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08c_aerials_feeders.md

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8c. Aerials and Feeders

Definitions

  • Electromagnetic Wave is created when radio frequency power from the transmitter causes a radio frequency current to flow in the aerial
  • Feeder is the cable connecting the aerial to the radio
  • Half wave dipole is a half wavelength wire cut in the middle and connected to a radio
  • Polar diagram is drawn by joining together all the points of equal field strength
  • Tuned circuit is an LC circuit, also called a resonant circuit, tank circuit
  • Reflector is an extra wire, slightly longer than the dipole, placed parallel to it at about λ/4 away
  • Director is a shorter wire on the other side of the dipole that will further increase the directional qualities
  • Coaxial cable (coax) has a centre conductor that is surrounded by insulation and a wire mesh screening
  • Screen is usually earthed and this cable is known as a unbalanced feeder
  • Ribbon cable feeder comprises of two parallel wires separated by an insulator
  • Balun can also be constructed to change the impedance
  • Standing waves occurs when there is the presence of both forward and reverse voltages & currents. If a feeder is incorrectly matched then some of the transmitted power will be reflected.
  • SWR is defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line. This in turn, tells you if the impedances between the load and source (transmitter) are matched.
  • Standing wave ratio meters (SWR meter or bridge) will determine the ratio between the forward and the reverse powers within the feeder cable and can indicate if the aerial is correctly matched to the feeder.
  • Aerial Matching unit (AMU) can improve this matching and can also (where necessary) perform the function of a balun when the impedance at the feed point of an aerial does not match the impedance of the of the feeder cable

Diagrams

  • Directional antenna

  • Yagi antenna

  • Yagi radiation pattern

Circuit diagrams

  • Current-voltage distribution at antenna

  • Tuned LC circuit

  • Aerial Matching unit (variable LC)

Notes

Electromagnetic waves

  • Vertical aerial will create an electromagnetic wave that is said to be vertically polarised
    • electric field will be vertical
    • magnetic field will be horizontal

Types of Aerials

Aerials Notes Length Impedance
1 Half wave dipole A feeder of that impedance can be used to give a good match λ/2 with λ/4 on each side low impedance at the centre feed point (70Ω / 80Ω)
2 Ground plane - The feeder cable is connected to the lower end and earth.
- The vertical aerial would normally be fed with coaxial cable from the radio.
- The inner is connected to the bottom of the aerial
- The outer screen is connected to earth (or ground plane wires or plate).		 λ/4 in vertical height
3 End Fed An Aerial Matching Unit (sometimes referred to as a Aerial Tuning Unit) would normally be used at the feed point length of the wire should not be an even number of λ/4 The AMU would be connected with 50Ω, 60Ω or 70Ω coaxial cable
4 Multiband For the aerial to function well on several Amateur Bands, it can be electrically achieved if tuned circuits are inserted into the aerial wires 2 x λ/4
5 Directional - designed to concentrate the transmitted RF energy transmitting in one direction
- If another wire, slightly longer than the dipole, is placed parallel to and about λ/4 away, then the aerial will have directional qualities. This extra wire is called a reflector
- Adding a shorter wire director on the other side will further increase the directional qualities.
6 Yagi - frequently used at VHF and UHF
- Adding directors dramatically reduces the impedance of the feed point of the driven element
- In order to combat this, the dipole is replaced by a loop.		 element spacing is typically λ/4
7 Quad - Have rod or wire elements that are formed into square loops
- Quad Aerial is compact and prevents high angle radiation		 square loops with λ/4 sides

Other concepts

  • Feeder cables
    • connects radio to the aerial
    • can be coaxial or ribbon cable
    • typical impedance is 50Ω / 70Ω
    • Advantage: does not have to be installed clear of building and other structures
    • Disadvantage:
      • not cheap
      • high loss for the signals passing through it
  • Ribbon cable
    • it can have a series of spacers, giving the appearance of a ladder or can have a continuous flat plastic spacing
    • wire spacing can range from 10mm to 100mm
    • gives impedance from about 70Ω to 600Ω
    • Advantage:
      • low loss
      • cheap
    • Disadvantage:
      • must be kept clear of walls and metal structures
  • Feeder
    • balanced twin or unbalanced coaxial cable
    • characteristic impedance that is dictated mainly by its physical dimensions
    • connected directly to the transmitter
    • should match its output impedance
    • transmitter output impedance is usually 50Ω or 75Ω
    • A short circuit at one end of a λ/4 cable (or odd number of λ/40) will appear to be an ∞ impedance as seen from the other end

Balanced / Unbalanced

Balanced Unbalanced
- feed-point of a dipole aerial
- twin feeder		 - Aerial socket on a radio
- Coaxial cable

Often necessary to "convert" from balanced to unbalanced

  • a 2:1 balun could be used to match a balanced aerial
  • feed impedance of 100Ω
  • coaxial feeder cable of 50Ω impedance

Standing wave ratio (SWR)

  • It is defined as the ratio of the maximum radio-frequency (RF) voltage to the minimum RF voltage along the line.
  • The SWR can also indicate if an impedance mismatch has occured, and by how much.
  • An open or short circuit will result in an infinite SWR.
  • If there is an impedance mismatch, it will result in an inefficient power transfer from source to load as the signal is reflected back towards the transmitting source.
  • Thus, a good impedance match is key to achieving a maximum power transfer to the load, and thus generaly leads to more of your signal being radiated out.

Standing wave ratio meters

  • A terminating resistor, equal to the characteristic impedance of the cable, is connected across the far end of the cable
  • A SWR meter is connected between the transmitter and the feeder cable
  • Low SWR reading indicates low power (radio frequency) is then transmitted
  • High SWR reading would indicate that there was likely to be a fault in the feeder cable (visual inspection required)