Eric:
I don't know what sort of resistor you got, but many larger wattage
resistors may be fine for 50 ohms at DC, but they can have significant
inductance and capacitance that causes them to have a very different
impedance at RF. If you have an antenna analyzer like the MFJ 259,
give it a scan. You are really much better off getting yourself an
honest dummy load designed to be 50 ohms up to 30MHz or better.
best wishes,
dave belsley, w1euy
On Jun 22, 2004, at 9:32 AM, Eric Ward wrote:
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.)
I had previously gone through the ATU alignment using, instead of a
dummy load, the G5RV routed through the external match (the LDG Z100)
tuned for that frequency. I figured that would be pretty close to 50
ohm, since it achieves an apparent 1:1 on that freq with my Icom 706
rig.
When I use the tuned G5RV as a "dummy load" and go through alignment,
I get what appear to me more reasonable values--i.e. when I
subsequently tune the same antenna with the KX1 ATU, I get 4.3W and
SWR of 1:1.6.
So something funky is going on and I cannot figure it out. Is there
some bloody obvious reason why the 10W resistor is not a good dummy
load? Is something else wrong with my rig, which otherwise is nominal
in all respects? (BTW, I have R4 maxed out clockwise.) I do not have
a wattmeter, so I cannot independently check output with and without
the ATU installed, but the rig is transmitting on 40 and 20m.
Thanks for any ideas...
73,
Eric
N0HHS
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