[Elecraft] Dumb(?) dummy load question

[David A. Belsley]

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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