EFHW match box, end fed half wave transformer, 49:1 unun and impedance transformer – the animal goes by many names. Try to Google one of them, and you will find many examples of home brew variations of them. They all seem very like (because they are) and nearly all of them share a common feature: Revolved windings. Why do people over complicate this simple device?
An unun is often called an impedance transformer, because that’s the feature we need in our installation. Nevertheless it is still a regular transformer with primary and secondary windings. The impedance changes because voltage and current also changes, just like in regular voltage transformers.
Transformers in general – principle of operation
From our preparations for the exam we know how a transformer works: We excite the primary windings which creates a magnetic field. The magnetic field in turn induces a current in the secondary windings. The voltage on secondary is proportional with the ratio of turns between primary and secondary: If secondary has twice as many turns as primary, the voltage doubles. And as a consequence the current halves (or vice versa).
If we insert an iron core inside the winding, we enhance the magnetic field. We often use this effect in transformers by winding primary and secondary windings on a toroid: The magnetic field gets stronger around the secondary winding and increases the induction.
How the impedance changes in a transformer
In all transformers (with a ratio different from 1:1) we see a change in impedance. In fact, the impedance transformation is proportional to the square of winding ratio. If we have a winding ratio of 1:2, the impedance ratio becomes 1:4. Why does this happen?
Let’s say you transmit 100 watts from your transceiver through a 1:4 unun to your antenna. You want your transceiver to “see” 50 ohms, so we assume that the impedance on the ununs primary is 50 ohms. Let us also assume that your transceivers output is 10 volts at the moment, just to keep things simple.
We remember that and . We can calculate with impedance the same way we do with resistance, so .
The current through the primary windings must be: .
With a winding ratio of 1:2, voltage doubles, and current therefore halves to retain the same power. Therefore and .
The impedance in the secondary windings must be .
We see that with a winding ratio of 1:2, the impedance ratio becomes 1:4. The impedance changes with the winding ratio squared. Note that this principle applies to all transformers.
The un-un-complicated unun
We saw that all transformers change the impedance as long as the ratio is something other than 1:1. Adjust the number of turns, and the impedance ratio will change accordingly. That’s actually all there is to it. But still we see those complicated designs with step-by-step guides on how to wind them. It’s time to un-complicate the un-un-complicated ununs. Let’s break down the main aspects that make the unun seem so complicated.
Reasons not to separate the windings
In pictures we see different variations of how to distribute the windings. The most regular is the one where the primary winding gets twisted together (revolved) with secondary. The primary and secondary windings could just as well be separated and located on opposite sides of the core.
Note: You should generally distribute the turns evenly to reduce stray capacitance. Enameled wire performs better than PVC insulated wire. The pictures above are winded for clarity.
As far as I see it there are three reasons for winding primary and secondary together on the same side of the core (not revolved):
- Even better coupling
- Easier to wind – exact same number of turns. (The current doesn’t count turns – even a quarter turns difference makes a (small) difference in coupling.)
- Wire ends located where you need to solder them
These are all valid reasons, even though it may make it harder to see what’s going on. But this doesn’t answer why so many revolve the conductors…
To revolve or not to revolve – that is the question
Revolving the windings on your torroid – It’s really like the woman I know who sawed the knee off the lamb thigh before steaking it in her large, modern stove: I asked her why she did, to which she replied: “My mom taught me to. I don’t know why but she always did so herself.” It turns out that her moms oven was too narrow to fit the thigh in its full length, so she simply removed the knee so it would fit.
Air is not a great magnetic conductor. Actually it’s quite terrible, because of its low permeability. A wires proximity to the core really matters. Revolving the wires increases the distance between the conductor and the core. -Especially if you use PVC insulated wire.
Reasons to revolve the conductors:
- Because the internetz told me to
- It’s so damn hard to know which wire should go where. (No, it’s not. Trust me.)
Reasons not to revolve the conductors
- Distance between core and conductor. (Less is more.)
- It’s fiddly
- Doesn’t look as good
Does it matter?
Winding primary and secondary together has its advantages. Distributing the turns to make the wire ends come out where you need them is practical. But revolving primary and secondary is bad practice*.
Does it make a difference? Probably not much. You would likely not be able to measure the difference between revolved and straight windings. But why bother when it’s just as easy not to?
*) Revolving “hot” and “cold” wires reduces unwanted emissions since they cancel each other out. Revolving the wires between the toroid and the connector can make some sense. But not on the toroid. Besides, if you worry about unwanted emissions there’s no way around a metal case if you build an unun.