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I'm getting started with my first (very basic) HF rig and for convenience and the experience I'd like to try an End Fed Half Wave antenna. Typical designs for a "matchbox" in this case include somewhere between a 6:1 and 10:1 turns ratio around a toroid.

Winding my own toroids is a new experience (though I guess I've built electromagnets and germanium radios as a kit) and I'm still pretty unfamiliar with the conventions. I understand that ferrites come in a variety of specifications beyond just size — permeability, flux density, etc. I need to learn more about these for winding inductors themselves, but how relevant are they for transformers?

For example, on the first link there's one short reference "the torroid [sic] is actually larger than a T-50-2". In this build "the toroid is a T130-2 core". I really don't know what those numbers mean to begin with, but I get the impression I could wind the right number of turns around pretty much any size/shape ferrite and get something that would work? Would certain options be less efficient? Or simply effect the maximum working power? What factors should I pay attention to, and what can I ignore for these purposes?

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  • $\begingroup$ How does one read a ferrite datasheet? $\endgroup$ – Phil Frost - W8II Nov 22 '15 at 14:19
  • $\begingroup$ Thanks, I noticed that while browsing the site. But my question is really: what is relevant for transformers vs. inductors. Are things like Q factor and permeability relevant when I'm not using the ferrite for its "Henries" but only for converting voltage? $\endgroup$ – natevw - AF7TB Nov 24 '15 at 2:53
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    $\begingroup$ Only an ideal transformer only transforms impedances. Real transformers introduce an impedance of their own to the circuit (like an inductor), have loss (and as such can handle only so much transmit power), can be non-linear (depending on core material), and may have a magnetic field that extends far away from the device (thus unintentionally passing through other materials with less desirable magnetic properties). So the material properties that govern inductors also very much apply to transformers. $\endgroup$ – Phil Frost - W8II Nov 24 '15 at 3:13
  • $\begingroup$ Great article you've linked to, it speaks some sense about "end fed" antennas. $\endgroup$ – tomnexus Nov 25 '15 at 21:05
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Transformers are inductors. They just happen to have mutual inductance with another inductor. Like inductors, they also have self-capacitance, and so a resonant frequency. They also have loss, and consequently some maximum power handling ability which if exceeded causes the transformer to fail by overheating.

As such, any parameter which is relevant to an inductor might also be relevant to a transformer. There is no general way to say what parameters are relevant or not: to answer that question requires some understanding of the circuit in question, and its operation and requirements.

For example, you'll need to consider the magnetizing inductance of the transformer. This is the inductance that the transmitter sees looking into the transformer as a result of the transformer itself. You can measure it by leaving the secondary disconnected and measuring the transformer as an inductor.

In your end-fed dipole example, this magnetizing inductance appears in parallel with the antenna. There are two ways to approach this problem.

One way is to make sure the magnetizing inductance is an impedance much greater than 50Ω (50Ω being, hopefully, the impedance of the antenna, as seen by the transmitter). This will mean that the magnetizing inductance has negligible effect on the overall impedance seen by the transmitter. (If that doesn't make sense, consider the impedance of a 1kΩ resistor: what happens if you add a 1MΩ resistor in parallel?)

If it's not practical to obtain a sufficiently high inductance, then a capacitor can be added in parallel with the transformer to make a parallel LC circuit, which at resonance has a very high impedance. The interwinding capacitance of the transformer also effectively contributes parallel capacitance, so it's possible to design a transformer which is self-resonant and does not require an additional capacitor.

Another way to deal with the magnetizing inductance is to incorporate it into the design of the circuit. If you needed an inductor anyway, perhaps as part of a filter or a matching network, then you can put the primary of an appropriately designed transformer there instead.

So what does this mean for your end-fed dipole example? What things might we change, and what effect would they have in that application?

We could increase the number of turns, while maintaining the same turns ratio. This will increase magnetizing inductance, which might be good since we want that impedance to be high. More turns will also increase the interwinding capacitance. But more turns will also increase the magnetic flux through the core, eventually hitting the limit of saturation.

We could use a different material. But we'd want to consider the losses inherent in that material, its permeability and the effect that will have on the magnetizing inductance, and its maximum flux density and how that will impact the peak current the transformer can handle.

The physical size of the core is also a concern. A larger core spreads the magnetic flux through it through more area, and so can handle more magnetic flux (and consequently, current and power) before reaching saturation. A larger core also has more area to lose heat from losses to its surroundings, and more thermal mass to absorb transient power peaks, and so can handle more power.

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  • $\begingroup$ Thanks, very helpful answer both to my original practical concern ("can I order just any random toroid to build this?") and some of the other things I haven't been able to really understand with EFHW matchboxes (especially: why most designs include a capacitor). I suspect it will help me a lot to think about transformers as transforming impedances rather than voltages. $\endgroup$ – natevw - AF7TB Nov 24 '15 at 18:53
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    $\begingroup$ Thank you for including core size and heat dissipation, which is often overlooked. It is far and away the most common cause of balun and transformer failure, particularly when people attempt to use incorrect core material (leading to saturation) or too small a core for a given power level/mismatch/imbalance. The T106-2 used in the typical EARCHI design cannot dissipate more than a few watts continuously unless it is in open air with excellent air flow. Even an FT240-43 will heat dramatically at 20W dissipation, which is not hard to do with 100W input on poorly designed antennas. $\endgroup$ – Hamsterdave Jan 19 '17 at 2:36

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