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Author Topic: Elecraft KPA-500 video overview...  (Read 11582 times)
AE5X
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Posts: 1454




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« on: December 05, 2011, 06:58:04 PM »

...is here for those interested:

http://www.ae5x.com/blog/

John AE5X
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K3RE
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Posts: 35




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« Reply #1 on: December 05, 2011, 07:51:56 PM »

Nice and informative video.
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ZENKI
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« Reply #2 on: December 06, 2011, 01:25:14 AM »

Anyone measure the IMD performance? If the IMD is poor its hard too justify such a high asking price.

...is here for those interested:

http://www.ae5x.com/blog/

John AE5X

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VE7RF
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Posts: 212




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« Reply #3 on: December 06, 2011, 04:13:03 AM »

Anyone measure the IMD performance? If the IMD is poor its hard too justify such a high asking price.

...is here for those interested:

http://www.ae5x.com/blog/

John AE5X


## apparently, somebody did measure it already...and it's not good !   Google TX IMD tests on KPA-500.  Now u know why Elecraft does NOT publish IMD specs. And if ur gonna do it right..at least quote the 3-5-7-9-11..and also do it at full power out, not 2/3 power, etc.  we want a real imd test. Folks have already done all this.   check it out.   You can have it...along with the lousy IMD on their K3.

Later... Jim  VE7RF
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AE5X
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« Reply #4 on: December 06, 2011, 05:40:09 AM »

Those tests (and more) have been done by Elecraft as part of their submission to the FCC for type approval. You can find them on my page here:
http://www.ae5x.com/blog/2011/03/15/elecraft-kpa-500-receives-fcc-acceptance/

The ARRL will publish its review in the Feb 2012 issue of QST.

John AE5X


« Last Edit: December 06, 2011, 06:31:32 AM by AE5X » Logged

VE7RF
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Posts: 212




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« Reply #5 on: December 06, 2011, 07:23:50 AM »

They cheated, and used a MK-V in Class A...and only ran the TX IMD tests on 160+20M.  Still it's not too bad an amp  for IMD.   I would have loved to have seen the tests repeated, still using the MK-V in class A...but on all 9 x HF bands.

The imd will be an absolute mess, when driven with the K3 on SSB !   The other problem is it only takes 25 watts to drive the amp.  The typ class AB  xcvr has lousy imd, when the power is turned down to 25 watts.  Ever seen the imd on a HF xcvr, when turned down to 2 watts, pure crap for imd.  What u need to run this new amp is a PAD, between the xcvr + new KPA-500 amp, and run the xcvr at it's... 'sweet spot'  for best xcvr imd....which is typ 70% of it's rated output. Check out the ARRL extended lab report on the kenwood TS-870. They not only test for imd on ALL bands... but also did it at 3 x different power levels on 20M only, 100/80/50 watts. The imd was a lousy -30db pep with 100w... and really cleaned up when power reduced to 80w.... and was very clean at 50 w.  But each xcvr is different, and they all change again on different bands. QST just reports the WORSE case TX IMD on any xcvr, which typ is 12m, but not always, it's all over the map.  The MK-V in Class A is also all over the map, huge differences between bands...and ditto with the same MK-V..when in Class AB mode.

Later...Jim  VE7RF
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W8JI
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« Reply #6 on: December 06, 2011, 08:05:03 AM »

They cheated, and used a MK-V in Class A...and only ran the TX IMD tests on 160+20M.  Still it's not too bad an amp  for IMD.   I would have loved to have seen the tests repeated, still using the MK-V in class A...but on all 9 x HF bands.

Why is that cheating? It might not be the most reliable way, depending on the MKV performance.

The proper way to do an IMD test is with two isolated rigs generating a carrier though a combiner, or the source has to be many times cleaner than the DUT. I have no idea how the MKV and tone generator combo tests, so I can't comment on the test validity, but the proper way to test is with two RF generators through a combiner.

If any stage in the system, other than the DUT, has non-linearity the non-linearities can correct or compensate each other.

I use two radios on CW, through 3 dB pads, into a combiner that has adjustable null, into a pad, into a class A 3CX1500A7 at 100 watts PEP output. My test setup has less IMD than I can measure.

By the way, the FCC does not require, nor do they consider, IMD tests on amplifiers.  They do not look at modulation byproducts. They look at things like gain, harmonics, and spurious unrelated to modulation. 

Quote
The imd will be an absolute mess, when driven with the K3 on SSB !   The other problem is it only takes 25 watts to drive the amp.
 


Unless the rules changed, the rule is 50 watts minimum drive for rated output. I think that may have recently changed.

The 25 watts is not a problem with the K3, because of the K3's ALC system. The 25 watts would be a problem with many other rigs, because of ALC overshoot. That could easily be cured with a pad, so I don't see the problem.

Quote
The MK-V in Class A is also all over the map, huge differences between bands...and ditto with the same MK-V..when in Class AB mode.

If that is the case, than any IMD tests using it are invalid. This is because distortion can correct distortion, or even make distortion worse, in the DUT. The source has to be clean.

73 Tom
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G4AON
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« Reply #7 on: December 06, 2011, 11:17:00 AM »

The typ class AB  xcvr has lousy imd, when the power is turned down to 25 watts

...Jim  VE7RF

The K3 IMD figures improve when it's running at 1/4 power. My own measurements, on 80m, show 3rd order IMD 33 dB down @ 100 Watts and 46 dB down @ 25 Watts (using ARRL measurements based on PEP being 6 dB down on a single tone).

Just got my KPA500 delivered today, it's up and running but I've not had chance to measure IMD with the K3/KPA500 combination yet.

73 Dave
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VE7RF
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Posts: 212




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« Reply #8 on: December 07, 2011, 05:50:10 AM »

Those tests (and more) have been done by Elecraft as part of their submission to the FCC for type approval. You can find them on my page here:
http://www.ae5x.com/blog/2011/03/15/elecraft-kpa-500-receives-fcc-acceptance/

The ARRL will publish its review in the Feb 2012 issue of QST.

John AE5X




Tom, check out the above link. It only needs 25 w to drive it. They list the MK-V in the 'test gear' section. They also show the 2 tone  driver IMD..which is identical to what a mk-V is..in class A.  The very last arrl extended lab report ever done was the MK-V..which depicts 18 x charts, 9 for AB..and 9 more for class A. The mk-V  goes a little screwy on 160m too.

Jim
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K6AER
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Posts: 5726




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« Reply #9 on: December 08, 2011, 01:12:28 PM »

The FCC 50 watts minimum drive requirement is for amplifiers out putting 1500 watts. The actual requirement from the FCC is the amplifier gain is limited to 15 dB or less. Most of the K3 users are into CW so IMD is not as important to them as the audio crowd.

Although good IMD is a very desirable goal the FCC doesn’t give a wit about the amplifiers IMD. All they care about is harmonic energy outside of the amateur band. Bad IMD or non linearity can contribute to higher levels of harmonic energy but low pass filtering or better tank circuit design can reduce those levels of harmonic energy.  There is some correlation between IMD and harmonic levels at 3,5,7,9 th harmonics but not as much as you think.

Most amplifiers have better IMD than the transceivers driving them. If you reduce power output the IMD will be better because you are operating the amplifier away from the nonlinear portion of the extreme voltage swing of the solid state devices.  This also holds true for tubes as well. IMD on the K3 is about 29 dB peak to third or about 23 dB from second to third. That is pretty bad. This explains the fuzzy audio I always hear from K3’s. Great IMD of an amplifier cannot clean up a poor signal from the exciter.

From my observation it is best to have at least 2 dB of amplifier head room. This means your THP 2.5 which maxes out at 1800 watts should be used on SSB at about 1000 watts. The amplifier is not a good choice if you want to have your average power bouncing around 1500 watts out.

This weekend I was on ten meters and some fella had a HLA-300 he claimed was getting 500 watts out. The amplifier has only 4 ea. 80 watt devices (SD1446). I had him to turn down the drive until he saw about 250 watts out of the amplifier. His signal cleaned up instantly. When I explained what was going on I was informed he had been an active CB’er for many years and he knew what he was doing. I bid him 73 and I noticed the signal got broad again. You can lead a horse to water…….

I realize this last statement might cause the original thread to be hijacked but having 700,000 hams in the hobby may not be a good thing if they don’t want to learn.
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VK4DD
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Posts: 79




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« Reply #10 on: December 08, 2011, 03:31:34 PM »


From my observation it is best to have at least 2 dB of amplifier head room. This means your THP 2.5 which maxes out at 1800 watts should be used on SSB at about 1000 watts. The amplifier is not a good choice if you want to have your average power bouncing around 1500 watts out.

This weekend I was on ten meters and some fella had a HLA-300 he claimed was getting 500 watts out. The amplifier has only 4 ea. 80 watt devices (SD1446). I had him to turn down the drive until he saw about 250 watts out of the amplifier. His signal cleaned up instantly. When I explained what was going on I was informed he had been an active CB’er for many years and he knew what he was doing. I bid him 73 and I noticed the signal got broad again. You can lead a horse to water…….


2 or 3dB spare is a good, because that's where the amplifier peaks with IMD and it avoids heat stress on the amplifier. However some see this as an illegal practice. In particular if the amplifier exceeds the legal limit. The argument of having 6 or even 10dB more power using particular non linear tubes does not make sense to me. This is different if these tubes are known to be very linear. But there is no amplifier which can compensate for a good antenna.

I once showed a ham that his watt meter was indicating too much and that he his other watt meter was indicating correct. He shaked his head and said that the watt meter which was indicating too much was a better one because he liked the fact that it was showing more power Embarrassed

May be the solution is to sell watt meters which will indicate twice the the output power?
I mean one that shows 500W with 250W out?

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G4AON
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Posts: 1407




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« Reply #11 on: December 08, 2011, 03:46:08 PM »

I found time to run a few 2 tone IMD tests with my K3 and KPA500 combination today, these were on 80m using a 50 Ohm Cantenna load with the spectrum measurements taken with a Microtelecom Perseus SDR. The SDR receiver was coupled via a loop of wire around the coax to the Cantenna. The two tones used in the test are from the internal 2 tone generator in the K3.

First the K3 running at 20 Watts, the approximate power level used to drive the KPA500 to 400 Watts output:
http://www.astromag.co.uk/images/K3_2tone_20W.jpg

Secondly the K3/KPA500 running 400 Watts:
http://www.astromag.co.uk/images/KPA500_2tone_400W.jpg

Thirdly, the K3 running 100 Watts output with a spoken "four" into the microphone:
http://www.astromag.co.uk/images/K3_voice_100W.jpg

Finally, the K3/KPA500 running 400 Watts with a "four" as above:
http://www.astromag.co.uk/images/KPA500_voice_400W.jpg

73 Dave
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K6AER
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« Reply #12 on: December 09, 2011, 12:51:25 PM »

Dave,

Your second to third look to be about 25 dB. OK but not fantastic. Although I admit the IMD is aout as good as most solid state hamd amplifiers.

It is about 20 dB under a good tube amp.

Mike
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G4AON
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« Reply #13 on: December 09, 2011, 02:16:38 PM »

Mike if you are comparing my measurement points with those of the ARRL you would need to add 6 dB to the difference as they need to be measured relative to peak which is 6 dB higher than two tone levels. This makes the 3rd order IMD -31 dB relative to peak.

Fed with a clean 2 tone signal, not what many will actually do in practice, the KPA500 is capable of 3rd order products around 35 dB down on peak.

73 Dave
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VE7RF
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Posts: 212




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« Reply #14 on: December 09, 2011, 09:01:14 PM »

Mike if you are comparing my measurement points with those of the ARRL you would need to add 6 dB to the difference as they need to be measured relative to peak which is 6 dB higher than two tone levels. This makes the 3rd order IMD -31 dB relative to peak.

Fed with a clean 2 tone signal, not what many will actually do in practice, the KPA500 is capable of 3rd order products around 35 dB down on peak.

73 Dave

## to pull that off, you require 2 x transmitter's, each with a cxr, feeding a combiner, then the class A buffer, etc, etc.  That way, the KPA-500 is being fed with with drive  from a souce that has virtually zero imd. begin with.  If you just feed the amp with a xcvr, and the xcvr is fed 2 x audio tones via the mic jack/phone patch input, then the total imd from the ouput of the amp will be no better than the xcvr that's  driving it.  Dirty xcvr driving a clean amp = dirty signal.  Still, that is a valid test, since that is the config u will be using on the air.

##  The story I got from the arrl was that RX S-meters are peak reading devices to begin with,
 [fast attack, slow decay] so IMD below pep is perfectly valid.   IE: You are 30 db over S-9 to station XXX.   Your imd happens to be -30db pep.  Your imd products are now S-9..when listening off freq a bit.

##  It's  a whole different story with multiplexed SSB. Before digital, all the telco's across NA used multiplexed ssb-cxr systems.  Like 200-4000 channel set ups.  They were all typ 300-3100hz wide each, and each suppressed cxr frq was 4 khz apart from the adjacent channel. They all started in groups of 12 x channels.  Several groups  would then be multiplexed together into a bigger group..then these groups got multiplexed into a super group...then into a master group, etc, etc. So you ended up with several hundred channels, all 4 khz apart, all running nose to tail. Now you end up with what's called a composite baseband signal..that could easily be 10 mhz wide.   Now toss in some imd further down stream and you can soon see, it will affect ALL the channels.  They will all end up with bad imd, and you end up with a dog's breakfast.

##  hams  don't run multiplex ,nor ISB, etc, so the imd  below pep is perfectly valid.  Smaller point to point HF ssb commercial setups will sometimes use 4 channel SSB.   The real problems start up with LOUD signals on crowded QUIET bands. If W6---  is say 40 over S-9..and ur on 10m, with a S-2 noise level, you are going to have to be a long way down the band, so you don't hear his imd. On 160-80-40m, imd is not such a big deal in urban area's...unless you are in the bush, with beverages etc.

##  commercial marine and aviation ssb is all channelized. It's not like a dx contest.

Later... Jim  VE7RF
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