Electronics > RF, Microwave, Ham Radio

Does my SWR power meter need to be calibrated?

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A.Z.:

--- Quote from: bob91343 on November 28, 2021, 05:57:09 am ---Well, 3.5W instead of 5W means a voltage error of maybe 15%.  You said HF so I am assuming short cables.  Most SWR meters (I am guessing) would have a calibration pot so maybe that's what needs adjusting.  If the frequency is much above 10 MHz I would be cautious about what to do about it.  At 30 MHz the dummy load may not be 50 Ohms so it's wise to check it.

I didn't do the math but 22.4 V seems not right.  Is that p-p?  And what kind of amplifier can be running at 5W out?  How much drive are you giving it?  Is the wave sinusoidal?

--- End quote ---

Which seems to match what's reported on eHam

https://www.eham.net/reviews/view-product?id=14770

they say the meter is off by about 20%, and if that's true with the meter set to 15W max input, then it's quite a lot imHo

[edit]

and here are some notes about correcting issues with a CN-101L, different from the CN-501H, but I wonder if the latter may use a similar schematic

https://pa0fri.home.xs4all.nl/Diversen/CN101L/CN101Leng.htm

I wonder if the 501 is just a new "incarnation" of the 101 with a slightly different front panel  :)

A.Z.:
as for calibration

open the meter

connect the meter to a tx and a dummy load

set the meter to 15w

drive it with 5 or 6w

adjust the fwd "lo" range trimmer

swap (reverse) the dx and dummy load connections

repeat the above, but this time adjust the ref "lo" trimmer

Bud:
It is not 100% clear from the photo but the reflected wave needle may need to be null adjusted using the screw on the front panel ,  as it appears to be below the null point.

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