One of my antennas is a commercial "G5RV" fed with 33' of 450 ohm ladder
line, terminated in a PL-259 pair, with coax from there to the shack.
Apparently this combination results in very high RF voltage at the
PL-259, and it arcs over at 1000 W (not from an Elecraft amp!). This
combo goes wild when I attempt high power on 80M, although it is stable
at 100 W. Just another thing to watch out for!
73 - George, W3HBM
On 6/1/2020 5:59 PM, Ted Edwards W3TB wrote:
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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 <[email protected]> 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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