Proper way to calibrate a watt meter

[kc2etm]

Hi want to calibrate my watt meter it is a powered pep reading meter I have the instructions from manufacture of the meter but in step 7 it says set to avg keydown a 20 watt cw transmission and adjust then it says switch to pep mode and do the same by doing this I would then with a non cw carrier a carrier with no tone won't the avg and pep resting carrier reading be very far apart

Thanks
Fred

[WD5JKO]

Fred,

    The PEP and Average level of a keyed carrier (no off on, or modulation) is the same. The meter most likely has some sort of peak reading circuit for PEP that is not part of the calibration steps you posted. Most PEP reading watt meters, if passive (no DC power required) will not read the full PEP. Then again, I think many PEP watt meters are deliberately set to average that peak a little such that a true 2KW or 2.5KW PEP signal registers as 1500W PEP.   

    The other big issue is the accuracy you might get. Consider that the industry standard Bird 43 is at best 5% accurate, and that is at full scale. Take the same meter that has slugs 20+ years old without a single NIST traceable calibration, and then perhaps 10% is the best you might get.

Jim
Wd5JKO

[KA0HCP]

Hi want to calibrate my watt meter it is a powered pep reading meter I have the instructions from manufacture of the meter but in step 7 it says set to avg keydown a 20 watt cw transmission and adjust then it says switch to pep mode and do the same by doing this I would then with a non cw carrier a carrier with no tone won't the avg and pep resting carrier reading be very far apart

Thanks
Fred

You must use a mode with a carrier, e.g. CW, RTTY, FM, AM when using a wattmeter/SWR meter.

[Jim, W5JO]

The big thing in this is having an accurate load.  I am not sure if the meter is designed for 50 ohms or not but, to be as accurate as possible, use what ever impedance the book specifies.  Then there is the issue of the reflected.  Most meters use a 150 ohm load and adjust the reflected (SWR) to 3.

[kc2etm]

Thanks everybody I used a 50ohm bird load and a bird 43 and followed the instructions and it did work properly of course only as accurate as the bird but close enough for reference I guess before calibration the three power scales weren't even close to each other

[N8ETQ]

Hey Fred,


    I've given up on watt meters, even Bird.
My old Cantenna even "Moves" in value and is
currently at 54.2 ohms. You need to measure
your load with a DMM.

    Then I use a Tektronics somethin' or other
and get a Peak to Peak, or just Peak voltage
across whatever value my Cantenna is at and
"Do the Math".

    Don't use one of the scope probes that
came with the scope. I use a chunk of RG-58
with a RCA jack on one end and a BNC on the
other end. Jump out, (or remove) that "R"/Diode
thing Heath put in there.

    Now you can get a Peak to Peak, or just Peak
"Voltage" across your load. Adjust your scope
for as "Full" of a screen you can get. Record
your voltage then I use this:

http://www.random-science-tools.com/electronics/dBm-Watts-volts.htm


    Plug in your values and your all set.
If you really want an accurate meter, Calibrate
at your frequency of interest and don't trust it
at any other Freq.

    Here, I have Tektronics 465, 475, 2215 and
a couple 2465's. Even have a Hitachi device
someone gave me. You can check the scope easy
enough with a battery and a DMM. You can check
your DMM with a resistor.

    About as close as "WE" can get in the basement!

GL es 73

/Dan