( ECE 5014 )Microwave Antenna and Radio Wave PropagationPresented by Daw Zin Mar OoDept:of Ec & ITY.T.U28.8.2008 :1 ( ECE 5014 )Microwave Antenna and Radio Wave PropagationPresented by Daw Zin Mar OoDept:of Ec & ITY.T.U28.8.2008


METHODS OF FEED AT THE ANTENNA :2 METHODS OF FEED AT THE ANTENNA In order to effect the maximum transfer of power the transmission line must be attached to the antenna in such a manner that its impedance sees an equivalent impedance at the point where it is connected.


METHODS OF FEED WITH RESONANT LINES :3 METHODS OF FEED WITH RESONANT LINES A resonant line is connected to an antenna at the highest or lowest impedance point, that is, at the voltage or current loop, respectively. It provides comparatively efficient transfer of power if its length is limited to one wave length. Two methods of feeding a half-wave antenna with resonant lines. (1) Half-wave length of resonant line connected directly to center of a half-wave antenna. At the transmitter end of the line is a series tuned circuit, which has a low impedance at resonance. The load, which is approximately 73 ohms at the center of the antenna.


Slide 4:4 (2) A half wave length of a resonant line connected directly to the end of a half-wave antenna. At the transmitter end of the line is a parallel-tuned circuit, which has a high impedance at resonance. An impedance of approximately 2500 ohms exists at the end of the λ/2 antenna. A center-fed and an end-fed half-wave antenna, using quarter-wave tuned lines. The center-fed half-wave antenna, has a parallel-tuned circuit, at the transmitter end of the feed line, which develops a high impedance. Thus the load or the center of the antenna which is a quarter wave length away sees a low impedance. The end-fed antenna has a series-tuned or low-impedance circuit at the transmitter end of its feed line.


Slide 5:5 Since a quarter-wave line inverts the impedance, the end of the antenna looking into the line sees a high impedance and, therefore, an approximate match is obtained. One advantage of the tuned-line system, when a half or quarter wave length line is used, is that the length of the line may vary as much as 20% from the calculated length for the antenna resonant frequency and still operate efficiently. Resonant or non-resonant condition of a transmission line is a direct function of the degree of mismatch between the antenna and the transmission line. When a standing-wave ratio of 1.5 to 1 is obtained, for all practical purposes the feed line approaches a non-resonant condition.


SINGLE-WIRE FEED SYSTEM :6 SINGLE-WIRE FEED SYSTEM Advantage of simplicity of adjustment and ease of construction. If the line is properly terminated at the antenna, it can be extended to any reasonable length. This type of feed system should be employed only when the site has a moist or highly conductive ground, because the capacity of the antenna to ground acts as the return path for the radio-frequency currents flowing in the antenna.


TWISTED-PAIR FEED SYSTEM :7 TWISTED-PAIR FEED SYSTEM This feed system is easy to install and has a low r-f potential due to its low impedance , but it is the least desirable type of non-resonant feed system from an efficiency standpoint. The Zo of a twisted-pair feeder approximates 70 ohms, it may be used to feed the center of a half-wave antenna. If the twisted-pair line is not weather-proofed, it is practically useless for damp-weather conditions and should be used only for emergencies.


COAXIAL-CABLE FEED SYSTEM :8 COAXIAL-CABLE FEED SYSTEM A coaxial cable connected to the center of a half-wave antenna; this type of feed system finds universal use. The advantage of this system lies in the fact that the low impedance of the cable matches the low impedance at the center of the antenna. The coaxial cable can be used also for feeding an antenna array, provided that a matching system is used between the antenna and the line.


DELTA-MATCHED FEED SYSTEM :9 DELTA-MATCHED FEED SYSTEM The characteristic impedance of the transmission line is too high to match by direct attachment of the line to the center of the antenna, an impedance-transformation method must be used. The impedance of the transmission line is gradually transformed into a higher value equal to the impedance encountered at the antenna. The major disadvantage of this type of feed system is the critical nature of the dimensions involved.


Slide 10:10 No matter how carefully the constructional details are followed, a slight readjustment of the dimensions will be necessary to improve efficiency. The principal advantage of delta-matched feed system, which employs a balanced transmission line, is that the radiation from the line is kept at a minimum at the frequency for which it was designed. With this type of antenna feed, therefore, two transmitters can be operated simultaneously on adjacent frequency without detrimental interference.


WAVE-GUIDE FEED SYSTEM :11 WAVE-GUIDE FEED SYSTEM Radiation is accomplished by means of two resonant windows formed by a capacitive iris and an inductive iris. This type of window is broadly resonant in much the same manner as a large-diameter dipole, and the feed is matched by means of a capacitive stub. This type of feed is essentially a low-impedance device, and since the wave guide used for transmission operates at a higher impedance, a matching section must be used. The matching section depends upon whether broad-band or narrow-band characteristics are required.


ATRIFICIAL-LINE MATCHING SYSTEM :12 ATRIFICIAL-LINE MATCHING SYSTEM One of the simplest methods of coupling a transmission line to an antenna at low, medium, or high frequencies is by the use of an artificial line. By proper design of this type of coupling network, a wide range of transmission line impedances can be coupled to any range of antenna input impedances in a narrow band of frequencies. The advantage of an artificial line over the various type of matching stubs previously mentioned is that the components can be lumped and built into a small weather-proofed enclosure and mounted at the antenna input; thus, it has negligible effect on the field radiation pattern.


Slide 13:13 The coupling network acts to cancel out the reactive components that would exist if the transmission lines were coupled directly to the antenna. Thus, the network must contain inductive and capacitive reactances of the proper values to make the antenna appear to the line as a pure resistance equal in value to the characteristic impedance of the line. The values of inductance and capacity required can be found by calculation, if the antenna input impedance and transmission-line characteristic impedance are known.


The inductive reactance can be found by the formula: :14 The inductive reactance can be found by the formula: Where R1 = antenna input impedance R2 = transmission-line characteristic impedance The capacitive reactance can be found by the formula:


Slide 15:15 Thank You