KC8AON's QRP PROJECTS
50 OHM DUMMY LOAD / RELATIVE READING POWER METER

GREAT FOR THE QRP HOMEBREWERS TEST BENCH !


Here's a neat little piece of equipment that I built while waiting for my SW-30+ kit to arrive. It's a combination 50 ohm load and relative reading wattmeter. I was wanting something that I could plug the SW-30+ directly into when I finished building it to give an indication of RF power to tune the transmitter up with, so this is what I built. The result is a handy little meter that provides a 50 ohm load to the transmitter during tuneup and shows an indication of forward RF power. I did not calibrate mine since all I wanted was something to give an indication and show me when I hit a peak, and this device provided just what I needed ! You may decide to calibrate yours if you decide to build one, and that would make it even better !
 


SCHEMATIC FOR THE "LOADMETER" AS I CALL IT.



The schematic above shows how simple this device is, and it starts out with the 50 ohm dummy load. The load resistor in this instrument is nothing more than 15 one watt 750 ohm metal oxide or carbon resistors in parallel. The ones I used were donated in kit form by someone that I don't remember their name or callsign (sorry !). But it consisted of 2 pieces of copper clad board (double sided), with a hole in one for a coax connector to be mounted, and all the holes for the resistors drilled in both boards plus a hole for the center conductor of the coax connector in one of the boards. The resistor leads are simply inserted into the holes between the 2 boards and soldered into place and the coax connector is mounted on one board (ground side) with the center conductor soldered to the other board. The meter I used is an old 1 mA meter that I had in my junk box (I use what I have because I'm cheap). The specs on the meter are not critical, and you could even use an old S meter from an old CB if you have any ! I salvage all of these that I can ! Other than that, the circuit is fairly straight forward. I did connect mine to my Yaesu FT-840 after I completed it, and with the 840 set to 15 watts CW, I adjusted the pot in the meter to read full scale. That way, I could use it not only with my 2 watts rigs, but I can also use it with my old Yaesu FT-7 that does about 12 watts out max, and my Icom 703 that does about 10 watts out.
 


PICTURE OF THE 50 OHM LOAD



This is a picture of the 50 ohm load itself incase you may want to try and duplicate it. There's really no need though, as you can simply wire the resistors together without the copperclad board. Remember, I just use what I have on hand if possible !
 


HAVE FUN BUILDING ! -->

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