Transmission line transformers using stranded wire.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AA7EJ, Mar 27, 2012.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    I just build a 2.25:1 balun using 14 gauge STANDED run of the mill insulated wire.
    Trifilary wound, close spaced winding to match around 100 Ohms to 50.
    Don’t have it handy, but something THW or similar type of wire.

    In his book Transmission line transformers, Mr Sevick uses solid wire and never really elaborates why.
    He does mention that the characteristic impedance of bifilary wound transformer is important and describes various insulation / spacing schemes for the wiring.
    Unfortunately he does not elaborate much on trifilary would toroid.

    Any thoughts on using stranded wire in these transmission line transformers??
    73 Vaclav
  2. VE3EKJ

    VE3EKJ Ham Member QRZ Page

    I'm not sure it would make much difference! Electrons travel on the skin of the wire anyway. What counts is the diameter of the wire and the spacing between turns caused by the thickness of the insulation. This will determine the characteristic impedance of the windings.

    If the wire diameter is the same along with that of the insulation then I can't see what difference changing from solid to stranded would make.

    Then again, I'm always learning new things! Perhaps someone else has something to chip in.
  3. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Does stranded wire have more surface area than solid?
  4. AA7EJ

    AA7EJ Ham Member QRZ Page

    If we use litz wire as a example of an attempt to reduce skin effect than yes , stranded wire must have more surface. At least as far as skin effect is concerned.
    But these wire strands are not insulated as litz wires are.
    In my next transformer I will use solid wire because stranded is little too hard to form on the toroid core. Also the plastic insulation is tough to handle and is messes up the impedance anyway,
  5. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Yes, Litz is insulated, so that's a whole different apple. You get lots more surface area that way. But, I wonder how much extra surface area you get with stranded non-Litz instead of solid. I'm guessing it's more, but that's a guess, and how much more, I don't know.
    I wouldn't use stranded for the same reason you're not, but, I just was wondering if anyone has any data to say how much effective surface area there is with stranded vs solid.
  6. KB1CKT

    KB1CKT Ham Member QRZ Page

    I would think the biggest problem is the insulation. Magnet wire uses enamel(?) for insulation, which offers, err, some very low dielectric constant (even if it was very high, it's very thin). While solid wire can use PVC insulation also, I don't think I've ever seen stranded wire with anything but Teflon or PVC insulation. That extra insulation will increase interwinding capacitance, thus limiting upper frequency usage.

    Also, under heating conditions (excessive losses or not) I suspect the thin enamel coated magnet wire is better at shedding heat than Teflon/PVC is. PVC can melt very easily also.

    Finally, I'm not sure what effect it has, but the wire isn't wrapped as tightly against the core due to the insulation. That may increase leakge inductance.
  7. N3OX

    N3OX Ham Member QRZ Page

    I think the "effective area" as measured by the actual RF resistance for a given diameter may end up LESS...

    When I get a chance I'll check the resistance of stranded vs. solid wire. I suspect the solid is better.

    I'd always heard that smooth strip was better than braid and I tried that out recently: it's at the bottom of this page:

    even beautiful shiny tight woven braid is MUCH higher resistance than copper strap, even when the strap is narrower...

    Now I think that the resistance is probably of negligible importance in transmission line transformers, but I would guess stranded is NOT better and may very well be worse... probably not as much difference as the difference between braid and strap, but still favoring solid wire.
  8. W0BTU

    W0BTU Ham Member QRZ Page


    What I would like to see is that tinned braid replaced with shiny untinned copper braid (maybe from a piece of coax shield), and see what happens then.

    I'm in no way advocating the use of braid over strap. I'm just curious as to what extent that the poor-conducting tin plating is contributing to these losses.
  9. AA7EJ

    AA7EJ Ham Member QRZ Page

    I do not have means to measure these creations, but I am considering trifilar transformer using coax. One of the windings would be the braid, but they are in parallel so the whatever effect would be even less noticeable.
    I tried it before and RG58 is really a pain to wind on 2 inch toroid.
    I am searching for something more flexible, but high braid coverage.
    I got some Rat Shack scraps, but that may be waste of time.
    I find it interesting that Mr Sevick used coax without the outside jacket.
    If I understand the theory of the transmission line transformer , the outside jacket insulation in this case should not have much effect on the characteristic impedance of the winding. Maybe he did have similar problems with winding the RG58 and without the outside jacket it is little easier HI HI HI
  10. N3OX

    N3OX Ham Member QRZ Page

    Yeah it would be interesting. Then I will leave it outside for a year and repeat :)
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