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`Ω.`

## 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.

# Remarks

On the Internet you will find a lot of articles discribing impedance transformers for End-Fed antennes. Some mixing terms and pictures.

## Terms

- 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

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