[Elecraft] WM-2 / 100 mod ?

[Don Brown]

Hi

Ok here is the original post:

OHR Dummy Load Modification

 

 

There was a question about modifying the OHR 100 watt dummy load so you could use the OHR WM-2 wattmeter to indicate up to 100 watts for testing and calibration of the KPA100. I have found a simpler and better way to do this. By converting the load into a pi network power attenuator with 20 db reduction the WM-2 will read 1 watt when the load is dissipating 100 watts. This can be done with just one resistor and a BNC chassis mount connector. 



You will need a 62 ohm 2 - 3 watt resistor MOS or carbon comp 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 board 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 now has a value of 62.5 ohms. Connect the 62 ohm resistor across the BNC connector and run a short wire from the isolated 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 network with a 62 ohm power resistor to ground and a 250 ohm resistor in series 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 make the load as accurate as possible then add a 2500 ohm 1 or 2 watt MOS resistor across one of the load resistors in the 62.5 ohm section. This will drop the 62.5 ohm resistor to 62 ohms, the correct value. This should not be necessary in most cases as 62.5 is fairly close. If you have done the mod right then you should still measure 50 ohms with an ohmmeter across the input connector if the output connector is terminated with 50 ohms.

 

To use the load connect the WM-2 input to the BNC connector with the correct 50 ohm cable and/or adapter. 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 on the meter and multiply by 10. The accuracy is quite good through 15 meters but falls off a little above 12 meters probably due to stray capacitance or the WM-2 response may fall off a little at the high end.

 

 

Don Brown

KD5NDB



  ----- Original Message ----- 
  From: Able2fly at aol.com<mailto:Able2fly at aol.com> 
  To: DolfinDon at msn.com<mailto:DolfinDon at msn.com> 
  Sent: Tuesday, May 18, 2004 10:02 AM
  Subject: Re: [Elecraft] WM-2 / 100 mod ?


  Don,

  It might be worth a re-post. I couldn't find it in the archives. Being a relative new comer I missed it the first go around.

  Thanks, Bill
  ~~~~~~~~~~~~~~~~~~~~~~~~~~

    I posted a write-up some time ago on how to modify the OHR 100 watt dummy load into a 100 watt 20 db attenuator. When connected to the WM-2 the 0-1 watt scale is multiplied by 100. The mod works quite well but it does roll off a little at 10 meters due to stray capacitance. If anyone is interested I will re-post the instructions.