Slotline
Understanding Slotline
Slotline is the complementary structure to microstrip: where microstrip is a strip conductor over a ground plane, slotline is the absence of conductor (a slot) in a ground plane. This duality (Babinet's principle) means slotline has unique properties that complement microstrip.
Slotline Properties
- Impedance range: 50-300 ohms. Higher impedance than microstrip, complementary impedance levels.
- Field distribution: Electric field crosses the slot; magnetic field encircles it. Opposite to microstrip.
- Radiation: Tends to radiate more than microstrip, especially at wide slots.
- Coupling: Naturally couples to slot antennas and waveguide slots.
Applications
- Balanced mixers: Slotline balun feeds provide a natural balanced input for diode pairs.
- Microstrip-to-slotline transitions: Used for 180-degree power dividers and baluns.
- Slot antennas: Fed by microstrip crossing the slot or CPW.
Frequently Asked Questions
What is a slotline?
A slotline is a planar transmission line formed by a slot (gap) in a ground plane on a dielectric substrate. It is the electromagnetic complement of microstrip. Slotlines are used for balanced mixers, slot antenna feeds, and transitions.
How is slotline different from microstrip?
Microstrip is a strip over ground plane; slotline is a gap in a ground plane. Microstrip has fields between strip and ground (different sides); slotline has fields across the slot (same side). Their impedance ranges are complementary.
When do you use slotline?
Slotline is used when a balanced structure is needed (mixers), for coupling to slot antennas, and as a transition element between microstrip and balanced circuits. It is rarely used as a primary transmission line due to higher radiation loss.