[Elecraft] Re: QRP Dummy Load

[Don Brown]

Hi

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.=20

I have the OHR 100 watt load and it is good until you get up around 12 =
meters then the SWR starts to climb. I don't think it would be too good =
at VHF, so not usable with the transverters the TenTec has this problem =
also although not as bad. It is still pretty good at 30 MHz but not much =
beyond that.

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

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

I prefer resistive attenuators rather than toroid pickoffs because they =
are inherently accurate at any frequency if the resistors are non =
inductive and the correct value. The accuracy can be tested with DC and =
a voltmeter with this type on attenuator.

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.


Don Brown
KD5NDB

  Tom and Ron,

  My two cents - a known accurate dummy load is important to me.  Most =
of the
  time 20% off is OK because I am just using it as a loading device.

  HOWEVER, when I want to make measurements, it must be accurate, and I =
really
  want it to be 50 ohms rather than 51 ohms. (Don Brown take note =
please).
  The math to calculate power, etc. is much simpler when the resistance =
is 50
  ohms, and IS the stated design point for most equipment.


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