Courtesy of Pasternack : Waveguide Frequencies and Geometries

Loss, whether due to radiation leakage or conduction resonance, is a common problem in RF microwave transmission lines, especially when high-powered frequency transmissions are involved. The solution? Waveguides.

Waveguide Basics

A waveguide is an electromagnetic feed line used for high frequency microwave signals in high-power transmitters and receivers and is used in radar equipment, in microwave ovens, in satellite dishes or in any RF microwave system where high-power transmission is needed. Waveguides are hollow metallic tubes or light carbon fiber composites constructed with high grade metals like copper, brass, or plated metals. Silver or other plating is used on the inside walls of a waveguide which acts to decrease the resistance loss by shielding and provides efficient isolation between adjacent signals. Transmission lines like microstrip, stripline, or coaxial cable may also be considered to be waveguides and they are usually referred to as dielectric waveguides with a solid center core. However, on high-powered microwave waveguides where the line may get too hot, air in the cavity may be pressurized, recirculated, or a Freon-like gas is used to keep the waveguide cool and also can prevent arcing.

While the disadvantages of using a waveguide include a high production cost, large size and mass of the guide, and the inability of running a DC current alongside the RF signal, the advantages in using a waveguide are that they are completely shielded, high-powered transmission lines that provide good isolation and very low loss that can bend without compromising performance.

Frequencies and Geometries

For the signal to propagate, waveguides need a minimum cross section relative to the wavelength of the signal; these cross sections can be either rectangular, circular, or elliptical. The dimensions of a waveguide determine the wavelengths it can support and in which modes. The lowest frequency range a waveguide will operate is where the cross section is large enough to fit one complete wavelength of the signal. In hollow waveguides, or waveguides using a single conductor, transverse-electromagnetic (TEM) mode of transmission waves are not possible, since Maxwell’s Equations demonstrates that an electric field must have zero divergence and zero curl and be equal to zero at boundaries, resulting in a zero field.

Comparatively, for two-conductor lower frequency transmission lines, like microstrip, stripline, or coaxial cable, TEM mode is possible. In rectangular and circular waveguides, the dominant modes are designated the TE₁₀ mode and TE₁₁ modes.

According to Maxwell’s equations, there are three rules that apply to waveguides:

1 )Electromagnetic waves are reflected by conductors,

2 )Electric field lines that make contact with a conductor must be perpendicular to it,

3 )Magnetic field lines close to a conductor must be parallel to it.

These rules allow for certain modes of propagation such that the TE₁₀ (transverse electric) mode is the mode in which energy propagates in rectangular waveguide. The mode with the lowest cutoff frequency is noted as the dominant mode of the guide.

Because waveguides are the transmission lines for super high frequency (SHF) systems, they must operate with only one mode propagating through the waveguide. Waveguide propagation modes depend on the operating wavelength, polarization, shape, and size of the guide.  Waveguides standards are based on rectangular waveguides and are designed with these characteristics:

one band starts where another band ends, with a band overlapping the two bands in order to allow for applications near band edges,

> the lower edge of the band is approximately 30% higher than a waveguide cutoff frequency thus limiting dispersion and loss per unit length,

>In order to avoid evanescent-wave coupling by way of higher order modes, the upper edge of the band is approximately 5% lower than the cutoff frequency of the next higher order mode,

>the waveguide height is half the waveguide width which allows a 2:1 operation bandwidth – having the height exactly half the width maximizes the power inside the waveguide.

For convenience, our waveguide calculator provides the cutoff frequency, operating frequency range and closest waveguide size for a rectangular waveguide.

Variations on the Waveguide

>The double-ridge rectangular waveguide is a type of waveguide used in RF microwave systems. The ridges in this waveguide design serve to increase the bandwidth but, on the downside, creates higher attenuation and lower power-handling capability.

>The single ridge waveguide is similar to the rectangular waveguide but noted for its large capacitive loading centered on its broad wall.  Compared to a rectangular waveguide, the single ridge waveguide has a lower cut-off frequency with a smaller cross section. However, when compared with the double ridge waveguide, the single ridge yields increased loss and reduced power handling capabilities.

>The slotted waveguide, generally used for radar and similar applications, serves as a feed path with each slot acting as a separate radiator. This antenna structure can generate an electromagnetic wave transmission in a specified narrow and targeted direction.

Courtesy of Pasternack : High-Reliability RF Cables

RF/microwave cable assemblies are critical components in many systems, essential for providing high-frequency signal interconnections with no room for failure.

Jack Browne
Technical Contributor Microwaves & RF Magazine

Coaxial cable assemblies are indispensable, signal-routing components in many RF/microwave systems, although they are often specified and added as an afterthought. For military applications, the durability of high-reliability (hi-rel) cables can mean the difference between life and death. In deep-space missions, such as for communications or surveillance satellites, cable failure is not an option. But military and aerospace applications are no longer the only requirements for hi-rel cables and connectors, and more and more commercial and industrial users come to depend on long operating lifetimes from cables installed in such applications as commercial communications systems and industrial manufacturing equipment.

Choosing a hi-rel RF/microwave cable assembly may be essential to the completion of a system design, whether it is for commercial, industrial, or military applications. But it is not just a simple matter of looking up a few specifications and making a cable fit a set of requirements. It is important to understand how reliability is defined for different applications and then how it can be achieved with different types of RF/microwave cable assemblies for reliability that stands the test of time and whatever an application can throw at it.

Coaxial RF/microwave cables (Fig. 1) have many uses in high-frequency applications, from “set-and-forget” interconnections in commercial or military communications equipment to applications in test systems where cable positions and connections are constantly changing.

What Constitutes Reliability?

Reliability is often defined in terms of product failures and lifetimes in the shape of a “bath-tub” curve as a function of time, with three different failure rates over a product’s lifetime. The first is a period of decreasing failures, followed by a period of constant failures, with a third period of increasing failures during the wear-out stage of the product. High-reliability cables are designed and assembled for a minimum number of failures along the bath-tub curve and lifetime of the cable assemblies so that subsequent system failures do not result from the failing interconnects.

Reliability refers to receiving the expected levels of performance from a cable assembly every time it is used, without degradation or loss in performance for the full lifetime of the application. Failure of a cable assembly can be expected when it exceeds the mechanical or electrical or environmental limits set for it, such as forming a flexible cable into a bend radius smaller than the recommended minimum or applying RF/microwave power levels higher than the rated maximum or operating the cable assembly at temperatures exceeding the specified operating temperature range. The definition of reliability becomes more complex when limits are set for performance degradation that is acceptable over time, and the performance of a coaxial cable assembly can be described by many different parameters, such as insertion loss, VSWR, and phase stability.

The reliability of a cable assembly may be linked to maintaining tight control on just one or two performance parameters or on a full set of parameters. The required tolerances for what describes a hi-rel cable assembly can differ from one application to another. For example, a variation of ±2 dB in insertion loss over certain temperature and frequency ranges may constitute excellent reliability for a system requiring ±3 dB insertion-loss stability but may result in the failure of a system requiring ±0.5 dB insertion-loss stability. The reliability of a coaxial cable assembly is very much a function of an application and its operating environment.

Military Specifications

The reliability of cables and connectors for military and aerospace applications is guided by many different military standards, such as MIL-DTL-17 coaxial cables and MIL-PRF-39012 RF coaxial connectors. MIL-DTL-17 covers many different applications, operating environments, and sets of performance specifications for different coaxial cables, with a great deal of attention paid to the center conductor dimensions and the materials chosen for the center conductor, such as tin, silver-coated copper, and copper-clad steel conductors.

Coaxial connectors for military and aerospace applications must often meet the standards contained within MIL-PRF-39012, which is used by all departments and agencies of the U. S. Department of Defense (DoD). It covers general requirements and test methods for characterizing RF connectors used with flexible and semirigid coaxial cables for military applications, and applies to many different types of connectors, including field-serviceable and non-field-serviceable connectors.

The reliability of these connectors is defined by critical interface materials and finishes, such as PTFE dielectric material, silver-plating thickness on brass-bodied connectors, passivation for steel-bodied connectors, and the thickness of gold plating on the connector center contacts. MIL-PRF-39012 even details the use of recycled, recovered, or environmentally friendly materials in the manufacture of military-grade connectors whenever possible and visual means of checking when connector pairs are properly mated.

Extending Beyond Milspace

Applications for high-rel cable assemblies include interconnections not just in military systems and subsystems, but in commercial, industrial, and a growing number of medical electronic systems relying on electromagnetic (EM) energy for treatment. These are systems where access may be limited, such as on a battlefield or in space, and the cost of failure (whether due to a cable or any other component) is prohibitive.

In many cases, reliability is important for maintaining service, such as in a wireless cellular communications network. The lack of reliability could lead to a costly loss of service in a high-frequency RF/microwave network.  Hi-rel cable assemblies are often used in applications in which performance degradation is unacceptable, such as in test-and-measurement facilities and in automatic-test-equipment (ATE) systems where the accuracy and reliability of the system of the measurements is part of the quality assurance used by a high-frequency component to validate the performance of products before they are shipped.

For such applications as in satellites or other space-based equipment, cable assemblies can be specified according to what are known as “S-level” requirements, characterized by extensive environmental mechanical and electrical testing to ensure the high reliability needed in space-based applications. S-level reliability is considered the highest level of reliability possible for a tested product. It is usually achieved by performing life-testing of a component, such as a coaxial cable assembly, in an environmental chamber in which the component can be operated and tested over the full range of environmental conditions, such as the full temperature range or the full shock and vibration range.

The level of reliability provided by S-level cable assemblies may be unmatched by any other military standard, but it is not without a price for the extensive processes and testing required to produce such failure-free cables. The price tag for such high reliability is considerably higher than for standard grade cables, although when compared to the cost of failure in a space-based application, the investment in S-level reliability is worth it. This leads to the question: What makes one cable assembly more reliable than another? It is due to several factors, including product design, choice of materials, the processes applied, and the level of process control maintained on those processes.

Withstanding the Conditions

As noted earlier, a cable assembly’s application and operating environment can have a great impact on its reliability and require that an RF cable be designed specifically for an application and operating environment for optimum long-term reliability. For example, a cable assembly lacking proper EMI shielding can provide a pathway to interference in a system with high sensitivity, such as a surveillance or radar receiver.

Recent applications for RF cable assemblies, such as in surveillance unmanned aerial vehicles (UAVs), have demanding requirements for environmental specifications, such as temperature, shock, and vibration, and can fail when equipped with standard-grade cable assemblies rather than hi-rel cables. At the same time, to ease the load requirements on a UAV, cable assemblies for UAV applications should be as compact and light in weight as possible, while still achieving the durability required for hard landings and takeoffs.

Depending upon the application, such as a cable assembly that may require constant movement, the cable assembly must be designed for the needs of the application and with materials that support the application, such as proper conductor, shield, and jacket materials for an application. High-quality materials, such as high-conductivity metals and low-loss dielectric materials, can deliver excellent performance in a cable assembly but in themselves will not ensure high reliability.

The connectors, cable, and cable-connector interfaces must be well designed to maintain high reliability under harsh operating conditions, and the manufacturing processes that assemble those different materials and components into a cable assembly must be well controlled and repeatable. In addition, when specialty materials are used in producing a cable assembly for an application, the manufacturer must have tight control over the supply chain to maintain adequate inventory of the required materials.

Test Phase

While testing is widely referred to as a “non-value-added” part of component manufacturing, testing and measurement results are critical for improving product reliability. A manufacturer will not know how reliable their product is if they are not making measurements on it. Furthermore, manufacturers of high-reliability cable assemblies are frequently working to improve reliability. This entails new ideas, new processes, and experiments. The testing is critical and will ultimately help determine if the new processes are yielding benefits or are inadvertently causing problems.

Typical measurements include insertion loss, return loss (VSWR), and phase stability with flexure and over temperature, and thermal cycling, performed on ATE systems built around a microwave vector network analyzer (VNA) with suitable frequency range for the cable assemblies under test. Analysis of the measured data reveals a great deal about the reliability of the cable assemblies, while a review of the historical test data provides insights into the efficiency and effectiveness of the manufacturing process, especially when attempting to create a manufacturing process that provides the highest reliability possible for all combinations of RF/microwave coaxial cables and connectors, such as semirigid and flexible cables and TNC, BNC, Type N, SMA, and BMA connectors (Fig. 2).

Regular measurements and analysis of test results are critical ports of any process for building components with ever-improving reliability, but they are also essential contributors to the product design process, working together with three-dimensional (3D) electromagnetic (EM) simulation software to develop new connector and cable configurations with higher reliability and improving performance at higher frequencies, as commercial, industrial, and military cable and connector specifiers seek ever lower interconnection losses at signal frequencies reaching into the millimeter-wave frequency range. The use of modern computer simulation software tools provides experimental looks at new connector, cable, and interface configurations under any number of operating conditions, as might be needed for high-reliability applications. Access to high-performance test systems and reliable test data helps to validate and refine those connectors and cable models.

Growing dependability on electronic devices at RF, microwave, and even millimeter-wave frequencies for commercial, industrial, and military applications, puts the onus on manufacturers of high-frequency components such as RF/microwave cable assemblies to deliver products with acceptable reliability, and this applies to many different environments and operating conditions. While the breakdown of a wireless base station due to an RF/microwave component failure may lead to lack of cellular communications service for customers in the area around that base station for some time, it can be reached and eventually repaired with a replacement part, even if it happens to be a coaxial cable assembly. The use of a cable assembly with proven track record for reliability in the base station will minimize the need for a repair due to a cable failure.

In contrast, should a cable assembly in an orbiting satellite fail, the repair is not quite so routine (or inexpensive). Because of the difficulty and cost of repairing the satellite, it is an application that truly calls for the state of the art in reliability for an RF/microwave coaxial cable assembly—cables designed, manufactured, and tested to S-level qualification. These are cable assemblies for applications where failure is not an option (Fig. 3), and where the added cost of this cable rating and qualification is insignificant compared to the cost of a cable failure.

Courtesy of Pasternack : Antenna Performance Criteria Part 2

Antennas are essential components of RF and microwave devices and are used in a wide variety of applications including radio and television broadcasting, radar, cellular transmission, and satellite communications to name a few. Antennas are designed to transmit and receive radio waves determined by the design of the application intended to receive the transmissions and can be in all horizontal directions equally as in omnidirectional antennas, or in a designated direction as in directional or high gain antennas.  An antenna in the receiving mode, in the form of a wire, horn, aperture, array, dielectric rod, for example, is used to collect electromagnetic waves and to extract power from them. Important properties related to the design of an antenna include gain and radiated efficiency, as discussed in an earlier article, aperture, directivity, bandwidth, polarization, radiation pattern, effective length, and resonance and are discussed here:

• Aperture

Power received by the antenna is associated with a collective area known as the effective aperture measured as the area of a circle to the incoming signal as the power density (watts per square meter) x aperture (square meters) = available power from antenna in watts. Antenna gain is proportional to aperture and gain is increased by focusing waves in a single direction while reducing other directions. Thus, the larger the aperture, the higher gain and narrower the beam-width. In most cases, larger antennas tend to have a higher maximum effective area.

• Directivity

Antenna directivity is the measure of concentrated energy radiated in a particular direction expressed as the ratio of radiation intensity in a given direction to the average radiation intensity. In other words, it is the ability of an antenna to focus energy in a specific direction when transmitting or receiving.

• Bandwidth

The bandwidth of an antenna refers to the range of frequencies over which the antenna can operate and is conceived of in terms of percentage of the center frequency of the band. Bandwidth is constant relative to frequency and antennas of different types have different bandwidth limitations.

• Polarization

Polarization is the orientation of the electric field of an electromagnetic wave, usually described as an ellipse. Electromagnetic waves emitted from an antenna can be polarized vertically and horizontally. The initial polarization of a radio wave is determined by the antenna. For example, if the wave is polarized in the vertical direction, then the E vector is vertical and it requires a vertical antenna. Circular polarization is a combination of both horizontal and vertical waves and, in the electric field vector, appear to be rotating with circular motion around the direction of propagation, making one full turn for each RF cycle.

• Radiation Pattern

Because antennas do not radiate power equally in all directions, antenna radiation patterns or polar diagrams are important tools to quickly evaluate the overall picture of antenna response. The radiation pattern of a transmitting antenna is a plot that describes the strength of the power field radiated by the antenna in various degrees. Radiation plots are often shown in the plane of the axis of the antenna (E plane) or the plane perpendicular to the axis (H-plane) and are usually shown in relative dB (decibels).

• Effective Length

The effective length describes the efficiency of an antennas in transmitting and receiving electromagnetic waves. It is used to determine the voltage induced on the open-circuit terminals of the antenna when a wave hits it. In a receiving antenna, the effective length is the length and orientation of a uniform current required to produce the same electric field as the transmitting antenna. It is a useful tool in determining the effect of polarization mismatch between the propagated waves of the transmitting antenna and the receiving antenna.

Courtesy of Pasternack : New Solderless Vertical Launch Connectors with Maximum Operating Frequency Up to 50 GHz

New Line of Removable Vertical Launch Connectors Deliver VSWR as Low as 1.3:1

IRVINE, Calif. – Pasternack, a leading provider of RF, microwave and millimeter wave products, has introduced a new line of solderless vertical launch connectors that are ideal for high-speed networking, high-speed computing and telecommunications applications.

Pasternack’s new series of vertical launch connectors consists of 12 models that provide VSWR as low as 1.3:1 and maximum operating frequency of up to 50 GHz, depending on the model. These launches boast a reusable clamp attachment and can be used for microstrip or stripline. They are offered in male and female versions, covering 2.4mm, 2.92mm and SMA interfaces, and all models provide solderless installation. These removable vertical launches feature a stainless steel outer conductor, gold-plated beryllium copper center contact and Polyetherimide (PEI) insulators. They are ideal for high-speed backplanes, signal integrity measurements, semiconductor verification boards, multi-channel tests and SERDES applications.

“The VSWR of these new vertical launch PCB connectors minimizes the performance trade off compared to end launches. This allows our customers to take advantage of additional PCB real estate and allows for easier access for their test cables,” said Dan Birch, Product Line Manager at Pasternack.

Pasternack’s new solderless vertical launch connectors are in-stock and ready for immediate shipment with no minimum order quantity. For detailed information on these products, please visit https://www.pasternack.com/pages/rf-microwave-and-millimeter-wave-products/vertical-launch-solderless-connectors.html.

For inquiries, Pasternack can be contacted at +1-949-261-1920.

About Pasternack:

A leader in RF products since 1972, Pasternack is an ISO 9001:2008 certified manufacturer and supplier offering the industry’s largest selection of active and passive RF, microwave and millimeter wave products available for same-day shipping. Pasternack is an Infinite Electronics company.

Press Contact:

Peter McNeil
Pasternack
17792 Fitch

Courtesy of Pasternack Blog : Coaxial Cable Loss due to Loss Tangent

Electrical losses in a coaxial cable create heat in the outer and center conductors and are the two main types of coaxial cable loss, skin-effect loss and dielectric loss, respectively. This heat, or loss, can be calculated with the understanding of the following concepts.

What is Skin Effect Loss?

In coaxial cable, skin effect is the movement of an alternating electric current (AC) whereby the current density is greater near the surface of the conductor and lessens within the conductor.  Skin effect is the decline in current density and the skin depth is a measure of the depth at which the current density falls to 1/e of its value near the surface. Over 98 percent of the current flows within a layer 4 times the skin depth from the surface. At high frequencies, the skin depth becomes much smaller. Skin effect loss usually occurs at high frequencies when the signal reaches and moves along the surface of the inner conductor, which causes additional RF losses at higher frequencies.

A skin depth calculator can be found here.

Resistance per unit length is the ratio of specific resistance or resistivity to the area of cross-section of given conductor in Ohm per meter. With skin-effect loss, the resistance per unit length, Rl, and the inductance per unit length, Ll, increase with the square root of the frequency.

Skin effect losses are resistive, caused by the narrowing of the conduction path. In calculating loss, loss per unit length includes the skin effect loss and dielectric loss.

What is Dielectric Loss?

Dielectric conduction loss is caused when the insulating material inside the transmission line absorbs energy from the electromagnetic field developed between the inner and outer conductors. Dielectric loss explains the amount of dissipation of electromagnetic energy or heat of a dielectric material. It is often described in terms of either the loss angle δ or the corresponding loss tangent tan δ. Both refer to the phasor in the complex plane whose real and imaginary parts are the resistive (lossy) component of an electromagnetic field and its reactive (lossless) counterpart. The ratio of two quantities is defined in term of Tan.

What is Loss Tangent?

Loss tangent is the ratio at any specified frequency between the real and imaginary parts of the impedance of the capacitor. A large loss tangent refers to a high degree of dielectric absorption. Loss tangent is the ratio between the imaginary and real parts of the complex permittivity where the permittivity of dielectric is given by:

ε =ε_re − jε_im

When this formula is drawn on an x-y plane, the tangent of the angle between the real and the imaginary quantity is discoverable which can be described as:

tanδ = ε_im/ε_re

Which means the ratio of the imaginary part to the real part of the permittivity is found to be another quantity, i.e. the loss tangent, which is used to express the losses in a dielectric material. In other words, it is the ratio of the imaginary part to the real part or the tangent of the angle between the complex number and the real axis.  This angle is the loss angle and the tangent is called the loss tangent.  Thus, the value of the loss tangent describes how lossy a material is, such that it either represents a very lossy material or a very good conductor.

Measurement of loss using sinusoidal excitation at a particular frequency yields the product of skin effect and dielectric loss functions or the sum of the skin effect and dielectric losses in units of dB. Once the skin-effect loss is derived, the portion of loss attributable to dielectric losses may be estimable.