RF Connectors & Coax Adapters
Connector
Connectors engineered to carry GHz signals without reflections: constant 50 Ω impedance right through the joint. SMA, U.FL, BNC, N — each with its niche, and each with a reverse-polarity trap for the unwary.
What it looks like
RF Connectors & Coax Adapters
A typical connector — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Impedance
Ω50 Ω for radio, 75 Ω for TV/video (F, some BNC). Mixing them 'works' at low frequency but mismatches at RF.
Frequency rating
GHzSMA to 18 GHz, N to 11 GHz, BNC to ~4 GHz, U.FL to 6 GHz — beyond it, loss and reflections grow.
Gender & polarity
—RP-SMA (WiFi) swaps the pin between genders vs standard SMA — the single most common wrong-part order in wireless.
Mating cycles
cyclesU.FL is rated ~30 matings — it's a build-time connector, not a plug-unplug one. SMA ~500.
Insertion loss
dBEach junction costs ~0.1–0.3 dB; adapter chains add up fast at GHz.
Markings
Rarely marked — identify by geometry: SMA male has the pin and inside thread on the barrel... except RP-SMA reverses the pin. N is the big weatherproof one; U.FL is the tiny snap-on inside laptops and modules.
Standard values
Common pigtails: U.FL to RP-SMA female bulkhead (WiFi modules), SMA male–male RG316 jumpers, BNC-to-banana for scopes/meters. Adapters exist between every pair — keep an adapter kit.
How to choose
1) Match what's on the module and antenna — check RP vs standard SMA by photo, not name. 2) N-type for outdoor and >1 W power, with self-amalgamating tape sealing. 3) U.FL for inside enclosures only; press straight down to mate, lever gently to unmate. 4) Keep coax runs short and adapters few — every joint eats signal.
Pinout & package
Coaxial: centre pin (signal) + shell (ground/shield). Cable-mount (crimp/solder), bulkhead (panel jack), and PCB edge/SMD footprints. Pigtails bridge board-level U.FL to panel connectors.
Example circuits
- ESP32 module U.FL pad → pigtail → RP-SMA bulkhead → external antenna
- NanoVNA (SMA) with cal kit measuring an antenna's VSWR
- Oscilloscope BNC input with 10:1 probe or BNC-to-BNC from a signal generator
- Roof antenna → N-type weatherproofed joint → LMR400 coax → radio
Common failures
U.FL centre socket deformed by careless mating (intermittent RF), cross-threaded SMA (permanent damage — start threads by hand), water-corroded outdoor connectors, broken crimp at the cable strain point, and RP/standard mismatch making no centre contact.
How to test
Continuity: centre-to-centre and shield-to-shield through the cable, and NO continuity centre-to-shield (unless the antenna is a DC-shorted design). Wiggle test at each end while monitoring RSSI. A VNA shows a bad connector as a return-loss spike.
Substitutes
Adapters convert between any series (with a small dB cost). For receive-only TV, 75/50 Ω mixing is tolerable; for transmitters, match properly. Replace U.FL pigtails as a unit — the connector isn't reusable many times.