Stripline

As a stripline, Eng. stripline is referred to a certain class of electric waveguides. All the strip lines have in common that they consist of one or more thin conductive strips which are applied to a dielectric. Can stripline structures eg from in a plane disposed conduit strips are made. Often they are isolated or placed in over a metallic surface.

Application is the high-frequency technology and there especially the microwave range of frequencies - with strip lines can be inexpensively and reproducibly defined impedances in circuits for propagation, coupling and filtering high signal frequencies produced.

And the power supply and the radiating elements of antenna may be formed as a strip conductor.

Synonyms and differentiation from other concepts

Frequently the English term stripline is used, sometimes the term microstrip, which, however, refers to a special design. The term stripline and the description of the design at first sight suggests that it generally in printed circuit boards ( PCBs ) is striplines. From setting up the interconnects same principle striplines, but only the latter are dimensioned and operated as a waveguide, which is not generally true for printed circuits and printed circuit boards.

Since the design of the microstrip line is the most frequent, the terms are often equated. In this case, however, other types were excluded as the coplanar line, for example. Therefore, a separation of terms is attached.

Properties as a waveguide

Strip lines are dimensioned so that in general only able to spread quasi-TEM wave. These can be viewed with some simplifications almost as TEM waves: both the electric and the magnetic fields are almost exclusively perpendicular to the propagation direction, as is the case in coaxial or twisted pair cables. Condition for this is that the cross - dimensions of the ducts are small compared to the wavelength. Strip lines are used only for short distances within assemblies.

The advantage of strip lines is that they can be inexpensive, reproducible and material-saving manufacture. This is particularly important in complex circuits in which there are also other, consisting of strip lines, components. Another advantage is the low field spreading out of the planar structure, and therefore only a small emission of waves occurring in the chamber. Therefore, high-frequency circuits fabricated in stripline technology can often be operated without a closed case or without individual, separate chambers.

The wave impedance of a stripline is determined by its width and through the thickness and Dielektriziätszahl the insulator substrate. Since the latter two quantities are generally constant, the calculation and simulation of stripline circuits is facilitated. A calculation tool can be found in.

Strip conductor structures are used not only for the processing of high frequency signals, in or as antennas. They often form on a common substrate to both the feeding and adjusting component and the radiating elements are themselves examples of the patch antenna, the helical antenna, the panel antenna and dipole antennas. All of these antennas can be made ​​entirely of planar stripline structures. Also, helical antennas are often made ​​of strip lines, but here they are wrapped around a cylinder or cone. In all these cases, the phase positions and impedances of the line-bound waves by varying the length and width of the strip lines are brought to the fact that their fields overlap such that a (often directed) held radiation than radio waves.

Designs

There are a variety of types, which can be used in combination under certain circumstances. These include:

• Microstrip line
• Balanced stripline
• Coplanar, balanced or unbalanced
• Double belt line
• Unshielded slot lines ( eg part of the Vivaldi antenna )
• Shielded, that is, into a waveguide built-in slot-lines (also called Finleitungen ): unilateral Finleitung
• Bilateral Finleitung
• Antipodal Finleitung

Microstrip line

As microstrip lines strip lines are referred to consist of a conductive strip, which are separated by a dielectric substrate of a conductive surface. They are usually used for transporting and processing of electromagnetic waves in the range of several hundred megahertz and 20 gigahertz.

A microstrip line is made of a non-conductive substrate ( PCB ), which is completely metallised on the lower side ( ground surface ). On top of a conductor in the form of a strip ( strip conductor ), so arranged with a defined cross-sectional area. This strip is usually made ​​by machining the upper metallization by etching or milling.

As the substrate are different dielectrics. Is widely used glass-fiber reinforced PTFE ( RT / Duroid ). For higher demands alumina is used in addition to other ceramic materials. That in the normal board manufacturing conventional FR4 ( glass-reinforced epoxy resin) is not suitable at frequencies above a few GHz, typically, as the loss tangent is too large.

The signal propagates on the one hand in the space between stripline and ground plane. On the other hand, the field lines also occur in the free space above the strip conductor, which is usually filled with air. One must therefore speak of an inhomogeneous dielectric.

If the strip conductor is interrupted, the signal can skip the gap under certain conditions and then spread further.

Characteristic impedance

For microstrip lines on printed circuit boards an exact solution for the characteristic impedance ( characteristic impedance ) can be specified for particular cases, the most common form of H. Wheeler in 1965 derived in the following form:

Weff is the effective width of the cable including a correction factor for the thickness of the metallization. This effective width is given by the following equation:

With

The above equation for the characteristic impedance under the following conditions provides asymptotically exact values:

For all other cases, the equal sign in the above equation is replaced by a ≈ and the error of the approximation is usually less than 1% and less than 2% guaranteed.

In addition, still exist in the literature a number of other, mostly simpler approximate equations with restricted scopes for the characteristic impedance of microstrip lines.

Balanced stripline

In contrast to the micro- strip line of the conductor strip is in the symmetrical strip line at top and bottom covered with a thick dielectric material is equal and parallel to two conductive layers (ground), which are applied to the dielectrics. Since the electric field lines can not up and down occur in the free space by the complete coverage with conductive material can be spoken by a homogeneous dielectric, which facilitates the simulation of circuits.

Symmetrical strip lines are more difficult to manufacture due to the higher number of layers and are therefore rarely used alone.

Coplanar

As a coplanar stripline are referred to the same layer as a metallized surface connected to ground and separated from it only by a gap. All conductive layers are on one side of a continuous dielectric constant density. Below and above the circuit is air passing through the electric field lines and, therefore, the medium must be considered as inhomogeneous.

Many circuits consist of coplanar lines, under whose dielectric as microstrip lines, a ground plane is present. The upper, dashed lines, and the lower, closed ground plane connected by vias. This combination circuits can be produced, in which only small interactions between the conductor structures as well as to the environment occur.

Components in stripline circuits

Simple components such as capacitors and inductors can be generated directly by specially dimensioned strip. So thin conductors have long, inductive, whereas short, broad, a capacitive effect. In addition to these classical elements can still be more typical for the high-frequency technology components implemented directly with stripes.

These include:

• Sump ( anechoic termination)
• Impedance matching, inductive or capacitive coupling
• Reflector, a series resonant circuit, the parallel resonant circuit

From these basic elements, more complex functional units can be produced:

• Directional coupler
• Power divider
• Filter (band pass, band reject, high pass, low pass)
• Crossovers
• Exchanger for initial and coupling, electrical isolation, impedance or symmetry adaptation

Discrete components, such as are used in normal circuit boards can also be soldered to a microstrip circuit, taking into account their dimensions and mutual interference. Particularly suitable are SMD components. Partly also SMD types are specifically designed for this case. This is especially the case with active elements such as transistors or diodes.