SOLVED - Wire type to wind balun question

[krupski]

Hi all,
I want to wind a 4:1 balun for use with an off center fed dipole. My question is this: Does it matter if I wind the balun using silicone insulated, fine stranded wire versus the typical solid (magnet wire in teflon sleeving)? Either type will be 16 ga, wound on an FT-240-61 core and will see 100 watts max. Does the wire type matter here?  Thanks!

[David Hess]

The wire type only matters insofar as the extra wire insulation will take up more room in the winding window.  At higher powers more or better insulation is needed to handle the higher voltages and higher temperatures.

[T3sl4co1l]

At what frequency?

Insulation thickness and dielectric constant determine impedance of the pair (assuming this is a TLT), which has impact on the matching and balance towards the high frequency cutoff (1/4 wave point) of the transformer/windings.

The dielectric should generally have low tan delta (silicone is probably okay?, PVC is bad, others are generally okay to great), and the wire spacing and k should be selected to give the desired characteristic impedance, give or take a modest margin.

As for its other properties, silicone I suppose doesn't do you any good here.  You can't run at high temperature (besides solder melting, resistance rising, and corrosion at temp, the ferrite core has a relatively low Tc being a NiZn material), and its chemical inertness / environmental resistance is hard to harness without potting the whole thing.  If you have it on hand---it's fine, you just aren't taking advantage of these properties.

Wire stranding probably makes no difference; it has a small advantage at medium frequencies where the contact resistance between strands is high enough to give a *little* bit of litz effect (generally in the 5-15% range), but I'm guessing from the core, this will be at frequencies where this doesn't matter.  Or maybe solid is in fact better, because of the smoother surface (whereas the rough surface of a stranded wire causes current to flow preferentially by the outer peaks, and over a longer distance along the valleys).  If nothing else, wind both kinds and measure the insertion loss!

Tim

[krupski]

The wire type only matters insofar as the extra wire insulation will take up more room in the winding window.  At higher powers more better insulation is needed to handle the higher voltages and higher temperatures.

Thanks for the answer. The core has more than enough room for the silicone insulated wire. The power level will be 100 watts max, so I don't think heating will be a problem... and silicone handles heat well so all should be OK.

[krupski]

At what frequency? The frequency will be US amateur radio bands... and only 40 meters to 10 meters.

Insulation thickness and dielectric constant determine impedance of the pair (assuming this is a TLT), which has impact on the matching and balance towards the high frequency cutoff (1/4 wave point) of the transformer/windings. The balun is a combination of a common mode choke and a 4:1 transformer (wound on 2 separate cores). Since the frequencies involved are relatively low (~28 MHz. max), I'm sure(?) that insulation thickness and dielectric constant are not important.

The dielectric should generally have low tan delta (silicone is probably okay?, PVC is bad, others are generally okay to great), and the wire spacing and k should be selected to give the desired characteristic impedance, give or take a modest margin. I assumed that PVC would be a bad idea since I used to be an engineer at a medical device manufacturer where we RF heat sealed vinyl (PVC) bags using RF in the 13 to 25 MHz. range, so I know it's lossy.

As for its other properties, silicone I suppose doesn't do you any good here.  You can't run at high temperature (besides solder melting, resistance rising, and corrosion at temp, the ferrite core has a relatively low Tc being a NiZn material), and its chemical inertness / environmental resistance is hard to harness without potting the whole thing.  If you have it on hand---it's fine, you just aren't taking advantage of these properties.

Wire stranding probably makes no difference; it has a small advantage at medium frequencies where the contact resistance between strands is high enough to give a *little* bit of litz effect (generally in the 5-15% range), but I'm guessing from the core, this will be at frequencies where this doesn't matter.  Or maybe solid is in fact better, because of the smoother surface (whereas the rough surface of a stranded wire causes current to flow preferentially by the outer peaks, and over a longer distance along the valleys).  If nothing else, wind both kinds and measure the insertion loss!

Stranded VS solid was actually my primary concern, so you saying it shouldn't make a difference makes me feel better about it!

Tim

My replies are added to your quote. And, thank you for the reply.

[nctnico]

I'd look at teflon insulated wire as well.  Don't know about the loss though but it has a very low dielectric constant so you get a lower capacitance between the wires.

BTW: don't mistake teflon for kynar. You need teflon

[T3sl4co1l]

Ah, good call, PVDF sucks:
https://www.professionalplastics.com/professionalplastics/Kynar740DataSheet.pdf
give or take frequency. Based on other data, sounds like it has a dipole relaxation time constant somewhere in the 10s-100s kHz, so, particularly lossy there.  Maybe there are others higher up, too.

Tim