FW: [AMRadio] bad antenna luck


Jim Candela jcandela at prodigy.net
Tue Jan 9 18:24:10 EST 2007




From: Geoff/W5OMR <ars.w5omr at gmail.com>
 Here's a question, in that regard... I've got a 640w 'dry' dummy load 
comprised of (4) 50 ohm, 160w NI resistors connected in series/parallel. 
I always ass/u/me'd that it was 50 ohms, but I -finally- put in one of 
those self-righteous, swr-protected, solid-state transceivers and 
watched the output meter while I tuned across the spectrum.

The SWR on my load didn't get below 2:1 (and only got down to 1.5:1) nor 
did the power output creep past 10w (up to around 75w) until the 
frequency readout showed around 2.5Mc.  Everywhere else was 3:1 or better.

Why would (4) 50ohm, 160w Non Inductive resistors lashed up in 
series/parallel show such behavior?


Reply by Jim, WD5JKO:

Jeff,

  I'd first measure that dummy load with a DVM on ohms. Overheating can result in resistance changes. My MFJ oil can dummy load has had too many rigs on it, and is now closer to 75 ohms from prolonged key down AM rigs, Linears, etc.

  I suspect you have a big issue with that load. Your statement about 2.5 Mc meant 2.5 mc or lower? Or 2.5 mc or higher?

I once was given a shopping bag of 47K 1/2 watt Carbon Comp resistors (CC). I made ten rings of 100 resistors in parallel totaling 1000 resistors. I should have had 47 ohms at 500 watts, and when in a gallon paint can full of ATF I was hoping for 1000 watts @ 47 ohms. What was wrong with my plan??

1.) CC resistors ratchet upward in value after being heated. My 47 ohms was closer to 63 ohms now.
2.) Each resistor had a few pf of capacity across the terminals. If that was a mere 3pf per resistor, multiply that by 1000, and the 63 ohms was shunted with 3000pf, or .003uf. I could only use this dummy load with a tuner because the SWR into it was terrible because of that shunt capacitance. It was neat project, but a total failure. Remember my comment about, " i can't make a resistor work'? This is where that saying came from.

I bet you have resistors that are not 50 ohms anymore, and the inter wiring capacitance, and inductance make it a crappy load at RF.
I have seen loads with wire wound resistors where the direction of turns changes to cancel out the inductance. This works up to 5-10 mc or so.

Regards,
Jim
JKO






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