[Elecraft] Re: QRP Dummy Load

[Tom Hammond]

Hi Don(s), et al:

>OK The 51 ohm is only 2% off of the 50 ohm mark but if you want more 
>accuracy then 2 100 ohm or 3 150 ohm may be better. Then the tolerances 
>may be more likely to cancel out or you could pad it with a higher value 
>resistor but then you may add some inductance. The several 51 ohm Ohmite 
>resistors I have are very close to 50 ohms. They all read about 50.04 to 
>50.06 and they still read close to 1:1 SWR out to 500 MHz. That is pretty 
>close for a piece of home built equipment. Most wattmeters or scopes are 
>only 5% if you are lucky.
>
>Some time back I posted the instructions to modify the OHR 100 dummy load 
>so it can be used with the WM-2 wattmeter. This changes it into a 20 db 
>power attenuator so the 1 watt scale on the WM-2 will read 0 to 100 watts. 
>As long as we are designing a new product maybe we should add this also. I 
>can repost the instructions if anyone is interested

Persinally, I'd prefer to have them keep it simple... maybe a tap and 
detector which could be used with a Hi-Z DMM/DVM and maybe a (printed) 
look-up table to calculate power into the dummyload, but nothing more 
'involved' that that.

>I also have a TenTec dummy load and I modified it with a 40 db tap off so 
>it can be connected to a scope. With this setup 2 volts pk to pk will give 
>you exactly 100 watts

OK, add an attenuated tap as well.

>As far as fan cooling this would only be required if the load was used for 
>an extended time at its full rating. Most of the time the fan would not be 
>needed. If the fan was included and a barrel connector was on the PC board 
>so a standard K2 power cable could be plugged in then you could us it if 
>you needed it by plugging in the power cable. At power levels up to about 
>30 watts a fan would not be required with two or three resistors and a 
>small heatsink or if the resistors were mounted to a metal box. BTY the 
>case of the resistors is isolated so no insulator is required.

Maybe the fan could be a user-applied option. I'm still trying to keep the 
$$$ down.

73,

Tom  N0SS