**1. Introduction**

46 Wireless Communications and Networks – Recent Advances

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Microstrip antennas are one of the most widely used types of antennas in the microwave frequency range, and they are often used in the millimeter-wave frequency range. Actually as the demand for high data rates grows and microwave frequency bands become congested, the millimeter-wave spectrum is becoming increasingly attractive for emerging wireless applications. The abundance of bandwidth and large propagation losses at millimeter-wave frequencies makes these bands best-suited for short-range or localized systems that provide broad bandwidth. Automotive radar systems including cruise control, collision avoidance and radiolocation with operation up to 10 GHz have a large market potential in the near future of millimetre wave applications.

One advantage of the microstrip antenna is easy matching, fabrication simplicity and low profile, in the sense that the substrate is fairly thin. If the substrate is thin enough, the antenna actually becomes conformal, meaning that the substrate can be bent to conform to a curved surface. Disadvantages of the microstrip antenna include the fact that it is usually narrowband, with bandwidths of a few percent being typical. Also, the radiation efficiency of the microstrip antenna tends to be lower than some other types of antennas, with efficiencies between 70% and 90% being typical. A microstrip antenna operating in a travelling wave configuration could provide the bandwidth and the efficiencies needed.

Travelling-wave antennas are a class of antennas that use a travelling wave on a guiding structure as the main radiating mechanism. It is well known that antennas with open-ended wires where the current must go to zero (dipoles, monopoles, etc.) can be characterized as standing wave antennas or resonant antennas. The current on these antennas can be written as a sum of waves traveling in opposite directions (waves which travel toward the end of the wire and are reflected in the opposite direction). For example, the current on a dipole of length l is given by:

$$I(\mathbf{z}) = I\_o \sin\left[k\left(\frac{I}{2} - \mathbf{z}'\right)\right]$$

$$= \frac{I\_o}{2f} \left[e^{\beta k\left(\frac{I}{2} - \mathbf{z}'\right)} - e^{-\beta k\left(\frac{I}{2} - \mathbf{z}'\right)}\right] \tag{1}$$

Traveling wave antennas are characterized by matched terminations (not open circuits) so that the current is defined in terms of waves traveling in only one direction (a complex exponential as opposed to a sine or cosine). A traveling wave antenna can be formed by a single wire transmission line (single wire over ground) which is terminated with a matched load (no reflection). Typically, the length of the transmission line is several wavelengths.

Fig. 1. Beverage or wave antenna.

The antenna shown in Fig. 1 above is commonly called a Beverage or wave antenna. This antenna can be analyzed as a rectangular loop, according to image theory. However, the effects of an imperfect ground may be significant and can be included using the reflection coefficient approach. The contribution to the far fields due to the vertical conductors is typically neglected since it is small if l >> h. Note that the antenna does not radiate efficiently if the height h is small relative to wavelength. In an alternative technique of analyzing this antenna, the far field produced by a long isolated wire of length l can be determined and the overall far field found using the 2 element array factor. Traveling wave antennas are commonly formed using wire segments with different geometries. Therefore, the antenna far field can be obtained by superposition using the far fields of the individual segments. Thus, the radiation characteristics of a long straight segment of wire carrying a traveling wave type of current are necessary to analyze the typical traveling wave antenna. Traveling-wave antennas are distinguished from other antennas by the presence of a traveling wave along the structure and by the propagation of power in a single direction. Linear wire antennas are the dominant type of traveling-wave antennas.

There are in general two types of traveling-wave antennas [1-2]. The first one is the surfacewave antenna, which is a slow-wave structure, where the phase velocity of the wave is smaller than the velocity of light in free space and the radiation occurs from discontinuities in the structure (typically the feed and the termination regions). The propagation wavenumber of the traveling wave is therefore a real number (ignoring conductors or other losses). Because the wave radiates only at the discontinuities, the radiation pattern physically arises from two equivalent sources, one at the beginning and one at the end of the

Traveling wave antennas are characterized by matched terminations (not open circuits) so that the current is defined in terms of waves traveling in only one direction (a complex exponential as opposed to a sine or cosine). A traveling wave antenna can be formed by a single wire transmission line (single wire over ground) which is terminated with a matched load (no reflection). Typically, the length of the transmission line is several wavelengths.

The antenna shown in Fig. 1 above is commonly called a Beverage or wave antenna. This antenna can be analyzed as a rectangular loop, according to image theory. However, the effects of an imperfect ground may be significant and can be included using the reflection coefficient approach. The contribution to the far fields due to the vertical conductors is typically neglected since it is small if l >> h. Note that the antenna does not radiate efficiently if the height h is small relative to wavelength. In an alternative technique of analyzing this antenna, the far field produced by a long isolated wire of length l can be determined and the overall far field found using the 2 element array factor. Traveling wave antennas are commonly formed using wire segments with different geometries. Therefore, the antenna far field can be obtained by superposition using the far fields of the individual segments. Thus, the radiation characteristics of a long straight segment of wire carrying a traveling wave type of current are necessary to analyze the typical traveling wave antenna. Traveling-wave antennas are distinguished from other antennas by the presence of a traveling wave along the structure and by the propagation of power in a single direction.

There are in general two types of traveling-wave antennas [1-2]. The first one is the surfacewave antenna, which is a slow-wave structure, where the phase velocity of the wave is smaller than the velocity of light in free space and the radiation occurs from discontinuities in the structure (typically the feed and the termination regions). The propagation wavenumber of the traveling wave is therefore a real number (ignoring conductors or other losses). Because the wave radiates only at the discontinuities, the radiation pattern physically arises from two equivalent sources, one at the beginning and one at the end of the

Linear wire antennas are the dominant type of traveling-wave antennas.

Fig. 1. Beverage or wave antenna.

structure. This makes it difficult to obtain highly-directive singlebeam radiation patterns. However, moderately directly patterns having a main beam near endfire can be achieved, although with a significant sidelobe level. For these antennas there is an optimum length depending on the desired location of the main beam. Examples include wires in free space or over a ground plane, helixes, dielectric slabs or rods, corrugated conductors, ''beverage'' antenna, or the V antenna. An independent control of the beam angle and the beam width is not possible.

The second type of the travelling wave antennas are a fast-wave structure as leaky-wave antenna (LWA) where the phase velocity of the wave is greater than the velocity of light in free space. The structure radiates all its power with the fields decaying in the direction of wave travel.

A popular and practical traveling-wave antenna is the Yagi–Uda antenna It uses an arrangement of parasitic elements around the feed element to act as reflectors and directors to produce an endfire beam. The elements are linear dipoles with a folded dipole used as the feed. The mutual coupling between the standing-wave current elements in the antenna is used to produce a traveling-wave unidirectional pattern. Recently has been developed a new simple analytical and technical design of meanderline antenna, taped leaky wave antenna (LWA) and taped composite right/left-handed transmission-line (CRLHTL) LWA. The meanderline antenna is a traveling-wave structure, which enables reduction of the antenna length. It has a periodical array structure of alternative square patterns. With this pattern, the extended wire can be made much longer than the initial antenna (dipole) length, so that the selfresonance can be attained. The resonance frequency is then lower and radiation resistance is higher than that of a dipole of the same length. This in turn implies that the antenna is effectively made small.
