Published on 10/13/2026
A box of unmarked cables arrives from a client's old lab. Some have threaded ends, some have bayonet pins, some have tiny snap fittings. Without a datasheet, you need a way to tell an SMA from a TNC or a BNC from an SMB, and whether a connector is male or female, standard or reverse polarity. This chart works from what you can measure: thread, coupling and pin.
Short answer: count the coupling style first (thread, bayonet or snap), then measure the thread diameter, then check whether the center is a pin or a socket. SMA, 3.5 mm and 2.92 mm all use a 1/4-36 thread and are the same size, so they look identical without a gauge or a datasheet.
The tables below give dimensions and identification rules. This guide uses text and measurements rather than photographs, so you can check each connector with calipers.
On This Page
| Connector | Coupling | Thread or size | Impedance | Notes |
|---|---|---|---|---|
| SMA | Threaded | 1/4-36 UNS (6.35 mm) | 50 ohm | Hex nut 5/16 in |
| 3.5 mm / 2.92 mm | Threaded | 1/4-36 UNS (6.35 mm) | 50 ohm | Looks like SMA, air dielectric |
| N | Threaded | 5/8-24 UNEF (15.9 mm) | 50 or 75 ohm | Large, hex nut 3/4 in |
| TNC | Threaded | 7/16-28 UNEF (11.1 mm) | 50 or 75 ohm | Threaded cousin of BNC |
| BNC | Bayonet | Two lugs, about 10 mm | 50 or 75 ohm | Quarter-turn twist |
| UHF (PL-259 / SO-239) | Threaded | 5/8-24 UNEF | Not constant | Common on HF and CB |
| F type | Threaded | 3/8-32 | 75 ohm | Satellite and TV |
| 7/16 DIN | Threaded | M29 x 1.5 (29 mm nut) | 50 ohm | Large base station connector |
| SMB | Snap-on | Snap-on body, about 5 mm | 50 or 75 ohm | Push-on |
| MCX | Snap-on | Smaller than SMB | 50 or 75 ohm | Push-on, 360 degree rotation |
| MMCX | Snap-on | Smaller than MCX | 50 ohm | Push-on, rotates |
| U.FL / MHF | Snap-on | About 2 to 3 mm square | 50 ohm | Tiny board connector |
Dimensions are nominal. Use calipers and a thread gauge for confirmation.
Rule 01
Look at the center contact
A male connector has a center pin. A female connector has a center socket (a small hole or slotted tube).
Rule 02
Check the outer shell
On threaded connectors, the male has the coupling nut with internal threads, and the female has external threads. For RP (reverse polarity) connectors, the center contact is swapped but the shell is not.
Rule 03
Check for RP
An RP-SMA plug has a coupling nut with internal threads like a standard plug, but a center socket instead of a pin. An RP-SMA jack has external threads but a center pin. This is common on Wi-Fi gear.
Rule 04
Check for 50 or 75 ohm
75 ohm center pins are thinner. Compare with a known 50 ohm part or check the dielectric size and markings.
Key idea: SMA and RP-SMA connectors fit together visually but will not mate electrically. If a threaded connector will not seat or the center contact does not engage, check for reverse polarity before forcing it.
Send us a photo or sample
Our engineers can identify the connector and build the assembly you need, then sweep it on a VNA for VSWR and insertion loss before shipment.
Talk to an EngineerStep 01
Coupling
Threaded, bayonet or snap. Bayonet means BNC. Snap means SMB, MCX, MMCX, U.FL or similar.
Step 02
Measure thread outer diameter
6.35 mm points to SMA family, 11.1 mm to TNC, 15.9 mm to N or UHF.
Step 03
Look at the dielectric
White PTFE means SMA or TNC. Air in the gap and an extra-thin pin points to 3.5 mm or 2.92 mm.
Step 04
Check gender and polarity
Pin or socket, then standard or RP.
Step 05
Check impedance
Compare with a known 50 ohm part or look for a 75 ohm marking.
Avoid
How do I identify an RF connector?
Check the coupling style (thread, bayonet or snap), measure the thread diameter, then check for a pin or socket.
How can I tell SMA from RP-SMA?
RP-SMA swaps the center contact. An RP-SMA plug has a nut with internal threads but a center socket, and an RP-SMA jack has external threads but a center pin.
What is the difference between SMA and 3.5 mm?
They look the same and mate mechanically. 3.5 mm has air dielectric and is rated higher in frequency.
How do I tell BNC from TNC?
BNC uses a bayonet twist. TNC has a 7/16-28 thread.
How do I tell 50 ohm from 75 ohm BNC?
75 ohm has a thinner center pin and a smaller dielectric.
Which connector has a 5/8-24 thread?
N type and UHF both use it, but they are not compatible.
Identify connectors by coupling, thread size and center contact. Measure rather than guess, and watch for RP versions and 75 ohm parts.
SigmaRF builds assemblies with all of these connectors in India (Vadodara) and the USA (Cleveland area), VNA-swept before shipment, with lead times as short as 2 days.
Need a cable with the right connector?
Send us the part or a measurement.
Standard AssembliesCustom AssembliesTalk to an Engineer