The AM Forum

What is the correct procedure for calibrating a directional wattmeter with the inductive coupling currrent sampler, and with trimmer cap voltage dividers on the input and output?

I'm thinking Dentron W-2, Collins 302C and their ilk.

Thanks,

Steve

I reverse the input and output into a 50 ohm load to set the reflected power backwards. I duplicated the Collins circuit but used 2 meters and calibrated for 2000 W forward and 200 watts reflected.

More specifically.....there is a trimmer cap AND an adjustment pot for each meter scale (forward and reflected).  Turning the cap moves the needle a little at a time, while turning the pot moves the needle quite a bit more.  My W-2 manual does not include calibration instructions.  I'm not sure what each adjustment is supposed to do since one affects the other. 

The pot is for power set and the cap is for frequency compensation.  Set the pot at proper power level for the center of the frequency spread of the meter, then check at a frequency above and below the center.

Set the cap for equal readings or as close as possible.  If your diodes are good they should be fairly close to accurate.  Use very short jumpers between your power source and your dummy load.  Be sure your dummy load is flat at the frequency.

Look at the Drake W4 meter manual. It has alignment instructions in it. Not exactly like the Dentron but should give you some good clues.

I think these caps are to null power in the reverse direction so when working into a 50 ohm load you null the reverse then flip the sensor and null the forward cap. I have ten turn trim pots in series with the two meters to set the calibration. This means you need a reference standard watt meter. A bird 43 in my case.

This dosn't apply to this exactly, but it is a good trick any how. If you want to check a SWR meter for calibration on the SWR reflected side. Just make up a 100 ohm dummy load and run the meter into that it should read 2:1. Then check it into a 50 ohm load and it should be flat.

I calibrated my HB unit in watts both directions 2:1 is about 10% reflected

The w-4 manual helped.  I think I got it calibrated as close as I can.  Guess I'll have to get a Bird, Coaxial Dynamics, or LPA-100 if I want to be more exacting.

Thanks all,

Steve

Those things have so many variables that have to be adjusted just right, plus critical fixed value components such as the frequency compensator, all of which probably drift out of calibration with time, that I wouldn't trust it for anything more than a relative power indicator.

I have a Mirage wattmeter that I calibrated using a Bird 43 several years ago, and on some ranges, the thing indicates more power output than I am running DC input!  Maybe I have found a solution to the energy crisis.  ;D  I just use it to indicate relative power for tuning purposes, and for measuring SWR.  The SWR indication still appears accurate when I load into my 2 kw continuous duty dummy load.

I'd just look for a good thermocouple rf ammeter. It will give a relative power indication, and you can calculate the actual power using ohm's law (assuming you can achieve a known, non-reactive, purely resistive load using the tuner).