The impedance for a half wave antenna is high at both ends. Around 3 kΩ.
For a 50Ω coax feed line a 50x (2,5kΩ) to 70x (3,5kΩ) impedance transformer is needed.
A winding ratio of 4 (for example) transforms the voltage a factor 4 up and the current a factor 4 down (equal power). This means the impedance transformation is the square of the turn / winding ratio. So in this example a impedance transformation of 16, giving a 800Ω output impedance.
Primary and secondary windings are bifilar wound (and loosely twisted).
Ferrite Toroid FT-140-43 should be enough for 100W PEP SSB.
3x24 windings (left) - 2x14 windings (right) - Bifilar wound - Common ground
Measurements SWR & impedance
I've experimented (and measured) with different transformation / winding ratios.
Impedance transformer 2 primary turns (1:1 ratio)
Most impedance transformers described on the internet use 2 primary windings.
Bifilar (and twisted) wound with the same number of secondary turns and dummy-loaded with 50
Impedance transformer 3 primary turns (1:1 ratio)
Same measurements now with 3x3 turns (see picture).
As you may notice the SWR is a little bit better.
The impedance curve is more flat. Especially in the low frequency range.
Impedance transformation calculation
The squared winding ratio gives the impedance transformation.
Include the bifilar (twisted) wound secundary windings in the counting.
On the Internet you will find a lot of articles discribing impedance transformers for End-Fed antennes. Some mixing terms and pictures.
- A bifilar coil *) is an electromagnetic coil that contains two closely spaced, parallel windings
- An Autotransformer is an electrical transformer with only one winding
- A Balun is an electrical device that converts between a balanced signal (two signals working against each other where ground is irrelevant) and an unbalanced signal (a single signal working against ground or pseudo-ground)
- A UnUn is an electrical device that converts between a unbalanced signal and an unbalanced signal (a single signal working against ground or pseudo-ground)
*) the parallel wires may be additionally (loosely) twisted