RF connectors

What it is

An RF connector joins a cable to a radio, antenna or another cable. A good one is part of the transmission line: it keeps the impedance steady, holds the shield continuous and survives the weather. A bad choice or a bad installation is a common cause of loss, odd SWR and intermittent faults.

The common families

Connector	Coupling	Typical use	Notes
UHF (PL-259 plug, SO-239 socket)	threaded	HF and VHF radios, amplifiers, mobile	cheap and robust, but the impedance is not held constant, so it is a poor choice at UHF and up; not weatherproof on its own
N	threaded	outdoor runs, repeater sites, UHF and microwave	holds a constant impedance; sealed versions resist the weather; larger and heavier
BNC	bayonet, quarter turn	test gear, handhelds, short jumpers	quick to connect; light duty
TNC	threaded	mobile and vibration-prone installs, low microwave	like a BNC but locked by threads
SMA	small threaded hex nut	handhelds, modules, microwave	small and light; delicate and low power; wears with repeated use

Other types you will meet: F connectors on 75 Ω TV cable, tiny MCX, MMCX and U.FL types inside equipment and on modules, and large DIN-type connectors on commercial and repeater cabling for high power.

What matters in practice

• Frequency. The cheaper threaded UHF connector is fine on HF and 2 m. Move to N (or TNC or SMA, if sized for the job) for 70 cm and above.
• Weather. None of these stays dry by itself. Wrap outdoor joints with self-amalgamating tape, then electrical tape over it, shaped so water sheds, and form a drip loop in the cable. A wet connector lets water wick into the cable.
• Installation. Most failures are in the wiring, not the connector: a cold-soldered shield, a centre pin that is not fully seated, or a crimp that is not tight. Use the right size plug for the cable.
• Adapters. Each one adds loss, a possible mismatch and one more joint to fail. Fewer is better.
• Gender and polarity. Check before ordering. Some small connectors come in "reverse-polarity" forms (RP-SMA is common on Wi-Fi gear) that look the same but will not mate.
• Impedance. Don't mix 50 Ω and 75 Ω versions of the same family: they can mate imperfectly and wear contacts.

Related

• Coaxial cableConstruction, types and choosing a coax.
• Feed line loss and velocity factorAttenuation by frequency and electrical length.
• Characteristic impedanceWhat Z0 means and why it must match.
• Common-mode chokesKeeping RF off the outside of the coax.
• SWR and reflectionsStanding waves, reflection coefficient and return loss.
• Baluns and ununsBalanced-unbalanced and impedance transformers.