[Elecraft] Re: QRP Dummy Load

Wed Aug 6 18:47:01 2003

Well, Don, it has been my experience that resistor groupings produced (and 
bought) together have values that are, in the words of the econometrician, 
highly autocorrelated.  So, if you buy a group of 10 27K ohm resistors, and 
you measure the first as being high at 28K, the odds are overwhelming that 
the great majority of the rest will also be high, near 28K ohms.  In order 
to get some reasonable averaging, you need resistors that are truly random 
in value around their nominal value.

best wishes,

dave belsley, w1euy

--On Wednesday, August 6, 2003 4:48 PM -0500 Don Brown <[email protected]> 
wrote:

> 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.
>
> 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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----------------------------------
David A. Belsley
Professor of Economics