Conventional transmission lines electronic symbol for a transmission line. A
transmission line is the material
medium or structure that forms all or part of a
path from one place to another for directing the
transmission of energy, such as
electromagnetic waves or
electric power transmission. Types of transmission line include
wires,
coaxial cables, dielectric slabs,
striplines,
optical fibers,
electric power lines and waveguides. A
microstrip is a type of transmission line that can be fabricated using
printed circuit board technology and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the
substrate. Microwave components such as
antennas,
couplers,
filters and
power dividers can be formed from a microstrip. From the simplified schematics to the right it can be seen that total impedance, conductance, reactance (capacitance and inductance) and the transmission medium (transmission line) can be represented by single components that give the overall value. With transmission line media it is important to match the load impedance ZL to the
characteristic impedance Z0 as closely as possible, because it is usually desirable that the load absorbs as much power as possible. : R is the
resistance per unit length, : L is the
inductance per unit length, : G is the
conductance of the dielectric per unit length, : C is the
capacitance per unit length, : j is the
imaginary unit, and : \omega is the
angular frequency.
Lumped circuit elements Often, because of the goal that moves physical metamaterial inclusions (or cells) to smaller sizes, discussion and implementation of
lumped LC circuits or
distributed LC networks are often examined. Lumped circuit elements are actually microscopic elements that effectively approximate their larger component counterparts. For example, circuit capacitance and inductance can be created with split rings, which are on the scale of nanometers at optical frequencies. The distributed LC model is related to the lumped LC model, however the
distributed-element model is more accurate but more complex than the
lumped-element model.
Metamaterial-loaded transmission-line configurations Some noted metamaterial antennas employ negative-refractive-index transmission-line metamaterials (NRI-TLM). These include
lenses that can overcome the
diffraction limit, narrowband and broadband phase-shifting lines, small antennas, low-profile antennas, antenna feed networks, novel power architectures, and high-directivity couplers. Loading a planar metamaterial network of TLs with series capacitors and shunt inductors produces higher performance. This results in a large operating
bandwidth while the refractive index is negative.
Negative refractive index metamaterials supporting 2-D waves In 2002, rather than using SRR-wire configuration, or other 3-D media, researchers looked at planar configurations that supported backward wave propagation, thus demonstrating negative refractive index and focusing as a consequence.
Growing evanescent waves in negative-refractive-index transmission-line media The periodic 2-D LC loaded transmission-line (
TL) was shown to exhibit NRI properties over a broad frequency range. This network will be referred to as a dual TL structure since it is of a high-pass configuration, as opposed to the low-pass representation of a conventional TL structure.
Backward wave antenna using an NRI loaded transmission line Grbic
et al. used one-dimensional LC loaded transmission line network, which supports fast backward-wave propagation to demonstrate characteristics analogous to "reversed Cherenkov radiation". Their proposed backward-wave radiating structure was inspired by negative refractive index LC materials. The simulated E-plane pattern at 15 GHz showed radiation towards the backfire direction in the far-field pattern, clearly indicating the excitation of a backward wave. Since the transverse dimension of the array is electrically short, the structure is backed by a long metallic trough. The trough acts as a waveguide below cut-off and recovers the back radiation, resulting in unidirectional far-field patterns.
Planar NIMs with periodic loaded transmission lines Planar media can be implemented with an effective negative refractive index. The underlying concept is based on appropriately loading a printed network of transmission lines periodically with inductors and capacitors. This technique results in effective permittivity and permeability material parameters that are both inherently and simultaneously negative, obviating the need to employ separate means. The proposed media possess other desirable features including very wide bandwidth over which the refractive index remains negative, the ability to guide 2-D TM waves, scalability from RF to millimeter-wave frequencies and low transmission losses, as well as the potential for tunability by inserting varactors and/or switches in the unit cell. The concept has been verified with circuit and full-wave simulations. A prototype focusing device has been tested experimentally. The experimental results demonstrated focusing of an incident cylindrical wave within an octave bandwidth and over an electrically short area; suggestive of near-field focusing. == Configurations ==