RF connectors
| RF connectors | |
|---|---|
| Purpose | Joining coax to radios, antennas, test gear and other cables |
| Most common in amateur radio | UHF (PL-259 plug and SO-239 socket) |
| Other common types | BNC, N, SMA |
| Gender | Male has a center pin; female has a socket |
| Practical upper frequency (approx.) | |
| UHF | A few hundred MHz |
| BNC | A few GHz |
| N | About 11 GHz |
| SMA | About 18 GHz |
RF connectors join lengths of coaxial cable to radios, antennas, meters and each other. A good connector keeps the signal inside the cable, keeps water out and adds no noticeable loss. A bad one, or a good one badly installed, is one of the first things to suspect when a station misbehaves. Several families are in common use, and most stations end up with at least three of them, along with a drawer of adapters.
Common types
UHF (PL-259 and SO-239). The PL-259 plug and SO-239 socket are the standard on HF transceivers, amplifiers and tuners and on most VHF mobile radios. The pair is rugged, handles high power and is easy to install with a soldering iron, but it isn't a constant-impedance design, so despite the name it is poor above a few hundred megahertz. It is fine for HF and the 2-meter band, and usable but not ideal on 70 centimeters. A standard PL-259 fits thick cable such as RG-213 or LMR-400; thinner cable needs a reducer, the UG-175 for RG-58 or the UG-176 for RG-8X.
BNC. The BNC has a bayonet coupling that locks with a quarter turn. It is quick, secure and constant impedance, good into the low gigahertz range, and is found on test equipment, scanners, older handhelds and many low-power radios. The name stands for bayonet Neill–Concelman, after its two designers.
N. The N connector, named after Paul Neill of Bell Labs, is threaded, constant impedance and handles high power. With its gasket it resists weather well, which makes it the preferred connector for VHF and UHF base antennas, repeaters and microwave work. It is larger and fussier to install than a PL-259.
SMA. The SMA is a small threaded connector rated well into the microwave range. It is used on modern handhelds, on many software-defined radio receivers and on low-cost network analyzers. It is precise but delicate: tighten it with the fingers, or with a torque wrench made for it, never with pliers, and don't use it to support a heavy antenna.
SMA genders and the handheld mix-up
Gender refers to the center contact, not to the threads: a male connector has a pin and a female connector has a socket. Judging gender by which part has the threads on the outside is a frequent source of mistakes.
Handhelds are where this gets confusing. Radios from the major Japanese manufacturers have traditionally used a female SMA on the radio, so their antennas are male. Many inexpensive imported handhelds use the reverse, with the pin on the radio and the socket on the antenna. An antenna made for one type won't connect properly to the other, so antennas and adapters are sold in both versions, usually labeled with the radio brands they fit. Wi-Fi equipment adds a third naming scheme, reverse-polarity SMA (RP-SMA), and handheld antenna listings sometimes borrow the term loosely. Look at the connector itself, pin or socket, before buying. See also Handheld antennas.
Adapters
Adapters between any two types are cheap and handy for testing and temporary setups. Each one adds a small loss, another joint that can work loose and, outdoors, another place for water to get in, so avoid building permanent chains of them. On a handheld, a BNC adapter makes it quick to swap antennas, but a long antenna on top of an adapter puts a lot of leverage on the radio's small SMA jack. A short flexible jumper cable avoids the strain.
Installing and weatherproofing
A connector is only as good as its installation. Follow the instructions for the specific connector and cable, keep stray strands of braid away from the center pin, solder or crimp with the proper tool, and check with an ohmmeter for continuity and shorts before the connector goes into service.
Outdoor connectors need sealing, because water that gets in wicks along the coax and ruins it well beyond the connector. A common method:
- Tighten the connector, then wrap it in a layer of vinyl electrical tape, which makes the later layers easier to remove.
- Cover that with self-fusing rubber tape or butyl mastic tape, stretched or pressed into place so it seals.
- Finish with a layer of UV-resistant vinyl tape to protect the rubber from sunlight.
Overlap each layer like shingles so water runs off. Where possible, point connectors downward and leave a drip loop in the cable below them. Avoid acetic-cure silicone sealant, the kind that smells of vinegar, because it can corrode the metal.