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Author Topic: toroid for balun  (Read 716 times)
W5DXP
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« on: June 13, 2005, 05:44:20 AM »

At $9, it is hard to beat the Amidon AB240 balun kit. Instructions come with it for making a 4:1 or 1:1 balun.

http://www.amidoncorp.com/aai_cost_experimenter.htm
--
73, Cecil, W5DXP
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AE7GL
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« Reply #1 on: June 12, 2005, 07:19:17 PM »

I want to make a 4:1 balun for an HF dipole. Something like what's shown here...

http://www.rason.org/Projects/balun/balun.htm

My questions are...

What wire can be used? I have some 20 gauge insulated, can that be used?

I have a toroid out of a computer power supply. Can it be used? Measurements are...

OD   about 15/16 inch or about 23.5 mm
ID   about 1/2 inch or about 12.7 mm
thickness about 3/8 inch or about 9.5 mm

And it's yellow with one white edge if that means anything. Otherwise, no markings on it.

And thoughts or suggestions would be appreciated.

Thanks,
Charlie
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KZ1X
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« Reply #2 on: June 12, 2005, 07:43:56 PM »

You can buy or build your own.  For the latter, see the several nice kits from The Wireman in Landrum SC, or try Amidon or Bytemark.
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AE7GL
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« Reply #3 on: June 12, 2005, 07:54:21 PM »

> You can buy or build your own.

I understand. What I'm asking is "will what I have work to build one"?

Thanks
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KZ1X
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« Reply #4 on: June 12, 2005, 08:02:20 PM »

"What I'm asking is "will what I have work to build one"?"

Oh.  I figured you knew; probably not.  Why?  A combination of these:  unknown permeability, wrong material, wrong frequency range, wrong size.

You pretty much have to have a known-type toroid and that's the problem with "surplus" units.  

Plus, size does matter, no way to make a legal-limit balun with less than about a 2" - 2.5" toroid and Thermaleze\Formvar enameled 14 AWG.
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AE7GL
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« Reply #5 on: June 12, 2005, 08:16:07 PM »

OK. That's what I needed to know. I'm not concerned about it being big enough to run a lot of power, but I do want it to be right for what I do run.

Thanks
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KE3HO
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« Reply #6 on: June 12, 2005, 09:07:28 PM »

The core that you have will NOT work for a balun. Yellow with one white edge is a very low frequency core, I don't remember the material number, but it is made for DC to something like 100kHz at best. For an HF balun, you probably want something like a 61 material ferrite core.

73 - Jim
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K0RFD
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« Reply #7 on: June 13, 2005, 05:17:17 AM »

Above answers are correct, but ALSO:

Why would you want a 4:1 balun for a dipole?
As a flat-top, a single-band resonant dipole will have an impedance of about 72 ohms.  As you bring the legs together as an inverted vee, it will get closer to 50 ohms.  You need a 1:1 balun.  Preferably a current balun. But in that application, you are only interested in keeping RF off the feedline.  So you can make a coil of coax and accomplish the same thing.

If you plan to use this dipole as a multiband doublet, feed it with ladder line and use a tuner.  If your tuner doesn't have a built-in balun, use a short jumper of coax to a 1:1 balun just outside the shack.  The impedance will vary all over the place depending on frequency.    The impedance you see at  the shack end will also depend on the feedline length at any given frequency.  4:1 will just be a number pulled out of the air, as good as 1:1 or any other number pulled out of the air.

There are many 1:1 balun kits available.  Somebody already suggested The Wireman, I'd start there.
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K0BG
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« Reply #8 on: June 13, 2005, 06:22:52 AM »

I looked at the page you referred to, and I'd take it with a grain of salt. While using a 1:4 (4:1 is incorrect in this context) with a built in tuner might work with some resonant antennas, it is not a substitute for a decent wide-range antenna tuner.

Further, the reference of the power handling capabilities of the various sizes of cores is erroneous. In other words, there is more to it than the size of the core.

The author didn't offer any building tips. With out the correct size of wire, the correct spacing between the windings, and a few other factors, you end up with nothing more than an RF heat source.

Alan, KØBG
www.k0bg.com
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W8JI
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« Reply #9 on: June 13, 2005, 03:41:56 PM »

by K0BG on June 13, 2005  Mail this to a friend!  
I looked at the page you referred to, and I'd take it with a grain of salt. While using a 1:4 (4:1 is incorrect in this context) with a built in tuner might work with some resonant antennas, it is not a substitute for a decent wide-range antenna tuner.>>


I'm confused. Why is 1:4 balun correct?

1:4 BALUN implies the the 1 ratio side is balanced, the 4 side is UN balanced.

I would think he needs a 1:1, but a 4:1 balun at the tuner would work....but NOT a 1:4 balun. Very few applications need the low Z side BALanced and the high Z side UNbalanced.

73 Tom

 

 
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AE7GL
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« Reply #10 on: June 13, 2005, 04:52:08 PM »

Thanks guys. I know what I have won't work now. I'll need to get the right pieces. But I'm not sure now about whether I need a 4:1(or 1:4) or 1:1. I thought dipoles were typically around 200 ohms at the feedpoint. A 1:1 wouldn't be right for that would it? I'm feeding it with coax from the radio because I don't have a tuner yet. I plan to trim the dipole to resonance on 40 meters.

Thanks,
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W8JI
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« Reply #11 on: June 13, 2005, 05:05:46 PM »

You need a 1:1 balun. When you feed it with ladder line you probably still need a 1:1 balun.
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W5DXP
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« Reply #12 on: June 14, 2005, 11:39:31 AM »

>W8JI wrote: I'm confused. Why is 1:4 balun correct? 1:4 BALUN implies the the 1 ratio side is balanced, the 4 side is UN balanced.<

You guys must be Christians. "Let not the right hand knoweth what the left hand doeth." :-) 73, Cecil, W5DXP
 
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
DD3LY
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Posts: 25




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« Reply #13 on: June 17, 2005, 02:23:06 PM »

If you want to match a simple center fed dipole with its natural resonance to a coaxial cable you need a 1:1 BalUn.
You may build one on your own by simply winding a part of your coaxial feed line trough a toroid. You need to make so many turns that the impedance of this inductivity has at least four times (or better ten times) the value of the feed lines impedance. If you use Amidon '43 material which is ferrite and NO iron powder, this will work with low losses for the higher frequencies (>10MHz), I recommend '77 material for the lower range.
If your feedline is not slinky enough to turn it around the toroid you may use thin Teflon-coaxial line or even double-enamelled copper wire. Do not woory too much about the losses in this short length of thin coax. If you use double-enamelled copper wire this might give some slight addtional but due to the short length this does no harm and you cut your dipole for best VSWR anyway. If you work QRO take care not to injure the isolation of the two enamelled copper wires with the sharp edges of the toroid while making the BalUn.
This simple-way-BalUn may be enhanced with an additional winding for better balance but you will never be able to see or feel the result in your QSOs.
Therefore...       ...just keep it simple!
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DD3LY
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« Reply #14 on: June 17, 2005, 02:26:48 PM »

If you want to match a simple center fed dipole with its natural resonance to a coaxial cable you need a 1:1 BalUn.
You may build one on your own by simply winding a part of your coaxial feed line trough a toroid. You need to make so many turns that the impedance of this inductivity has at least four times (or better ten times) the value of the feed lines impedance. If you use Amidon '43 material which is ferrite and NO iron powder, this will work with low losses for the higher frequencies (>10MHz), I recommend '77 material for the lower range.
If your feedline is not slinky enough to turn it around the toroid you may use thin Teflon-coaxial line or even double-enamelled copper wire. Do not woory too much about the losses in this short length of thin coax. If you use double-enamelled copper wire this might give some slight addtional but due to the short length this does no harm and you cut your dipole for best VSWR anyway. If you work QRO take care not to injure the isolation of the two enamelled copper wires with the sharp edges of the toroid while making the BalUn.
This simple-way-BalUn may be enhanced with an additional winding for better balance but you will never be able to see or feel the result in your QSOs.
Therefore...       ...just keep it simple!
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