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RF Connectors

RF connectors are connectors specifically designed to carry high-frequency signals, typically in the 100MHz-30GHz range with minimal reflection and attenuation.

BNC Connectors

BNC (Bayonet Neill-Concelman) connectors are quick connect connectors designed for RF signals. It is named after it’s Bayonet locking mechanism and it’s inventors, Paul Neill and Carl Concelman. It is “standardized” in MIL-STD-348A1.

Four BNC connectors on the front of a Keysight InfiniiVision DSOX2024A oscilloscope to connect the probes to.

Specifications

  • Passband: 0-4GHz
  • Impedance: 50 Ω50\,\Omega (standard) or 75 Ω75\,\Omega (video).

Uses

They were originally designed for military use, but are now commonly used for a variety of RF applications including:

  • Oscilloscope probes: Used for lower frequency oscilloscope probes (0-200MHz). The top-end modern oscilloscopes tend to have proprietary connections to the probes, with added pins for extra functionality.
  • Analog video signals
  • Radio antennas
Photo of a BNC adapter, showing the male BNC connector.

The USSR version of the BNC connector is called the SR connector. However, the connector dimensions are slightly different due to the conversion from imperial to metric. The SR connectors can be mated with BNC connectors, although sometimes it requires a decent amount of force.

U.FL

Also known as UMC or XFL connectors.

A photo of a male, SMD mount UFL connector. Image from www.digikey.com.

Schematic Symbol

There is no “standard” schematic symbol for the male, SMD mount UFL connector, so I normally decide to go with some that is visually resembles the actual connector part, as shown below:

A schematic symbol for the male, SMD, UFL (a.k.a. UMC, UMCC) style RF connector.

The height of the male SMD connector can vary, but something around 1.2mm is common.

SMA Connectors

SMA (SubMiniature version A) connectors are small (although probably medium-sized by today’s standards) 50 Ω50\,\Omega RF coaxial connectors with a screw-style connection mechanism. The gender name refers to the innermost electrical component (e.g. a male SMA connector has the pin in the middle, whilst the female has the socket to accept the pin). They typically have a passband between 0−16 GHz0-16\,GHz, although some specialized versions extends in the 30+ GHz30{+}\,GHz territory2.

One male and one female inline (connector to cable) SMA connector next to each other.

The original SMA connectors were developed in the 1960s3.

  • Standard Male SMA connector: Central 0.9 mm diameter pin surrounded by a barrel with inside threads.
  • Standard Female SMA connector: Central sleeve (made from the di-electric) surrounded by a barrel with outside threads.
A coaxial cable with SMA connectors at both ends.

Reverse-polarity and Reverse-thread SMA Connectors

Be wary, there are two additional variations (aside from the normal SMA connector) of SMA connectors which are hard to recognise on first glance.

Reverse-polarity (RP) SMA connectors have the pin and sleeve swapped between the male and female parts. Also known as RP-SMA or R-SMA. Reverse-polarity was designed to prevent the unauthorized connection of a antenna with a larger gain to WiFi equipment3, to distinguish them from cellular equipment involving LTE/GSM/UMTS which was utilizing the standard SMA connector. These days, RP-SMA connectors are now commonplace, somewhat defeating the protection through obscurity. However in general, standard SMA connectors are still used for cellular applications whilst RP-SMA for WiFi/WLAN.

There are also reverse-thread (RT) SMA connectors which have the threads in the opposite direction to the standard SMA connectors. One such example is the Pasternack “RT SMA Male Connector Crimp/Solder Attachment for RG55, RG141, RG142, RG223, RG400”4.

N Connectors

N connectors (a.k.a. N-type or Type N connectors) are a larger type of threaded RF connector. It was one of the first connectors capable of carrying microwave frequency signals (originally at about 1 GHz). It was invented in the 1940s by Paul Neill of Bell Labs and the connected is named after him.5

Close-up of a N Connector.

MMCX Connectors

Micro-miniature coaxial (MMCX) connectors are a RF connector type similar to MCX (micro coaxial) but smaller. They allow signals from DC to about 6 GHz. It was developed by Huber+Suhner in the 1990s (the creators of the original MCX connector).

PropertyValueComment
Frequency RangeDC-6 GHz
Impedance50Ω
Size2.5x3.6mm (approx.) Smaller than SMA, larger than U.FL.
Fastening TypeLock snap
A photo of a through-hole female MMCX connector.6
A photo of a cable with MMCX male connectors on both ends. One is straight and the other is a right-angle.7

They fasten with a lock snap mechanism (no threads) which allows them to be easily connected and disconnected, and for them to rotate 360 degrees. However this makes them less suitable than threaded connectors for rugged environments in where there might be significant vibration or force which would cause them to detach.

MMCX connectors are smaller than SMA connectors, but larger than U.FL.

There are plug (male) and jack (female) genders of this connector. As usual, the gender is determined by the shape of the conductive part of the antenna. In a plug it has a protruding pin, and in a jack it has a receiving socket.

Like the MCX connector, the MMCX conforms to the European CECC 22000 specification.8

Uses

MMCX connectors are commonly used on cellular modules for connecting an external antenna. They are not normally used in smartphones as they are too large, instead the smaller U.FL is normally used.

Different Connector Diameters

Be careful, there are different diameters for the MMCX connector housing (the parts that mate are still standard). I have noticed this with the inline MMCX connectors. Most I had on hand had a max. diameter of about 4.4-4.6 mm, but there was a weird one which had a smaller max. diameter of about 3.5 mm.

A photo showing the varying housing diameters of MMCX connectors. The style on the left seemed far more common.

Footnotes

  1. Department of Defence. Interface Standard: Radio Frequency Connector Interfaces for MIL-C-3643, MIL-C-3650, MIL-C-3655, MIL-C-25516, MIL-C-26637, MIL-C-39012, MIL-C-49142, MIL-A-55339, MIL-C-83517. Retrieved 2021-12-10, from http://everyspec.com/MIL-STD/MIL-STD-0300-0499/MIL-STD-348A_420/. ↩

  2. Amphenol RF. SMA Connectors. Retrieved 2021-11-22, from https://www.amphenolrf.com/connectors/sma-connectors.html. ↩

  3. Reichelt Elektronik. SMA and RP-SMA - What You Need to Know About Coaxial Connectors. Retrieved 2021-11-22, from https://www.reichelt.com/magazin/en/sma-and-rp-sma-what-you-need-to-know-about-coaxial-connectors/. ↩ ↩2

  4. Pasternack. RT SMA Male Connector Crimp/Solder Attachment for RG55, RG141, RG142, RG223, RG400 [product page]. Retrieved 2024-10-27, from https://www.pasternack.com/sma-male-reverse-thread-rg55-rg141-rg142-rg223-rg400-connector-pe44182-p.aspx. ↩

  5. Wikipedia (2025, Feb 6). N-connector. Retrieved 2025-04-03, from https://en.wikipedia.org/wiki/N_connector. ↩

  6. Digikey. Connectors, Interconnects > Coaxial Connectors (RF) > Coaxial Connector (RF) Assemblies > Molex 0734151471 [product page]. Retrieved 2025-06-10, from https://www.digikey.co.nz/en/products/detail/molex/0734151471/1465179. ↩

  7. Pasternack. MMCX Plug to MMCX Plug Right Angle Cable Using RG174 Coax, RoHS [product page]. Retrieved 2025-06-10, from https://www.pasternack.com/mmcx-plug-mmcx-plug-rg174au-cable-assembly-pe34881lf-p.aspx. ↩

  8. Amphenol RF. MMCX Connectors. Retrieved 2025-06-10, from https://www.amphenolrf.com/rf-connectors/mmcx-connectors.html. ↩