CTU Presents

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CTU Presents
Contest Antenna Systems Steve Knowles
G3UFY
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Performance measurement

• If we know the power fed to an antenna we can
  quantify its performance by measuring the field
  strength produced, and comparing it with a
  standard reference.
• Comparison with an Isotropic Radiator
• The field strength due to the test antenna is
  compared with that which would be produced by an
  Isotropic Radiator (dBi)
• Comparison with a Reference Dipole
• The test antenna is compared with the maximum
  radiation of a half-wave dipole (dBd)

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Visual Representation
A solid figure where all points on the surface
receive the same field strength from the antenna
at the centre
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A Polar Diagram
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Software modelling of antennas
For most of these slides I used EZNEC (by
W7EL) Downloadable from the Web. Reasonably
priced around 100. As well as antennas this
software models feeders, transmission lines,
traps, ATUs copes with sloping ground, variable
ground quality and annular terrain
characteristics. Produces Polar Diagrams and
3-Dimensional plots.
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Horizontal Doublet(over perfect ground)
Up 1/8 wavelength
Up 1/4 wavelength
Up 1/2 wavelength
Up 3/4 wavelength
Up 1 wavelength
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Horizontal Doublet(over real ground)
Up 1/8 wavelength
Up 1/4 wavelength
Up 1/2 wavelength
Up 3/4 wavelength
Up 1 wavelength
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½ Wave Doublet, up 2 Wavelengths
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Vertical Monopole(over perfect ground)
¼ Wavelength Monopole
½ Wavelength Monopole
¾ Wavelength Monopole
Ground Plane (45o radials) at ground level
GP (45o radials) up 1 wavelength
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Vertical Monopole(over real ground)
¼ Wavelength Monopole
½ Wavelength Monopole
¾ Wavelength Monopole
Ground Plane (45o radials) at ground level
GP (45o radials) up 1 wavelength
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Horizontal / Vertical hybrids

• End-fed wire
• Tee (or doublet with the feeders strapped as
  commonly used on 160m)
• Inverted L
• Sloper (sloping quarter-wave)
• All will perform better if operating over a good
  earth system

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Significance of Wave Angle
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Significance of Wave Angle
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Horizontal (Azimuth) Directivity
3.5MHz / ½ wavelength
7 MHz / 1 wavelength
14 MHz / 2 wavelengths
21 MHz / 3 wavelengths
28 MHz / 4 wavelengths
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Beam Antennas

• Beam antennas work by combining the radiation
  patterns of multiple elements in such a way that
  they reinforce in one direction and cancel in
  others.
• Arrays may be Driven, where RF is supplied to
  every element -
• or Parasitic where only one element is supplied
  with RF the other elements extract the energy
  they need from the primary radiation-
• or a combination of the two.

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Beam Antennas
W8JK driven 2-ele
ZL-Special driven 2-ele
Four-Square driven equivalent 2-ele vertical
Yagi parasitic 3-ele
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About Large Loops

• The most popular loop antenna is one wavelength
  in circumference, used in the vertical plane.
• Square, triangular and circular loops are almost
  identical in performance.
• If fed in the centre of the bottom or top, the
  antenna is horizontally polarized with about
  1.5dBd gain over a doublet and with similar
  characteristics.
• If fed half-way up one side, the antenna is
  vertically polarized with about 1.5dB gain over a
  ¼ wave monopole and with similar characteristics.

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Receive-only Antennas
Shielded Loop (K9FD)
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Receive-only Antennas
Beverage
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What to aim for (HF Bands)

• 14MHz A 3- or 4-element monoband yagi at 130ft
  ANDa 3-element monoband yagi at 45ft
• 21MHz A 4- or 5-element monoband yagi at 100ft
  ANDA 4-element monoband yagi at 35ft.
• 28MHz A 6-element monoband yagi at 70ft ANDa
  6-element monoband yagi at 25ft

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What to aim for (LF Bands)

• 160m A large vertical (80ft) over a good set
  of radials Doublet as high as possible. Tuned
  rotary loop or Beverages for directional
  reception.
• 80m Four-square phased verticals over a good
  set of radials Doublet at 70ft.
• 40m 2- or 3-ele monoband yagi at 130ft
  ORFour-square phased verticals over good set of
  radialsANDDoublet at 40ft OR Full-wave loop
  with top wire at 40-45 ft

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What is the minimum I need?
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CTU Presents
Contest Antenna Systems The End