Loop antenna

Figure 3-3 Worst Way to Measure Anything—If Ground Clips Are at Different Grounds, this Can Create a Huge Loop Antenna of Circulating Current... 

FIGURE 8.11 (a) A two-element circular loop antenna backed by a rectangular cavity beneath the ground plane, (b) Radiation pattern at the x—y plane... 

FIGURE 8.12 (a) Return loss and (b) AR and gain for diverse cavity depths, regarding the two-element circular loop antenna... 

B. M. Kent, Impedance Properties of an Infinite Array of Non-planar Rectangular Loop Antennas Embedded in a General Stratified Medinm, Ph.D. Dissertation, Ohio State University, Department of Electrical Engineering, Columbus, OH, 1984. 

A thin wire bent into a closed contour is called a loop antenna. The terminals of a loop antenna are formed by a small discontinuity or gap in the conducting wire. Loops have been used in such diverse applications as radio receiving antennas, direction finding, magnetic field-strength probes, as well as array elements. 

There are various types of loop antennas including rectangular, triangular, rhombic, and circular. In this section we will focus on the thin circular loop antenna, which is one of the most popular and commonly used configurations. 

Figure 13.16 illustrates the coordinate system that will be adopted for the circular loop antenna of radius a. The source point and field point are designated by the spherical coordinates (r = a,9 = 90°, (p ) and r,9,(p), respectively. Hence, using the geometry depicted in Fig. 13.16 it may be shown that the distance from the source point on the loop to the field point at some arbitrary location in space is... 

Circular loop antennas in which the radius of the loop is less than about 0.047. maybe considered electrically small. Electrically small-loop antennas are not usually used in transmitting applications because of their very low-radiation resistance, making them inefficient radiators. On the other hand, small circular loops... 

FIGURE 13.17 Three-dimensional radiation pattern produced by an electrically small-loop antenna with a uniform current distribution. 

Hence, the far-field radiation pattern of an electrically smaU-loop antenna has a sin 0 variation as demonstrated by Eq. (13.78) and Eq. (13.79). Figure 13.17 illustrates the three-dimensional radiation pattern that would be produced by a small loop. Finally, by using Eq. (13.15) with f 9,(p) = sin 0, it follows that the directivity of a small loop is... 

This form of the current distribution is very general and may be appHed to the analysis of the radiation characteristics of loop antennas for a wide variety of circumstances (King and Smith, 1981 King, 1969). 

The actual form of the current distribution on a particular loop antenna maybe determined by performing a method of moments analysis, as discussed in two earlier sections. It is a straightforward procedure to find the input impedance, radiation pattern, and gain of a loop once the current distribution is known. Method of moments generated plots of the input impedance vs. loop radius may be found in Fig. 13.6 of Harrington (1968). 

Loop antenna A type of antenna that is constructed by bending a thin wire into a closed contour. 

King, R.W.P. 1969. The loop antenna for transmission and reception. In Antenna Theory, Pt. I, McGraw-Hill, New York. 

Lindsay, J.E., Jr. 1960. A circular loop antenna with non uniform current distribution. IRE Trans. Antennas Propagat. AP-8(4) 439M 41. 

Storer, J.E. 1956. Impedance ofthin-wire loop antennas. AIEE Trans. Parti. Communication and Electronics... 

Werner, D.H. 1996. An exact integration procedure for vector potentials of thin circular loop antennas. 

If there are several cables running from the preamplifier to the amplifier, the lie of the cables could form a form of loop-antenna , which can be a particularly efficient way to pick up RFI. This can be minimized by bundling all the cables together using tape or cable ties to keep them in place. Tennelec ran these signal and power supply lines within one shielded bundle as a matter of course, as do modem instruments, such as the OKTEC DSpec. 

Recently, a loop antenna for the 915 MHz device was assessed in an in vivo porcine liver model by Shock et al. (2004). Loop antennae were described in combination with an electrocautery device for wire deployment. The loop antenna was reported to create a larger ablation zone (mean ablation diameter 4.3+-0.6 cm) as compared to a single straight MW antenna. However, viable tissue was found in the center of the loop for the single configuration. Complete ablation in the center was described for an orthogonal configuration of two loop antennae. 

Shock SA, Meredith K, Warner TF, Sampson LA, Wright AS, Winter TC 3 , Mahvi DM, Fine JP, Lee FT Jr (2004) Micro-wave ablation with loop antenna in vivo porcine liver model. Radiology 231 143-149... 

Silicone-Based Elastic, Liquid Metal, Unbalanced Loop Antenna... 

Figure 18.4 Photographs of the foldable, stretchable, fluidic unbalanced loop antenna enclosed in a silicone substrate a) in its relaxed state, b) w/ 40% vertical strain, and c) horizontal elongation, d) in its folded state, e) w/ severe twisting, and f) biaxial stretching.

Figure 18.5 Simulated and measured reflection coefficients, of a silicone-based elastic unbalanced loop antenna, a) Simulated and measured (in logarithm scale) of the relaxed and flexed antenna b) Measured of the flexed antenna w/ 20% x-axis (solid), 40% x-axis...

Figure 18.16 Numerical and experimental results of the standalone submodules prior to the final integration, a) Measured output DC voltages versus varying input power, of the RF power detector, w/ the LED ON/OFF states marked, b) Simulated and measured reflection coefficients, S j, of the standalone silicone-rubber-based fluidic unbalanced loop antenna in its relaxed state, c) Measured Sjj of the flexed antenna with varying strain along its x-axis. d) Measured Sjj of the flexed antenna with varying strain along itsy-axis. The corresponding coordinate system is shown in Figure 18.15.

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