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Author Topic: Wire Ant. Balun Hook up  (Read 928 times)
N2UNL
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Posts: 15




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« on: February 16, 2008, 03:37:33 PM »

I'm home brewing a offset ant. fed with the 22'coax (RG8)vert radiator that is line insulated at the feed point.  Total lenght is 135'  my short leg is 50' and the long leg is 85'. How do i know what side of the Balun to connect the short side or the long side ?  What side does the shield go to or the center input go to from  the Balun ?  It's probley easier than what i think but rather solder them on once than twice.  Thanks to all..  N2UNL
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N1QOQ
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Posts: 188




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« Reply #1 on: February 16, 2008, 04:16:35 PM »

Use an ohm meter

73
Paul
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N2UNL
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Posts: 15




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« Reply #2 on: February 16, 2008, 04:24:58 PM »

Ohm meter ? I don't see how that tells me what side goes where. Am I missing something ?
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N2UNL
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« Reply #3 on: February 16, 2008, 04:30:48 PM »

A Ohm meter doesn't answer the question of , where does the input of the center pin come out ? does it go to the short side of the wire or the long side of the offset wire ant.
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AC5E
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« Reply #4 on: February 16, 2008, 05:04:31 PM »

The coax center conductor goes to the long leg of your off center fed dipole. The shield goes to the short leg. and there should be a "current balun" at the feedpoint to keep the outside of the shield from becoming part of the antenna.

If you want a "step up" balun at the feedpoint, you can easily construct a current balun of coax that will force equal currents on the two legs of the dipole.

73  Pete Allen  AC5E
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W6BR
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Posts: 59




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« Reply #5 on: February 16, 2008, 05:05:02 PM »

I'm confused as to what you are trying to do - Do you have a offset diopole type antenna feeding it with RG-8??

If so, either side of the balun goes to the sides of the balun, does'nt matter which side. Then screw the RG-8 coax to the bottom RG-239 connector.

If it is different let me know and I'll try and help you - or Google "Off Set Center Antennas" and see how they are built. do you have an ARRL Handbook - it will also tell you how to build one.

Good Luck, 73, de W6BR
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KC0DBY
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« Reply #6 on: February 16, 2008, 05:12:39 PM »

I completely agree that the first response was not very enlightening...  Unfortunately I don't have the answer either- and google didn't reveal much to me either.  I have found that it doesn't really matter much with a dipole, but your setup is slightly different.

I'm assuming that you'll be using a tuner with this antenna, and if its not TOO much of a bother, you can always try it one way, and turn the power on the radio down to 5 watts, and try to tune it, and check the other polarity if you aren't satisfied.  I've found that doing things "the wrong way" has eventually paid off big time when experimenting with antenna designs- sometimes a mis-measurement or reverse polarity will result in MUCH better performance than expected.

I'm on a small lot, and have found my favorite past-time to be putzing around with antennas, so I got an analyzer, which was really the best investment I ever made.  They are a TON of fun- just making slight changes, checking them out, and trying them on air.

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N2UNL
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« Reply #7 on: February 16, 2008, 05:41:48 PM »

W6BR.. Yes that's right it is a offset feed with rg8x.  just like the windom.  you say it doesn't matter what leg goes on what side, and conterdicting that answer is AC5E that says that "the center conducter goes to the long side". WOW  where do I go from here ?  can you give me supporting theory on youre finding ?  Thanks,  Brian
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N5RMS
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Posts: 37




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« Reply #8 on: February 16, 2008, 09:50:10 PM »

Well, it depends on what type of balun you have. Some are just ferrite beads around a small piece of coax, covered by PVC pipe.  The other type may have a ferrite donut with wire wrapped around it.  This type can be a 1:1 or 4:1.  
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N3JBH
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« Reply #9 on: February 17, 2008, 05:45:22 AM »

I should assume you will be using a 4:1 or 6:1 balun for this antenna. In the Basic Design of these types of Baluns it should be come evident that it would not matter what side of the Balun is on what side of the wire.

That being said use a Quailty Balun. It never ends to amaze me the tales i here from the anti balun crew's about how balun's are no good yet i find out the used a balun the was cheaper then there afternoon lunch at McDonalds. Then complain the the balun did not work etc etc.

Spend your money on quailty stuff and you should be happy.
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N1QOQ
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Posts: 188




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« Reply #10 on: February 17, 2008, 07:14:37 AM »

You asked, How do I know what side of the balun the center conductor goes to. Thats why I said use an ohm meter. Every diagram of a off set dipole I have seen shows the center conductor on the long side the sheild on the short. Maybe there is more than one way to do it.
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AB8SG
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Posts: 136




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« Reply #11 on: February 17, 2008, 07:56:11 AM »

If you are using a pre made balun, it shouldn't matter which output wire goes to which antenna wire. Suggestions to use an ohm meter will not be very helpful.
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N3OX
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« Reply #12 on: February 17, 2008, 10:18:03 AM »

"If you are using a pre made balun, it shouldn't matter which output wire goes to which antenna wire."

That's getting closer but not quite.

If you are using a VERY GOOD balun, it won't matter at all which side goes to which wire.  Not all commercial pre made baluns are good.  Some are pretty bad at being baluns.

A perfect voltage balun would force the voltages to be equal on each terminal.

A perfect current balun would force the currents to be equal at each terminal.

So, it wouldn't matter which way you hooked it up.  In practice, on an off-center fed antenna with a *real* balun, there will be some difference between hooking it up in some particular way.

With a good quality, high impedance current (choke) balun like a DX Engineering or Array Solutions or a homemade type of good design (the most affordable way to get such a balun) it really won't matter much which way you hook it up.  

The very high impedance of the balun makes it so that neither antenna wire can make currents flow on the coax, so there's no distinction between the wire that winds around the core and connects to the center conductor and the wire that winds around the core and connects to the shield.  A good current or "choke" balun makes the output terminals "float" with respect to the input terminals.

Here's a page on testing baluns that should give some perspective:

http://www.w8ji.com/Baluns/balun_test.htm

You can ground either of the output terminals and you'll get the same result if the balun is perfect.  If it's good, there should be very little difference.  The output terminals don't "know" which coax side they're connected to!  That's how a balun SHOULD work.

Now, the degree of "floating" of the output terminals depends on the construction of the balun and the type of antenna.  On a random, unequal leg antenna even a good balun will show *more* difference if you hook it up in the two different directions.

In this case, the only way to know is to *try* it both ways, especially if you don't have a hundred dollar commercial or homebrewed monster balun !

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
G0GQK
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Posts: 634




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« Reply #13 on: February 17, 2008, 11:25:13 AM »

I'll say one thing, you've had a selection of strange answers ! Why do you need a balun ? Who suggested you need a balun ? A balun is used to transform balanced to unbalanced isn't it ?  A dipole is balanced, both legs are the same length, the feeder you are using is coax, which is unbalanced. The off centre fed antenna is unbalanced, one leg is longer than the other.

I suggest that you search around and find construction details of a windom which works and make one exactly the same, because if you keep getting dodgy advise, you will never construct one which works properly.

G0GQK
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WB6BYU
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Posts: 13356




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« Reply #14 on: February 18, 2008, 10:05:20 AM »

The purpose of a balun is to isolate the antenna from the feedline so
there is no RF flowing on the outside of the coax.  If you truly are using
a balun it should make no difference how it is connected.  But if your
balun is doing its job there is no RF flowing on the vertical 22' section
because it is isolated from the antenna by the balun.

Some designs may use a 4 : 1 UN-UN at the feedpoint instead.  The
difference is that now the coax shield is connected to one side of the
antenna.  (With a 4 : 1 balun the shield is connected to the center tap
rather than to one end of the winding.)  In that case it may make a
difference how the transformer is connected.  The main reason for
using the unbalanced connection and letting some of the coax radiate
appears to be to improve the impedance match on 15m, where the
original Windom design tends to have a high impedance.  In this case
the length of the coax section (which becomes one side of the antenna)
plus the length of the other side of the antenna needs to be resonant
on 15m, and that detail will depend on the actual wire lengths of the
design you are building.

Note that the actual current flowing on the 22' vertical section is pretty
small, and contributes little to the radiation pattern on most bands.
(Contrary to the claims of some manufacturers.)

A good article on the Off-Center Fed Dipole from W4RNL is here:

http://www.cebik.com/gup/gup9.html
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