Impedance antenna

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]. 

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]... 

We next seek the reflection coefficient T between the antenna impedance Za and Z. If one of them is real, the usual simple expression for T is valid (see later). However, if they are both complex as shown in Fig. 2.3, top, we... 

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. 

We now add a groundplane to our array of dipoles as shown in Fig. 2.9. This will distinctly change the antenna impedance Za, but that is of no particular concern for our present purpose. 

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. 

Fig. 2.13 The Thevenin equivalent circuit for a receiving antenna always yields the correct power delivered to the load impedance Zl. The power lost in the antenna impedance Za represents power scattered somewhere, not necessarily in the back direction. Additional scattering not accounted for by Za will in general take place.

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). 

Fig. 6.15 Typical antenna impedance = 2Rao +JXa in the negative direction for a long wire antenna like a flat spiral without the effect of a groundplane. At lower frequencies the groundplane impedance Zi+ is inductive as shown on the rim of the Smith chart. The total antenna may improve if jXa is capacitive but get worse If Inductive. Thus, no broadband compensation will take place.

This situation in no way violates Foster s reaction theorem [101], which applies only to lossless reactances while antenna impedances of radiating antennas are (or should be) lossy to represent radiated power. 

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

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. 

A mechanical analogy can be done we must anticipate that there will be very little coupling between GW and ordinary matter. In fact, we know that sound waves in common materials couple efficiently from one body of density p, and sound velocity vx to another of density p2 and sound velocity v2 if the product p, p, = p2v2. The product pv is called the sound impedance per unit area and has dimension of kg/m2s. For an antenna made of copper with a surface area of 1 m2, the impedance is 3 x 107 kg/s. For space-time this would be ... 

K. S. Yee and J. S. Chen. Impedance boundary condition simulation in the FDTD/FVTD hybrid leee Transactions on Antennas and Propagation, 1997, 45, 921-925. 

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... 

M. K. Karkkainen, FDTD model of electrically thick frequency-dispersive coatings on metals and semiconductors based on surface impedance boundary conditions, IEEE Trans. Antennas Propag., vol. 53, no. 3, pp. 1174-1186, Mar. 2005. doi 10.1109/TAP.2004.842655... 

FIGURE 8.10 (a) Input impedance for the two equilateral-triangular microstrip antennas, (b) Optimum return loss frequency /rl and AR frequency /ar for different values of lengths h and g, concerning the 6 -inclined antenna... 

FIGURE 8.24 (a) Transmitted power of a 7x7 antenna array, (b) Characteristic impedance of a high-pass filter for a 6 x 6 and 9x9 configuration... 

C. Antenna. The antenna is used to make a transition from a guided wave (from the transmission line) to a radiated electromagnetic wave. The design of the antenna is influenced by many factors such as size, frequency, and electrical impedance. Antennas are normally of two types - omnidirectional and directional. The omnidirectional antennas are element type antennas such as monopoles or dipoles. The directional are horn-type antennas, parabolic dish type antennas such as a satellite communications antenna (SATCOM), or a phased-array antenna which can emit many beams at once. The characteristics of the antenna are a very important aspect of hazard evaluation. 

It was mentioned in sub-section 13.5.2 that the cut-off frequency for effective screening is in the range of 0.5 kHz to 2.0 kHz for external interference. At frequencies higher than about 1 MHz it is useful to consider the coupling between the screen and the core as an impedance that relates the screen current to the core open-circuit voltage. In such a case it is not specified how the current appears in the screen. It could be by mutual induction from nearby cables, but more often by radio waves received from local radio transmitters, radio telephones, or a radar antenna. The impedance is called the shield transfer impedance Zj and it can be measured by a relatively simple test procedure. The expression for the impedance Zj is -... 

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