RE: Experiment of reproducible results PART II

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From: Nick Kennedy (nkennedy@tcainternet.com)
Date: Mon Jul 15 2002 - 20:13:57 CDT


Whoa--looks like missed out on a good one by being on a canoe trip (my
first) this weekend. But I'll pitch in my 0.02 Euros worth anyway--my
small contribution to The Discussion That Can Never End.

On what matching does for you--I don't necessarily agree that it allows the
TX to give its Maximum Output. I think Rated Output would be more like it.
 Reason--you're not necessarily matching the source resistance when you
provide the desired 50 ohm load. I think we've discussed this and sort of
agreed on it before.

On mismatches caused by measurements ... I agree that you can't measure
anything without disturbing it a bit; and that SWR meters are going to
introduce a small mismatch, but it's still hard to believe that this is the
cause of the big bad discrepancies some people are reporting. Unless the
meter design is really poor.

Let's see--I'm right now trying my MFJ-259B into a 50 ohm BNC
terminator--SWR at 21 MHz is 1.0 to 1. Now shooting thru my homebrew W7EL
wattmeter / SWR meter at 21 MHz to the same load, SWR is 1.1 to 1. OK it
had an effect, but it's not huge. And I freely admit that my construction
techniques on this SWR meter weren't the best.

Concerning the original issue on Reproducible Results Part I (if I remember
correctly)--the SWR would jump up when power was reduced. I used to see
that on some rigs I owned previously. My theory (not too rigorous
scientifically) was that the amplifier liked to go into oscillation, but
sufficient drive would swamp or otherwise overwhelm that tendency.
 Reducing drive below a critical value would allow the evil spirits to come
to the fore? What do the text books say about that one?

72--Nick, WA5BDU


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