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Author Topic: Air Wound Balun  (Read 2774 times)
K3DC
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« on: October 11, 2009, 05:24:33 PM »

Tonight a bunch of hams go together and built air wound 4:1 voltage baluns.

Based on this design:

http://www.rogertango.com/articleread.aspx?idnumber=34232411

I tested one using a 259B and a 200ohm 1w 2% resistor as the load.

It seemed to be most 1:1 in the 17m band.  At 20mhz it as 2:1 and in 1.8 it was over 3.5:1.

Is it difficult to create one of these PVC baluns that is flat?  Every balun that I've came across and the two I bought both use cores.


k3dc (x ki4ymd)
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W8JI
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« Reply #1 on: October 11, 2009, 05:50:53 PM »

The problems are flux leakage, lack of mutual coupling, and low inductance in the coils when they are air wound.

You will have to be very good at picking a winding style when using air to get something that works from 3 to 15 MHz, let alone all of HF.

I would use ferrite cores.
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K3DC
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« Reply #2 on: October 11, 2009, 05:58:40 PM »

Pretty much what I was thinking.

I wanted to build one as a learning exercise and replace the LDG 4:1 I have in my attic at the feed point of my loop.  I need good response on 3.5 -> 14.350.  

k3dc
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N3OX
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« Reply #3 on: October 11, 2009, 06:02:54 PM »

"
Is it difficult to create one of these PVC baluns that is flat?"

Yup.

I always suspected that thing didn't work.

You'll notice the author put it at the coax-twinlead junction of an antenna that's presumably used with a tuner.  So who cares if it gives bad SWR?  :-)

A 4:1 voltage balun isn't what I'd use in that service anyway.  I'd use a 1:1 current balun that could handle high differential voltages and currents, which is basically just a small, close spaced twinlead made of nice fat wires good for pretty high voltage (I  like that silver plated teflon stuff) wound on a suitable ferrite core, just like you'd wind a ferrite coax choke.

Here's that type:

http://www.io.com/~n5fc/balun_1-1_open.jpg

You can probably get a reasonable choking impedance that way on a few bands with an air core, but there's a reason we use cores in broadband baluns.

The same goes for actually needing a 4:1 broadband impedance transformation.  Low power 4:1 ferrite baluns are pretty non-critical objects.  They usually work OK for me on a random core out of the junkbox.  When I build a balun I sometimes build a pair if I have two of the same core.  That way you can not only do a SWR sweep with a known resistor but you can double-check at full power that they work OK by wiring 'em back to back to match your dummy load up by the ratio and back down to the transmitter.  Then you can just check the power at the input and output of the double balun stack with a meter calibrated for 50 ohms.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
G8JNJ
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« Reply #4 on: October 12, 2009, 01:22:54 AM »

Hi All,

Air wound baluns only provide acceptable performance over a very narrow frequency range.

You have to treat them with the same care and attention that you would have to apply when building a tuned circuit. It's very easy for them to become detuned from the desired operating frequency, when connected in circuit to an antenna system.

The much broader bandwidth offered by ferrite cored baluns and transformers makes this much less critical.

You can see an experiment showing the transition from air cored transformer to ferrite cored one in this document.

http://g8jnj.webs.com/Balun%20construction.pdf

Although the text mainly applies to iron powder cored Unun construction, there is a lot of other stuff in it which is relevant to broadband transformers and baluns.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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K0BG
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« Reply #5 on: October 12, 2009, 05:30:43 AM »

Years ago, E.F. Johnson made one of those. I had one once, but never used it. If my memory hasn't failed, it was rated for 1 to 15 Megacycles (yes, I know it's MHz now).

Alan, KØBG
www.k0bg.com
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KC9Q
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« Reply #6 on: October 12, 2009, 08:28:06 AM »

Chris,

Like most said here, use a torrid wound balun.  One of the better baluns to build is a 4:1 Guanella as described by Dr. Jerry Sevick in his book: Building and Using Baluns and Ununs.

Go to: http://www.i1wqrlinkradio.com/antype/ch11/chiave1821.htm

This shows a switchable version.  I made one using two 7-turn windings  on an AMIDON FT-240-K core.  For easy mechanical construction connect the red and green wires together, and use the blue and violet wires to form the balanced output.  I used ordinary #14 gauge household insulated wiring.  This should produce a balun capable of 300 Watts PEP.  For higher power use #14 gauge enameled wire with #12 teflon sleeving.  Measured VSWR using a 220 ohm resistor was as follows:

VSWR    Band
 2:1     160
 1.5      80
 1.3      60
 1.3      40
 1.2      30
 1.2      20
 1.2      17
 1.2      15
 1.2      12
 1.2      10
 1.1       6

This should put you in the ballpark, Chris.

73,
Mike
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KC9Q
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Posts: 48




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« Reply #7 on: October 12, 2009, 08:28:17 AM »

Chris,

Like most said here, use a torrid wound balun.  One of the better baluns to build is a 4:1 Guanella as described by Dr. Jerry Sevick in his book: Building and Using Baluns and Ununs.

Go to: http://www.i1wqrlinkradio.com/antype/ch11/chiave1821.htm

This shows a switchable version.  I made one using two 7-turn windings  on an AMIDON FT-240-K core.  For easy mechanical construction connect the red and green wires together, and use the blue and violet wires to form the balanced output.  I used ordinary #14 gauge household insulated wiring.  This should produce a balun capable of 300 Watts PEP.  For higher power use #14 gauge enameled wire with #12 teflon sleeving.  Measured VSWR using a 220 ohm resistor was as follows:

VSWR    Band
 2:1     160
 1.5      80
 1.3      60
 1.3      40
 1.2      30
 1.2      20
 1.2      17
 1.2      15
 1.2      12
 1.2      10
 1.1       6

This should put you in the ballpark, Chris.

73,
Mike
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G8JNJ
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« Reply #8 on: October 12, 2009, 09:07:32 AM »

Hi All,

The basic design quoted in the mail above is a good starting point and type K (or 61) is a good choice of material. But for the best results you need to use two separate cores, as there is far too much interaction between the two sets of windings on one core.

I also notice that the design doesn't seem to specify the direction in which the pairs of wires forming each half of the balun are wound relative to each other, which is another warning sign about the validity of the design.

See about half way down this page for details of a 4:1 balun on two cores.

http://g8jnj.webs.com/baluns.htm

Regards,

Martin - G8JNJ

www.g8jnj.webs.com
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W5WSS
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Posts: 1657




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« Reply #9 on: October 12, 2009, 10:36:30 AM »

A single band(a distinction) air wound coaxial coil can effectively choke common mode return signal when constructed properly, and is a good application for a mono band Yagi. Whether one wants to call this construction technique a Balun or not is questionable. Perhaps this string pertains to purely a Balun. Seems some would benefit from clarification....but not me lol.
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VK1OD
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Posts: 1697




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« Reply #10 on: October 12, 2009, 01:30:58 PM »

Notwithstanding the above recommendations, the balun design at Go to: http://www.i1wqrlinkradio.com/antype/ch11/chiave1821.htm is quite flawed.

For an explanation of why, see my article http://www.vk1od.net/balun/gsc/index.htm .

I think Tom (W8JI) has also written an article on the same issue.

Both the article referenced above, and the one in the original posting are examples of 'designs' published without supporting quantitative analysis or measurement data... but that is not unusual.

Owen
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