[Elecraft] [QRO] KPA500 variation of PA current band-by-band
Brian Alsop
alsopb at nc.rr.com
Wed Feb 8 07:04:14 EST 2012
Guys,
On the other hand, there nothing wrong with trying to characterize the
beastie you have as best you can. If for no other reason to see if
something changes with time and how much.
Also, how does my kit comparing to other kits which presumable have
similar but not identical characteristics? Note with the exception of
the dummy load impedance, all instrumentation used is within the
KPA-500. For what it is worth, my measurements were made with a WATERS
1.5 kW dummy load which the KPA-500 thinks is 1.1 to 1 across the 9 HF
bands. Even if it were 1.5 :1 you're talking about introducing 20 watts
of reflected power. I assume the KPA-500 is reading forward power and
not forward-reflected. Thus these kinds of error would make the output
power read higher than reality, not lower.
Also look it at a learning experience. This is my first solid state
amp. I know what to expect from tube amps but not solid state ones.
How does a solid state amp behave in the efficiency department vs
frequency? When one sees it takes 16 amps on one band to produce "500"
watts and 11 on another, there is plenty reason to raise questions and
want to understand why.
One thing uncovered already is that current is measured by a Hall effect
device instead of the time honored low value series resistor/voltmeter
combo. What is it's accuracy? Could that device's accuracy explain a
significant amount some of the variations seen.
To me it's about understaning my beastie.
Certainly people should not be castigated for asking questions. We are
not all EE's with 50 years experience and a shop full of precision
measuring equipment.
73 de Brian/K3KO
On 2/8/2012 03:19, Don Wilhelm wrote:
> Guy and all,
>
> You are so correct. This is ham radio, and precision instrumentation
> for power output levels is not the norm - a 20% error potential should
> always be considered.
> Yes, I personally have power instrumentation that is calibrated to the 5
> % level up to 30 MHz. That level of precision has come at a goodly cost
> and effort.
>
> In general, amateur quality power measurement gear should be given a +/-
> 20% range of error. There are variations due to frequency, variations
> due to changes in the dummy load actual impedance, and if one is using
> an antenna, any common mode RF current can substantially influence the
> readings on the meters.
>
> In other words, what you see on your power meter may or may not reflect
> reality. If there is any doubt about your results, tell me what your
> results are when driving a known good 50 ohm non-reactive dummy load
> (measure it with your antenna analyzer), and if a problem is indicated
> with that load, then you can conclude that you have a transceiver
> problem. If it is OK into that dummy load, but gives problems driving
> your antenna, then you will have to solve the problem in your antenna field.
>
> These are generalized statements and not directed at any particular
> installation. If you and your indications are suspect, then investigate
> your particular situation carefully (it is possible that we can help).
>
> 73,
> Don W3FPR
>
> On 2/7/2012 9:40 PM, Guy Olinger K2AV wrote:
>> 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
>>> blanket."
>>>
>>> -- 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
>>>>
>>>>
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