The tremendous growth of wireless communication around the world has spawned the development of more types of RF connectors than was ever imaginable a mere twenty years ago. Radio frequency connectors today are providing critical links to a substantial amount of equipment, including networking, cellular communications, radio frequency identification, global positioning systems, mobile radio systems and many more. Understanding the factors that impact the selection as well as the design of RF connectors is essential knowledge for a design engineer who works with RF circuits. This learning module explores the features and applications of the most common types of RF connectors.
The objective of this learning module is to provide you with the essentials of RF connector technology. You will first review the basics of electromagnetic wave theory. In the later sections, you will gain an understanding of radio waves, coaxial transmission lines, and the common types of RF connectors and their applications.
Upon completion of this module, you will be able to:
3. The Electromagnetic SpectrumBack to Top
Radio frequency and wireless communication devices have been built for over a century. Many scientists contributed to their development, including Sir Oliver Lodge, Alexander Stepanovich Popov, Sir Jagadish Chandra Bose, and many others. But it was Guglielmo Marconi’s experimental broadcast of the first transatlantic radio signal in 1901 that ultimately led to the commercial viability and world-wide adoption of RF technology. Devices based on RF technology ranging from radio, TV, cellular and the Internet are so common today that we can hardly imagine a world without them. Before discussing the different types of RF connectors in this learning module, let’s begin with a brief review of the theory behind radio frequency devices, beginning with the Electromagnetic (EM) Spectrum.
Decades before Marconi's first transatlantic radio broadcast, Scottish scientist James C. Maxwell demonstrated, in the 1860s, that time-varying electric and magnetic fields cause electromagnetic waves to travel in a vacuum at the speed of light. Twenty years later, Heinrich Hertz went on to apply Maxwell's theories and experimentally prove the existence of radio waves in the late 1880s.
The discovery of electromagnetic waves led to the identification of an entire range of EM waves that are together called the Electromagnetic Spectrum (EM). These waves are radiant energy, resulting from electromagnetic interactions. While EM waves are continuous in nature, scientists typically divide the EM spectrum into discrete segments called bands. The following table describes the EM spectrum's frequency bands:
|Frequency (f)||Wavelength (λ)||Band||Description|
|30-300 Hz||104 to 103 km||ELF||Extremely Low Frequency|
|300-3000 Hz||103 to 102 km||VF||Voice Frequency|
|3-30 KHz||100 to 10 km||VLF||Very Low Frequency|
|30-300 KHz||10 to 1 km||LF||Low Frequency|
|0.3-3 MHz||1 to 0.1 km||MF||Medium Frequency|
|3-30 MHz||100 to 10 m||HF||High Frequency|
|30-300 MHz||10 to 1 m||VHF||Very High Frequency|
|300-3000 MHz||100 to 10 cm||UHF||Ultra High Frequency|
|3-30 GHz||10 to 1 cm||SHF||Super High Frequency|
|30-300 GHZ||10 to 1 mm||EHF||Extremely High Frequency (Millimeter Waves)|
Electromagnetic waves travel at the speed of light in a vacuum. These waves are described by their relationship between frequency (f) and wavelength (λ). Their relationship is shown in the following equation:
(with c = 3.0 x 108 m/sec)
4. Radio Frequency Waves: AM/FM Bands Back to Top
Radio waves are one of the many types of electromagnetic (EM) radiation or energy; they appear at the lowest frequency portion of the electromagnetic spectrum. They have wavelengths between 1 millimeter and 100 kilometers (or 300 GHz and 3 kHz in frequency). While radio waves occur naturally in the world, radio waves can be generated by RF devices used in fixed and mobile radio communication, broadcasting, radar, communications satellites, computer networks and more.
AM and FM bands are the part of the electromagnetic spectrum used for radio broadcasting. In the AM band, radio waves, called "carrier waves," are used to broadcast commercial radio signals in the frequency range from 540 to 1600 kHz. The abbreviation AM stands for "amplitude modulation," which is the method for broadcasting radio waves by varying the amplitude of the carrier signal to be transmitted. The resulting wave has a constant frequency, but a varying amplitude.
In the FM band, the carrier waves are used to broadcast commercial radio signals in the frequency range from 88 to 108 MHz. The abbreviation FM stands for "frequency modulation," which is the method for broadcasting radio waves by varying the frequency of the carrier signal to be transmitted. The resulting wave has a constant amplitude, but a varying frequency.
RF technology continues to demonstrate itself as a durable technology with its growing impact on IoT wireless connectivity. Much of today's most popular wireless equipment operate in the 2.4-GHz radio band, including Wi-Fi hot spots and home wireless routers, ZigBee, Bluetooth, some cordless phones, among others. Sub-1GHz radio bands are even finding application in the transceivers and microcontrollers designed for this radio band and destined for use in long-range, wireless IoT networks.
5. Coaxial Transmission Lines Back to Top
A transmission line is a two-conductor structure that supports the propagation of transverse electromagnetic (TEM) waves (i.e., an electromagnetic wave where both the electric and magnetic fields are perpendicular to the direction of wave propagation). It is designed to transmit RF power in the most efficient manner from one point to another. An RF transmission line consists of a source, a load, and the combination of RF cable and connectors. Coaxial lines are the most common type of RF transmission line.
- 5.1 Coaxial Cables
Coaxial cables are specially constructed cables with a core center conductor, surrounded by a tubular non-conductive insulator (dielectric), which is then enclosed by an braided copper shielding (the outer conductor). The dielectric separates the core inner conductor from the outer shielding conductor. The coaxial cable's outer shielding is typically protected by PVC material.
Coaxial cable is designed to keep the RF waves inside the cable while keeping out magnetic interference. It does so by effectively placing two wires in one structure, each with current moving in opposite directions that cancel out the currents due to the proximity effect thereby preventing RF radiation. Coaxial cable is ideal for applications where attenuation must be kept to a minimum and the elimination of outside interference is critical.
- 5.2 Impedance Matching
Transmission lines must be impedance matched over the entire coaxial structure in order to prevent reflections of the signal at the line terminations. If the impedances aren’t matched, maximum power will not be delivered (from source to load) and standing waves will develop along the line.
|In a transmission line, the source, line and load impedances must be matched for maximum power transfer to occur. In other words, ZS = Z0 = ZL|
- 5.3 PCB Transmission Lines
Transmission lines can also be constructed utilizing printed circuit board technology. The substrate of the printed circuit board functions as the dielectric and separates the two conductors. The first conductor is typically a narrow etch, while the second conductor is the ground plane. The most popular types of printed circuit board transmission lines are: microstrip, stripline, coplanar waveguide and slotline.
Coaxial RF connectors allow a cable to be connected to another cable or component (e.g, printed circuit board) of the RF transmission line structure. There is a wide variety of coaxial connectors designed to fit with different types of coaxial cable. Coaxial connectors will be discussed in the next section.
6. Coaxial Connectors Back to Top
In this section, let's discuss the components, coupling styles, and termination types of a typical coaxial connector.
- 6.1 Components
Shielding (Outer conductor): provides shielding that keeps interference outside the connector while keeping desirable currents inside. It’s held at ground or reference potential for a circuit path return.
- 6.2 Coupling Style
The coupling style is the method used to mate a coaxial plug to a coaxial connector or receptacle. There are four types of coupling mechanisms used with RF connectors, including: Threaded, Bayonet, Snap-On and Slide-On.
In general, threaded styles maximize mating security. They are used in high vibration environments. Snap-on devices provide the quickest mating. Bayonet couplings offer a compromise between ease of use and mating security. Slide on coupling provides quick and easy installation.
The coupling style also impacts performance. As with electrical connectors, higher performance RF products require exceptionally tight, stable mating interfaces to minimize noise and optimize energy transfer. Traditionally, threaded couplings have provided this integrity. Today, however, non-threaded coupling technology have advanced significantly to provide the same benefit and often in a small footprint.
- 6.3 Terminations
Coaxial cables are typical terminated with RF connectors. Coaxial connectors are designed to maintain a coaxial form across the connection and have the same impedance as the attached cable. Coaxial connectors can be terminated in the following ways: Plugging, Soldering, Crimping, Clamping, Pressing and Threading.
7. Performance SpecificationsBack to Top
Now that you understand the physical features of coaxial connectors, let's consider their performance. There are many factors that determine the performance of RF coaxial connectors, but the four main specifications are:
Frequency range: Each coaxial connector series supports a specific frequency range. A connector's frequency rating is typically determined by the internal geometry of the coaxial structure with which it is used. Conductor sizes and the type and amount of insulation between them determine this spec. In general, higher frequency connectors tend to be smaller in diameter and have tighter tolerances.
Impedance: Impedance can be thought of as the total resistance a system offers to the flow of RF waves. For maximum power transfer of RF energy, impedance should be matched throughout the entire coaxial structure. If the coaxial structure is not impedance matched, parts of the RF waves reflect back toward their source, decreasing energy transfer and flow efficiency. In addition, the reflections may distort the waves to the point where the information they carry cannot be accurately interpreted. Most RF application manufacturers design their products for either of the two industry-standard cable impedance ratings: 50 Ohm or 75 Ohm. 50 Ohm cable is commonly used for radio transmitters and receivers, laboratory equipment, and data communications. 75 Ohm cable is typically used in video applications, CATV networks, TV antenna wiring, and telecommunications.
Voltage standing wave ratio (VSWR): While it's theoretically possible to achieve perfect impedance matching, it's not usually cost effective. There will always be reflections. How much reflection an RF connector introduces is stated in its VSWR (Voltage Standing Wave Ratio) specification. VSWR expresses the reflected signal as a ratio to the pure signal; it measures the impedance matching of loads to the characteristic impedance of a transmission line. Impedance mismatches result in standing waves along the transmission line. An ideal VSWR would be 1.00, indicating that all RF energy passes through the connector. The VSWR should be as low as possible.
Intermodulation distortion: intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities in a system. This expresses the signal distortion from factors caused by poor design and manufacturing, bad grounds, corrosion, low contact pressure, etc. These problems generate unwanted frequencies that distort the original signal, inhibit its ability to carry data without errors, and impede accurate demodulation. Some typical contributions to intermodulation distortion are: oxidized metal contact surfaces, current saturation, and oil or grease layers between contacts.
8. Types of RF ConnectorsBack to Top
There are many types of coaxial connectors. This learning module will discuss on the most common types, categorized by size.
- 8.1 Miniature RF Connectors
- 8.2 Subminiature RF Connectors
The ever increasing need for higher density and lighter weight electronics within today's systems requires compact connections. SMPM RF Blind-Mate board-to-board and cable connections deliver the needed density in a high-performance connector. Providing superior frequency performance from DC to 65 GHz, the SMPM connector also compensates for the axial and radial misalignment issues inherent with board-to-board mating.
SMP subminiature connectors offer excellent performance from DC to 40 GHz. PCB mount, cable mount and in-series adapters provide an interconnect solution for board-to-board and blind mate applications while maintaining package density. SMP connectors are also available in multi-port solutions. Interface styles include smooth, limited detent and full detent to cover a wide range of applications.
- 8.3 Microminiature RF Connectors
- 8.4 Ultra-microminiature RF connectors
- 8.5 Medium RF Connectors
- 8.6 Large RF Connectors
7/16 DIN connectors are designed for rugged, low-loss, high-power wireless infrastructure systems. They are designed for a wide range of cables, including corrugated styles. Silver or tri-metal (Copper, Tin and Zinc) plating is used for low Inter-modulation Distortion (IMD). The tri-metal plating is tarnish resistant and durable and provides abrasion resistance comparable to nickel with far better electrical performance. Tri-metal plating alloys are non-magnetic which is critical for low IMD. They are rated from DC to 7 GHz. They have an easy-Hex coupling nut to allow toolless application.
- 8.7 Multi-Port (Ganged) RF Connectors
*Trademark. Molex® is a trademark of Molex Corporation. Other logos, product and/or company names may be trademarks of their respective owners.
Test Your KnowledgeBack to Top
Are you ready to demonstrate your RF Connectors knowledge? Then take a quick 20-question multiple choice quiz to see how much you've learned from this Essentials Connectors 3 module.
To earn the Connectors 3 badge, read through the module to learn all about RF connectors, complete the quiz at the bottom, leave us some feedback in the comments section and then download the attached Connectors 3 PDF (below) for future reference.
1) The TNC connector is a threaded version of the BNC RF connector. The threaded coupling provides what benefit?
2) For maximum transfer of RF energy, impedance should be matched throughout the entire coaxial structure (source, line and load).
3) What feature of a QMA RF connector enables easy cable routing?
4) True or False: Radio waves are a form of energy in the modification of velocity-dependent electric and magnetic fields.
5) The enOcean Pi features a radio transmitter that transmits radio messages at either 868 MHz, 315 MHz or 902 MHz. What EM band are these frequencies?
6) You need to select an RF connector for a densely populated electronic module that's destined to be installed in an environmentally controlled environment. What category of RF connector is the best choice for this application?
7) RF is an electromagnetic wave that travels at the speed of light in a vacuum. RF waves are described by their relationship between _________ and _________ .
8) What is the most efficient method of transporting RF signals from one point to another point?
9) True or False: The Voltage Standing Wave Ratio (VSWR) is a numerical measure that describes how well an antenna is impedance matched to the radio or transmission line.
10) RF Connectors have which of the following coupling mechanisms?
11) What type of RF connector has a self-aligning feature that compensates for a small misalignment when mating?
12) What component of an SMA connector minimize reflections and attenuation at higher frequencies while providing a high degree of mechanical strength and durability?
13) 7/16 DIN connectors are ideal for what type of application?
14) True or False: Fakra RF connectors are designed for on-board automotive telematics.
15) In a PCB transmission line, what is the function of the PCB substrate?
16) What is the wavelength of an EM wave with a frequency of 3 MHz?
17) True or False: Electromagnetic waves are the result of oscillation of electric and magnetic fields. These fields are in opposition to each other and to the direction of the wave.
18) Why does coaxial cable prevent the radiation of RF waves outside the cable?
19) True or False: A transmission line must be impedance matched to prevent magnetic interference.