RF Connectors for Ham Radio: PL-259, N, BNC and SMA Compared

Every signal in a ham station passes through RF connectors, usually several of them, between the radio and the antenna. Most of the time nobody thinks about them. Then an intermittent SWR reading, a crackle on receive or a sudden drop in signal turns out to be a corroded plug at the top of the mast or a cheap adapter behind the radio. Choosing the right connector for each job, and fitting it properly, is one of the least glamorous and most effective things you can do for your station.

This guide compares the connector families radio amateurs meet most often: the PL-259 and SO-239 pair, the type N, the BNC and the SMA. It explains where each one belongs, how frequency, power and weather change the choice, how to use adapters without creating new problems, and how to spot and fix connector faults.

What Makes a Good RF Connector

An RF connector has to do more than make electrical contact. It has to carry the signal from one piece of coax to another without disturbing it. Four properties matter:

• Impedance: most amateur coax is 50 ohms, and the ideal connector keeps that impedance constant through the joint. A connector that is not designed for constant impedance creates a small mismatch, negligible on HF and increasingly significant as frequency rises.
• Loss: every connection adds a little loss. A few good connectors on HF make no measurable difference; many poor connectors on UHF can.
• Power handling: the connector must handle the voltage and current at your power level without heating or arcing, especially into a mismatched load.
• Mechanical robustness and weather resistance: outdoor connectors must survive rain, temperature cycles and the weight of the cable.

Different connector families make different trade-offs between these properties, which is why a typical station uses more than one kind.

PL-259 and SO-239: The HF Standard

The PL-259 plug and its matching SO-239 socket form what is known as the UHF connector family. The design dates from the 1930s, and the name is a historical accident: it was named when “UHF” meant frequencies above 30 MHz. Today it is the most common connector on full-size HF radios, amplifiers, tuners and SWR meters.

The UHF connector is not a constant impedance design, and there is no single active standard governing its dimensions, so quality varies widely between manufacturers. On HF this hardly matters; a good PL-259 performs perfectly well there. On VHF it is usually acceptable for short runs. On 70 centimetres and above, the impedance bump and variable quality start to matter, and better connectors are preferred.

The PL-259 has two big advantages: it is robust and it can handle high power. Its weak points are fitting and quality. A poorly soldered PL-259, or a cheap one with a loose centre pin or thin plating, causes many of the intermittent faults hams chase. Our step-by-step guide on how to solder a PL-259 connector covers the right technique.

Reducers and cable size

The standard PL-259 is sized for thick coax such as RG-8 and RG-213. For thinner cable such as RG-58 or RG-8X, a reducer adapts the plug to the smaller diameter. Using the right reducer for the cable, rather than packing the gap with tape, is essential for a mechanically sound joint.

Type N: The Weatherproof All-Rounder

The N connector was invented in the 1940s by Paul Neill of Bell Labs and was one of the first connectors capable of carrying microwave frequencies. It is a threaded, medium-size, constant impedance design, and modern versions handle frequencies far above anything most amateurs use. Unlike the UHF connector, the N connector has a proper interface specification, so good examples from different makers mate consistently.

Several features make the N connector a favourite for outdoor and VHF/UHF work:

• Constant impedance through the joint, so it adds very little mismatch on VHF, UHF and microwave bands.
• A gasket on many versions that helps keep moisture out, which is why it is common on antennas, masthead preamplifiers and outdoor equipment.
• Good power handling for amateur use at VHF and UHF.

The N connector is harder to fit than a PL-259, because the centre pin must sit at the correct depth. Crimp versions with the correct crimp tool make this much easier and more repeatable. The coupling nut is normally hand-tightened; manufacturers publish torque recommendations, but in practice firm hand tightening on clean threads is what most amateurs use.

BNC: Quick Connect for Small Equipment

The BNC connector is a miniature quick-connect design with a twist-to-lock bayonet coupling. It was introduced on military radio equipment in the 1940s and is now everywhere: test equipment, older handheld radios, receivers, QRP rigs and video systems. It maintains the cable’s characteristic impedance and is made in 50 ohm and 75 ohm versions.

The bayonet lock makes BNC very fast to connect and disconnect, which is ideal for test leads, dummy loads and portable setups that are assembled and taken apart often. It is not designed for high power, and it is not weatherproof without additional protection, so it is rarely the right choice for a permanent outdoor connection or an amplifier output.

50 and 75 ohm BNC

The 50 ohm and 75 ohm BNC versions look almost identical, but the centre pin and insulation differ. Mixing them can damage the female socket or produce a poor contact. Video and broadcast equipment often uses 75 ohm BNC, so be careful when buying used cables and adapters.

SMA: Small, Precise and Delicate

SMA connectors are small threaded semi-precision connectors with a 50 ohm impedance, developed in the 1960s for microwave work. In amateur radio they are most familiar on handheld radios and their antennas, USB SDR dongles, antenna analysers such as the NanoVNA and many small modules.

SMA connectors perform very well electrically, but they are small and mechanically delicate. They are designed for a limited number of mating cycles, and overtightening or bending a cable at the connector can damage them. A few precautions help:

• Do not hang weight on them. Use a short flexible jumper rather than connecting heavy coax or a large antenna directly to an SMA socket.
• Tighten by the nut only. Turning the body of the connector instead of the nut wears the mating surfaces.
• Check the gender. Handheld radios and antennas use both SMA male and SMA female on the radio side, depending on the manufacturer. Check before you buy an antenna or adapter.

RF Connectors Compared

The table summarises where each connector type fits in a typical amateur station.

Other connector families appear occasionally: TNC, a threaded relative of BNC; F connectors on 75 ohm satellite and TV systems; and large high-power connectors in commercial installations. For most amateur stations, the four types above cover everything.

Adapters: Useful but Not Free

Adapters let you connect different connector types, and every station has a drawer full of them. They are useful, but each adapter adds two more connections, a little loss, and another potential point of failure. Some practical rules:

• Minimise adapters in the main feed line. If you need an adapter at the radio, use one good adapter rather than a chain of three.
• Never put a stack of adapters outdoors. Each joint is a place for water to enter. Fit the correct connector to the cable instead.
• Buy quality adapters for anything that carries power. Cheap adapters with poor plating or loose centre pins are a common cause of intermittent faults.
• Do not hang weight on small connectors. A heavy PL-259 adapter on an SMA socket can break the radio’s connector.

Weatherproofing Outdoor Connectors

Water is the enemy of every outdoor RF connector. Once moisture gets into a connector or coax, it causes corrosion, rising loss and erratic SWR, and it can travel along the cable. Good weatherproofing practice:

1. Choose connectors suited to outdoor use, such as type N with a gasket, where possible.
2. Make sure the connector is clean and dry before sealing it.
3. Wrap the joint with a layer of quality electrical tape, then self-amalgamating tape, then another layer of electrical tape for UV protection. The first layer makes the seal easier to remove later.
4. Form a drip loop so water runs off the cable before it reaches the connector.
5. Point connectors downwards where possible, so water does not collect in the joint.

Connectors at the bottom of a feed line also matter for safety. The entry point to the house is where lightning protection and grounding belong, as explained in our guide to station grounding and lightning protection.

Tools for Fitting Connectors

Most connector faults start at the workbench, so the right tools make a real difference. A good coax stripper set to the correct dimensions for your cable and connector gives clean, repeatable cuts and avoids nicking the centre conductor or braid. For solder-type PL-259s, a soldering iron with enough heat capacity to heat the connector body quickly prevents melted dielectric. For crimp connectors, use the crimp tool and die set specified for that connector and cable; a generic crimper rarely gives a reliable joint.

After fitting a connector, check it before it goes into service. Measure continuity between the centre pins at both ends, confirm there is no short between centre and shield, and inspect the joint for stray braid strands. Then sweep the finished cable into a dummy load with an analyser. Our NanoVNA guide explains how to calibrate and read such a sweep. A few minutes of testing indoors saves hours of fault-finding at the top of a mast.

Finding Connector Faults

Connector problems often show up as symptoms that look like antenna or radio faults. Signs to watch for:

• SWR that changes when you move or touch the cable. A loose connection or broken shield near a connector is likely.
• Intermittent receive noise or crackling, especially in wind or after rain.
• A gradual rise in SWR or loss over months, typical of water ingress.
• A connector that feels warm after transmitting, a sign of a poor contact or a connector being used beyond its rating.

An antenna analyser helps isolate the problem. Measure at the radio end, then at intermediate points if you can, and compare with a known good dummy load. Choosing low-loss cable in the first place also matters; our coaxial cable comparison explains how cable type and length affect loss on each band.

The Bottom Line

RF connectors are small parts with a big effect on station reliability. The PL-259 and SO-239 remain the right choice for most HF equipment, provided they are good quality and properly fitted. The type N is the best general choice for VHF and UHF feed lines and outdoor connections, thanks to its constant impedance and weather resistance. BNC is ideal where you connect and disconnect often, and SMA is excellent but delicate on handhelds, SDRs and analysers. Use as few adapters as possible, weatherproof every outdoor joint properly, and treat any SWR that changes when you touch the cable as a connector fault until proven otherwise.

Frequently Asked Questions

Why is the PL-259 called a UHF connector?

The design was named in the 1930s, when “UHF” meant frequencies above 30 MHz. The name stuck even though the frequency bands were later renamed, and today the connector is mainly used on HF.

Can I use PL-259 connectors on 2 meters and 70 centimetres?

On 2 meters, good PL-259 connectors are usually acceptable for short runs. On 70 centimetres and higher, type N connectors are generally preferred because they keep a constant impedance and their quality is more consistent.

Which RF connectors are best outdoors?

Type N connectors with a gasket are a common choice outdoors. Whatever connector you use, seal it properly with tape, form a drip loop and avoid stacking adapters at the antenna.

Is BNC suitable for high power?

BNC is designed as a small quick-connect connector and is not intended for high power. It is best for test equipment, receivers, QRP rigs and portable setups.

Why does my handheld antenna not fit my radio?

Handhelds use both SMA male and SMA female on the radio, depending on the manufacturer. Check the gender of your radio’s connector before buying an antenna or adapter.

Do adapters reduce performance?

Each adapter adds two connections, a little loss and another possible point of failure. One good adapter is usually fine, but avoid chains of adapters in the main feed line and outdoors.