[Elecraft] Modifying the OHR 100W dummy load for the WM-2

[Don Brown]

Hi

There was a question about modifying the OHR 100 watt dummy load so you c=
ould use the OHR WM-2 wattmeter to indicate up to 100 watts for testing a=
nd calibration of the KPA100. I think I have found a simpler and better w=
ay to do this. By converting the load into a pi network attenuator with 2=
0 db reduction the WM-2 will read 100 watts when the load is dissipating =
100 watts. This can be done with just one resistor and a BNC chassis moun=
t connector. You will need a 62 ohm 2 - 3 watt resistor MOS or carbon com=
p type (you can use a 51 ohm 2 watt and a 10 ohm 1/4 watt in series) and =
a chassis mount BNC connector. Mount the chassis mount connector next to =
the SO239 connector on the back of the dummy load. Remove the circuit boa=
rd and make a cut in the circuit trace between the last resistor and the =
ground screw at the back right of the board. Then make a cut in the trace=
 between the 4th and 5th resistor from the back right. This will isolate =
4 - 1K resistors in parallel from ground forming a 250 ohm resistor with =
one side connected to the dummy load that is now 62.5 ohms. connect the 6=
2 ohm resistor across the BNC connector and run a short wire from the iso=
lated side of the 250 ohm resistor (the outside end of the last resistor =
in the string on the right. This completes the mod. You now have a pi net=
work with a 62 ohm power resistor to ground and a 250 ohm resistor in ser=
ies with another 62 ohm to ground on the output. The input and output is =
50 ohms and a 20 db reduction in power. If you are using the load without=
 the WM-2 then you will need to terminate the output BNC with a 50 ohm 2 =
watt resistor or the load will be a couple of ohms high If you want to ma=
ke the load as accurate as possible then add a 2500 ohm 1 or 2 watt MOS r=
esistor across one of the load resistors but this should not be necessary=
 in most cases.

To use the load connect the WM-2 to the BNC connector and place a 2 watt =
or higher 50 ohm load on the WM-2 antenna connector. set the WM-2 to the =
1 watt full scale range and it will read from 0 to 100 watts. Use the 10 =
watt scale and multiply by 10. The accuracy is quite good through 15 mete=
rs but falls off a little above 12 meters probably due to stray capacitan=
ce or the WM-2 response may fall off a little at the high end.


Don Brown
KD5NDB


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