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Transcript of Antenna Report
METALLIC TRANSMISSION LINE EQUIVALENT CIRCUIT Transmission Characteristics
- called as secondary constants - Characteristic Impedance and Propagation Constant
- complex quantity that is expressed in - ideally independent of line length and cannot be directly measured - sometimes called surge impedance *If a transmission line is infinitely long, it can store energy infinitely; energy from the source enters the line, and none of it is returned.*
For simplicity, only the series resistance Rs are considered.
Mathematically, Z1 is
If the process shown were continued, the impedance seen looking into the line will decrease asymptotically towards 37 , which is the characteristic impedance of the line. If the transmission line shown were terminated in a load resistance ZL = 37 , the impedance seen looking into any number of sections would equal 37 , the characteristic impedance. For a single section of line, Z0 is
Adding a second section, Z2 is
Therefore, if this line were terminated into a load
resistance ZL = 37, Z0 = 37 no matter how many
sections are included.
The characteristic impedance is simply the ratio of the
voltage (E0) to the line current (I0), mathematically, Z0 is
The characteristic impedance of a 2-wire parallel
transmission line with an air dielectric can be determined from
its physical dimensions (see Figure 12-17a) and the formula
Determine the characteristics impedance for an RG-59A
coaxial cable with the following specifications: L = 0.118H/ft
and C = 21pF/ft.
Transmission lines can be summarized thus far as follows: 1. The input impedance of an infinitely long line at radio frequencies is resistive and equal to Z0. 2. Electromagnetic waves travel down the line without reflections; such a line is called nonresonant. 3. The ratio of voltage to current in any point along the line is equal to Z0. 4. The incident voltage and current at any point along the line are in phase. 5. Line losses on a nonresonant line are minimum per unit length. 6. Any transmission line that is terminated in a purely resistive load equal to Z0 acts as if it were an infinite line. a. Zi = Z0 b. There are no reflected waves. c. V and I are in phase. d. There is maximum transfer of power from the source load. PROPAGATION CONSTANT - sometimes called propagation coefficient - is used to express the attenuation (signal loss) and the phase shift per unit length of transmission line. - with an infinitely long line or a line that looks infinitely long, such as a finite line terminated in a matched load (Z0 = ZL), no energy is returned or reflected back toward the source. Mathematically, the propagation constant is
where = propagation constant (unitless) = attenuation coefficient (nepers per unit length) = phase shift coefficient (radians per unit length) The propagation constant is a complex quantity defined by
The neper (unit symbol Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms.
Attenuation is a general term that refers to any reduction in the strength of a signal. Attenuation occurs with any type of signal, whether digital or analog. Sometimes called loss, attenuation is a natural consequence of signal transmission over long distances. The extent of attenuation is usually expressed in units called decibels (dBs).
The primary line constants are parameters that describe the characteristics of conductive transmission lines, such as pairs of copper wires, in terms the physical electrical properties of the line. The primary line constants are only relevant to such lines and are to be contrasted with the secondary line constants, which can be derived from them, and are more generally applicable. The secondary line constants can be used, for instance, to compare the characteristics of a waveguide to a copper line, whereas the primary constants have no meaning for a waveguide.
resistance (R) inductance(L) capacitance(C) shunt conductance(G) - a low-resistance connection between two points in an electric circuit that forms an alternative path for a portion of the current; also called bypass; conductance of a shunt is high as the resistance is low.