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MECA Electronics

Transmission Lines and Coaxial Connectors
Wireless Networking in the Developing World

Wireless Networking in the Developing World

Many thanks to the authors for making this very informative works available per the Creative Commons ShareAlike license.

"The overall goal of this book is to help you build affordable communication technology in your local community by making best use of whatever resources are available." - The Authors

Some pertinent parts of the book are excerpted here. To access the complete document, please visit http://wndw.net.

Coaxial Connector Frequency Ranges

Transmission Line Equations

Transmission Lines, Waveguide, Coaxial Connectors

The transmitter that generates the RF power to drive the antenna is usually located at some distance from the antenna terminals. The connecting link between the two is the RF transmission line. Its purpose is to carry RF power from one place to another, and to do this as efficiently as possible. From the receiver side, the antenna is responsible for picking up any radio signals in the air and passing them to the receiver with the minimum amount of distortion and maximum efficiency, so that the radio has its best chance to decode the signal. For these reasons, the RF cable has a very important role in radio systems: it must maintain the integrity of the signals in both directions.

The simplest transmission line one can envisage is the bifilar or twin lead, consisting of two conductors separated by a dielectric or insulator. The dielectric can be air or a plastic like the one used for flat transmission lines used in TV antennas. A bifilar transmission line open at one end will not radiate because the current in each wire has the same value but opposite direction, so that the fields created on a given point at some distance from the line cancel.

Radio, transmission line and antenna - RF Cafe

Figure ATL 1 - Radio, transmission line and antenna.

Radio, transmission line and antenna - RF Cafe

Figure ATL 2 - Bifilar transmission line.

Antenna from transmission line - RF Cafe

Figure ATL 3 - Antenna from transmission line.

Coaxial cable with jacket, shield, dielectric, and core conductor - RF Cafe

Figure ATL 4 - Coaxial cable with jacket, shield, dielectric, and core conductor.

If we bend the open ends of the transmission line in opposite directions, the currents will now generate electric fields that are in phase and reinforce each other and will therefore radiate and propagate at a distance. We now have an antenna at the end of the transmission line.

The length of the bent portion of the transmission line will determine the antenna feature. If this length corresponds to a quarter of a wavelength we will have a half wave dipole antenna with a gain of 2.15 dBi.

The functioning of the bifilar transmission line just described is strongly affected by any metal in its proximity, so a better solution is to confine the electrical fields by means of an external conductor that shields the internal one. This constitutes a coaxial cable. Alternatively, a hollow metallic pipe of the proper dimensions will also effectively carry RF energy in what is known as a waveguide.

Cables

For frequencies higher than HF the coaxial cables (or coax for short, derived from the words "of common axis") are used almost exclusively. Coax cables have a core conductor wire surrounded by a non-conductive material called dielectric, or simply insulation.

The dielectric is then surrounded by an encompassing shielding which is often made of braided wires. The dielectric prevents an electrical connection between the core and the shielding. Finally, the coax is protected by an outer casing which is generally made from a PVC material.

The inner conductor carries the RF signal, and the outer shield prevents the RF signal from radiating to the atmosphere, and also prevents outside signals from interfering with the signal carried by the core. Another interesting fact is that high frequency electrical signal travels only along the outer layer of a conductor, the inside material does not contribute to the conduction, hence the larger the central conductor, the better the signal will flow. This is called the "skin effect."

Even though the coaxial construction is good at transporting the signal, there is always resistance to the electrical flow: as the signal travels along, it will fade away.

This fading is known as attenuation, and for transmission lines it is measured in decibels per meter (dB/m).

The rate of attenuation is a function of the signal frequency and the physical construction of the cable itself. As the signal frequency increases, so does its attenuation.

Obviously, we need to minimize the cable attenuation as much as possible by keeping the cable very short and using high quality cables.

Here are some points to consider when choosing a cable for use with microwave devices:

  1. "The shorter the better!" The first rule when you install a piece of cable is to try to keep it as short as possible. The power loss is not linear, so doubling the cable length means that you are going to lose much more than twice the power. In the same way, reducing the cable length by half gives you more than twice the power at the antenna. The best solution is to place the transmitter as close as possible to the antenna, even when this means placing it on a tower.
  2. "The cheaper the worse!" The second golden rule is that any money you invest in buying a good quality cable is a bargain. Cheap cables can be used at low frequencies, such as VHF. Microwaves require the highest quality cables available.
  3. Avoid RG-58. It is intended for thin Ethernet networking, CB or VHF radio, not for microwave.
  4. Avoid RG-213 or RG-8. They are intended for CB and HF radio. In this case even if the diameter is large the attenuation is significant due to the cheap insulator used.
  5. Whenever possible, use the best rated LMR cable or equivalent you can find. LMR is a brand of coax cable available in various diameters that works well at microwave frequencies. The most commonly used are LMR-400 and LMR-600. Heliax cables are also very good, but expensive and difficult to use.
  6. Whenever possible, use cables that are pre-crimped and tested in a proper lab. Installing connectors to cable is a tricky business, and is difficult to do properly even with the specific tools. Never step over a cable, bend it too much, or try to unplug a connector by pulling the cable directly. All of these behaviors may change the mechanical characteristic of the cable and therefore its impedance, short the inner conductor to the shield, or even break the line.
  7. Those problems are difficult to track and recognize and can lead to unpredictable behavior on the radio link.
  8. For very short distances, a thin cable of good quality maybe adequate since it will not introduce too much attenuation.

Waveguides

The X, Y, and Z axis of a rectangular waveguide - RF Cafe

Figure ATL 5 - The X, Y, and Z axis of a rectangular waveguide.

Waveguide type table - RF Cafe

Waveguide specifications.

Above 2 GHz, the wavelength is short enough to allow practical, efficient energy transfer by different means. A waveguide is a conducting tube through which energy is transmitted in the form of electromagnetic waves. The tube acts as a boundary that confines the waves in the enclosed space. The Faraday cage phenomenon prevents electromagnetic effects from being evident outside the guide. The electromagnetic fields are propagated through the waveguide by means of reflections against its inner walls, which are considered perfect conductors. The intensity of the fields is greatest at the center along the X dimension, and must diminish to zero at the end walls because the existence of any field parallel to the walls at the surface would cause an infinite current to flow in a perfect conductor.

The X, Y and Z axis of a rectangular waveguide can be seen in the following figure:

There are an infinite number of ways in which the electric and magnetic fields can arrange themselves in a waveguide for frequencies above the low cutoff. Each of these field configurations is called a mode. The modes may be separated into two general groups. One group, designated TM (Transverse Magnetic), has the magnetic field entirely transverse to the direction of propagation, but has a component of the electric field in the direction of propagation. The other type, designated TE (Transverse Electric) has the electric field entirely transverse, but has a component of magnetic field in the direction of propagation.

The mode of propagation is identified by the group letters followed by two subscript numerals. For example, TE 10, TM 11, etc.

The number of possible modes increases with the frequency for a given size of guide, and there is only one possible mode, called the dominant mode, for the lowest frequency that can be transmitted. In a rectangular guide, the critical dimension is X. This dimension must be more than 0.5 λ at the lowest frequency to be transmitted. In practice, the Y dimension is usually about 0.5 X to avoid the possibility of operation in other than the dominant mode. Cross-sectional shapes other than the rectangle can be used, the most important being the circular pipe. Much the same considerations apply as in the rectangular case. Wavelength dimensions for rectangular and circular guides are given in the following table, where X is the width of a rectangular guide and r is the radius of a circular guide. All figures apply to the dominant mode.

Energy may be introduced into or extracted from a waveguide by means of either the electric or magnetic field. The energy transfer typically happens through a coaxial line. Two possible methods for coupling to a coaxial line are using the inner conductor of the coaxial line, or through a loop. A probe which is simply a short extension of the inner conductor of the coaxial line can be oriented so that it is parallel to the electric lines of force. A loop can be arranged so that it encloses some of the magnetic lines of force. The point at which maximum coupling is obtained depends upon the mode of propagation in the guide or cavity. Coupling is maximum when the coupling device is in the most intense field.

If a waveguide is left open at one end, it will radiate energy (that is, it can be used as an antenna rather than a transmission line).

This radiation can be enhanced by flaring the waveguide to form a pyramidal horn antenna. T here are examples of practical waveguide antennas for WiFi shown in Appendix A called Antenna Construction. 60 PHYSICS

Connectors and Adapters

Connectors allow a cable to be connected to another cable or to a component in the RF chain. There are a wide variety of fittings and connectors designed to go with various sizes and types of coaxial lines. We will describe some of the most popular ones.

BNC connectors were developed in the late 40s. BNC stands for Bayonet Neill Concelman, named after the men who invented it: Paul Neill and Carl Concelman. The BNC product line is a miniature quick connect/disconnect connector. It features two bayonet lugs on the female connector, and mating is achieved with only a quarter turn of the coupling nut. BNCs are ideally suited for cable termination for miniature to subminiature coaxial cable (RG-58 to RG-179, RG-316, etc.). They are most commonly found on test equipment and 10base2 coaxial Ethernet cables.

TNC connectors were also invented by Neill and Concelman, and are a threaded variation of the BNC. Due to the better interconnect provided by the threaded connector, TNC connectors work well through about 12 GHz. TNC stands for Threaded Neill Concelman.

N female barrel adapter - RF Cafe

Figure ATL 6 - An N female barrel adapter.

Type N (again for Neill, although sometimes attributed to "Navy") connectors were originally developed during the Second World War. They are usable up to 18 GHz, and very commonly used for microwave applications. They are available for almost all types of cable. Both the plug / cable and plug / socket joints are supposedly waterproof, providing an effective cable clamp. Nevertheless for outdoor use they should be wrapped in self agglomerating tape to prevent water from seeping in.

SMA is an acronym for Sub Miniature version A, and was developed in the 60s. SMA connectors are precision, subminiature units that provide excellent electrical performance up to 18 GHz. These threaded high-performance connectors are compact in size and mechanically have outstanding durability. The SMB name derives from Sub Miniature B, and it is the second subminiature design.

The SMB is a smaller version of the SMA with snap-on coupling. It provides broadband capability through 4 GHz with a snap-on connector design. 5.

MCX connectors were introduced in the 80s.

While the MCX uses identical inner contact and insulator dimensions as the SMB, the outer diameter of the plug is 30% smaller than the SMB. This series provides designers with options where weight and physical space are limited. MCX provides broadband capability though 6 GHz with a snap-on connector design.

In addition to these standard connectors, most WiFi devices use a variety of proprietary connectors. Often, these are simply standard microwave connectors with the center conductor parts reversed, or the thread cut in the opposite direction. These parts are often integrated into a microwave system using a short, flexible jumper called a pigtail that converts the non-standard connector into something more robust and commonly available. Some of these connectors include:

RP-TNC. This is a TNC connector with the genders reversed.

U.FL (also known as MHF). This is possibly the smallest microwave connector currently in wide use. The U.FL/MHF is typically used to connect a mini-PCI radio card to an antenna or larger connector (such as an N or TNC) using a thin cable in what is known as a pigtail.

The MMCX series, which is also called a MicroMate, is one of the smallest RF connector line and was developed in the 90s. MMCX is a micro-miniature connector series with a lock-snap mechanism allowing for 360 degrees rotation enabling flexibility.

MC-Card connectors are even smaller and more fragile than MMCX. They have a split outer connector that breaks easily after just a few interconnects.

Adapters are short, two-sided devices which are used to join two cables or components which cannot be connected directly. For example, an adapter can be used to connect an SMA connector to a BNC.

Adapters may also be used to fit together connectors of the same type, but of different gender. Figure ATL 6: An N female barrel adapter.

For example a very useful adapter is the one which enables to join two Type N connectors, having socket (female) connectors on both sides.

Choosing the Proper Connector

"The gender question." Most connectors have a well defined gender. Male connectors have an external housing or sleeve (frequently with an inner thread) that is meant to surround the body of the female connector. They normally have a pin that inserts in the corresponding socket of the female connector, which has a housing threaded on the outer surface or two bayonet studs protruding from a cylinder. Beware of reverse polarity connectors, in which the male has an inner socket and the female an inner pin. Usually cables have male connectors on both ends, while RF devices (i.e. transmitters and antennas) have female connectors. Lightning arrestors, directional couplers and line-through measuring devices may have both male and female connectors. Be sure that every male connector in your system mates with a female connector.

"Less is best!" Try to minimize the number of connectors and adapters in the RF chain. Each connector introduces some additional loss (up to a dB for each connection, depending on the connector!)

"Buy, don’t build!" As mentioned earlier, buy cables that are already terminated with the connectors you need whenever possible. Soldering connectors is not an easy task, and to do this job properly is almost impossible for small connectors as U.FL and MMCX. Even terminating "Foam" cables is not an easy task. Don’t use BNC for 2.4 GHz or higher. Use N type connectors (or SMA, SMB, TNC, etc.)

Microwave connectors are precision-made parts, and can be easily damaged by mistreatment. As a general rule, you should rotate the outer sleeve to tighten the connector, leaving the rest of the connector (and cable) stationary. If other parts of the connector are twisted while tightening or loosening, damage can easily occur.

Never step over connectors, or drop connectors on the floor when disconnecting cables (this happens more often than you may imagine, especially when working on a mast over a roof).

Never use tools like pliers to tighten connectors. Always use your hands. When working outside, remember that metals expand at high temperatures and contract at low temperatures: connector too tight in the summer can bind or even break in winter.

 

 

Posted March 4, 2020

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