[Elecraft] NOT the feedline (was KPA-1500 faulting on 6m; SWR issues)

Sun May 31 22:06:14 EDT 2020

A few things I observe on my KPA 1500:

On 6m the KPA1500 reads 1.3/1.4:1 into a perfect load.  FYI it reads 
1.2:1 in to a perfect load on 10M also, but 1.0:1 on all other 
bands.Elecraft have various excuses for this.

1500W its really pushing it on 6M. Way to much compression. I run 
mine at 1100W out with 38W drive. as you state 90W, that's way too much drive.

As the amp warms up the output drops (on all bands), but I never 
exceed 38W drive on 6M.

On FT8 I drive it to 500W out only, and the amp stays reasonably cool.

On 6M I bypass the tuner as the real antenna SWR is less than 1.5:1, 
and I ignore what the KP1500 says it is.

Try the amp the way I use it and see how it goes.

Good luck

- Paul KW7Y

At 06:29 PM 5/31/2020, you wrote:
>Many here suggested that it was most likely a feedline issue, and I figured
>this was indeed a distinct possibility.
>
>So this afternoon I replaced the feedline. I now have about 80 feet of brand
>new LMR-400, new 83-SP1 connectors (and an Amphenol adaptor at the balun
>since I don't have any type-N connectors here). The AA-230 ZOOM analyzer
>shows about 1.2:1 at 50.313, with resonance just below the band. I'll fix
>that tomorrow, but 1.2 or 1.3:1 should be nothing to the KPA.
>
>The TDR screen looks very clean, just a tiny bump where the feedline goes
>through the (new) bulkhead connector to get into the house. See:
>https://www.dropbox.com/s/izcwvece7nt5f42/2020-05-31%2019.32.44.jpg?dl=0
>
>The SWR chart between 50.000 and 50.500:
>https://www.dropbox.com/s/y2xh4l1lihy2ujr/2020-05-31%2021.23.29.jpg?dl=0
>
>Again, I know the antenna is resonant a bit low, but this should NOT be a
>factor. So here's what's now going on.
>
>1) Into a Cantenna dummy load, with 25W of drive from the M3s, the KPA-1500
>is delivering 1075W and showing 1.4:1. No problem delivering power and no
>faults.
>
>2) With input power increased, 35W of K3s drive gives 1375W at 1.5 into the
>Cantenna. Again, no problem delivering power to the purely resistive load.
>Why a 1.5:1, I don't know, but fine, whatever. I can TX for 15-20 second
>periods without issue.
>
>3) When I switch over to the live antenna, with25 Watts of drive from the
>K3s into the actual antenna and the amp faults out instantly in bypass. When
>the amplifier is in bypass and the tuner is in bypass, and when the K3s is
>delivering about 90W in FT8 mode, the front panel on the KPA-1500 says the
>SWR is 1.9:1. When I press the TUNE button on the radio, it's showing an SWR
>of 1.4:1.
>
>4) Amp in bypass, tuner inline, press TUNE button on the K3s and the front
>panel of the KPA-1500 is showing a perfect 1.0:1 match. It SHOULD work fine
>like this, right?
>
>5) Pressing ONLY the OPER/STBY button to put the amp in operate (KPA tuner
>in and untouched since step 4), the amp SOMETIMES delivers power, sometimes
>faults out, shows an SWR of 1.7:1. Pressing the OPER/STBY button again to
>use only exciter power, and touching NOTHING ELSE, it's back to 1.1:1 with
>90W of K3s drive.
>
>6) There is basically no rhyme or reason why this is happening and it's
>*somewhat* repeatable. The incredibly frustrating part is if I see a needed
>station come up, I'll flick the amp's tuner in line and it says it's tuned.
>Once WSJT-X keys the radio, 90 out of 100 times the amp will hard-fault. If
>it doesn't fault on the first transmission, it absolutely WILL on the second
>transmission.
>
>So I'm now at a complete loss to understand what's going on here. Amp is
>fine into a resistive load. Brand-new antenna, brand new LMR-400 feedline,
>brand new connectors, the KPA-1500 is faulting out the same as it did on the
>10+ year old 6M5 with a patchwork of feedlines. I can't see how this is
>anything but a problem in the KPA-1500 (S/N 0398).
>
>  - pjd
>
>-----Original Message-----
>From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
>Behalf Of Jim Brown
>Sent: Sunday, May 31, 2020 8:22 PM
>To: elecraft at mailman.qth.net
>Subject: Re: [Elecraft] KPA-1500 faulting on 6m; SWR issues
>
>On 5/31/2020 9:54 AM, Jim Miller wrote:
> > Have you vetted your feedline at 1500w? Many times something might be
> > good at low power and fail at 1500.
>
>This is good advice. Guys who do lots of portable setup say to always check
>the coax first, which mostly means check for a connector that's bad or badly
>installed.
>
>In general, if it doesn't say Amphenol or isn't stamped with a MIL spec
>number, it's very likely to be poor quality. This includes barrels,
>adapters, etc.  Also make sure that every connector is" wrench tight."
>
>The advice to run it straight to a dummy load is right on -- if you have one
>big enough. :)
>
>73, Jim K9YC
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