On 4/17/2013 5:06 PM, Jim Brown wrote:
Measured how? By the rig, or by the amp? The SWR indicator in the RIG
will measure the match between the rig and the amp. The SWR indicator in
the AMP will measure the match between the rig and the feedline.
Right. That is what I am talking about.
It is quite normal for different SWR indicators to give different
readings on the same antenna. Don't be alarmed unless one of them reads
VERY high (much greater than 3:1).
OK... I am not as worried as I am curious why
the rig sees a higher SWR with the amp on than
with it off. The amp tuning circuit plus antenna
must present the rig with a different SWR than the
antenna alone. I am wondering why that happens
- especially when it happens on 17m but not on
15 or 10m when I size the same exact antenna in
proportion up or down in size for the different
bands?
* Does this have anything to do with working 17m
opposed to another band?
No.
Shoot... another WAG misses the mark ! ;-)
BTW -- FWIW, my Titan 425 amps, designed in the late '70s, work best on
the WARC bands when I set the amp to the band BELOW the WARC band I want
to work.
OK... maybe I should try that, instead, and
see what I get.
What could be the harm in doing that? After all,
there is only so much smoke, so once it is out,
the danger is over, right? ;-)
Another BTW -- the right way to tune a triode power amp is to drive it a
bit below the power level needed for rated output, tune Tune and Load
for maximum output, increase the drive only enough to get rated output
(or lower power if you want) and re-peak Load and Tune.
OK... I get that.
NEVER use ALC
between the rig and the amp to set power level -- that is a recipe for
massive splatter.
I will remember that. As this is Centurion
and an Omni VII I am spared all the fuss and
bother of ALC circuits.
AND -- a great way to tune a power amp is to use your
paddle to send a series of dits, which reduces the stress on the tubes
and the power supply, while still causing the amp to operate at its peak
output level (and thus provide the cleanest tuning).
I should do that. I have just gotten into
the easy habit of hitting the tune button
and it lowers power to 17 watts so I guess
I figured that was going easy on it.
I do know pulse tuning is a preferred method.
Before I got my KPA500, I occasionally used the Titan to get me to 200W
on 30M. Two of my Titans are happy on 30M, but the oldest one, probably
built before WARC bands were authorized, is not. I suspect that the
input circuit is not sufficiently broadband to pass 10 MHz (I tried both
40M and 20M settings of the bandswitch), and in either position it
seemed like something was overheating, even at only 200W out. That would
be consistent with trying to run power through a resonant circuit
outside of its design range.
This was the sort of thing I was guessing
at above. I figured it was a WARC band and
the amp would not tune as cleanly as on 20m
and 15m.
Remember that SWR is a very limited way to describe what's happening in
a system -- as the late Dick Heyser said of frequency response, "it's
like trying to write poetry with only one word in your vocabulary."
Agreed. And the examination prep books and other
materials do a lousy job of explaining this.
So many of my friends think the antenna is
"resonant" when SWR is lowest, and that is all
they ever look at.
I know better, but I honestly don't know enough
to say I fully understand all the rest that is
involved. Someday I will. If I can learn to
put the seat down, I can learn this, too, right?
;-)
Thanks for the uptake.
------------------------- K8JHR -------------------
73, Jim K9YC
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