Field Guide · hardware

Also known as: SMA, SubMiniature version A, RP-SMA

SMA (SubMiniature version A) is a small threaded coaxial connector with a nominal 50 Ω impedance, usable from DC to about 18 GHz.1 It is the de-facto RF port on software-defined-radio receivers, low-noise amplifiers, filters, and compact antennas, so almost every SDR accessory mates to it. The male plug carries a captive pin and an internal thread; the female jack has a centre socket and an external thread that the plug screws onto.

coaxplug (thread)jackcoax
An SMA plug screws onto a jack; the threaded coupling holds a repeatable 50 Ω interface up to 18 GHz.

Overview

SMA was developed in the 1960s for miniature semi-rigid cable and has since become one of the most common RF interfaces in test equipment and hobby radio. The thread is a 1/4-36 UNS form, and the coupling nut is torqued (about 0.5–1 N·m on a precision joint) to guarantee a consistent, low-reflection contact. Because the connector is physically small, it suits the crowded edge of an SDR board where a bulkier BNC or N-type would not fit.

What it is

Mechanically, SMA trades quick handling for bandwidth and repeatability. Screwing the nut takes longer than the bayonet twist of a BNC, but the threaded interface stays put under vibration and holds its match at microwave frequencies where a BNC has long since become lossy and reflective. Standard SMA uses a PTFE dielectric and is specified to roughly 500 mating cycles — far fewer than a BNC — so it is meant for connections you make and then leave alone, not for constant swapping.

Variants

  • RP-SMA (reverse polarity SMA) swaps the genders of the centre contacts: the plug that would carry a pin instead carries a socket, and vice versa. The shell and thread are unchanged, so an RP-SMA plug will thread onto a standard SMA jack but the centre conductors never touch and no signal passes. RP-SMA was mandated on much consumer Wi-Fi gear to discourage swapping to higher-gain antennas; it is electrically identical to SMA, just keyed differently. Mixing RP-SMA and standard SMA is the most common cause of a cable that mates mechanically yet passes no RF — always check the pin/socket, not just the thread.
  • 3.5 mm and 2.92 mm (K) connectors are precision air-dielectric relatives that mate with SMA but extend usable range to 26–40 GHz.
  • 75 Ω SMA exists but is rare and not interchangeable with the 50 Ω type.

Relevance to SDR

The SMA jack is the antenna port on nearly every SDR that matters to a trunking listener: RTL-SDR V3/V4 dongles, Airspy, HackRF, and the bias-tee-fed LNAs and filters between antenna and radio. Adapters to BNC, N-type, and older ports are cheap, but each adapter adds a small reflection and insertion loss, so a clean run uses the fewest transitions possible. GopherTrunk is decode software and touches no connectors directly, but the quality and correctness of the SMA joints in front of the receiver set the signal-to-noise ratio it has to work with — a loose nut or an RP-SMA mismatch shows up as missing or weak captures long before any DSP tuning matters.

Sources

  1. SMA connector — Wikipedia, on SMA construction, 50 Ω impedance, 18 GHz rating, thread form, and the RP-SMA variant. ↩

See also