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

Stripline is one of the transmission-line options facing high-frequency circuit designers, especially for circuits where minimal electromagnetic (EM) radiation is important. Stripline can be thought of as a flat conductor suspended between two ground planes, with dielectric material separating the conductor from the ground planes. The configuration results in less EM radiation than circuits with microstrip transmission lines, with greater isolation between adjacent circuit traces compared to microstrip, but in a buried configuration that is more difficult to fabricate and service than microstrip. In spite of the challenges, the high performance possible has encouraged designers to implement high-frequency multilayer stripline constructions in extremely compact configurations. The choice of printed-circuit-board (PCB) material can contribute a great deal to the success of a single-layer or multilayer stripline circuit assembly.

PrintStripline circuits can be assembled in numerous forms, including as balanced, offset, and suspended stripline versions. The characteristics of the circuit-board material, such as its dielectric constant (Dk), can contribute much to the performance and behavior of these different stripline versions. Stripline circuits have even been fabricated with different dielectric materials (with their different Dk values) on either side of the conductor, for added design flexibility.

Ground and signal paths in stripline are typically created by forming plated viaholes through the conductive and dielectric materials, and the use of plated viaholes also lends itself to forming signal paths in multilayer circuit constructions. The EM fields within a stripline circuit assembly are strongly contained near the center conductor and the top and bottom ground planes of each layer, and it is important to closely match the top and bottom ground planes to the same potential to prevent propagation of any unwanted parallel-plate modes between the two ground planes.

Ideally, high-frequency EM signals are contained entirely within a stripline PCB, with no leakage or emissions and with excellent shielding against spurious signals.

Stripline is considered a transverse-electromagnetic (TEM) medium in contrast to microstrip, which is a quasi-TEM medium. Most stripline designs aim for a characteristic impedance of 50 Ω, which is determined by the width of the conductive strip, the thickness of the circuit substrate material, and the dielectric constant of the substrate material. For a 50-Ω characteristic impedance, and a given thickness of circuit dielectric material, a stripline circuit will employ a narrower conductive strip than a microstrip circuit, and it will suffer greater loss through the dielectric material than microstrip, which uses the lossless air above the circuit in part for its signal propagation. For a comparison of stripline and microstrip, please visit the December 20, 2010 ROG Blog: “Microstrip Versus Stripline: How To Make The Choice.

In terms of selecting circuit materials for stripline, a smooth copper conductor surface is important for minimizing loss and the condition of the copper conductor is often overlooked when assessing the loss characteristics of a PCB. A copper conductor with rough surface will exhibit more loss than a copper conductor with smooth surface, so circuit materials intended for low-loss circuits should include smooth copper conductor surfaces. Since stripline has four copper-substrate interfaces, in addition, there is the opportunity for the roughness of the copper surface to be inconsistent across the different interfaces, resulting in differences in the conductor loss characteristics across the circuit board. Particularly in thin stripline circuits (where the ground-to-ground plane separation is 20 mils or less), copper conductor surface roughness can be a major contributor to the insertion loss of the circuit.

Stripline circuit fabricators may pay great attention to using a substrate with smooth conductive copper for the circuitry but may use a copper foil with much rougher surface for the other conductive layer. The rougher surface helps achieve increased bond strength when bonding the different circuit layers, resulting in a reliable multilayer circuit assembly. Unfortunately, it sacrifices some of the electrical performance possible through the use of the smoothest possible surfaces for all copper surfaces.

This difference in the surface roughness of the copper layers can impact the performance of offset stripline circuit assemblies, where the signal is not in the geometric center of the cross-sectional view of a circuit board. In such an assembly, the surface roughness of the copper plane that is closest to the signal plane will have the greatest effect on the circuit’s electrical performance. If this copper layer has the rougher surface, it can have an impact on the loss of the circuit. Even if the circuitry is fabricated on a copper layer with extremely smooth surface, the presence of another copper layer with rougher copper surface that is closer to the signal plane can override any benefits from the smooth copper surface used for the circuitry.

Stripline circuits are generally thought to be nondispersive in nature, but this may not always be the case. Circuits that incorporate a bonding layer with Dk that is different than the rest of the stripline structure may exhibit dispersive characteristics. To achieve a nondispersive circuit structure, the Dk should be fairly close throughout the stripline circuit. As examples, the RT/duroid® 6002 or RO3003™ circuit laminates are closely matched in Dk with the 2929 bondply materials, all from Rogers Corp. This consistency in Dk throughout a stripline circuit structure based on these materials will minimize dispersion.

The variations in stripline circuits include offset stripline and suspended stripline. Offset stripline can be formed by gluing together two stripline substrates of unequal heights. Suspended stripline makes use of air as part of the dielectric material, supporting pure TEM mode propagation. It is usually a circuit suspended within a metallic structure, with the entire structure enclosed. For low loss, suspended stripline technology has been used with the same conductive circuit pattern on both sides of the circuit assembly, electrically connected by plated viaholes. The entire assembly is then protected within a metallic enclosure with air cavities top and bottom serving as air substrates, where the lids are the ground planes. Suspended stripline circuits are capable of wide bandwidths with low loss and minimal spurious radiation, but assembly can also be complex and expensive.

Screen shot 2014-08-08 at 1.33.54 PMIn terms of propagation speed, stripline in its various forms will be slower than microstrip. That is, the propagation time through a microstrip circuit will be a fraction of the propagation time required for a similar stripline circuit on the same substrate material. Microstrip benefits from the use of air as a dielectric while stripline’s propagation characteristics are based solely on the dielectric material surrounding the signal trace. Propagation delays will increase in both cases with increased value of Dk for the dielectric material.

Stripline circuits in their various forms represent numerous design tradeoffs, including the difficulty of assembling, accessing, and testing stripline’s buried circuit traces versus the benefits of minimal signal leakage and only minor effects from external interference. Stripline may suffer somewhat higher losses than a microstrip circuit fabricated on a similar circuit material, but the stripline circuits will also be less affected by external interference signals and will exhibit less radiation of its own.

Download the ROG Mobile app to 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.

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.


RogersCorporation logoRogers Corporation (NYSE: ROG) plans to release results for its 2014 third quarter after the close of trading on Tuesday, October 28, 2014. A copy of the announcement will be available on the Rogers website at

All interested parties are invited to participate in Rogers’ quarterly teleconference to be held on Wednesday, October 29 at 9:00 am ET. Bruce D. Hoechner, President and CEO, and members of senior management will review the results and respond to questions.

To participate in the teleconference please call 1-800-574-8929, toll-free in the U.S., or 1-973-935-8524, outside the U.S. There is no pass code for the teleconference.

For interested parties who do not wish to ask questions, the call is being webcast live by Thomson Reuters and may be accessed through a link on the Rogers web site at

A slide presentation will be made available prior to the start of the call. The slide presentation may be accessed on the Rogers Corporation website under the Investor Relations section.

If you are unable to participate during the live teleconference, the call will be archived until Wednesday, November 5, 2014. The audio archive can be accessed by calling 1-855-859-2056 in the U.S. or 1-404-537-3406 outside the U.S. The pass code for the audio replay is 98226626. To access the archived audio online, please visit the Investor Relations section of the Rogers website and select the webcast link.

Technical Education Webinar Series
Title: High Frequency Materials and Characterization up to Millimeter Wave Frequencies
Date: October 23, 2014
Time: 8am PT/ 11am ET/ 3pm UTC
Sponsored by: Rogers Corp.
Presented by: John Coonrod, Sr. Market Development Engineer, Rogers Corporation, Advanced Circuit Materials Division
Screen shot 2014-10-17 at 12.58.15 PMOverview:
Microwave circuit designers have many powerful tools. However most are strongly dependent on the accuracy of the input data. High frequency printed circuit board (PCB) material properties are often assumed to be very simple, but understanding the details can increase model accuracy and minimize design time.
Those familiar with high frequency behavior of circuit materials realize the term for relative permittivity of “dielectric constant” is a misnomer and is not a constant. The dielectric constant of any PCB material will vary with frequency. Additionally wave propagation properties can be altered by a relationship of substrate thickness to copper surface roughness and this can vary the dielectric constant as perceived by circuit performance, even when using homogenous material.This webinar will give a simple overview of common test methods used to determine the dielectric constant for high frequency materials, followed by a description of methods that are best for characterizing material properties for microwave modeling and design.Agenda:

  • Description of Common Test Methods
  • Optimal Test Procedures for Circuit Characterization
  • Real World Application of RF Circuit Design Best Practices

Why join this webcast?
The actual circuit defined dielectric constant is an elusive property for designers to properly incorporate into their design models. This webinar will help designers define the dielectric constant parameters of a circuit as related to frequency, circuit thickness and copper surface roughness.

Register Now

Sponsored by:

Rogers Corporation



The Rogers Corporation Board of Directors has appointed Helene Simonet to serve as a member of the Company’s Board. This addition fills a vacancy on the Board due to a recent director retirement.  Ms. Simonet will be a member of the Audit Committee.

RogersCorporation logoSince April 2002, Ms. Simonet has served as the Executive Vice President and Chief Financial Officer of Coherent, Inc. (NASDAQ: COHR). Coherent is a leading global supplier of photonics technology products and solutions for a wide range of commercial and scientific research applications. Prior to joining Coherent in 1999, Ms. Simonet had previously spent more than twenty years in senior finance positions at Raychem Corporation.

“We are very pleased to have Helene join the Rogers Board of Directors,” said Bruce Hoechner, President and Chief Executive Officer of Rogers Corporation. “Her extensive background with global technology companies, as well as her finance leadership, makes her an excellent addition to our Board.”

Find more Rogers Corp. news here.



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

High frequency signals must survive many transitions in an RF/microwave system, with one of the more challenging being the point at which signals are “launched” from a coaxial connector to a printed-circuit board (PCB). Managing that transition without interruptions to the signals requires not only proper mechanical alignment but careful electrical optimization. Signals can propagate through many different kinds of coaxial connectors and many types of PCB materials, and the combinations require proper preparation to form the most seamless transitions. Following some general guidelines can help improve the effectiveness of an RF/microwave signal launch in double-copper-layer and multilayer PCBs, even when they contain different types of transmission-line formats, such as microstrip, stripline, and coplanar-waveguide (CPW) transmission lines.

PrintSuccessful high-frequency signal launches require a tight match between a coaxial connector and a PCB. Because of the wide number of choices for each, there are no automatic combinations that ensure this tight match. But following some basic design guidelines and, hopefully, access to computer simulation software, such as a commercial three-dimensional (3D) electromagnetic (EM) simulator, can help in optimizing the signal launch from a coaxial connector to a PCB.

In general, signal launches require good impedance matches between a coaxial connector and a PCB’s transmission lines. Good signal launches are easier to achieve at lower frequencies, and generally easier to achieve for narrowband designs than for broadband designs.

Many different coaxial connector types are used at high frequencies, including BNC, Type N, and SMA connectors, each with its own frequency range and mechanical and electrical characteristics. Connectors are differentiated by gender (male or female) as well as by different styles, such as standard straight connectors, edge-launch connectors for PCBs, and right-angle connectors for PCBs. These connectors and, often, their connected coaxial cables, will have a characteristic impedance, such as 50 or 75 Ω. A key to achieving a good signal launch to a PCB is to minimize disruptions in the impedance from the connector’s center conductor to the metal transmission line on the PCB, so that high-frequency signals can launch or flow without insertion loss or reflections (return loss) from the connector to the PCB’s transmission lines.

Signals and their electromagnetic (EM) fields propagating through a cable and connector have a cylindrical orientation, compared to the signals and EM fields in a PCB which have a planar or rectangular orientation. In changing from a connector to a PCB, the signals change orientation to adapt to the new propagation medium, and anomalies can occur in the form of signal loss or reflections. Different types of transmission lines on the PCB can present different challenges in making the connector-to-PCB transition, with grounded-coplanar-waveguide (GCPW) transmission lines providing the easiest transition, followed by microstrip transmission lines, and then stripline transmission lines being the most difficult to make the transition because of their “buried-within-the-substrate” nature.

Circuit designers have learned a few tricks in launching signals from different connectors to different types of transmission lines and PCBs. For one thing, the ground path on a PCB is a very important part of any successful signal launch from a connector to a PCB, since a continuous ground return path is essential to the uninterrupted, low-loss propagation of high-frequency signals from a connector through the PCB. The length of the ground path can also affect the quality of a signal launch from a coaxial connector to a PCB. Even such things as minimizing differences in conductivity between the solder used to join a coaxial connector’s metal parts to a PCB’s conductor metal can make an impact in improving the transition and the performance, especially at higher frequencies. These small losses and impedance mismatches are increasingly noticeable at higher frequencies.

One of the most basic practices in achieving a good signal launch is to minimize dimensional differences between a connector’s conductor and the circuit’s conductor on the PCB to which it is connected. At higher frequencies, connector dimensions shrink, and there is more of a tendency for the PCB’s conductor to be much wider, resulting in a capacitive spike at the transition from the connector conductor to the PCB conductor. Circuit designers have learned that by tapering the circuit conductor to create a more narrow transition where it meets the coaxial connector’s conductive pin, the transition becomes more inductive and less capacitive in nature, and the capacitive spike at the transition can be reduced or minimized. Impedance mismatches in the signal launch interface between a connector and a PCB are due to changes in the electrical characteristics of the circuit. An increase in impedance is due to a rise in inductance at the transition while a decrease in impedance is the result of an increase in capacitance at the transition.

Modifying the inductive or capacitive nature of the transition from the coaxial connector to the PCB will result in frequency-dependent changes to the nature of the signal launch. The PCB’s ground-plane spacing can also play a role in these frequency-dependent changes depending on how it changes the inductive/capacitive characteristics of the PCB and the transition. The length of the taper used to narrow the PCB’s transmission lines closer to the dimensions of the coaxial connector’s conductor can also impact the frequency response of the circuit

How does the choice of PCB material impact the quest for a high-performance signal launch? One of the more important characteristics of any circuit material for achieving a good signal launch is consistent dielectric constant (Dk) value throughout the material. Not only will this ensure consistent impedance for the transmission lines on the PCB but it will aid in achieving the desired impedance match at the transition from the coaxial connector to the PCB. The choice of circuit material Dk value will impact circuit dimensions at a given frequency, with higher Dk values resulting in narrow conductor widths and smaller circuits for a given frequency compared to circuit materials with lower Dk values. The thickness of the PCB material will also affect the transition from connector to PCB, since thicker PCB materials will yield wider transmission-line conductor widths for the same impedance, which may require additional tapering on the PCB in order to achieve a conductor width that is more closely matched to the coaxial connector’s conductor width.

Multilayer circuits (with layers usually numbered according to the number of conductor layers) can produce their own set of headaches for achieving good signal launches with coaxial connectors, since they offer added complexity and may even include different transmission-line technologies together. Coaxial connectors are typically mounted on the top of a multilayer assembly, and plated through holes (PTHs) are used to create electrical paths to the different PCB conductor layers. A connector’s signal launch typical employs a PTH for electrical connections to the PCB’s inner conductor layers. Access to inner layer conductors may be more difficult, but the essential guidelines referenced for achieving good signal launch, such as trying to match connector and PCB conductor dimensions, still hold.

Download the ROG Mobile app to 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.

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.

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