Hi-
There are no dumb questions.
On Mon, 17 Feb 1997 20:19:22 GMT mdwatt@usit.net (Marty Watt) writes:
>I found two each 50 Ohm and 100 Ohm 10W power resistors at Radio
>Shack. One measured 48.9 on the DMM. The 100 Ohms I put in
>parallel to get 50 Ohms, measured at 49.0 Ohms.
>
>How much difference is 1 Ohm going to make as to SWR?
Not much! SWR = 50/49 = 1.020408
Yeah, it really is that easy. For 100 ohms, 100/50 = 2. For 25
ohms, 50/25 = 2.
Same swr whether your 50 ohm line looks at 25 or 100 ohm loads.
> I looked in the 1990 handbook, but can find no way
> to calculate SWR based on two readings (ideal vs. actual).
It is there.
> One is mounted to a coax stub, the
>parallel 100 Ohms is in a box. I know I'm adding some reactance by
>using the parallel, but will the SWR still be OK?
NO, probably not. These are POWER resistors- they are almost
certainly wire wound. Each one is a lossy inductor made of lots of
turns. Unless you have some way of measuring the inductance, I
wouldn't mess with them. They MIGHT be ok for the lower frequencies
(160 and 80 meters), but above that, no.
>Also, how much power out can I reasonably expect to use these two
>dummy loads? I would suppose the 50 Ohm 10W would be good for 10W
>continuous. Will the parallel 100 Ohm 10W give me 20W power
>maximums?
Yes. And the amount of extra indutcance or capacitance you add by
paralleling resistors is negligible in the HF bands (well, maybe at 10m,
if you're sloppy with lead length, you'll have a problem.) So the way
to make a good dummy load is to take 20 1 Kohm one or two watt
carbon-composition or metal film resistors, and parallel them. Use some plates of PC board material and run the resistors between 'em. Yeah,
there will be some inductance and stray capacitance- it will still be
ok for HF.
Now why would you ever need a dummy load for more than 5 watts???
Six 300 ohm/ 1 watt resistors will do the job, or three 150 ohm/ 2
watt resistors, or...
Note that the metal film resistors are made my cutting a spiral out
of a thin resistive cylindrical metal film, and so they also have some inductance, but not much. Somewhere I read that it is negligible. The
problem with the carbon composition resistors is that their tolerance is
poor, they can change their resistance when you solder them in place,
and no one makes them anymore, so they can be hard to find.
Hope that helps.
73, ; D DWink@Juno.com Dan Winkler N7IVR Seattle, WA
----------whom the gods would destroy, they first make proud ----------
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