ECE 5014 Microwave Antenna and Radio Wave Propagation Presented by Daw Zin Mar Oo Dept:of Ec

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( ECE 5014 )Microwave Antenna and Radio Wave
PropagationPresented by Daw Zin Mar OoDeptof
Ec ITY.T.U28.8.2008
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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.

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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.

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• (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 ?/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.

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• 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.

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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.

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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.

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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.

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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.

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• 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.

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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.

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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.

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• 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.

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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
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Thank You