[Elecraft] Re: QRP Dummy Load

[Tom Hammond]

Hi Ron:

>Now that worked GREAT for hollow-state (tube) equipment with pi-net outputs
>and simple solid-state gear also with tunable outputs in which we only
>wanted something "in the ballpark". We didn't care too much what the REAL RF
>resistance was as long as it was sort of 50 ohm-ish and sort of
>low-reactance.

Long live the pi-net.

>One question I don't see answered here is, "How accurate do we need a dummy
>load to be?" It's a different world not that out rigs are "pre-tuned" and
>expect a certain non-reactive impedance at the antenna terminal.
>
>Since so many people use commercial dummy loads without a problem, I'd guess
>the current crop of dummy loads is just fine. They seem to be accurate to
>within only about 20% or so! They claim an SWR of only 1.3:1 or so.
>
>Is that adequate?

A question I've been considering as well.

Since many builders will be using their dummyload for both SWR and Power 
calibrations, I'd think we'd like to have RF resistance accuracy as good as 
possible, GIVEN reasonable pricing. And, we'd certainly want to reduce 
residual reactance as much as possible.

Given the short leads of these resistors, I'd bet that reactance would not 
be much of problem. And the published default accuracy of resistance is 
+/-5%, so that should put us to within +/- 2.5 Ohms for the 51 Ohm 
resistor. +/- 5 Ohms for 100 Ohm resistors, and if the resistances of two 
just happened to be on opposite sides of 100 Ohms, they'd tend a wash out 
when paralleled. I worst case SWR (47.5 Ohms) would still provide a 1.07:1 
SWR, all other factors being equal. Looks pretty good to me.

73,

Tom  N0SS