Posted by
Ted Edwards W3TB on
Jun 01, 2020; 9:59pm
URL: http://elecraft.85.s1.nabble.com/Re-NOT-the-feedline-was-KPA-1500-faulting-on-6m-SWR-issues-tp7661502p7661542.html
I am following this with great interest.
Like Alan G0GNX, I also use an OCF, RG-8X out to the current balun in this
case, 300 ohm to the antenna. K3, KPA-500 and KAT-500.
If I am running stations in a contest on 40m and also 80m CW, It "appears"
that my VSWR rises after a half hour and then the KAT-500 starts to try to
spontaneously retune. Doesn't happen on 20m and up. This past weekend in
CQ WPX, I reduced drive so that output was about 300 watts and it all
became tame.
I had thought that it was a heating of the RG-8X; then I changed my mind to
the current balun from Radiowavz that is rated for 1.5 KW. I think it is
the balun just getting hot out there. I had used a W2AU 4:1 balun with my
OCF, which is a voltage balun but I didn't know about that -- for upwards
of 40 years and with the Elecraft equipment for about 4 years since I got
the amp/tuner. I just switched to a current balun last year with one that
I bought at Dayton.
I would have expected more problem with the voltage balun than with the
current balun. I could change back to the W2AU if needed.
Interesting stuff, so I am reading along. And my thanks to all of you.
On Mon, Jun 1, 2020 at 4:42 PM Fred Jensen <
[hidden email]> wrote:
> Nearly same experience Bob: Sloping V, 135 ft legs, from top of 80 ft
> tower fed with homemade 600 ohm open wire using a DX Engineering 4:1
> "balun" [a strange, usually misunderstood piece of electronic apparatus
> often used for the wrong reasons] rated at 10 KW. It warmed up
> noticeably at 1.2 KW RTTY use. It helps to remember that one can
> saturate a ferrite core [especially when very hot] which creates a
> racket reminiscent of a non-synchronous spark gap TX.
>
> 73,
> Fred ["Skip"] K6DGW
> Sparks NV DM09dn
> Washoe County
>
> On 6/1/2020 1:48 PM, Bob McGraw K4TAX wrote:
> > Based on my experience, balun power ratings are for MATCHED
> > conditions. It is rare that hams use a balun in a matched condition.
> > Thus a 1:1 balun should see 50 ohms on the input and 50 ohms on the
> > output, while a 4:1 balun should see 200 ohms on the output and 50
> > ohms on the input. In the case of a resonant folded dipole, a 4:1
> > balun is typically operating in a nearly matched condition. All others
> > combinations are unknown and random.
> >
> > I run about 500 watts on all bands. My baluns are rated at 5KW! It
> > takes 3 or 4 big hunkin' pieces of ferrite to attain this power
> > level. My 6 meter balun is a 1/2 wavelength electrically of RG-213.
> > No ferrite!
> >
> > Buy or build a balun of your choice. Using an IR temperature gun,
> > measure the ambient temperature of the core. Run about 1/2 rated
> > power carrier for 30 to 60 seconds. Measure the temperature again.
> > If it is warm to hot, this is RF producing heat. And likely
> > continuing will produce core failure. This is not a good balun for
> > your application.
> >
> > One of my baluns work between the output of my KAT500 and the balanced
> > feed line connected to the center of a 256 ft wire. That antenna
> > works 160M - 6M with zero issues. Now, I do run a hybrid balun being
> > a 4:1 Guanella balun as a transformer, and it is fed with a 1:1 balun
> > for common mode rejection.
> >
> > Most single core, i.e. 2 or 3 cores stacked with 2 to 4 windings are
> > not at all a proper balun design A Guanella balun will have 2 cores
> > with 2 windings and then another 2 separate cores with another 2
> > windings. These are then wired to produce a 4:1 balun with good
> > common mode rejection. Most "factory" 4:1 baluns are poorly
> > designed and built junk.
> >
> > See
https://www.dj0ip.de/balun-stuff/ for further references.
> >
> > 73
> >
> > Bob, K4TAX
>
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--
73 de Ted Edwards, W3TB and GØPWW
and thinking about operating CW:
"Do today what others won't,
so you can do tomorrow what others can't."
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