Chelmsford Amateur Radio Society Advanced Course (5) Antennas

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Chelmsford Amateur Radio Society Advanced
Course(5) Antennas
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Frequency Wavelength

• The size of an antenna is determined by the
  wavelength of operation
• In free space Frequency x Wavelength
  Speed of Light, 3x108m/s
• v F x ? 3 x 108 or ? 3 x 108 /
  F where F is in Hertz and ? in metres
• A wavelength for 14MHz would be 3 x 108 / 14.05 x
  106 21.35 metres.
• Therefore a half wavelength 10.67 metres and as
  the feed-point for a DIPOLE is in the centre,
  each wire should be 5.33 metres long.
• BUT . . .
• The velocity of a RF wave in wire is slight lower
  than that in free space.
• This means you can cut your wire approximately 5
  shorter

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Antenna Environment

• The environment around an antenna affects its
  radiation pattern
• Thus commercial stations have HF/MF antennas in
  open countryside.
• Ground effects
• The type of ground beneath the antenna modifies
  the radiation pattern.
• The environment affects vertical antennas more
  than horizontal ones
• Only a vertical polarised antenna produces a
  ground wave.
• Height above ground and its conductivity affects
  the radiation pattern.
• The better the ground conductivity the better
  (lower) the radiation angle.
• Salt Water is best, whilst urban concrete not so
  good, which is one reason to ensure you have a
  good RF earth mat.

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Polar Diagrams Radiation Angle

• Polar diagrams give the radiation patterns for an
  antenna, and graphically describe the direction
  of radiation.
• The Radiation Angle is that angle at which the
  main lobes radiate.
• This angle can be affected by the height of the
  antenna above ground.
• Long distance communication requires a low angle
  of radiation.
• Only under certain circumstances will you require
  a high angle radiation.

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Feed Point Impedance

• Feed Point Impedance is the impedance measured at
  the point at which the feeder connects to the
  antenna.
• It is only resistive at the resonate frequency of
  the antenna,
• At all other frequencies it will have some
  reactive quantity.
• Feed point impedance for various ideal antennas,
  in the clear-

Antenna Feed Impedance Half Wave Dipole Centre
fed 70 ohms Quarter Wave Vertical Fed at base,
radials 35 ohms /- x 5/8 Vertical (¾? with
loading coil) Fed at base 50 ohms Folded
Dipoles Centre fed 300 ohms Full Wave Loops
(Watch These on Tx) 100 ohms Quarter wave End
fed 15 ohms Reactance Half wave End fed High
Reactance
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Yagi Antenna Elements

• Beams consist of a Reflector and Directors spaced
  at a fraction of a wavelength, usually fed by
  coax.
• Their presence reduce the Driven Dipole element
  feed point impedance to 15 to 30 ohms, which can
  be matched with a BALUN transformer.
• One matching method is to make the driven element
  a folded dipole whose 300 ohm impedance will be
  reduced to 70/80 ohms which is easier to match
  to the 50 ohm feeder.

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Folded Dipoles

• Folded Dipoles are Dipoles with an extra
  conductor placed alongside and connected at the
  ends - equivalent to two in parallel
• Current is shared(halved) between the two
  elements for a given power which quadruples the
  input impedance relative to a standard dipole
• Folded dipoles therefore have a close match to
  300 Ohm feeder
• The higher impedance enables elimination of
  Matching Transformers which otherwise would
  restrict bandwidth. For example-
• A 14MHz Dipole antenna exhibits up to 21 VSWR
  over the band.
• A folded dipole would have a flat VSWR over band.

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Trap Dipoles

• One dipole can be turned into two or three by the
  addition of parallel tuned circuits called TRAPS
• The impedance of the traps is about 500 ohms.
• As XL dominates then Z 2? FL.
• The trap principle can also be used in beam
  antennas to construct two or more bands out of
  one set of rod elements

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The Quad Antenna

• Normally used at 14MHz and above and requires a
  balanced feed
• Consists of a driven element in the shape of a
  square loop, placed in close proximity to a
  square of the same size which acts as a
  reflector.
• Can be considered as two closely stacked two
  element aerials which are end fed. Like a Yagi,
  it can have extra elements - especially at VHF
• Has a very low radiation angle is, making it good
  for HF DX.

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Antenna Matching Units

• Antenna matching units are used to convert the
  impedance of the antenna system to the output
  impedance of the transmitter.
• They are installed as close to the antenna as
  possible with the VSWR meter as close to the
  Transmitter as possible.
• Only at resonance will the antenna system
  Reactance, X, be zero.
• The AMU therefore cancels the X part of the
  impedance and converts the R to a value that
  equals the Z of the Transmitter.

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Matching Networks-1

• Wide Impedance Matching Range with Tapped Coil
  can cover a good Frequency Range

• Good for Matching Lower band Quarter Waves

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Matching Networks-2
Tank Matching
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Baluns

• Remember BALUN Balanced Unbalanced
• Many antennas are balanced devices, such as
  dipoles etc.
• Connecting a dipole to an unbalanced coax cable
  causes currents to flow in the outer sheath.
• These currents give rise to unwanted radiation
  which may cause EMC problems.
• A solution is to match the balanced antenna to
  the unbalanced line using a BALUN
• There are three basic types and we will deal with
  each in turn.

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Baluns - Basic Format

• Below is the basic format of The Transformer
  BALUN
• These are normally wound on ferrite cores and are
  used to match a balanced system such as ladder
  line or a dipole to an unbalanced line such as a
  coaxial cable.

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Transformer Baluns

• Recall Zp Zs . ( Np / Ns ) 2
• 12 Turns Ratio will create a 14 Impedance
  Transformation

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Sleeve Balun

• A ?/4 long braided or solid extra outer conductor
  is located around and insulated from the coax
  screen and connected to the screen at the rear
• The high impedance of the open circuit present a
  low impedance to the currents on the coax screen
  ? /4 on to the rear
• The sleeve acts as choke coil to isolate
  remaining line from antenna.
• As it is based upon ? /4 on one band, this is a
  single band device

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Choke Baluns

• Current or Choke Baluns prevent current flowing
  on the screen of the coaxial feeder cable.
• Such currents may cause unwanted radiation which
  might lead to EMC problems.