Transformers
A transformer transfers electrical energy from one circuit to another using magnetic coupling — without any direct electrical connection between the two. Think of it like a magnetic gearbox: you can trade voltage for current or current for voltage, keeping power roughly constant. This ability to change voltage levels safely and efficiently makes transformers essential in power supplies, antenna systems, and RF circuits throughout ham radio.
- How a transformer works: mutual inductance
- The turns ratio
- Voltage and current transformation
- Impedance transformation
- Transformer efficiency and losses
- Types of transformers in ham radio
- Baluns and ununs
How a Transformer Works: Mutual Inductance
A basic transformer consists of two coils wound on a shared core. When alternating current flows through the first coil (the primary), it creates a constantly changing magnetic field in the core. That changing field passes through the second coil (the secondary) and, by Faraday's law of electromagnetic induction, induces a voltage across it. No electrical connection between the two circuits is needed — the energy transfer is entirely magnetic.
This only works with AC or changing currents. A steady DC through the primary creates a static magnetic field — no change, no induction, no output from the secondary. Transformers are fundamentally AC devices.
The primary winding drives alternating flux through the shared core. The secondary winding intercepts that flux and has a voltage induced across it proportional to its number of turns.
View LargerThe Turns Ratio
The turns ratio (n) is the ratio of the number of turns on the primary winding (Np) to the number of turns on the secondary winding (Ns). This single number determines everything: the voltage ratio, the current ratio, and the impedance ratio.
n = Np / Ns
- n > 1: step-down transformer (secondary voltage lower than primary)
- n < 1: step-up transformer (secondary voltage higher than primary)
- n = 1: isolation transformer (same voltage, but circuits isolated)
Turns Ratio Calculator
Enter the number of primary and secondary turns to find the turns ratio, or enter the turns ratio directly.
Voltage and Current Transformation
The secondary voltage is proportional to the turns ratio. If you have twice as many turns on the secondary as the primary, you get twice the voltage. Current behaves inversely: double the voltage means half the current, because power is conserved (in an ideal transformer).
- Vs = Vp × (Ns / Np)
- Is = Ip × (Np / Ns)
- Power in = Power out: Vp × Ip = Vs × Is (ideal)
Secondary Voltage and Current Calculator
Enter primary voltage, primary current, and turns ratio (Np/Ns) to find secondary values.
Impedance Transformation
One of the most important jobs a transformer can do in radio work is transform impedances. If a transmitter output stage needs to see a 50 Ω load but the antenna presents 200 Ω, a transformer with the right turns ratio bridges the gap. The impedance ratio changes as the square of the turns ratio:
Zp / Zs = (Np / Ns)² = n²
- A 4:1 impedance transformer has a turns ratio of 2:1
- A 9:1 impedance transformer has a turns ratio of 3:1
- Commonly used for antenna matching: 200 Ω balanced to 50 Ω coax = 4:1 balun
Required impedance ratio: 200 / 50 = 4:1
Required turns ratio: √4 = 2:1
A 2:1 turns ratio balun (4:1 impedance balun) makes this match.
Transformer Efficiency and Losses
Real transformers are not perfectly efficient. Energy is lost through:
- Copper losses (I²R): Heat generated in the resistance of the winding wire. Heavier gauge wire reduces this.
- Core losses (eddy currents and hysteresis): Circulating currents induced in the core material produce heat. Laminated iron cores and ferrite materials reduce this.
- Flux leakage: Not all the magnetic flux links both windings, especially in loosely coupled designs.
Good power transformers achieve 95–99% efficiency. Small signal RF transformers wound on ferrite cores also achieve high efficiency across a broad frequency range. Efficiency becomes critical in high-power RF applications such as PA output networks.
Types of Transformers in Ham Radio
| Type | Purpose | Where found |
|---|---|---|
| Utility power transformer | Step 120/240 V AC down to working voltages | Linear power supplies, linear amplifiers |
| Audio transformer | Impedance matching between stages; isolation | Microphone circuits, phone patches |
| IF transformer | Bandpass filter and impedance matching at IF frequency | Superheterodyne receivers (455 kHz, 10.7 MHz) |
| RF broadband transformer | Broadband impedance matching, 1.8–54 MHz | Driver stages, antenna matching, baluns |
| Balun/unun | Convert balanced ↔ unbalanced; impedance transform | Antenna feedpoints, coax-to-antenna interfaces |
Baluns and Ununs
A balun (balanced-to-unbalanced) is a specialized transformer that connects a balanced circuit (such as a dipole antenna with two equal arms) to an unbalanced circuit (such as coaxial cable, which has its shield at ground potential). Without a balun at a dipole feedpoint, RF currents flow on the outside of the coax shield, causing radiation from the feedline, pattern distortion, and RF in the shack.
An unun (unbalanced-to-unbalanced) is used when both sides are unbalanced but have different impedances — for example, transforming from 450 Ω open-wire line to 50 Ω coax.
A 1:1 current balun provides balance without impedance transformation. A 4:1 voltage balun transforms the 200 Ω balanced feedpoint of a dipole to 50 Ω for coaxial cable.
View LargerCommon balun types used in ham radio:
- 1:1 current balun (choke balun): Suppresses common-mode current on the coax shield without changing impedance. Most commonly used balun for a 50 Ω dipole.
- 4:1 voltage balun: Transforms a 200 Ω balanced load to 50 Ω unbalanced, used with folded dipoles and some multi-band antennas.
- 9:1 unun: Transforms a high-impedance end-fed wire (typically 400–600 Ω) to 50 Ω for coax feeding.
Frequently Asked Questions
Why does a transformer only work with AC, not DC?
Electromagnetic induction only occurs when the magnetic field is changing. A steady DC current through the primary creates a constant magnetic field — no change means no induction in the secondary and no output voltage. Alternating current continuously changes direction and magnitude, so the magnetic field is always changing, continuously inducing voltage in the secondary. Some DC transformer equivalents (such as inverter-based SMPS) rapidly switch DC on and off to create a changing field.
What is a 1:1 balun and why do I need one?
A 1:1 balun does not transform impedance — both sides are the same impedance (typically 50 Ω). What it does is prevent common-mode current from flowing on the outside of the coax shield. Without a balun at a dipole feedpoint, the coax outer conductor becomes part of the antenna, carrying RF into the shack, distorting the radiation pattern, and potentially causing RF burns or audio interference. A 1:1 choke balun presents a very high impedance to common-mode current while passing differential (wanted) current without impedance change.
How does impedance transformation relate to maximum power transfer?
Maximum power is transferred when the source impedance equals the load impedance. A transformer allows you to match mismatched impedances — effectively making the load appear to be the correct impedance from the source's perspective. In RF work, the transmitter output stage is designed to drive 50 Ω, so everything in the signal path (including the antenna system) must present 50 Ω for maximum power transfer and minimum reflected power (low SWR).
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.