This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Microwave circuit dimensions are related to their wavelengths / frequencies and to the dielectric constant (Dk) of their substrates. Quite simply, higher-frequency signals have smaller wavelengths and their electromagnetic (EM) energy of those smaller wavelengths will propagate through circuits with smaller dimensions. Phase velocity is related to wavelength, with slower EM waves having shorter wavelengths which propagate through circuit structures with smaller dimensions than faster waves with their longer wavelengths.

For a given frequency / wavelength, the circuit dimensions can also be reduced in size by fabricating the circuits on laminates with high dielectric constants, such as RO3010™ and RT/duroid® 6010.2LM circuit materials from Rogers Corp. The high Dk values of these circuit materials results in shorter wavelengths and smaller circuit dimensions, but also slows the propagation of the EM waves along the conductors of a printed circuit board (PCB).

Transmission-line conductors on PCBs for RF/microwave circuitry take on various forms, such as microstrip, stripline, coplanar waveguide (CPW), and even substrate-integrated-waveguide (SIW) transmission lines for circuits at millimeter-wave frequencies. For a conventional microstrip transmission line with solid conductors, for example, the phase velocity has relatively high speed. It is related to the current density, with most of the current running along the edges of the solid microstrip conductors in a circuit. If the solid conductors are modified into a conductor pattern that is not solid, such as a ladder pattern, the phase velocity will be slowed and the wavelength will be reduced.

The phase velocity is a function of the separation distance between the storage locations in a conductor for the EM energy’s electric and magnetic field components, resulting from capacitive loading (and storage capabilities) within a transmission line’s conductors. When the magnetic and electric energy storage locations are in close proximity, the phase velocity is higher or faster than when the magnetic and electric energy storage locations within the conductor are further apart. For the slower phase velocity, the wavelength of the transmission-line’s conductors is shorter than for the conductors with the faster phase velocity. Slowing the phase velocity of a microstrip transmission-line’s conductors is a way to miniaturize circuits fabricated on PCB materials with a given Dk value.

By modifying conventional microstrip conductors into unconventional patterns, such as a ladder configuration, distances between stored magnetic and electrical energy field components are increased beyond the separation distances for standard solid microstrip conductors. A microstrip ladder pattern effectively conducts EM energy, but with slower phase velocity than a solid microstrip transmission-line conductor because of its greater distance between magnetic and electric energy storage locations. Just how much slower the phase velocity is than a standard solid microstrip conductor will depend upon the geometry of the microstrip ladder pattern.

Such slow wave structures reduce the group velocity of a transmission line, effectively increasing its group delay compared to a conventional solid microstrip transmission line, and reducing its wavelength compared to the wavelength of a conventional microstrip transmission line. As an example, consider a half-wavelength gap-coupled resonator constructed from microstrip transmission lines. When fabricated with a standard solid conductor pattern on 20-mil-thick RO4835™ laminate from Rogers Corp., with typical Dk of 3.48 at 10 GHz in the z-direction of the material, the circuit structure will resonate at 4 GHz. When using the appropriate microstrip ladder conductor pattern on the same material, the resonant frequency of the gap-coupled resonator circuit can be reduced by 25%, to 3 GHz.

Coupled resonators are often used to realize various high-frequency filter functions, so this gap-coupled-resonator example with its microstrip ladder pattern of conductors can be applied to these types of filters to achieve a reduction in the size of the filter circuitry for a given design frequency. An even greater reduction in filter size can be gained by combining the ladder conductor pattern approach with high-Dk PCB material, such as RO3010 ceramic-filled polytetrafluoroethylene (PTFE) laminate with a Dk of 10.2 measured in the z-direction of the material at 10 GHz and RT/duroid 6010LM ceramic-PTFE composite material with Dk of 10.2 in the z-direction at 10 GHz.

What is the impact on circuit size when using one of these high-Dk PCB materials? If a circuit material such as RO3003™ laminate with a Dk of 3.0 in the z-direction at 10 GHz is used as a starting point for a filter circuit with microstrip edge-coupled structure, including edge-coupled resonators, and the same filter is designed on a PCB material with much higher Dk, such as RT/duroid 6010LM laminate with its Dk of 10.2, a healthy reduction in size is possible. The size of the same microstrip edge-coupled filter function will be about 30% smaller on the higher-Dk material.

By combining slow-wave circuit patterns such as the microstrip ladder pattern with high-Dk circuit laminates such as the 10.2 Dk RO3010 or RT/duroid 6010LM PCB materials, the circuit-shrinking effects of both approaches can be applied to an RF/microwave circuit design for a significant reduction in circuit size. Both of these low-loss circuit materials feature tightly controlled Dk values for excellent high-frequency performance and reduced circuit sizes.

Screen shot 2014-08-08 at 1.33.54 PMLadders are just one type of microstrip conductor pattern that can be used to create slow-wave structures, with many different patterns capable of achieving slow-wave propagation and contributing to a reduction of circuit size for a given design frequency and circuit laminate Dk value. The key to success for forming any slow-wave pattern (and shrinking the dimensions of a high-frequency circuit) lies in separating the electric and magnetic energy storage within the structure, thereby slowing the phase velocity and reducing the wavelength of the transmission line for a given circuit material and Dk value.

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Product Life Cycle Cost Analysis, Cost of Ownership, LCA (Life Cost Analysis), Whole Life Cost, and “Cradle to Grave” Costing – what do these names and acronyms have in common? The answer is they all are used to define the calculation method that evaluates the cost of a product throughout its time in service.

This is certainly nothing new to the mass transportation industry. In fact, many governments and municipalities require an LCA as part of a tender offering; and some transit authorities have expanded the criteria to include end-of-life costing and the triple bottom line (financial, ecological, social performance) of the cost impact upon environmental sustainability. Surprisingly, LCA is oftentimes misunderstood or only partially considered in tender awards.

In defense of the transit authorities (and from their perspective), many vendors enhance or misrepresent their respective products’ life performance claims and either omit substantiated time-simulated data or exaggerate future savings. Typical cost models have the tendency to be over-simplified and lack the flexibility needed to accommodate specific and customized variables not shared among transit authorities. Lastly, there are hidden, intangible and questionable costs that may be difficult or impossible to estimate.

Seating Life Cycle Analysis

While life cost analysis can be assessed for any rail car or vehicle component, seat cushion tenders offer the best opportunity to scrutinize the validity of an LCA model. Within the interior, seating is one of the five most costly elements of the rail car and is at the top of the list for future refurbishments. Assessing new build costs with forecast refurbishment estimates is seemingly straightforward. As a start, a Whole Cost Analysis would include the following factors:

  1. The cost of seating for the initial new-build delivery,
  2. Assumptions of how far into the future the transit authority will keep the seats in service before a full or partial refurbishment,
  3. Estimated costs of the refurbishment, and
  4. Historical costs for replacements between refurbishments.

Other considerations may include the cost of labor and lost revenue while the seat or rail car is in refurbishment or out of service. Ridership dissatisfaction due to seating discomfort or style could also be identified as an intangible cost. But, regardless of the depth of such an analysis, this list is relatively common and generally used in the decision matrix. There are, however, hidden costs that rarely come to the surface.

Hidden Costs

In the development of a seating life cycle cost analysis, Time-to-Refurbishment is most commonly coupled to Loss-of-Comfort. An argument can be made, however, suggesting Compromised Safety as another criterion, which could become a significant source of hidden cost. The notion of Compromised Safety is predicated upon the global rail standards that address flame, smoke, and toxicity (FST), such as ASTM D 3675, BS 6853, NFF 16-101, and DIN 5510. Full seat assemblies or the individual materials in the construction of a seat and seat cushion are mandated to conform to specific FST measurements dependent upon the category of the train. In practice, certified third-party FST test reports must accompany a seating tender as verification of conformance. Thus, the qualification testing of a material or full seat is of the utmost importance. Conversely, there are significant consequences if the test report comes back as a NO PASS or at a rating lower than intended.

The language in the standards, as well as the testing methods, is the result of professional expertise and years of collaboration. They are well-defined, sophisticated and strictly reviewed. Thus, the preparation of a vendor sample designated for testing submission will be of the highest level of engineering and craftsmanship. For example, a seat, which is to be submitted for a British Spec (BS) 6853 Category 1a burn test, will be meticulously assembled, with special attention to the wrapping of the fire barriers and upholstery fabric over the foam seat cushion. The tighter the fit of the upholstery folds and tucks, the lower the risk of air paths between the fabric and foam cushion. An air path has the potential of becoming an oxygen source for accelerated flame spread. Objectively, the entire process is logical, legitimate, and of the utmost regard for the safety of the ridership.

Why, then, is this a discussion on hidden life cycle costs? The answer to this question is built upon cycle-testing data, field observations, and a hypothesis. It has been established that achieving good FST results is partially dependent upon the best-of-the-best material samples and assembly practices. What would be the FST results if a mass-production seat were pulled out of service after thousands of cycles and months or years of usage? Depending on the type of foam specified for the cushioning, the cycling of a seat can result in a loss in foam thickness, reduced spring-back force (the resultant force the foam will have upon the load of the passenger, which is directly related to comfort), and compromised weight distribution, which directly affects comfort performance. This deterioration will cause the fit between the upholstered fabric, fire barriers, and foam to become loose and crumpled.

In a lab study performed in the UK, a 100,000 cycle test was conducted on two upholstered foam cushions: fire-retardant open cell polyurethane foam and open cell silicone foam. The empirical data is astounding. The fire retardant polyurethane material diminished in thickness by >10% with a spring force loss of >50%, while the silicone foam deterioration was negligible in both thickness and spring force. In addition, the polyurethane cushion took on a compression set with a concave shape replicating the shape of the Jounce and Squirm impact apparatus. The discoloration, as seen in the photo, is the remnants of the fire retardant filler after having eroded away the cell walls of the polyurethane foam. (See Figure 1).

Screen Shot 2015-12-31 at 11.31.27 AM

In complete contrast, the silicone foam shows no shaped compression set and has no visual defects. While this is test data, it has been observed in the field as well. The photo of the cycle tested upholstered polyurethane cushion is comparable to worn and loose seats found in service. (See Figure 2).

Screen Shot 2015-12-31 at 11.34.14 AM

Back to LCA!

Would this 100,000 cycle tested seat still pass the FST requirements as did its hand-crafted predecessor without any cycling or wear? Is this a compromise of safety? Also, is this a hidden cost that should be included into an LCA? None of these questions is to be answered in this discussion, but only offered up as a question for serious thought. Comprehensively, a thorough life cycle cost analysis should include a systems approach – analyzing potential costs, with probabilities, of a system or module failure due to performance degradation of the component under evaluation.

In the case of the seats, there may be a point in time when loss of thickness, spring back force, or a shaping from compression set could cause a system failure. The system failure could range from acceptable comfort to something as severe as non-compliance to an FST standard.

Rogers Corporation, the manufacturer of BISCO MF1 open cell silicone foam specially formulated for rail car seating cushions, has launched a Seat Cushion Cost of Ownership Tool (See Figure 3) that allows the user to simulate a specific scenario: size of seat cushion, number of seats per car, number of cars in the fleet, average number of years between refurbishments, cost of competitive materials, labor rates, estimated number of replacements, need for fire barriers, and revenue loss.

Screen Shot 2015-12-31 at 11.25.05 AM

The calculator presents a picture of the total cost of ownership for each material along with metric tons of material that will be designated for landfill over the time period.

It will be up to the user to add in the hidden costs – if there are any – as suggested in this discussion. Hidden costs can be difficult to measure, but that doesn’t make them any less significant, especially when it comes to any potential compromise of safety.


It is an understatement to say that the design, development, and eventual tender and sale of passenger rail cars ranks among the most cost competitive products in the world. Designers, engineers, and supply chain managers are under extreme pressure to deliver high quality, high-performing, and very long-lasting products to the market. Meeting cost budgets is made possible by ingenuity, technology, and strategic manufacturing and marketing.Rail_Window

As with any purchasing decision, the price of any one rail car component or module has a cost-value relationship. And more often than not, there is a significant amount of emphasis and evaluation placed upon the selection of the materials used within the more prominent features of the interiors. This includes composite materials for side wall and ceiling liners, seat cushion foams and upholstery, and flooring structures, to name a few.

In addition, specialty materials are also specified in combination with highly sophisticated HVAC and control system hardware. Buyers and sellers will negotiate the integrity and different levels of performance among the choices (and prices) available – balancing budget with cost and value.

However, what is frequently ignored is the hidden value and importance of the multitude of gaskets and seals that protect, preserve and actually integrate the various interior systems and modules together. While one of the main functions of a rail gasket or seal is to keep moisture or water from leaking into an undesired place, there are also many other considerations that should go into gasket selection. A great deal of engineering effort is spent determining vibration isolation and damping, acoustic blocking and absorption, as well as contribution to EMI/RFI protection and conformance to flame, smoke and toxicity requirements.

The cost of a rail gasket is driven mainly by the selection of the material that will be used for fabrication. And being that the full range of a gasket’s importance is often unknown, the materials for gaskets have occasionally been identified as a potential cost-cutting initiative. This is where one must fully appreciate that the cost of a gasket is much more than its price. Depending on its application, a specific sealing strip could be the main vibration isolator between two expensive wall panels. Should the sealing strip be fabricated from a less expensive material, it will most likely have a shorter life.

The life of a strip or gasket is determined by many characteristics. One is its ability to not take a “compression set,” to experience a permanent decrease in thickness over time. Another is its “degradation of force deflection” over time, which relates to a material’s resultant force when in compression. In this example, a decline in the vibration isolation performance capabilities could allow for the wall panels to fatigue at a much quicker rate than design intent. This could result in many scenarios such as a noisier and more uncomfortable ride, displeasing passengers – or potentially opening up the need to replace failing wall panels earlier than according to the MRO budget.

There are other and more critical examples, such as the protection of controls equipment in each car of an EMU train or the requirement to meet a BS 6853 Category 1a or NFF 16-101 M1F1 FST standard. In any of these hypothetical situations, the cost of the seal or gasket can be extrapolated into multiples of the price.

Screen Shot 2015-12-18 at 10.43.45 AMToday, there are countless train builds that have embraced the need for high-performing, long life materials for seal and gasket design. Solutions designed from silicone materials are either in final design or in service with many high-profile projects, including the many CHR (China 380 km high speed rail) projects, the Sydney, Australia PPP project, as well as leading designs throughout Europe.

BISCO Silicones from Rogers Corporation, which offer a portfolio of materials specific to the rail industry, offer application-specific solutions to meet all objectives, and can assist in determining the full cost impact that a poorly designed gasket or specified material could have.

For more information on flame standards, read “Don’t Get Burned by Flame Standards” in Machine Design.


This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Millimeter-wave circuits were once considered exotic and only used for specialized applications, typically in the military space. For one thing, frequencies with such small wavelengths, from about 30 to 300 GHz, required special components and circuits scaled to those diminutive wavelengths. But lower-frequency bands are being consumed by a growing number of wireless applications, and millimeter-wave frequency bands are looking more and more attractive for communications systems of the future. Such high frequencies have even been proposed as part of an emerging fifth-generation (5G) wireless standard that will be challenged to connect billions of global Internet of Things (IoT) devices by means of available wireless bandwidths. Millimeter-wave bandwidths have long been employed for military radar systems and are increasingly being used in commercial automotive collision-avoidance radar systems. Achieving millimeter-wave circuit designs on reliable printed-circuit-board (PCB) materials in a practical manner will be the challenge in making these higher frequencies affordable. Substrate-Integrated-Waveguide (SIW) circuit technology may just be the solution.

As noted in an earlier blog (“Make Waveguide in Planar PCB Form”), SIW structures are essentially waveguide in planar form, with the capability to support millimeter-wave signals with relatively low loss even at those higher frequencies. SIW technology offers improved performance at millimeter-wave frequencies compared to traditional transmission-line technologies, such as microstrip, stripline, and even grounded coplanar-waveguide (GCPW) approaches, with limitations at millimeter-wave frequencies.

SIW has often been described as a form of transition between microstrip and dielectric filled waveguide (DFW). SIW can be fabricated with many of the same methods as microstrip. At millimeter-wave frequencies, however, microstrip circuits require small features and extremely tight machined tolerances to support the transmission of such high frequencies. In addition, at millimeter-wave frequencies, SIW circuits do not exhibit radiation losses suffered by microstrip. In fact, SIW circuits in general do not have the potential problems with electromagnetic interference (EMI) of the other transmission-line formats. SIW technology provides the means to realize extremely compact components; it is suitable for passive components, such as filters, but has also been used as active components, such as oscillators, at microwave through millimeter-wave frequencies. Commercial EM simulation software is most often used to aid in the design, simulation, and optimization of SIW circuitry, and such software programs can effectively model the effects of the dielectric substrates used as the foundations for SIW circuitry.

In forming SIW transmission lines, a rectangular waveguide is created within a substrate, usually on circuit-board material such as RO4350B™ LoPro® laminates from Rogers Corp. which has a low relative dielectric constant of 3.48 in the z-axis (through the thickness) measured at 10 GHz. This low-loss circuit material, which is widely used as the foundation for wireless base-station power amplifiers, features properties well suited to SIW circuits. It can be fabricated with the methods used for FR-4 circuit materials, to maintain low production costs.

SIW circuits and their dielectric-filled waveguide transmission lines are formed on a circuit material such as RO4350B LoPro laminate by adding a top metal plane over a laminate with a ground plane, then fabricating rows of conductive plated viaholes on both sides along the length of the substrate material. These plated-through-hole (PTH) viaholes are used to make the sidewalls of the rectangular waveguide structure formed on the PCB material. In forming the SIW embedded waveguide structure, more conductive metal is actually used than in stripline or microstrip transmission lines for similar wavelengths, resulting in less conduction loss at microwave and millimeter-wave frequencies.

What is critical in the fabrication of SIW circuits is the formation and spacing of the viaholes. Close spacing yields less conduction loss through the use of more conductive metal to form the waveguide structures, but also results in longer and more complex production times in fabricating the SIW PCBs. Wider spacing can save production time, but can also raise conduction losses and can result in higher EM leakage losses because of the wide spacing. SIW circuits will also suffer dielectric losses (as will all high-frequency circuit formats), which are dependent upon the choice of circuit laminate material. For SIW circuits, whether at microwave or millimeter-wave frequencies, and really with all high-frequency circuits, PCB materials should be chosen wisely for optimum balance between performance and cost.

One PCB material parameter that is critical for SIW reliability is coefficient of thermal expansion (CTE) which gauges the expansion of a circuit material with elevated temperatures. The SIW viaholes are plated through holes (PTHs) through the dielectric PCB material, and high values of CTE, which denote excessive expansion with temperature, will result in undue stress on the sidewalls of the PTHs. Circuit materials, such as RO4350B LoPro noted previously, and RO4835™ LoPro material from Rogers Corp., with stable CTE characteristics, are ideal candidates for high-reliability SIW circuits. RO4835 LoPro circuit materials have been used for years in the fabrication of multilayer circuits with high layer counts, relying on PTHs for interconnection of those many layers. As an added benefit for creating cost-effective SIW circuits, both RO4835B LoPro and RO4835 LoPro materials can be fabricated with standard FR-4 epoxy/glass processes to help minimize production costs.

At millimeter-wave frequencies, SIW circuits exhibit low loss similar to their larger mechanical waveguide descendants, and considerably less than the other, more conventional transmission-line formats, such as microstrip, stripline, and GCPW. But SIW circuits also share other traits of larger mechanical waveguide, including a lower-frequency cutoff point. As with mechanical waveguide, SIW circuits are designed for particular operating frequencies and bandwidths depending upon the circuit dimensions, and designers must be aware that they will be working with a lower-frequency (and upper-frequency) cutoff point and a target low-loss passband. But SIW circuits can also work quite well with traditional transmission-line formats, with microstrip and GCPW transmission lines serving as fairly simple and effective feedlines from other parts of a circuit to the SIW circuitry.

With the expected growth in the demand for millimeter-wave circuits, especially with the expansion of IoT and higher-frequency automotive applications, SIW technology appears quite attractive as an effective and practical solution for designing and fabricating affordable millimeter-wave circuits. In particular, SIW technology provides the means to realize miniaturized planar antennas as millimeter-wScreen shot 2014-08-08 at 1.33.54 PMave frequencies, perhaps in volumes reaching millions of components as needed for these many emerging wireless applications.

ROG Mobile App

Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

Ask an Engineer

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

For more than 45 years, PORON® technologies have provided long-term comfort and performance to brands in the work, outdoor, athletic, casual, and fashion industries. Through our close partnerships with designers and developers, we continue to innovate new technology advancements for underfoot cushioning.

PORON Comfort Mens footwear image web

For those more focused on sporting goods, you have seen the rapid growth of the PORON sub-brand — PORON® XRD® Extreme Impact Protection. PORON® XRD® Impact Protection innovations and brand partnerships can be found in American football, work wear, baseball, mountain biking, and more.

As we continue to serve our brand partners and establish connections with athletes, we have recognized a need to establish two separate brands: PORON Comfort and XRD Impact Protection. Working as PORON Comfort and XRD Impact Protection will allow us to better serve the distinct needs of our footwear and impact protection partners and end-users.

Over the next couple of months, you will see some exciting news and updates from both the PORON Comfort and XRD Impact Protection brands. Some of these updates are starting to evolve today via our social and online media channels.002962_PORON_OR Daily Show Ad-1_v5.indd

But one thing will not change, you can rest assured the PORON Comfort and XRD Extreme Impact Protection brands will continue to stand for innovation and quality, and our partnerships with the industry’s leading brands will continue to lead the way for advancements in product design.

We hope you will continue to take part in both the PORON Comfort and XRD Impact Protection brands. Stay tuned for new developments, partnership announcements and brand engagement opportunities from both PORON Comfort and XRD Extreme Impact Protection!

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