[Elecraft] [K3] Issue with HI CUR on 12m

Tue Oct 13 10:47:21 EDT 2020

This happened to me on a multi-band antenna, with SWR changing, but no 
Hi Cur warning.

I would be transmitting for months, using only 100W with no issues. I 
added in a KPA500/KAT500 combination and apart from a short test at 
400W, I continued to use it at 200W (SSB) max.
After a month or so, the SWR would change - mid QSO. A quick retune and 
I was able to continue. Over a few weeks the occurrence was more 
frequent. Using a Rig Expert analyser, no fault or bad SWR was indicated 
and I (wrongly) guessed that the 400W Guanella balun was breaking down 
at power over 100W. In the meantime I ordered a new 1KW balun made for 
that antenna, but before it arrived, during a QSO there was a 
catastrophic "occurrence" with all kinds of warnings and the SWR was off 
the scale.

A check with a multimeter indicated a dead short somewhere in the 
feedline (RG58). I also took the balun apart and found no fault. As I'd 
ordered a new balun, I decided to change it anyway. I don't have the 
power to exceed the new balun's spec, so it should last forever - RF 
wise. ;-)

Obviously I replaced the co-ax feeder - I now use Westflex 103 with much 
larger ferrites (to fit the new cable size) as a common mode choke. All 
now works as it should and shows no sign of breakdown, even at 400W (max 
legal for the UK).

The old co-ax is a different matter. With both ends disconnected (open) 
connecting one end to the analyser showed a short circuit about 15 feet 
along its length. I cut out a section either side and the remaining two 
pieces showed 'open' as they should. On visual inspection I find that 
the braid of the co-ax has migrated through the center insulation and is 
touching the inner conductor, indicating that a fair amount of heat has 
been generated. The area in question was right where the common mode 
ferrites had been fitted, although there were no tell-tale marks on the 
ferrites or the outer casing of the co-ax. It's fairly obvious that a 
high voltage node has caused an internal flashover which in turn has 
changed the characteristic which has resulted in a high current node 
which has generated the heat.

You may well be right that your balun is not faulty, but you should 
check it anyway as well as checking out all the other aspects of the 
installation. If you can, replace the feeder. You may be surprised at 
what you find.

73,

Alan. G4GNX

------ Original Message ------
From: "Bill Coleman" <aa4lr at arrl.net>
To: "Don Wilhelm" <donwilh at embarqmail.com>
Cc: "Elecraft" <elecraft at mailman.qth.net>
Sent: 13/10/2020 01:55:16
Subject: Re: [Elecraft] [K3] Issue with HI CUR on 12m

>SWR on the problem antenna is below 1.2:1. It’s a trap dipole - designed for that band.
>
>It is unlikely the balun is “breaking down”. It’s a current balun - #43 ferrite beads on RG-313 coax. It can easily handle a kW or more.
>
>>  On Oct 12, 2020, at 2:12 AM, Don Wilhelm <donwilh at embarqmail.com> wrote:
>>
>>  Bill,
>>
>>  I would suggest that you check the SWR on the antenna before concluding it is an RF feedback problem.
>>  Don't forget that the balun may be breaking down with power and show no problem at lower power (like from an antenna analyzer).
>>
>>  Don W3FPR
>>
>>  On 10/11/2020 8:53 PM, Bill Coleman wrote:
>>>  Don,
>>>
>>>  I finally got around to running my K3 into a dummy load.
>>>
>>>  I had no problem running 100w or even 110w with a dummy load. Current draw from the power supply was 22 A and 23 A, respectively, on the K3 display. No HI CUR indications.
>>>
>>>  External wattmeter validates the output.
>>>
>>>  I was getting the HI CUR indication on a trap 30/17/12m inverted V. I had to take it down, and now I’m using a trap 80/40/20m dipole through the tuner. No HI CUR indication on that antenna, either.
>>>
>>>  Even though I have current baluns on both antennas, I’m thinking that my HI CUR problem may be related to RF feedback in the shack.
>>>
>>>  Do you concur?
>>>
>>>  I’m in the process of a number of shack improvements, including antenna replacements and grounding and bonding, so that might take care of it.
>>>
>>>>  On Jan 11, 2020, at 11:29 AM, Don Wilhelm <donwilh at embarqmail.com> wrote:
>>>>
>>>>  Bill,
>>>>
>>>>  Is this into a dummy load (bypass the ATU or tune the ATU into the dummy load).
>>>>  If it is into an antenna, there are a whole lot of guesses about how the antenna behaves with higher power levels than your antenna analyzer shows.  So check it with a dummy load to eliminate the possibility that the problem is in the antenna.
>>>>
>>>>  Put an external wattmeter in line with the dummy load to eliminate questions about the calibration on the K3 wattmeter.
>>>>
>>>>  If you can measure the actual current drawn, that would also be helpful.
>>>>
>>>>  73,
>>>>  Don W3FPR
>>>>
>>>>  On 1/11/2020 11:18 AM, Bill Coleman wrote:
>>>>>  Back in early December, I installed two KSYN3As in my K3 SN 7071. Sometime after that, I noticed that on 12m, I’m seeing a HI CUR indication when I set the power level to more than about 80-85 watts. The K3 will back off on the power level, then slowly ramp back up until it hits HI CUR again.
>>>>>  I don’t remember seeing this before. After the KSYNC3A install, I ran the transmit gain calibration at both 5 and 50 watts, as specified in the instructions. Just in case something went awry, I ran it again.
>>>>>  I’m still seeing this problem. What causes HI CUR at 85 watts? The K3 ought to be able to produce over 110 watts on 12m.
>>>>>  I’m not seeing this issue on other bands.
>>>>>  No issues with calibration
>>>  Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
>>>  Web: http://boringhamradiopart.blogspot.com
>>>  Quote: "Not within a thousand years will man ever fly!"
>>>              -- Wilbur Wright, 1901
>>>
>>
>
>Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
>Web: http://boringhamradiopart.blogspot.com
>Quote: "Not within a thousand years will man ever fly!"
>             -- Wilbur Wright, 1901
>
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