An RF choke is an inductor designed to present a high impedance to radio-frequency currents while allowing DC or low-frequency currents to pass. The best design depends on the frequency range and power level.
Common RF Choke Designs
1. Air-Core Solenoid Choke
Simple and effective at VHF and UHF.
Advantages
- No core losses
- High Q
- Suitable for transmitters and receivers
Typical construction
- Enamelled copper wire
- 5–20 turns
- Wound on a former then removed or left in place
The inductance can be estimated by:
where and are coil radius and length in inches.
2. Ferrite-Bead Choke
Common in receiver front ends and power leads.
Construction
- Wire passed through one or more ferrite beads.
- Multiple turns through the bead increase impedance approximately as .
Excellent for suppressing common-mode currents and noise.
3. Toroidal RF Choke
Widely used in amateur radio equipment.
Advantages
- Compact
- High inductance
- Minimal external magnetic field
For HF applications, ferrite materials such as type 43 or 31 are popular.
4. Coaxial RF Choke (Current Balun)
Particularly useful for antennas and feedlines.
Examples:
- Several turns of coax wound into a coil.
- Coax passed through ferrite cores.
Common on dipoles, Yagis, and radio astronomy antennas to reduce feedline radiation.
Choke Design by Frequency – recommendations from ChatGPT:
| Frequency | Typical Choke |
|---|---|
| LF (30–300 kHz) | Iron-core or ferrite-core |
| MF (300 kHz–3 MHz) | Ferrite-core or toroidal |
| HF (3–30 MHz) | Toroidal ferrite or air-core |
| VHF (30–300 MHz) | Air-core, ferrite beads |
| UHF (>300 MHz) | Ferrite beads, transmission-line chokes |
For Your 1420 MHz Hydrogen-Line Work – recommended by ChatGPT:
At 1420 MHz, ordinary wound inductors often become self-resonant and cease behaving as chokes. Instead, designers usually use:
- Ferrite beads on power leads.
- Quarter-wave transmission-line stubs.
- Feed-through capacitors combined with ferrite beads.
- Coaxial common-mode chokes near the antenna feedpoint.
For a 1420 MHz receiver front end, a few ferrite beads on the DC supply lead are often more effective than a conventional wound choke.