Special Measurement Techniques for Printed Antennas
This section describes some specific measurement techniques that are basically helpful in the development of structure and fabrication procedure of printed antennas rather than standard measurement techniques. There are two basic categories
Roots of J'„ (k,ma) = 0
Mode (n, m)
Root (X„„ or k„,„a)
in which the importance of these measurements exists [59, 60]. The first one is the impedance measurements, and the second one is the pattern measurement of the printed antenna. The first one covers the complex value of reflection coefficient or equivalent input impedance characteristics at the antenna terminals. Other various radiation characteristics such as gain, beam width, side lobe levels and polarization effects are describes under the second category. The noise figure and efficiency measurements are achieved by both categories. Some specific measurement techniques are recommended only to supplement the measurements of the input impedance and the radiated field. Various factors are present to stimulate the use of these techniques. One of them is the use of undefined dielectric materials or the use of multi-layer substrates with dissimilar properties. Other one is the complex analytical problems that are produced due to transition occurs into the printed network by various transmission lines or a waveguide to main radiator. The next one is applied to the network of large arrays with a complicated feed. Some specific measurement techniques such as time-domain reflectometry (TDR), probing the near field and direct efficiency measurement are used to determine the properties of such arrays.
The dielectric constant ‘e/ and the loss tangent ‘tan S’ are the two basic measurable physical properties based on which substrates are commercially supplied. All physical properties are generally specified at a lower value of frequency, e.g. I MHz, and at a specified frequency of operation such as 10GHz. The surface resistivity ‘Rs’ of the metallic cladding is one more important physical property that is formulated with respect to the conductivity ac,
where the frequency is denoted by/and the vacuum permeability is denoted by /j0 (= 4 ttx l()~7 in SI units). There are different cases in which the measurements of the printed antenna are achieved. The first case arises at higher frequencies; at that time, the substrate is tested by the manufacturer. The second case arises in mass production that requires a high accuracy and reproducibility. Another case arises in the production of multi-layer substrates that are arranged by placing foam plates with a space between the dielectric layers.
The losses by generation of radiation or by surface waves occur due to the discontinuity of the currents in the transition area of the printed networks. Those transitions are formed between the printed antenna to the transmitter or receiver modules or to the coaxial or waveguide terminals which is working as a measurement tools. The main disadvantage of the transition is its reactive nature that gives unwanted reflections because this resonant frequency may be altered. However, a degradation in the performance of antenna occurs due to the passing of all radiated power through the transition. The most commonly used transition occurs in between the coaxial line and the microstrip line, and is achieved by the soldering of the extended inner conductor of the coaxial probe to the printed radiator. This type of transition is appropriate for thin substrates that contain less discontinuity, and many connectors are available commercially for this application. Some other transitions such as striplines, slotlines and transition between microstrip lines and waveguides are also of our interest.
Measurements of Printed Lines and Networks
Previous sections presented the properties of the antenna substrate and the effects of the feed connector briefly. This section describes the measurement of the printed lines and basic configurations of feed networks. Initially, three fundamental parameters of the printed lines as described in  are presented as follows:
a. The relation between the effective dielectric constant eeft and the propagation constant (3 is formulated by
where the wave number of the free space is represented by kn.
b. ac and ad are the ohmic and the dielectric parts of the attenuation factor «, respectively.
c. Zc is the characteristic impedance of the transmission line.
After that, analyse all the characteristics related to the structures of the printed antenna applied in the feed network, such as bends, T-junctions, variation in width and cross-junctions. All these structures actually show different types of discontinuity that produces reflections and losses. These effects are generally measured by quantitative or by equivalent electrical circuit modelling. The power splitters and delay lines are the fundamental elements that are frequently considered. The main disadvantage of the measurement by network analyser is its performance achieved with or without automatic error correction in the frequency domain. The network analysis of the printed circuits consists of the main problem of coaxial or waveguide forms of test ports and standard calibration units. Therefore, printed lines are measured through transitions.
There are various solutions available for network analysis, of which one is the use of outstanding connectors (VSWR of about 1.01). To separate the resonance of connectors and the device under test, the use of a pair of tested devices is another solution. The next practical solution is the use of resonators consisting of devices under test.
The near-field measurement of a printed antenna is a particular type of pattern measurement that is applied in the region of the radiating aperture of the printed antenna. In case of expensive and impossible execution of far-field measurements such as high-gain antennas and mounted antennas and secure measurement for all weather conditions, this type of measurement is applicable. In this measurement, a scanning probe identifies the near field in a given surface (planar, cylindrical or spherical). To achieve the required far-field results, analytical or numerical methods are used for the analysis of the results of near-field measurement. Some common reviews based on near-field measurements are presented in [62-65]. They consist of scanning methods, data analysis methods and error correction methods that are used to measure the errors generated by probes. To understand the distribution of the field on the aperture of an antenna, near-field scanning is essential, which means near-field measurement must be precise and probe effects are not ignored on the measured field. The near-field probing of printed antennas is one such application, which is applied at a distance of millimetres from the printed antenna. It also works as a diagnostic tool for the designing and manufacturing of the antenna. To find out local defects, asymmetric feed networks and excitation points of the radiator and to develop a report of the distribution of current, nearfield probing is very helpful.
The main causes of the low efficiency of printed antennas are the dissipation losses, ohmic losses and radiation losses in the feeding network, inherent properties of the dielectric, and surface wave excitation in the substrate material. These constraints are not good for large arrays operated at high frequencies with a long and complex feed network. It is difficult to know the contribution of fraction of different losses that are responsible for the reduction in the gain of the printed antenna. A receiver with low noise is the best solution for maintaining variation between dissipation and radiation losses. At the time of dissipation losses, it works like an attenuator and decreases the overall noise figure of the system. However, radiation losses do not affect the noise figure, but noise is generated by the side lobes. Another solution to create differences between the losses is the use of a plastic cover that is placed in front of the antenna.
Here we focus on the simple technique used for describing the measurement of antenna efficiency that is achieved by power efficiency measurement. The overall power efficiency is stated as the ratio of the total radiated power to the input power at the terminals of the antenna. The formulated expression of efficiency is given in terms of gain and efficiency:
This chapter elaborated on the basics of printed antennas with a concise presentation on various feeding techniques and their essential parameters that are used in the postulation. The objective of the chapter was to discuss what new printed antennas contribute compared to others. This chapter also described the characteristics of printed antennas, features of basic patches, and advantages and disadvantages of printed antennas with their general applications. Techniques developed for low-profile printed antennas and the features of printed antenna technology were explained in detail. Fundamental issues and design limitations of printed antennas were also explored in this chapter. After explaining the design methodology of a printed antenna, this chapter was completed with a short report on the special measurement techniques for printed antennas. The techniques reported for measurement in this chapter will be very helpful in the processes of designing and manufacturing of printed antennas with different issues. These techniques are recommended for the enhancement of the basic process of measurement of far field and for improved analysis of networks.
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