The Saga of the Current Balun
May 2008
After attending the 2008 DX Convention in Visalia, and listening to K9YC talk about high current baluns, I sent in an order to Olympix for their Ferrite 311000 core to build a better current balun for my Windom. ( It's a Fair-Rite© 0431177081 ) Below is the result
In the photos it looks like the ones K9YC built? It should have something like 2300 ohms of common mode resistance at 10 Mhz. ( see the chart on page 40 of the link above. ) The antenna still seems to listen and talk well but the funny thing is, both my LDG auto tuners, AT-897 and AT-7000, put up a huge fuss when I tune at 160 meters when the antenna is 2 to 1 SWR or more? They didn't do that with the 9 in line toroids. I tried 6 turns in 2ea torroids but it only got worse.
Calling for help I email'ed K9YC. The conversation begins below.
On Mon, 26 May 2008 20:46:10 -0700, Craig A. Williams wrote:
Hello Jim;
Aside from winning a Yaesu IC-7000 with $60.00 in drawing tickets,
listening to your presentation on balms was my high point at the 2008
DX convention. After your presentation I went home and ordered the
"correct" parts for the current balun on my 160 Meter Carolina Windom.
The before and after is at http://www.craigwilliams.com/radio/shack/index.html
about 1/2 way down the page.
My question is, if sometime, at your convenience, could you look at my
page and see if you think I did it correctly or, suggest anything else
I should do to this antenna? Except for my LDG auto tuners doing a lot
more clunking on 160M with the big torrid the antenna seems to TX and
RX great.
Thanks
Craig Williams W6CAW
Hello Craig,
Thanks for the kind words. And congrats on winning the radio!
I'm not hot on Carolina Windom antennas, but as I understand, it's dipole fed
off center so that the feed point is at a fairly high impedance, which is why
you are using the 4:1 balun.
Several thoughts. First, the choke you've installed doesn't have much Z on
160M -- it isn't big enough. But it does have inductance. If you're running
power on 160, it may overheat, because the Z isn't high enough. See the data
for that choke -- it's in the tutorial just before the "cookbook." It isn't
clear to me what you're trying to accomplish with the choke. It's too small to
do much on 160 -- you need a much larger one. The choke you've wound is tuned
way up on 10M, and it's at the wrong place in your antenna.
Off-center fed antennas are inherently unbalanced, so they are inherently
noisy on RX. But that off center feed has another important contribution -- it
affects the tuning of the antenna. When you added the choke at the bottom, you
changed the current distribution on the feed line by adding that choke at the
base. That's why your antenna tuners are spitting up on you. Moving the choke
to the feed point will also irritate the antenna tuners, because you're
disconnecting the feed line from the antenna. In other words, the feed line of
an OCF antenna is intentionally part of the antenna!
The best location for a coaxial choke (at least the first one) is right at the
antenna feed point (that is, up in the air).
If your concern is 160, you need a MUCH bigger choke, it needs to be at the
feed point, and it's going to seriously detune your antenna.
If your primary objective is 160M, I would turn it into 1) a center-fed dipole
or 2) a top-loaded vertical with radials. In both cases, I would use a BIG
coax choke at the feed point -- something like 7 turns through five #31 torrid
cores. If you want it to work on both 160 and 80, I would either build it as a
fan dipole (two elements on the same feed point) or as a loaded dipole like
those sold by Hypower Antenna Co. I've done both, and both work very well.
Here at home, I have a 160/80 loaded dipole with a 40M fan element up at 100
ft, and an 86 ft top-loaded Tee vertical with 60 radials. I have each hooked
up to its own tuner, and can switch between them. Both work well, but the
vertical usually works better. All of that, of course, depends on the height
of both antennas, which is determined by the available sky hooks. :) I've got
these big redwoods. And it depends on what you want to work -- locals or DX or
both. I run 1.4 kW to the vertical most of the time, and locals tell me I'm
LOUD. :)
Hope this helps,
73,
Jim K9YC
Feeling crushed I responded.
Thanks for your extensive reply. I better order some more torrid!.
The Carolina part of the Windom, according to the internet that is, is the vertical feed line from the 4:1 balun
to the bottom of the antenna is suppose to act as a vertical radiator. This is suppose to fill in the multiple
lobes you get as you work the antenna on 80 through 10M. At the base of the antenna you put a current balun in
the feed line to stop the radiation / common mode at that point. The best discussion of this was at http://www.w5fc.org/files/QRP%20Expressions.pdf
. Today the antenna is resonate, VSWR 1.5-,1 at 1.8Mhz and my tuner does a quick tune and is happy. At 2.0Mhz,
VSWR at 3.5-1, the tuner does a long tune every time like it didn't remember it just tuned there.
I am only using 100 watts so assume I won't have a heating problem. The funny thing about noise is there is about
1/2 as much on 20 M as my R7000 vertical. A little less signal on the Windom sometimes but 1/2 the time more signal.
Guess it depends on the polarization on the received signal. The Windom puts a great signal into the Left Coast
and Arizona on 80M. I do a lot of RACES/ARES stuff on 80M.
Thanks again for the excellent suggestions. Maybe when my Tech. Class YL gets used to the one long wire I can put
up some more. With 8 acres it's not like I don't have room. Only problem is in the high desert here all our trees
are around 20 feet high! Can you say NVIS?
Craig Williams W6CAW
Well, the end of the story for now, is Jim was right. The more I tried to build a current choke the worse it got
so, I tore it all out and put my little ones back in. The VSWR in 1.9 Mhz is 1.2 to 1 and my tuners are happy.
I ASSUME, since I have the feed line buried from the antenna to the shack mother earth is my common mode choke
as I don't appear to have any RF in the shack. Hopefully this little exercise will help others.
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