When it comes to transmitting radio frequencies over cable, you can’t just hook up to any old wire. Coaxial cables are specially shielded to efficiently carry radio signals and are used for everything from Wi-Fi to Cable TV. Because of the way they’re constructed, coaxial cables require special connectors that maintain the shielding from cable to board, and these connectors come in a variety of flavors. This guide will help you figure out which is best for your application!
First, A Little Background…
Surely there are other ways to carry alternating current of radio frequency: Ladder-Line, Cage-Line, Stripline, Twisted Pairs; but the most common in the multi-megahertz range is the humble coaxial. Coaxial cable, so named because the conductor and shield share a geometric axis, was patented in 1880 by English engineer Oliver Heaviside. The advantage to using coaxial cable, and the reason it’s frequently used in consumer electronics, is because they can be run near metal objects, twisted and bent without drastically affecting the signal. The reason behind that is the way that coaxial cable is constructed. By separating the central conductor from the shielding using a dielectric insulator, coaxial cables maintain a carefully controlled distance between elements regardless of the orientation of the cable. Being geometrically coaxial has its advantages as well, including the confinement of magnetic fields to the dielectric which allows for very little leakage outside the cable.
The one downside to using a coaxial cable is that you can’t simply strip a couple of “coax” cables and solder them together. Okay, you can, but you create a sort of radio-frequency “kink” in the line that can interfere with your signal. To maintain the shielding effect of coaxial cable, you need to use coaxial connectors. Coaxial connectors range from millimeter-sized Hirose U. FL to (now outdated) EIA high-power connectors the size of plumbing pipe. Connectors are often patented by companies in the RF business and used to enforce standards. When those “standards” break down, connectors will sometimes be changed or deprecated. Over the years, this process along with the miniaturization of electronic devices has produced a diverse ecosystem of RF (radio frequency) connectors and cables. There are 3 or 4 different types that we deal with a lot here at SparkFun, so this guide is an attempt to give you enough info so that you can navigate to the connectors that you need.
The SMA (or Sub-Miniature A) connector has a 50Ω impedance and can handle frequencies up to 17 GHz. They’re small enough for most consumer RF applications and they use threaded housing to create strong mechanical connections. Male SMA connectors have a center pin and inner threads whereas female SMA connectors have a center sleeve and outer threads. SMA connectors are usually used for GPS and Cellular signal connections although they are used in other RF applications as well. The most common problem when working with SMA connectors is the possibility of confusion with Reverse Polarity SMA connectors.
Reverse Polarity SMA (RPSMA)
RPSMA connectors were initially introduced to prevent high-gain, professional grade antennas from being connected to commercial grade wireless equipment. Despite this, adapters and RPSMA antennas are both readily available now. Honestly, it wasn’t a brilliant idea and it left consumers with a mess. But hey, hindsight’s 20/20 right? RPSMA connectors are used mainly on WiFi and other consumer grade wireless equipment. Reverse Polarity SMA connectors have the same outer housing as SMA connectors, but the polarity of the connector is reversed. Male RPSMA connectors have a center sleeve and inside threads whereas female RPSMA connectors sport a center pin and outside threads. The term “Reverse Polarity” refers to this difference in connector polarity, not in any way to the signal polarity.
The Hirose U.FL (Known by a variety of names including AMC, UMCC, IPX) is the smallest RF connector that we carry… and it’s tiny! A mated connection only stands 2.5mm high! U.FL is good up to 6GHz and is usually used in applications where nothing else would fit. Often times they’re used in laptop WiFi cards and embedded systems. Male U.FL connectors are surface-mount parts soldered directly into the host. Female connectors are usually manufactured as a cable assembly with a high-quality 1.32mm doubly shielded cable. Female U.FL connectors rely on pressure from the shape of the connector to keep it secure. This does form a very tight ‘snap in’ connection. As a result, female U.FL connectors will wear out after a several connections and will need to be replaced as they aren’t generally intended to be connected and disconnected multiple times.
The BNC connector, named after its bayonet-style connector and its two inventors (Neill and Concelman) is a little bigger than the other connectors we’ve mentioned here. Also, because the BNC connector uses a slotted conductor, it can become an accidental radiator at at frequencies above 4GHz so its usually used for radio applications below 3GHz. You may have seen BNC connectors used for composite video on things like older studio equipment or CCTV cameras. They were also used on 10base2 this Ethernet networks. Today, they’re used largely on amateur radio equipment and test equipment. You’ll see these on things like oscilloscopes and different kinds of sensor connections.
Quick Reference Table
Here’s a table that shows each connector, its attributes and a few products you’ll find it on.