[Elecraft] [QRO] KPA500 variation of PA current band-by-band
Guy Olinger K2AV
olinger at bellsouth.net
Tue Feb 7 21:40:47 EST 2012
Hmmm,
Given that ham-affordable components are full of minor and harmless
frequency dependent characteristics, INCLUDING the antennas and dummy loads
used, is there really any good reason to EXPECT 1.8-30 power output
uniformity beyond what is commonly measured? Quite contrary to amps which
were designed, built and certified to a broadband tight uniformity (and
appropriately priced), if one WANTS exactly 500.0 watts from a KPA500 for
some purpose, and you actually trust your output measuring instrument to
deliver an accurate measurement to four figures, one just adjusts the
drive, does one not?
This is ham gear, not lab instruments, but I would be intrigued to hear
what purpose this debated level of accuracy serves, other than light
entertainment. I have sometimes heard some very interesting stuff coming
from the other side of what I thought was the left field foul line. One
never knows.
???
73, Guy.
On Tue, Feb 7, 2012 at 3:23 PM, <n5ge at n5ge.com> wrote:
>
> Is your dummy load absolutely a flat 1.0:1 from 1.0MHz to 30MHz?
>
> 73,
> Tom
> Amateur Radio Operator N5GE
> ARRL Lifetime Member
> QCWA Lifetime Member
>
> "Only the white eyes would believe they could cut the top
> off a blanket, sew it to the bottom and have a longer
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>
> -- American Indian comment about Daylight Saving Time --
>
>
> On Tue, 07 Feb 2012 13:35:47 +0000, Brian Alsop <alsopb at nc.rr.com> wrote:
>
> >Guys,
> >
> >I'm also curious about this-- especially what's going on on 15M.
> >My values at 500 W out into a dummy load are:
> > (amp), [volts], {efficiency}
> > 1.8MHz DC (11.5) [63.1] {70}
> > 3.5MHz (11.2) [63.5] {71}
> > 7MHz (12.2) [62.5] {67}
> > 10.1MHz (not measured)
> > 14MHz (13.4)[61.8] {60}
> > 18.1MHz (12.9)[62.8] (63}
> > 21MHz (15.8)[61.2] {52}
> > 24.9MHz (14.6) [60.1] {56}
> > 29.7MHz (14.7)[60.4] {56}
> >
> >These are in reasonable agreement with David's except for perhaps 10M.
> >
> >The problem with these measurements are the wattmeter accuracy (10%?)
> >and the current measurement inaccuracy. Default wattmeter calibration
> >factors were used above. Elecraft is using a Hall effect device to
> >measure PA current. It would be nice to know just what accuracy to
> >expect from it.
> >
> >The efficiencies appear to be in the right ballpark. They tend to fall
> >on a straight line log plot vs frequency, decreasing at higher freq's.
> >
> >If nothing else, the above numbers serve as a benchmark for future
> >comparisons down the road for my amp.
> >
> >Note: The current measurement can be significantly affected by RF on the
> >outside shield of the coax. Until a choke was employed on the feedline,
> >the 24.9 MHz current with a real antenna measured 3 amps too low. With
> >the choke it agreed with the above dummy load value. I suspect lots of
> >KPA-500 users do have this condition-- given the types of
> >antennas/tuners et al described by uses on this reflector.
> >
> >73 de Brian/K3KO
> >
> >On 2/4/2012 20:27, david park wrote:
> >> I have recently assembled KPA500 serial number 641. When testing into
> a dummy load I find that I get a large variation in the DC power input when
> driving for 500W CW when I go from band to band. My worst band is 21MHz,
> where I get the highest current (lowest efficiency). The drive was
> adjusted to give 500W as indicated by the LCD display. Here are my
> results:-
> >>
> >> 1.8MHz DC current 10.0A fan hardly ever comes on
> >> 3.5MHz DC current 10.0A fan hardly ever comes on
> >> 7MHz 12.0A
> >> 10.1MHz 12.4A
> >> 14MHz 14.3A
> >> 18.1MHz 13.8A
> >> 21MHz 16.4A temperature rises rapidly and
> fan soon gets to full speed
> >> 24.9MHz 13.5A
> >> 29.7MHz 12.3A
> >>
> >> I have no results for 50MHz band as I have no transceiver covering that
> band.
> >>
> >> The readings drift as the heatsink (and the dummy load) warm up, and so
> I took readings quickly and allowed time for the heatsink to cool down
> between each band. I find that the KPA500 current sensor tends to give low
> readings.
> >> I would be interested in knowing how other amplifiers perform.
> >>
> >> David Park G3PSV
> >>
> >
> >
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