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 Baluns, UnUns and T200-2 cores 
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Post Baluns, UnUns and T200-2 cores
Regular readers of my postings will know that I frequently warn that Type-2 iron dust cores just don't have enough permeability to work well in wideband HF applications like Baluns and UnUns. This morning I thought I'd wind one and measure its performance.

The UnUn comprised 14 bifilar turns on a T200-2 core wired to provide a 4:1 step down in impedance. I placed a 200Ω load across the secondary and measured the impedance looking into the primary at different frequencies. If it worked as designed we would expect to measure 50Ω - a VSWR of 1:1.

Here are the results. For comparison, shown in brackets are the VSWRs I measured on a UnUn wound with 10 bifilar turns on a FT240-43 ferrite core:

1.8 MHz: 4.5 (1.06)
3.6 MHz: 2.3 (1.06)
7.0 MHz: 1.6 (1.07)
14.1 MHz: 1.3 (1.1)
21.2 MHz: 1.3 (1.16)
28.5 MHz: 1.3 (1.24)
51 MHz: 2.5 (1.79)

As you can see it only begins to operate correctly above 40m whereas the ferrite core is reasonable throughout the HF range.

As I said in a recent posting, depending on your application a T200-2 UnUn may still "work"; what it wont do is provide the expected impedance transformation at the low end of the HF range.

73,
Steve


Sat Jan 22, 2011 1:47 pm
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 Baluns, UnUns and T200-2 cores 
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Post Re: Baluns, UnUns and T200-2 cores
I was at a local rally recently and had a wish list of T240-43 and T240-61 cores, I managed to get a couple of the -43 and came across a seller with some T200-2 cores, I remembered they were discussed in this section some time back, I just couldn't remember whether it was in praise of or not, it was cheap enough so I bought it, at least it wasn't a complete waste as you point out, it not being too bad above 40m means it has it's uses :lol:

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Sat Jan 22, 2011 7:44 pm
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 Baluns, UnUns and T200-2 cores 
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Post Re: Baluns, UnUns and T200-2 cores
Hi Steve,

Interesting post. I think I can confirm your findings in the field so to speak. My multitrap dipole seems to follow the trend you suggest of lowest SWR's that are achievable across the bands.

I have a 1:1 balun in the centre of my multiband trap dipole made with 17 bifular turns of enameled wire (not sure of wire gauge) on a t200-2 torroid. This is supposed to handle 400w SSB.

If i was to swap it for a balun made with an FT240-43. What size enameled wire and how many turns would you recommend for a 400w, 1:1 balun working from 80m to 10m

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Tue Jan 25, 2011 10:05 am
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 Baluns, UnUns and T200-2 cores 
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Post Re: Baluns, UnUns and T200-2 cores
Mark,

I'm slightly confused!

If you have a multiband trap dipole I would expect it to show a reasonable VSWR on all bands; once you insert a balun wound with bifilar turns you risk messing up those VSWRs because the bifilar winding will likely have a characteristic impedance significantly higher than 50Ω.

To preserve the VSWRs you need to wind the balun with 50Ω coax. 8 turns of RG58 on a FT240-43 core is about right for 80m thru 10m.

73,
Steve G3TXQ


Tue Jan 25, 2011 5:59 pm
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 Baluns, UnUns and T200-2 cores 
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Post Re: Baluns, UnUns and T200-2 cores
G3TXQ wrote:
Mark,

I'm slightly confused!

If you have a multiband trap dipole I would expect it to show a reasonable VSWR on all bands; once you insert a balun wound with bifilar turns you risk messing up those VSWRs because the bifilar winding will likely have a characteristic impedance significantly higher than 50Ω.

To preserve the VSWRs you need to wind the balun with 50Ω coax. 8 turns of RG58 on a FT240-43 core is about right for 80m thru 10m.

73,
Steve G3TXQ



Thanks for the advice Steve. I still have so much to learn. The VSWRs are pretty good as it is now,but I'll try the RG58 when my FT240-43 arrives.

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Tue Jan 25, 2011 9:05 pm
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