Why 24 GHz Is Different
At 24 GHz, standard 0402 SMD components (L, C) have self-resonant frequencies of 4–12 GHz — far below operating frequency. Lumped elements are unusable for matching at 24 GHz. Distributed (microstrip) elements are the only practical approach.
Substrate Selection for 24 GHz
| Substrate | Loss at 24 GHz (100mm) | Trace Width for 50Ω |
|---|---|---|
| FR4 (H=0.5mm) | ~15 dB (unusable) | 0.92 mm |
| Rogers RO4003C (H=0.254mm) | ~4 dB | 0.44 mm |
| RT/Duroid 5880 (H=0.127mm) | ~1.5 dB | 0.38 mm |
Microstrip Matching at 24 GHz (RT/Duroid 5880)
Device output impedance at 24 GHz: Z_out = 15 + j10 Ω (from S22) λg at 24 GHz on RT5880 (εe=1.88): λg = c/(24GHz×√1.88) = 9.12 mm Single-stub match: Distance d: on Smith chart, move from z=0.3+j0.2 to where Re(y)=1 d ≈ 0.08λg = 0.73 mm Open stub length l: cancel susceptance at distance d l ≈ 0.20λg = 1.82 mm Both traces: 50Ω microstrip (W=0.38mm for H=0.127mm Duroid 5880) PCB tolerance: ±0.025mm → at 24 GHz this shifts frequency by ~3%
RF View 24 GHz Simulation
- Load 24 GHz MMIC .s2p as DUT block
- Add Microstrip T-line: W=0.38mm, L=0.73mm, εr=2.20, H=0.127mm
- Add open stub shunt T-line: W=0.38mm, L=1.82mm (open at far end)
- Simulate → S11 at 24 GHz should show <−10 dB
RF View 24 GHz: Circuit Simulator microstrip T-line element supports 24 GHz design on RT/Duroid 5880. Microstrip Calculator provides W and λ/4 dimensions. Free on Android.