Antenna impedance matching

Besides the application of micromirror arrays, nickel surface micromachining with a copper sacrificial layer is a technology that can be used for various microfabrication concepts. Only recently, the method has been applied for the construction of capacitive RF switches for antenna impedance matching in multiband mobile phones [26]. 

W. N. Caron, Antenna Impedance Matching, American Radio Relay League, 1994. 

More specifically, let us now consider an antenna exposed to an incident plane wave propagating in the direction Sj and with power density as shown in Fig. 2.2. If the load impedance Z is conjugate-matched to the antenna impedance Z.4, the received power is maximum and given by [36]... 

Next we consider the case where Zl is conjugate-matched to the antenna impedance Z. Thus, the reflection coefficient at the array terminals is F = 0, and the backscatter from the array is simply proportional to C = — 1. 

Let us first examine the backward sector. If we assume that the load impedance Zl is conjugate-matched to the antenna impedance Z, all the energy incident upon this array with a groundplane will (as shown in Section 2.6.2) basically be absorbed. Thus, for this load condition the scattering pattern will simply be given by a number of low-level sidelobes due to the finiteness of the array. For an actual calculated example of what happened when Zl Z, see Section 5.3. 

Another approach is to use a circulator as shown in Fig. 2.19. Here the transmitter with internal impedance Zq is connected to port 1, the antenna with antenna impedance Za to port 2, and finally the receiver with input impedance Zr to port 3. The workings of a circulator is now such that a signal applied to port 1 will occur only at port 2, while a signal applied to ports 2 and 3 will occur only on ports 3 and 1, respectively. Thus a signal incident upon the antenna will see only the receiver input impedance Zr rather than the transmitter impedance Zq. Since the former in general is matched well to the antenna impedance Z, a low reradiation will occur in contrast to seeing Zq that may be quite different from Za (see Section B.9.1). 

We have so far considered RCS in-band reduction only. The success of this approach depends strongly upon how well the antenna is matched to the load (i.e., the terminal impedances of receivers or transmitters). Since a good match... 

The input impedance of an antenna plays an important role in the matching of the source to the antenna. Knowledge of the impedance over the operating bandwidth is of concern. The real part of the impedance is primarily due to the radiation resistance, and in part due to the ohmic loss of the conductors. The radiation resistance is the equivalent resistance, which if connected to the source in place of the antenna absorbs the same power as radiated by the antenna. Impedance can be determined a number of ways. Use of the method of moments gives the most definitive results subject to modeling limitations. Method of moments software was discussed in the first part of Sec. 13.1.3. 

Most practical antennas require some form of impedance matching between the transmission line and the radiating elements. The implementation of a matching network can take on many forms, depending on the operating frequency and output power. 

The azimuth pattern of each panel antenna is unidirectional, and three or four such panels are mounted on the sides of a triangular or square tower to achieve an omnidirectional pattern. The panels can be fed in-phase, with each one centered on the face of the tower, or fed in rotating phase with the proper mechanical offset. In the latter case, the input impedance match is considerably better. 

Angular frequency (co), 119 Antenna ("aerial"), 79, 161, 183 impedance matching, 213 symbol, 161 Anticipator, 236 Armature, 133, 138, 219 Armstrong, Edwin H., 181 Assembly language, 258 Astable multivibrator, 187 Attenuation, 122, 165 Audio (see CD, sound, XLR), amplifier, 174, 204,... 

Figures 18.9 and 18.10 present simulated and measured reflection coefficients of the microfluidic soft PICA in various states. The relaxed antenna achieves good impedance match (Sjj < -10 dB), within 3-11 GHz, both in simulations and experiments.

The antenna consists of a resonant if circuit with parallel and series capacitors, Cp and Cs, for tuning of the resonance frequency and for matching of the impedance, respectively (Fig. 2.3.5(a)). The impedance is a complex quantity which needs to be adjusted to 50 Q magnitude and 0° phase for optimum transfer of rf power. Depending on the equivalent resistance R, inductance L, and capacitance C of the components of the antenna (Fig. 2.3.5(b)), the quality factor... 

This observation is quite noteworthy. It shows that by matching an antenna in the neighborhood of maximum power transfer (i.e., conjugate matching) we obtain an added benefit, namely a potential strong reduction of the ripples of the scan impedance even at a frequency where the surface waves are dominating. 

We finally in Fig. 2.20, bottom, show the same array as in the middle, but this time we have replaced all the T connectors with the same type of hybrids as used earlier in Fig. 2.18. We will receive the same amount of energy in the load impedance Zl as in the middle case above however, the excess energy will be absorbed by the hybrid loads Zh rather than being reradiated (Recall The antenna sections will always be matched so nothing can be reradiated.). 

We note that the measnred RCS level is as much as 40 dB below the foil-covered antenna at one freqnency. That is in fact fairly close to what Jay calculated. Furthermore, the relative 13-dB bandwidth is abont 5.2/2.S = 1.9—that is, approximately an octave wide. Note that this result is obtained for fixed load impedances without any broadband matching as discussed in Appendix B. Further increase in bandwidth can be obtained by using more elements. 

Resonance frequency(ies) the application for which the antenna is intended will specify the frequency or multiple frequencies of resonance of the antenna. For example, industrial, scientific, and medical (ISM) band antennas are required to resonate at one (or more) of the ISM frequencies (868 MHz, 2.45 GHz, 5.8 GHz). An antenna operation requires that the antenna s input impedance is optimally matched to the source impedance, thus the injected power is maximally converted to the form of radiated field. The concept of resonance is strictly related to the reflection coefficient, which is described next. 

A folded dipole can be fashioned as shown in Fig. 13.44. Such a configuration results in increased bandwidth and impedance. Impedance can be further increased by using rods of different diameter and by varying the spacing of the elements. The 1/4-wave dipole elements connected to the closely-coupled 1/2-wave element act as a matching stub between the transmission line and the single-piece 1 /2-wave element. This broadbands the folded dipole antenna by a factor of 2. 

Figure 13.56 shows a shunt-fed slanted dipole antenna that consists of two half-wave dipoles offset 90°. The two sets of dipoles are rotated 22.5° (from their normal plane) and are delta-matched to provide a 50- 2 impedance at the radiator input flange. The lengths of all four dipole arms can be matched to... 

Antenna tuning unit (ATU). The tower s complex impedance at the feed point must be matched to the characteristic impedance of the transmission line so that maximum transfer of power may occur. Elements of inductive and capacitive reactance (coils and capacitors) are combined into a T or L network to accomplish this task. The ATU will also contain provisions for measuring base RF antenna current. 

Antenna matching The process of adjusting an antenna matching circuit (or the antenna itself) so that the input impedance of the antenna is equal to the characteristic impedance of the transmission line. 

Salonen, P., Rahmat-Samii, Y., 2006. Textile antennas effects of antenna bending on input matching and impedance bandwidth. In Antennas and Propagation, EuCAP 2006, First European Conference on 2006. IEEE. 

The factor m has the dimension of a conductance or, equivalently, of a reciprocal resistance. This resistance is called the wave resistance or, more generally, the optical wave impedance of the medium. For free space the wave resistance is 377 Q. For maximum efficiency transmitting and receiving antennas must be matched to that impedance. Similarly, electrical transmission lines must be terminated by their conjugate wave impedances to avoid reflections, hi optics an analogous situation exists. No reflection takes place at the interface of two media if their wave impedances are matched, a consideration important for the design of antireflection coatings. 

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