Microstrip Slot Antenna Calculator

Mode Given Frequency → Compute Slot Length Given Length → Compute Resonant Frequency Slot Type Microstrip Slot (λ/2) Slotline (λ/2 in slot) Dielectric Constant (ε_r) Substrate Thickness h (mm) Slot Width Ws (mm)

Top View of Rectangular Microstrip Slot

Theory (Rectangular Slot Resonance)

1. Effective permittivity for slot region

ε_eff,slot = (ε_r + 1)/2 − (ε_r − 1)/2 · (h / √(h² + Ws²))

2. Resonant condition (λ/2 slot)

Ls ≈ λ_g/2

λ_g = λ₀ / √ε_eff,slot

Ls ≈ c / (2·f_r·√ε_eff,slot)

3. Inverse (given Ls → f_r)

f_r ≈ c / (2·Ls·√ε_eff,slot)

Symbol Glossary

• Ls – slot length (mm)
• Ws – slot width (mm)
• h – substrate thickness (mm)
• εr – dielectric constant of substrate
• εeff,slot – effective permittivity seen by slot fields
• c – speed of light (3×10⁸ m/s)

Design Notes

• Slots radiate mainly from the **edges**, not from surface currents like patches.
• Slot width influences **bandwidth** and **Q-factor**.
• Complementary structure (slot ↔ dipole) follows Babinet’s principle approximately.
• Finite ground plane size alters fringing; this calculator assumes “infinite” ground.
• Final validation should be done using FEKO, CST or HFSS.