Winding input transformer for HF linear amplifier
This is as per VU2EVQ design. Take 25 cm and 10 cm of 25 SWG winding wire used in submersible motor. Make 3 turns using the 25 cm long wire by passing through BN43-202 core. This is the primary winding of the input transformer. Wind a single turn using the 10 cm long piece. That becomes the secondary winding. Make sure the winding is tight and you have not damaged the insulation of wire. After winding check that the primary and secondary are isolated by using multimeter set in continuity mode.
The BN43-202 is a popular “binocular” (two-hole) ferrite core made from 43 Material. This core is a “gold standard” for amateur radio and RF hobbyists because it is highly versatile for broadband applications.
Key Specifications
- Material: 43 (Nickel-Zinc ferrite).
- Permeability (μi): 800–850.
- Dimensions:
- Length: 14.35 mm (0.565″)
- Width: 13.30 mm (0.525″)
- Height: 7.50 mm (0.295″)
- Hole Diameter: 3.80 mm (0.150″)
- Inductance Factor (AL): Typically 2200–2890 mH/1000 turns (depending on the specific manufacturer test methods).
Common Applications
The binocular shape allows for high coupling between windings, making it ideal for:
- Broadband Transformers: Used in the 1–500 MHz range. It is the go-to core for small QRP (low power) antenna baluns and ununs.
- RF Power Amplifiers: Frequently used in the output stages of HF (High Frequency) amplifiers to match low-impedance transistors to 50 ohm systems.
- EMI/RFI Suppression: Effectively suppresses common-mode noise on data or power lines in the 25–300 MHz range.
Performance Ranges
| Application Type | Frequency Range |
|---|---|
| Wideband Transformers | 5 – 400 MHz |
| Power Transformers | 0.5 – 30 MHz |
| RFI Suppression | 5 – 500 MHz |
Design Tip
When winding this core, one “turn” is defined as the wire passing through both holes (forming a U-shape). Because of the high AL value, you often only need 1 or 2 turns for HF applications.