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Spencer Streeter ECE 3300 Portfolio - 9/29/06-10/04/06 Portfolio - 9/29/06 Antenna parameters: input impedance “The impedance presented by an antenna at its terminals or the ratio of the voltage to current at a pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a point”. An antenna’s input impedance is generally a function of frequency. Thus the antenna will be matched to the interconnecting transmission line and other associated equipment only within a bandwidth. The impedance is important for determining antenna efficiency. All power sources have internal impedance. Maximum power is transferred to an external device when its impedance matches the source internal impedance. resonant frequency Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. For example, an antenna tuned for 440MHz has a resonant frequency of 440MHz. bandwidth is defined as “the range of frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard”. The considered characteristic can be input impedance, pattern, beamwidth, polarization, side lobe level, gain, beam direction or radiation efficiency. The bandwidth can be considered as the range of frequencies on either side of a center frequency. For broadband antennas, the bandwidth is usually expressed as the ratio of the upper-to-lower frequencies (e.g. 10:1). For narrowband antennas, the bandwidth is expressed as a percentage of the frequency difference (upper minus lower) over the center frequency of the bandwidth (e.g. 5%). All the characteristics of an antenna do not necessarily vary in the same manner, or are even critically affected by the frequency, so the specifications are set in each case to meet the needs of the particular application. radiation pattern is the directional function characterizing the relative distribution of power radiated by an antenna. Also known as the antenna pattern. Antenna pattern – (pg. 380) describes the far-field directional properties of an antenna when measured at a fixed distance from the antenna. In general is a three-dimensional plot that displays the strength of the radiated field of power density as a function of direction (zenith angle and azimuth angle ) Gain "Gain" as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the Gain, Directive gain or Power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by an hypothetical isotropic antenna: G 4R 2Smax 4R 2 Smax P Sant D Pt Prad P Siso Gain accounts for ohmic losses in the antenna material, directivity (D) does not. The gain of an antenna is a passive phenomena - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a greater than one gain in some directions, it must have a less than one gain in other directions since energy is conserved by the antenna. How would you measure them? Input impedence can be measured by using a network analyzer. Resonant frequency can be found by using a network analyzer to find a “null”. Bandwidth can also be found using a network analyzer to find the width of the gap at 3 dB which ultimately defines the bandwidth. 1-2 UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING 50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu Spencer Streeter ECE 3300 Portfolio - 9/29/06-10/04/06 Figure 1 - Bandwidth Reciprocity allows you to compute the radiation pattern in the transmission mode. (pg.374). Another way to measure the radiation pattern would be to measure the power at each angle as you turned the antenna 360º’s. Gain could be measured by taking the maximum power found of all the power measurements made at each angle and dividing by the minimum power found of the same power measurements. Portfolio - 10/2/06 What is instantaneous power and time-averaged power? Instantaneous power is the product of instantaneous voltage and instantaneous current. For instantaneous incident power or power as it arrives to the load we have… P (t) i V0 2 cos 2 t (W) Z0 For the instantaneous power reflected by the load (negative sign signifies –z direction of travel) we have… P r (t) 2 V0 2 cos2 t r (W) Z0 Time-averaged power is instantaneous power averaged over one time period. Pav P P i av r av V0 1 Pav V˜ I˜ * 2 2 2Z 0 Portfolio - 10/2/06 No portfolio for today. 1 2 (remember that z z* z ) 2-2 UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING 50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu