SMA, BNC, PL-259 and N connectors: what goes where, and how not to mix them up

You buy a great antenna, it arrives — and it won't screw on. Or it screws on, but there's no signal. Nine times out of ten the culprit isn't the product but the connector: you mixed up the socket with the plug, or grabbed the wrong series. An antenna connector is a small thing worth a dollar or two, yet your entire signal passes through it. Let's go through the four main types you'll meet on radios, antennas and hotspots, and learn how to stop getting them wrong.

Plug and socket: the main source of confusion

Every connector is made of two halves. One has a central contact that is a pin (this is the male, the plug); the other has a socket to receive it (this is the female, the jack). For the joint to work, the pin has to go into the socket: male mates with female, and no other way.

Remember the one key rule and half your problems disappear: don't look at "male/female" in the abstract — look at the central contact. There's a pin sticking out in the middle — that's male. There's a little hole-socket — that's female. The thread, meanwhile, can be on the outside or the inside: it's misleading, so always go by the centre.

SMA — the most common one on handhelds

SMA (SubMiniature version A) is a small threaded connector found on almost every modern handheld radio. The thread is fine, the diameter is about a centimetre, and it holds securely. SMA is the source of the classic beginner's confusion, because different manufacturers arrange male and female differently.

Baofeng and similar "budget" radios

On many cheap Chinese radios (a large part of the Baofeng line — the UV-5R and relatives) the body has an SMA-male on top — the pin points upward. That means the antenna needs the matching part — an SMA-female (socket on the bottom). If you see an accessory listed as "SMA-F for Baofeng", that's exactly an antenna with a socket.

Kenwood, TYT, many AnyTone-compatible radios

On "Kenwood-type" radios the connectors are arranged the other way round: the radio body carries an SMA-female (socket), and the antenna has an SMA-male (pin). This covers some TYT and Wouxun models and units compatible with the Kenwood layout. It's not a hard "by brand" law — there are exceptions within a single make, so before buying an antenna look at your own radio: is there a pin sticking out, or do you see a socket?

BNC — a bayonet with a quarter-turn

BNC is the connector that doesn't screw on but snaps into place with a quarter turn. The socket has two protruding "ears" (bayonet lugs), and the mating part has a slot with a spring: push it on, turn it 90°, and it locks. Fast, convenient, no thread to turn.

BNC used to be fitted en masse to old handhelds and scanners; on new radios it has now been largely displaced by SMA, but it lives on in older base stations, test gear and scanners. An important nuance: BNC comes in 50- and 75-ohm versions. For radio you need the 50-ohm one — it's rated to work up to roughly 4 GHz. The 75-ohm version comes from video/TV equipment, and you shouldn't mix them up: the different characteristic impedance causes a mismatch.

PL-259 / SO-239 — the "UHF" series that, ironically, isn't great at UHF

This is the classic pair of "fat" connectors on base stations and cables: the PL-259 is the plug that screws onto its mating socket, the SO-239. Their historical name is the "UHF connector", and that's misleading: the series appeared long before "UHF" came to mean today's frequencies.

• Pros: cheap, ubiquitous, easy to solder, a thick central contact, and it handles high power.
• Cons: the design doesn't maintain a constant 50-ohm impedance at the connector, so formally it's not the best choice for the upper part of UHF.

In practice, though, on our bands (70 cm, ~430–440 MHz) the difference isn't as scary as forums make out. Measurements on an analyser show that at ~431 MHz the losses of a good PL-259/SO-239 pair and an N-type pair differ by literally hundredths of a decibel — within measurement error. So on 70 cm a good "UHF" connector with PTFE insulation works perfectly well; the problems start noticeably higher in frequency.

N-type — when you need reliability and the outdoors

N-type (Type N) is a medium-sized threaded connector that holds a constant 50 ohms and works comfortably at high frequencies. According to reference data its working range stretches to roughly 11 GHz, the coupling thread is 5/8″–24, and when mated correctly the joint is weatherproof (some versions reach IP67). That makes the N-type the de facto standard for outdoor antennas, masts and repeaters.

If a connector is going to live out in the rain, carry significant power, or work high in frequency — go for N-type. For a handheld in your pocket it's overkill (big and heavy), but for a fixed antenna on the roof it's exactly what you want.

Adapters: every joint is a hit to reliability

The temptation of "I'll buy an SMA→PL-259 adapter and connect anything" is understandable, but there are two sides to the coin. Every extra connector in the path means:

1. extra signal loss (a little, but it adds up);
2. a new point of failure — a spot where the contact can oxidise, the thread can work loose, and moisture can get in;
3. a mechanical lever — a long "garland" of adapters rocks the radio's socket and wears it out over time.

One neat adapter in a dry room is acceptable. A garland of three adapters on an antenna out in the rain is a road to unstable comms. If you need an adapter permanently, it's better to change the connector on the cable to the right one than to build a chain.

Cheat sheet: "what goes with what"

• Handheld → antenna: usually SMA; check male/female against your own radio.
• Hotspot (MMDVM): the board usually has SMA; a short "rubber duck" of the same type is ideal — no adapters.
• Old station / scanner: may be BNC (50 ohm!).
• Base station, cable in the house: PL-259/SO-239 — fine for 70 cm.
• Outdoor antenna, mast, power: N-type.