Posted by
Don Wilhelm-2 on
Jun 22, 2004; 3:07pm
URL: http://elecraft.85.s1.nabble.com/Dumb-dummy-load-question-tp368028p368034.html
Eric and all,
That 50 ohm power resistor you are using will be highly inductive - it is a
wirewound type, meaning it is a coil of resistive wire. Yes, it measures 50
ohms at DC, but for AC, the resistive (and reactive) value depend on the
frequency of operation.
For those wishing to save a buck on a dummy load, check out the power film
resistors by Caddock (MP series), they are non-inductive up to 500 mHz -
Mouser carries them priced from about $3 to $9 depending on the rating.
They are available in power ratings from 15 to 100 watts, but must be used
with an adequate heatsink - the 15 watt is derated to 1.5 watts without a
heat sink and the 60 watt version is derated to 3.5 watts. An easy to find
source of heat sink material is computer CPU coolers. See Tom Hammond's
website www.qsl.net/n0ss for one implementation. You can see the reisitors
themselves at www.mouser.com/caddock.
Another alternative is the Elecraft DL1 mini-kit - a good 50 ohm load with a
built in tap for connecting the DMM to measure voltage and calculate the
power.
73,
Don W3FPR
Life is what happens when you are making other plans
----- Original Message -----
In an attempt to save pennies, I got a 10W 50 ohm resistor ($1.79 for two)
at the Shack and soldered it to a male BNC--I figured this would make a
serviceable QRP dummy load. The resistor checked out at 50.0 ohm on my DMM.
When I hooked this to my KX1 and went through the ATU alignment procedure
(nulling C9, etc), I only get an apparent out put of about 0.3W (according
to the display on the radio). Ditto when I tune my G5RV--I get to 1:1.1,
but with only 0.3W. (This antenna/feedline system is tunable by my LDG Z100
to 1:1; see below.)
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