFJ unit it becomes all but impossible to read off the 'red' SWR reading lines.

This I think you've already figured out.


You could go hell for leather and recalibrate the power measuring capability of your MFJ but you either have to go the "chicken & egg" route requiring a decently calibrated secondary meter or go straight to source and read the voltages off directly then utilise Ohm's Law. (Also allowing for any voltage drops in your measurements at low power such as on the rectification diodes.)

The other option is to do exactly what you have been doing and use your radios internal SWR meter to make sure all is good.  


Knowing a bit of maths is one thing, simple solutions are quite another.

Quite often bogging yourself down with mathematics and theory can blind people of the obvious choices available.


Well done and I hope your intrigue is satisfied.

All the best,
Victor

About the only equation I can remember is: The squaw of the hippopotamus is equal to the sum of the squaws of the other two hides.

All over my head that lot lol.
So glad i have auto tuners.