RF Connectors Reference
Connector choice often determines system performance at microwave frequencies. Every connector junction contributes insertion loss, a small impedance discontinuity (which worsens with frequency), and a potential point of failure. Choosing the right connector requires balancing frequency range, power handling, mechanical robustness, and cost.
Connector Comparison Table
| Connector | Max frequency | Impedance | Power (CW) | Torque spec | Notes |
|---|---|---|---|---|---|
| BNC | 4 GHz | 50 Ω / 75 Ω | 80 W @ 1 GHz | Bayonet lock | Quick-connect; RF lab, oscilloscopes, video (75 Ω) |
| TNC | 12 GHz | 50 Ω / 75 Ω | 100 W | 1.13 N·m | BNC with threaded coupling; more rugged |
| N-type | 18 GHz | 50 Ω / 75 Ω | 300 W @ 1 GHz | 1.36 N·m | Workhorse for VHF/UHF/microwave; weatherproof versions |
| SMA | 18 GHz (26 GHz semi-precision) | 50 Ω | 100 W @ 1 GHz | 0.9 N·m | Most common RF connector; keep to 12 GHz in production |
| SMB | 4 GHz | 50 Ω | 25 W | Snap-on | Compact snap-on; consumer electronics |
| MMCX | 6 GHz | 50 Ω | 50 W | Snap-on | Miniature; Wi-Fi modules, GPS receivers |
| U.FL / IPEX | 6 GHz | 50 Ω | 10 W | Snap-on | Very small; < 30 insertion cycles; PCB to antenna pigtail |
| 2.92 mm (K) | 40 GHz | 50 Ω | 10 W | 0.9 N·m | SMA-compatible mating; 26–40 GHz range |
| 2.4 mm | 50 GHz | 50 Ω | 8 W | 0.9 N·m | Fragile; Ka-band test and mmWave |
| 1.85 mm (V) | 67 GHz | 50 Ω | 5 W | 0.9 N·m | Very fragile; V-band measurements |
| 1.0 mm | 110 GHz | 50 Ω | 2 W | 0.45 N·m | W-band; lab use only |
Insertion Loss Rules of Thumb
Every connector pair (male + female) contributes roughly:
- SMA pair: 0.1 dB at 10 GHz, 0.2 dB at 18 GHz
- N-type pair: 0.05 dB at 10 GHz
- 2.92 mm pair: 0.15 dB at 30 GHz, 0.3 dB at 40 GHz
- Each adapter (e.g. SMA-to-N): doubles the connector insertion loss
In a 10-connector measurement path at 18 GHz, connector losses alone can exceed 2 dB — easily dominating the DUT's own loss.
Torque and Connector Longevity
Over-torquing damages centre pin contacts. Under-torquing causes intermittent connections and unpredictable impedance discontinuities. Always use a torque wrench for precision measurements. SMA connectors are rated for ~500 mating cycles; precision SMA (0.9 N·m spec) for ~2000 cycles. 2.4 mm and 1.85 mm connectors are easily damaged by cross-threading — always align carefully before tightening.
PCB Launch Connectors
The transition from a PCB trace to a connector is a discontinuity. Edge-launch SMA connectors must be carefully matched — the launch geometry, board thickness, and connector pin dimension must all be designed together. At 20+ GHz, a poorly designed SMA launch can add 0.5–1 dB of loss and create a visible ripple in S₁₁ measurements. Some vendors provide field-solver-optimised footprints for their connectors at specific frequencies.
75 Ω Systems
Cable television, broadcast video, and some telecommunications infrastructure use 75 Ω impedance (derived from the impedance of a centre-loaded dipole in free space). BNC-75, TNC-75, N-75, and F-connectors are used. Never mate 50 Ω and 75 Ω connectors — the impedance mismatch causes a permanent reflection, and physically incompatible types (N-50 vs N-75) can be damaged if forced.