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Author Topic: 4:1 Balun for Windom Antenna  (Read 3255 times)
KI4HNN
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Posts: 63




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« on: May 20, 2007, 03:02:46 PM »

I have a question.  How can I make a 4:1 balun out of coax for the Windom antenna.  I am thinking of trying to build one of these but need a balun which I don't have.  Can I make it out of coax and can I make it out of 12 guage solid wire? Would anyone know of any websites that have detailed pages with construction details of how these baluns are made?  I am going to purchase a antenna tuner soon.  
Thanks,
Joey
KI4HNN
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WB6BYU
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Posts: 13120




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« Reply #1 on: May 21, 2007, 09:13:59 PM »

There is a common 4 : 1 balun circuit that uses a half wavelength of
coax, but that won't work for an Off-Center Fed dipole if you want to
use it on several bands (since the balun is only a half wavelength on
one frequency.)  There are a few designs for wideband 4 : 1 coax
baluns, but they aren't common.  I'll have to search my records to see
if I have a copy of a magazine article somewhere.  I suspect one reason
why they are not more common is the relative ease of winding one on
a ferrite rod or toroid core.  (You should find plenty of internet sites
that describe how to do that.)

One approach you can try, though, is to wind the coax into a pair of
choke coils.  There are some commonly-suggested numbers of turns
and former diameters that should work.  (The exact values aren't
particularly critical, but there is a trade-off between having enough
inductance for the lower frequencies without exceeding the self
resonant frequency on the higher bands.)  If you wound two pieces of
coax into identical coils (or perhaps wound a single coil using two
pieces of coax in parallel?) then you can connect the two pieces of coax
in parallel at one end and in series (that is, center conductor of one piece
to the shield of the other, and using the unconnected center and shield
as the balun output) and that should give you a 4 : 1 impedance
step-up.  (The parallel end would go to the feedline and the series
side to the antenna.)

Personally, I'd probably try winding a balun using hookup wire before
I tried to make one out of coax, but the preceeding approach should
work, though it may be cumbersome.
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AE6WA
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Posts: 59




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« Reply #2 on: June 10, 2007, 08:30:54 PM »

A windom antenna is an unbalanced antenna because it is off-center fed. Baluns are designed to feed a balanced load. Baluns are also lossy when feeding poor SWR loads. You should reserve Balun use for tuned center fed antennas.
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NA0AA
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Posts: 1043




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« Reply #3 on: June 17, 2007, 10:27:40 PM »

The ARRL Handbook, current edition has a good section on balun, and a diagram on how to wind a high quality 4:1 balun, although to be fair, at the cost of cores it's almost cheaper to buy one today.

I homebuild 9:1's for matching since that's the correct ratio to match a 450 window line.  It's really amazing how much the core material matters to performance - I have 3 balun, identical except for the core composition and on the same antenna, each one behaves completely differently with respect to the SWR response.  I can better tune the antenna by selecting the best match for the bands I want to work.

Very cool since I can get to the balun at ground level as well so it's easy to do.
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WB6BYU
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Posts: 13120




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« Reply #4 on: June 18, 2007, 08:53:56 PM »

KG6WOU wrote:
> I homebuild 9:1's for matching since that's the correct ratio to match a 450 window line.


This is ONLY the case when your antenna impedance is also 450 ohms,
which is rarely the case.  Otherwise, the impedance seen by the balun
depends on what is connected to the OTHER end of the transmission
line, not the characteristic impedance of the line itself.  (And I've been
told that many commerical "450 ohm" lines are closer to 400 ohms
instead.)

If you know that the impedance of the antenna is usually going to be
over 100 ohms or so, then a 4 : 1 balun is  a good choice.  It is much
harder to make a good 9 : 1 balun than a good 4 : 1 balun.  This is
because two wires couple together better than 3 do, and with just 3
windings the shield of the coax cable must be connected off-center
because there is no connection at the middle - unless you are going to
use 6 windings, which gets quite cumbersome.

If the impedance is sometimes below 100 ohms (as is often the case
when a single antenna and feedline are used on multiple bands) then
a 1 : 1 balun will often make it easier to match the loads.  For example,
if the impedance at the balun is 18 ohms, a 9 : 1 balun will step it down
to 2 ohms, while with a 1 : 1 balun the tuner sees 18 ohms.  Generally
tuner efficiency is worst when matching very low impedance loads
(especially on the lower bands).

In the case of the Off-Center Fed Dipole (OCFD), commonly called a
Windom by those who don't remember the single-wire feed version,
the impedance tends to be around 100 to 600 ohms, and a 4 : 1 balun
is a good choice.    You'll probably still need a tuner on most bands,
however.  If you feed the antenna with 450 ohm line down to the ground
and put the balun there, the impedances may be outside of this range
and a 1 : 1 balun may work better on some bands.
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NA0AA
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Posts: 1043




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« Reply #5 on: June 19, 2007, 06:33:22 PM »

I guess I could have prefaced that by saying in my antenna installation they work.

I put up a random flattop fed with 34' of '450 window line'.  I've fed it with 10 different combinations of baluns and tested performance across the bands.

I've tested direct coax, 1:1 [two flavors], 4:1, and four flavors of 9:1.  Each gives me a different best band response.  So I change 'em as required by where I want to work.

A 9:1 is a trifilar winding and is easy to fabricate.  Amidon FT-240 cores in 43, and 77 compositon worked best for my antenna, and oddly, so did direct coax feed to the window line.

Obviously, I'm using a transmatch to feed all of this.
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NE5C
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Posts: 320


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« Reply #6 on: June 27, 2007, 04:45:40 PM »

For my 80 Meter Carolina Windom I've used the "Radio Works Brand" model B4-2k Current Balun 4:1 and it worked fine also using with my Nye Viking Tuner. I honestly have to say though, that I found greater satisfaction with my Loop Antenna, well over, the Windom. 73 & God Bless - Jerry
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NE5C
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Posts: 320


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« Reply #7 on: June 27, 2007, 04:48:32 PM »

Now...I have not tried this Home-Made Balun, but check this out for home brew 4:1, try this link:

http://www.bloomington.in.us/~wh2t/balun.html
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PA0FRI
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Posts: 31


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« Reply #8 on: July 02, 2007, 06:02:28 PM »

See http://www.xs4all.nl/~pa0fri/Ant/Balun/balun.htm for a coax 1 : 4 balun
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