On 4/17/2013 12:52 PM, Richards wrote:
When I briefly transmit a CW tone barefoot on 17 m, the rig meter says
it sees 1.2 SWR, but when I put the amp in line, and run exciter power
(maybe only 2-30w) I get 2.1:1 SWR.
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.
I have not tried it on higher power. I am sorry I don't recall what
it did into my dummy load, and I cannot test that again right now
because a humongous thunder storm is rolling in off Lake Michigan and
...well... you understand my reticence to be operating while that is
brewing in the neighborhood.
The Inevitable Questions -
* So... why does this happen?
* Is this a normal occurrence?
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).
* Does this have anything to do with working 17m
opposed to another band?
No.
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. So I work 17M with the 20M setting, 12M with the 15M setting.
In both cases, the Tune knob is very close to the low end of its range,
and the Load knob is in the range of 2-3 on the scale of 1 - 10. The
Titans require a bit more drive on the WARC bands (typically 50-55 W)
for 1.5kW output, as compared to 35-45 W for the other bands, but the
Tune and Load controls DO provide a clearly defined peak.
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. NEVER use ALC
between the rig and the amp to set power level -- that is a recipe for
massive splatter. 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).
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.
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."
73, Jim K9YC
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