What is a hollow coil?

A hollow coil, also known as an air-core coil or non-solid coil, is a type of coil that does not have a solid core made of magnetic material such as iron or ferrite. Instead, the center of the coil is hollow or filled with air. This design is used for various applications in electronics and electrical engineering, and it has several characteristics:
1. **Inductance**: The inductance of a hollow coil is typically lower than that of a coil with a magnetic core because the air does not increase the magnetic flux as effectively as a magnetic material would.
2. **Magnetic Permeability**: Since air has a magnetic permeability close to that of a vacuum (approximately μ₀), the hollow coil will have a lower magnetic permeability compared to a coil with a magnetic core.
3. **Resonant Frequency**: In applications where the coil is used in resonant circuits, such as in radio frequency (RF) circuits, the hollow coil can help achieve higher resonant frequencies due to its lower inductance.
4. **Core Losses**: Hollow coils have minimal core losses because there is no material to cause hysteresis or eddy current losses. This makes them more efficient for certain applications.
5. **Physical Size**: To achieve a certain level of inductance without a magnetic core, a hollow coil might need to be larger in physical size compared to a coil with a core.
Applications of hollow coils include:
- **RF Transformers**: In radio frequency circuits, hollow coils can be used as transformers or inductors without the core losses that can occur at high frequencies.
- **Antennas**: Hollow coils can be used in the construction of certain types of antennas, such as solenoid antennas or inductors in matching networks.
- **Filters**: They can be used in filter circuits where high-frequency performance is required and core losses are undesirable.
- **Sensors**: In sensors that detect magnetic fields, a hollow coil can be used to avoid interference from the core material.
Hollow coils are chosen over coils with magnetic cores when the specific properties of air-core coils are desirable for the intended application.