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Converting CB Watt/SWR meter to 40m

Jim Dickey

Jim Dickey

That's my real name.
Jun 17, 2017
80
7
8
35
Sandpoint, ID
I have a ASTATIC 10/100w meter that was orginally designed for the 11m band. I have a better one for CB now, and would like to use this one to test 40m QRP amps and transceivers. Super accurate isn't important, I just need to be able to prove that I am making positive changes when I build or modify something.

I'm thinking this should be as easy as swapping a coil inside?
 

Is it going to be far enough off that the needle wont move? I have a few QRP rigs that put out 5w CW that could be used to calibrate.

Like I said I don't need any real accuracy, I thought the home brew forum was the right place to post this.
 
Just try it on 40m. You might be pleasantly surprised. Most CB power meters have a small pc board with a straight thru strip from input to output and a strip on either side for forward and reverse pickup. There are no coils to sub out. If you do not need great accuracy it my be functional for what you want already.
 
It should work.But you might need more drive to get FSD . I.E you might need 20w to get it to read 10. .

Thinking if it is a dual range 10 and 100 you will have 4 trim pots ( variable resistors ) 2 for each range fwd and reverse. Would be real easy to recalibrate.

Depends on the layout of your particular meter.
 
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Try it and see what happens. It'll probably work fine. If it's off a bit, do you really think it's going to make much/any practical difference? I don't. If you recalibrate, what do you recalibrate against? If that meter is right to about 10%, that's as good as it gets (even those Birds).
 
It worked great. Made some adapters for BNC to PL259 and plugged it in. The 40-Niner was reading about 300mW, the Super Rockmite was reading about a watt and a quarter. The amp I built (that IRF840 diagram that claims 60W out) is putting out zero watts. Gave me the info I needed at this point.

Just need to figure out why the amp no worky.
 
Almost all meter's should be considered relative measure of power not absolute. Even with 1% resistors inside as they age with out a means to accurately calibrate it fairly often you can not take it as anything remotely absolute. For home use using them to check function and for trending data is really keen.

Checking it against voltage output from final converted to watts is a good start to check the meter.
 
If intended for CB use it usually has silicon diodes @ .7 volts forward drop. A bit more sensitivity can be gained by replacing them with like 1N34/1N60 germanium diodes.
Past that you need a powered meter to bias your input voltage up at or near the .7 or .4 volts and adjust zero as necessary. You can also get a sensitive field strength meter this way which can be an issue. I have several reference oscillators that show up on my sensitive meter . Fluorescent ballasts drive it crazy ... :)

Seriously, for micro measurements I use a 'scope but not everyone has or needs one
 

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