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 UnUn's again 
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Post UnUn's again
Question for you Steve, or any one else...

I have seen schematics for 9:1 UNUN's and they seem to have an equal windings of the two wires, around 13-19 turns.
Having just looked at another site, the author is stating that the transformation ratio of the UNUN is determined by the square of the turns ration.. Ie, 30:10 will give a 9:1 Unun. Now if this is the case, then wouldnt the equal turns just be a 1:1 unun?

For the task that I am making a unun the turns ratio isnt really important... I just want to use it to bleed static charge from a kite antenna.

Can you confirm this please?

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Jason M0MGF
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Sat May 29, 2010 7:12 am
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 UnUn's again 
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Post Re: UnUn's again
Hi Jason,

UnUns are usually wound with either a bifilar or a trifilar winding.

If we wound an UnUn with 10 bifilar turns we would create an "autotransformer" with a 20 turn secondary and a 10 turn primary. The impedance transformation ratio is the square of the turns ratio, so in this case we've created a 4:1 UnUn.

If we wound it with 10 trifilar turns we would have a 30 turn secondary and a 10 turn primary, giving an impedance transformation ratio of 9:1.

Hope that's clear - it's easier to make than describe :)

73,
Steve G3TXQ


Sat May 29, 2010 8:06 am
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 UnUn's again 
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Post Re: UnUn's again
Thanks for that Steve,
So for what I have been looking at, Bifilar windings, it is just an autotransformer, a 1:1. The more windings I put on will depending on number of windings will alter the transformer ratio, to 1: (Windings ^ 2)

Clear as murky mud :)

As I said, I am only making one up so that I can use it to bleed of all that static from my kite antenna before it gets to my randon wire tuner and the radio. I am looking it suspending around 40m of wire from a kite, and hopefully work the low bands ... fingers crossed that it goes well and some wind appears.

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Jason M0MGF
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Sat May 29, 2010 8:17 am
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 UnUn's again 
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If instead of winding the unun like
Image

I was to wind it like (ignoring the 2:1 turns ratio)
Image

The performance would be the same? And I could use this method of winding for increasing the transformer ratio?

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Jason M0MGF
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Sat May 29, 2010 9:02 am
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Silent Key

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Jason,

The top photo is a 4:1 UnUn or "autotransformer". It comprises 12 bifilar turns (it shows 12 even though he says 14). The input connects across the primary winding of 12 green turns; the load is connected across a secondary of 24 turns made up of the 12 mauve turns plus the 12 green turns. So the turns ratio is 2:1 and the impedance transformation is the square of that: 4:1.

This construction is known as a Transmission Line Transformer because the green/mauve bifilar pair form a transmission line. Coupling between primary and secondary does not depend on flux flowing through the core; in fact at high frequencies you could take away the core and it would still work!

The bottom photo is a different beast. It has a 10 turn primary and a completely separate 20 turn secondary. It relies on flux in the core to couple energy from primary to secondary, unlike the UnUn which relied on transmission line effects. Because of that it will have much less bandwidth than the UnUn. The turns ratio is again 2:1 so it will have an impedance transformation of 4:1. Its only advantage is that input and output sides are totally isolated.

Any clearer?

73,
Steve G3TXQ


Sat May 29, 2010 9:34 am
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 UnUn's again 
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Post Re: UnUn's again
Yes, clearer. Thank you.


One last question on the this subject (I hope). What would be the effect of using more turns on the windings? I have just managed to wrap 22 turns on a 2inch core. How will this behave compared to the 12/14 turns as mentioned on the drawing?


Regards.
Jase

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Jason M0MGF
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Sat May 29, 2010 9:50 am
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 UnUn's again 
Silent Key

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Post Re: UnUn's again
Jase,

What's the core material? Is it #2 mix (red)?

Steve G3TXQ


Sat May 29, 2010 10:49 am
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 UnUn's again 
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Yes steve, T200-2 the red one.

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Jason M0MGF
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Sat May 29, 2010 10:57 am
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 UnUn's again 
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M0MGF wrote:
Yes steve, T200-2 the red one.


In that case, increasing to 22 bifilar turns (44 total) would be a good move - it will extend the frequency at which it will work, down to about 7MHz.

With the original 14 turns it wouldn't work as a 200 ohm / 50 ohm transformer below about 17MHz.

73,
Steve G3TXQ


Sat May 29, 2010 11:30 am
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 UnUn's again 
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Many thanks for that steve, Not quite as low a frequency. I was hoping to use it on topband.

If I was to use a T300-2, how many turns would I need for 160m operation?


Regards, Jase

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Sat May 29, 2010 12:11 pm
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 UnUn's again 
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Jase,

A T300-2 is actually worse than a T200-2; its dimensions mean that it has less inductance per turn. If you can get a T300A-2, that would need 62 turns total for 160m operation - that's 31 bifilar turns.

But please not that it depends on the application. When I say the UnUn would only "work" down to 7MHz, by "work" I mean that you can put 200 ohms across its secondary and get something close to 50 ohms at the primary. That's what you would want to happen if you had a 200 ohm antenna and wanted to get a close 50 ohm match.

But if you are using a UnUn to "tame" the extreme VSWRs on a "random wire", you probably don't care too much whether the impedance transformation is exactly 4:1, because at the end of the day you are going to use a tuner to produce a match. As long as the UnUn brings the impedance down, you may not care if it is 3:1, 4:1 or 5:1; then you can probably operate a particular balun at lower frequencies.

A pragmatic approach might be to wind as many bifilar turns as you can on two stacked T200-2 cores and see how it goes.

73,
Steve G3TXQ


Sat May 29, 2010 12:44 pm
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 UnUn's again 
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Steve,
Thank you very much for the lesson. Lots has been learnt, and a further understanding of the UnUn has been achieved.

As you correctly said, I am not too concerned about the 50ohm matching because I will be using the 40m wire for multiband operation, and will have my trusty SGC-237 taking care of any matching that is required. It would be nice to have a wire cut for each band to maximise efficiency, but that would not be too practicle. I will endeavour to get as many counterpoises out as practical, and then see what I can work.

Thanks again for the information.

Jase

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Sat May 29, 2010 12:59 pm
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Silent Key

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Jase,

Do you realise that if you can get the SGC tuner close to the base of the antenna you really don't need an UnUn?

73,
Steve G3TXQ


Sat May 29, 2010 1:31 pm
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 UnUn's again 
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But what about the static electricity that will build up on the wire? Wouldnt that do damage to the tuner?

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Jason M0MGF
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Sat May 29, 2010 1:48 pm
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 UnUn's again 
Silent Key

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Jase,

If that's a concern, simply connect a "bleeder resistor" between the antenna terminal and ground; something like 100k at a few Watts would do.

73,
Steve G3TXQ


Sat May 29, 2010 2:01 pm
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