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Strange K3 & KPA500 Issues fixed with a longer coax cable?

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Jim Kvochick

Strange K3 & KPA500 Issues fixed with a longer coax cable?

I've stumbled into a strange problem.

I have 4 friends with similar K3 stations to my own; K3 >>> KPA500 >>>
KAT500. The stations are powered with similar power sources.

One station sounds fine, works great. The other 4 had strange issues that
manifested themselves as audio distortion when running the amplifier. After
looking all over for RF and grounding issues, we swapped the coax cable from
the K3 to the amplifier, with a longer cable (about 6 to 7 feet - largely
because that was the only free cable we had handy).

Making the cable from the K3 to the amplifier longer eliminated the issue.
putting the cable back brought the problem back. Making a new short cable
still left us with audio distortion. The only thing that seemed to solve
the issue in all 4 cases was making the K3 to amplifier cable longer.

This got me to investigate my own station. It turns out that the cable I
used between the amplifier and the radio was about 6 feet long. Swapping
this cable for a shorter length brought about audio distortion symptoms.

Does anyone have any thoughts about this?

Cheers

Jim K8JK

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G8BYB

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

Hi Jim,
We had a very similar situation a couple of years ago at our local club
station which was using Yaesu kit and a MFJ ATU. We had relocated the
station and used different (shorter) coax to wire it all up. We encountered
various problems the main one being a high SWR. The cables were checked but
nothing untoward was found. Then one member said that we shouldn't use short
cables and so we found some longer cables, about 2m long, (not the
originals) and everything worked fine.
We never did find out the reasoning behind not using the short cable
lengths.
Andrew
G8BYB

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N2TK

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

In reply to this post by Jim Kvochick

I had a similar problem many years ago with an L-4B amp.
I wonder if adding ferrite to the coax would solve this problem?
GL
N2TK, Tony

-----Original Message-----
From: Elecraft [mailto:[hidden email]] On Behalf Of Jim
Kvochick
Sent: Tuesday, November 18, 2014 9:16 PM
To: [hidden email]
Cc: [hidden email]
Subject: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax
cable?

I've stumbled into a strange problem.

I have 4 friends with similar K3 stations to my own; K3 >>> KPA500 >>>
KAT500. The stations are powered with similar power sources.

One station sounds fine, works great. The other 4 had strange issues that
manifested themselves as audio distortion when running the amplifier. After
looking all over for RF and grounding issues, we swapped the coax cable from
the K3 to the amplifier, with a longer cable (about 6 to 7 feet - largely
because that was the only free cable we had handy).

Making the cable from the K3 to the amplifier longer eliminated the issue.
putting the cable back brought the problem back. Making a new short cable
still left us with audio distortion. The only thing that seemed to solve
the issue in all 4 cases was making the K3 to amplifier cable longer.

This got me to investigate my own station. It turns out that the cable I
used between the amplifier and the radio was about 6 feet long. Swapping
this cable for a shorter length brought about audio distortion symptoms.

Does anyone have any thoughts about this?

Cheers

Jim K8JK

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James kvochick

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

We tried all the normal "common mode" tricks, but the feedline length appeared to be the most effective.

Jim K8JK

Sent from my iPad

> On Nov 19, 2014, at 7:32 AM, N2TK, Tony <[hidden email]> wrote:
>
> I had a similar problem many years ago with an L-4B amp.
> I wonder if adding ferrite to the coax would solve this problem?
> GL
> N2TK, Tony
>
> -----Original Message-----
> From: Elecraft [mailto:[hidden email]] On Behalf Of Jim
> Kvochick
> Sent: Tuesday, November 18, 2014 9:16 PM
> To: [hidden email]
> Cc: [hidden email]
> Subject: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax
> cable?
>
> I've stumbled into a strange problem.
>
> I have 4 friends with similar K3 stations to my own; K3 >>> KPA500 >>>
> KAT500. The stations are powered with similar power sources.
>
> One station sounds fine, works great. The other 4 had strange issues that
> manifested themselves as audio distortion when running the amplifier. After
> looking all over for RF and grounding issues, we swapped the coax cable from
> the K3 to the amplifier, with a longer cable (about 6 to 7 feet - largely
> because that was the only free cable we had handy).
>
> Making the cable from the K3 to the amplifier longer eliminated the issue.
> putting the cable back brought the problem back. Making a new short cable
> still left us with audio distortion. The only thing that seemed to solve
> the issue in all 4 cases was making the K3 to amplifier cable longer.
>
> This got me to investigate my own station. It turns out that the cable I
> used between the amplifier and the radio was about 6 feet long. Swapping
> this cable for a shorter length brought about audio distortion symptoms.
>
> Does anyone have any thoughts about this?
>
> Cheers
>
> Jim K8JK
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
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> delivered to [hidden email]
>
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Jim AB3CV

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

Your problem isn't the length of the coax between the units. It is RF
getting into the shack in the first place via your feedline. What antennas
and feedlines do you have connected? What sort of common mode chokes have
you applied to the antennas which are connected.

73

jim ab3cv

On Wed, Nov 19, 2014 at 8:19 AM, James kvochick <[hidden email]>
wrote:

> We tried all the normal "common mode" tricks, but the feedline length
> appeared to be the most effective.
>
> Jim K8JK
>
>
> Sent from my iPad
>
> > On Nov 19, 2014, at 7:32 AM, N2TK, Tony <[hidden email]> wrote:
> >
> > I had a similar problem many years ago with an L-4B amp.
> > I wonder if adding ferrite to the coax would solve this problem?
> > GL
> > N2TK, Tony
> >
> > -----Original Message-----
> > From: Elecraft [mailto:[hidden email]] On Behalf Of
> Jim
> > Kvochick
> > Sent: Tuesday, November 18, 2014 9:16 PM
> > To: [hidden email]
> > Cc: [hidden email]
> > Subject: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax
> > cable?
> >
> > I've stumbled into a strange problem.
> >
> > I have 4 friends with similar K3 stations to my own; K3 >>> KPA500 >>>
> > KAT500. The stations are powered with similar power sources.
> >
> > One station sounds fine, works great. The other 4 had strange issues
> that
> > manifested themselves as audio distortion when running the amplifier.
> After
> > looking all over for RF and grounding issues, we swapped the coax cable
> from
> > the K3 to the amplifier, with a longer cable (about 6 to 7 feet - largely
> > because that was the only free cable we had handy).
> >
> > Making the cable from the K3 to the amplifier longer eliminated the
> issue.
> > putting the cable back brought the problem back. Making a new short
> cable
> > still left us with audio distortion. The only thing that seemed to
> solve
> > the issue in all 4 cases was making the K3 to amplifier cable longer.
> >
> > This got me to investigate my own station. It turns out that the cable I
> > used between the amplifier and the radio was about 6 feet long. Swapping
> > this cable for a shorter length brought about audio distortion symptoms.
> >
> > Does anyone have any thoughts about this?
> >
> > Cheers
> >
> > Jim K8JK
> >
> > ______________________________________________________________
> > Elecraft mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:[hidden email]
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> Message
> > delivered to [hidden email]
> >
> > ______________________________________________________________
> > Elecraft mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> > Post: mailto:[hidden email]
> >
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> > Please help support this email list: http://www.qsl.net/donate.html
> > Message delivered to [hidden email]
> ______________________________________________________________
> Elecraft mailing list
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>
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> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>
>

Elecraft mailing list

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

In reply to this post by James kvochick

Jim, RFI is not the issue here.What is happening is a matter of " Inductance "
By adjusting the length of the Jumper, the Impedance is adjusted accordingly.
((((73)))) Milverton.

From: James kvochick <[hidden email]>
To: "N2TK, Tony" <[hidden email]>
Cc: "[hidden email]" <[hidden email]>
Sent: Wednesday, November 19, 2014 7:19 AM
Subject: Re: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax cable?

We tried all the normal "common mode" tricks, but the feedline length appeared to be the most effective.

Jim K8JK

Sent from my iPad

ke9uw

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

In reply to this post by Jim AB3CV

The RF in the shack answer always makes me think that if I put RF in the shack of someone on the other side of the country, then I surely have RF in my shack too.
I think the real question is if the RF in the shack, everybody's shack, getting into something like the mic wiring or it's preamp.
When I read the longer cable response, I thought that it's just a change to the RF getting into something. If it is more inductance that helps, then what effect does RF Ferrite clipped onto a short coax have? Just a thought.

Chuck, KE9UW

Jim Miller

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

In reply to this post by Elecraft mailing list

A properly sourced and terminated length of coax is not inductive.

You have classic symptoms of common mode current problems.

J

On Nov 19, 2014, at 9:31 AM, Milverton M. Swire via Elecraft <[hidden email]> wrote:

Jim, RFI is not the issue here.What is happening is a matter of " Inductance "
By adjusting the length of the Jumper, the Impedance is adjusted accordingly.
((((73)))) Milverton.

From: James kvochick <[hidden email]>
To: "N2TK, Tony" <[hidden email]>
Cc: "[hidden email]" <[hidden email]>
Sent: Wednesday, November 19, 2014 7:19 AM
Subject: Re: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax cable?

We tried all the normal "common mode" tricks, but the feedline length appeared to be the most effective.

Jim K8JK

Sent from my iPad

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Richard Squire - HB9ANM

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

In reply to this post by Jim AB3CV

Well, this is surprising, to say the least. Inasmuch as it seems to have
happened in several places. I normally don't post remarks of the
"no-problem-here" sort but I don't think this has anything to do with
RFI or inductance. Neither are ferrite chokes likely to solve the
problem. My K3->KPA500 cable is only 65 cm (26") long (the only piece of
RG58 in my setup) but nobody mentioned any distortion.

BTW: Both the K3 AND the KPA500 are earthed (grounded, if you prefer)
separately to a common earth... I mean: grounding point, as well as the
(manual) tuner, the SteppIR controller, etc.

If a longer cable can solve the problem, it's OK but the main question
(the "WHY?") remains unanswered... Strange, indeed.

73
Richard - HB9ANM

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Richard - HB9ANM

Re: Strange K3 & KPA500 Issues fixed with a longer coax cable?

There are certainly other possibilities.

Making an amplifier unconditionally stable with any combination of source
impedance ,load impedance , drive level , and supply voltage and achieving
any reasonable gain , efficiency , bandwidth , and able to withstand about
anything a customer can dream up at the same time is a real challenge . And
what these source and load impedances are in band or well out of band have a
strong bearing on stability.

Then to add to the fun, if you are not a high quantity buyer of the
semiconductors , and therefore cannot demand there be no changes in
processing, you can do a design and a couple years later all of a sudden you
have a stability problem that never existed because the vendor decided to
make a process change such that the device still met the typical specs ,but
had a much stronger propensity for half freq oscillations. Some process
changes that raise low frequency gain without much effect on high frequency
gain can make an amp have a tendency for low frequency oscillations because
the decoupling /low frequency circuitry loading is now not sufficient to
suppress that .

Enough of that , what CAN be happening here is with certain cable lengths
between the amp and the K3 , in band or out of band impedances presented to
the K3 could well make the K3 PA want to have a spurious output like half
frequency oscillation or low frequency oscillations . I do not recall if the
KPA500 has any resistive broadband padding at the input in normal operation
, I believe they say they switch in a 3 dB pad when extreme overdrive is
applied. Resistive pads are a great way to limit in band or out of band
impedance excursions - BUT you give up amp gain dB for dB.

Same is true at the output of the amplifier -(or combinations of in and
output) certain loads may make the amp more prone to a spurious output at
half the driving frequency - even though the load may look like a perfect 50
ohms in band at the fundamental frequency . This is another joyous feature
of semiconductor power amp designers get to experience whether bipolar or
FET .

The designer has to be careful what the out of band impedances of the output
filters are - and just because you have a 1:1 SWR in band with your antenna
, the out of band impedances can be darn near anything .

As I recall , in this case , he had the same problem even into a dummy load
on the output. So just as an experiment it might be interesting to see of a
3 dB resistive pad between the amp and the K3 made the whole thing totally
independent of cable length. I am m not talking about cable loss , I am
talking about limiting the possible impedance excursions especially out of
band .

Enough of this - I spent 30 years of my life designing and putting into
production these beasts , I enjoy the fruits of the Elecraft designers
efforts .have no interest whatever in doing that again. I like antennas
much better they don't half f or oscillate at low frequencies- at least so
far .

Hank K7HP

-----Original Message-----
From: Richard - HB9ANM
Sent: Wednesday, November 19, 2014 12:58 PM
To: [hidden email]
Subject: Re: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax
cable?

Well, this is surprising, to say the least. Inasmuch as it seems to have
happened in several places. I normally don't post remarks of the
"no-problem-here" sort but I don't think this has anything to do with
RFI or inductance. Neither are ferrite chokes likely to solve the
problem. My K3->KPA500 cable is only 65 cm (26") long (the only piece of
RG58 in my setup) but nobody mentioned any distortion.

BTW: Both the K3 AND the KPA500 are earthed (grounded, if you prefer)
separately to a common earth... I mean: grounding point, as well as the
(manual) tuner, the SteppIR controller, etc.

If a longer cable can solve the problem, it's OK but the main question
(the "WHY?") remains unanswered... Strange, indeed.

73
Richard - HB9ANM

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