RF Connector Buying Guide

When it comes to transmitting radio frequencies over cable, it's essential to use the right kind of wire. Coaxial cables are specially shielded to carry radio signals efficiently and are employed in applications ranging from Wi-Fi to cable television. Due to their unique construction, coaxial cables require specific connectors that preserve the cable's shielding integrity from cable to board. This guide will assist you in selecting the best connectors for your application!

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Understanding Coaxial Cables

Alternative methods for carrying alternating current of radio frequency include Ladder-Line, Cage-Line, Stripline, and Twisted Pairs. However, the most common choice in the multi-megahertz range is the versatile coaxial cable. Patented by English engineer Oliver Heaviside, coaxial cables feature a conductor and shield that share a geometric axis, which minimizes signal degradation. This design is particularly useful in consumer electronics since coaxial cables can be positioned close to metal objects, twisted, and bent without significantly impacting the signal quality. This resilience is due to the separation of the central conductor from the shielding by a dielectric insulator, which maintains a controlled distance between components, regardless of cable orientation. The coaxial geometry also confines magnetic fields to the dielectric, limiting external leakage.

One drawback of coaxial cables is that they cannot simply be stripped and soldered together. While it's technically possible, doing so creates a "kink" in the radio-frequency line that can disrupt the signal. To avoid this issue, coaxial connectors are necessary. They range from the tiny Hirose U.FL, which is millimeter-sized, to high-power connectors the size of plumbing pipes. Many connectors in the RF industry are patented, and manufacturers may modify or discontinue certain standards over time. As electronic devices continue to shrink, a diverse array of RF connectors and cables has emerged, with 3 or 4 types commonly used at SparkFun. This guide aims to provide you with the information needed to navigate the available connectors effectively.

SMA Connectors

The SMA (Sub-Miniature A) connector features a 50Ω impedance and can support frequencies up to 17 GHz. These connectors are compact enough for most consumer RF applications and utilize a threaded housing to create secure mechanical connections. Male SMA connectors include a center pin and inner threads, while female SMA connectors have a center sleeve and outer threads. Typically, SMA connectors are used for GPS and cellular signal connections, although they have a broader range of applications. A common issue when working with SMA connectors is differentiating them from Reverse Polarity SMA connectors.

Reverse Polarity SMA (RPSMA)

RPSMA connectors were first introduced to prevent high-gain, professional-grade antennas from connecting to consumer-level wireless devices. Despite this initial intent, both adapters and RPSMA antennas are now widely available. In retrospect, this idea was not well-conceived, leading to some confusion for consumers. RPSMA connectors are primarily utilized in Wi-Fi and other consumer-level wireless devices. While their outer housing matches that of standard SMA connectors, the polarity is reversed. Male RPSMA connectors include a center sleeve and inner threads, whereas female RPSMA connectors feature a center pin and outer threads. The term "Reverse Polarity" pertains to this difference in connector polarity and does not reference the signal polarity.

U.FL Connectors

The Hirose U.FL connector, known by various names such as AMC, UMCC, and IPX, is the smallest RF connector available, measuring just 2.5mm in height when connected! Capable of handling frequencies up to 6 GHz, U.FL connectors are often employed in applications that require minimal space, such as laptop Wi-Fi cards and embedded systems. Male U.FL connectors are surface mount components soldered directly onto the host. Female connectors are typically part of a cable assembly made with high-quality, doubly-shielded 1.32mm cables. The design relies on the shape of the connector, providing a strong "snap-in" connection. However, the female U.FL connectors can wear out after several uses and often need replacement, as they're not intended for frequent reconnection.

BNC Connectors

The BNC connector, identifiable by its bayonet-style locking mechanism and named after its inventors Neill and Concelman, is larger than the previously discussed connectors. BNC connectors use a slotted conductor, which can inadvertently cause radiation interference at frequencies above 4 GHz. Consequently, they are most commonly applied in applications below 3 GHz. BNC connectors are often found in composite video connections, used in older studio equipment or CCTV cameras, and were once utilized in 10base2 Ethernet networks. Today, BNC connectors are prevalent in amateur radio equipment and various testing instruments, such as oscilloscopes and sensor connections.

Quick Reference Table

The table below summarizes each connector's attributes, including specific applications.