Chelmsford Amateur Radio Society Intermediate Course (5) Antennas and Feeders

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Chelmsford Amateur Radio Society Intermediate
Course(5) Antennas and Feeders
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Basic System
50 Ohms Output
Antenna Matching Unit
Transmitter
Receiver
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Feeders

• Feeder types Coaxial, Twin Conductors

Coax Inner Conductor is shrouded by
dielectric, with outer (braided) screen. For
Amateur Radio 50? Coax is used (whereas Video/TV
uses 75?)
Twin FeederTwo conductors kept at constant
separation by insulation - no screen Balanced
Feeder is available in 75-600 ?
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Feeder Impedance

• All Feeders have a Characteristic Impedance, Z0
  (eg 50 or 75 Ohm Coax, 300 Ohm twin feeder etc)
• Z0 is based on the Ratio of A and B (and the
  nature of the spacing dielectric)
• If correctly terminated by a resistive load then
  Z0 determines the ratio of Vrms / Irms in the
  feeder

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Balanced/Unbalanced/Match

• Coax is unbalanced - Inner has signal, Outer is
  at ground
• Twin Feeder is balanced - conductors have equal
  and opposite voltages/currents/fields
• Mounting Twin Feeder near to conducting objects
  will cause an imbalance in the conductors, a
  change in its Z0 and unwanted radiation (or loss
  of immunity from external interference)
• Similarly, severe bends or crushing coax changes
  its Z0 and causes internal mismatches
• Any mismatch within a feeder, or its termination,
  will result in its input impedance no longer
  being its characteristic impedance

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Decibels

• Gains and Losses may be expressed in dBs
• 3 dB is half steps, 6dB is quarter steps, 10dB is
  tenth
• You will need to remember this table for exam
• 3dB x2 or a half
• 6dB x4 or a quarter
• 9dB x8 or an eighth
• 10dB x10 or a tenth
• Example 3dB Gain doubles power, whilst 3dB Loss
  halves it

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Feeder Losses

• ALL feeders have loss - the longer the feeder the
  greater the loss. Twin Feeder has a lower loss
  than Coaxial cable
• This loss affects both the Transmit and Receive
  paths
• For some standard cables the loss is
• Per 100m RG58 RG213
• 10 MHz 4.8 dB 2.0 dB
• 30 MHz 8.2 dB 3.2 dB
• 144 MHz 21 dB 8.6 dB

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Antennas

• All Antennas have a feed point impedance.
• This is determined by the dimensions which will
  relate to the wavelength of the applied signal
  and the height of antenna
• Dipoles are a half wave length long and are a
  resistive match at only one frequency.
• If you replace the antenna by a resistor of the
  same value as the feed point impedance the
  transmitter will not be able to tell the
  difference
• Dipoles in theory are 73 ohms but in practice
  approx 65 ohms so close enough to the nominal
  course value of 50 ohms

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Voltage Standing Wave Ratio

• If the feed point impedance is incorrect then it
  will not match the impedance of the feeder and
  some energy will be reflected back down the
  feeder.
• When this reflected energy is returned to the
  Transmitter it is again reflected back to the
  antenna and is radiated.
• The combined energy is known as the forward and
  reflected power and gives rise to the Standing
  Waves on the feeder.

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VSWR Impedance

• If we have a mismatched antenna, energy is
  reflected back to the Transmitter where it is
  again reflected back to the antenna and is
  eventually radiated - Energy is not lost
• The combined forward and reflected power and
  gives rise to the Standing Waves on the feeder.
• The reflected signal will change the input
  impedance of the feeder so that it is no longer
  the characteristic impedance and the feeder no
  longer presents the correct impedance to the
  transmitter.
• Antenna Matching Units transform this effective
  impedance back to nominal so that the radio
  operates correctly - even though the antenna
  remains imperfect

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Electromagnetic Waves

• Electromagnetic radiation comprises both an
  Electric and a Magnetic Field.
• The two fields are at right-angles to each other
  and the direction of propagation is at
  right-angles to both fields.
• The Plane of the Electric Field defines the
  Polarisation of the wave.

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Polarisation

• Polarisation is the plane of the antennas
  radiating electric field.
• Common polarisations are Horizontal and Vertical.
• Transmitter and receiving antenna polarisation
  need to match for optimum signal strength,
  especially at VHF/UHF
• Verticals (?/4, 5?/8) give vertical polarisation.
• Yagi and Dipoles antennas may be either
  horizontal or vertical depending on their
  mounting.

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Dipole Radiation Pattern
Radiation Pattern for a Vertical Dipole-
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Yagis
Boom
Radiation Pattern
Feeder
Directors
Driven Element
Reflector