Decibel

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1. Decibel
2. Transmission lines, SWR
3. Antenna properties
4. Some common antennas

Tiiti Kellomäki, OH3HNY
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The decibel
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Decibel scale

• All circuits either amplify or attenuate power.
• A Po/Pi 100000
• B Po/Pi 0.0002
• As the power ratios can be very large or very
  small, it is handy to use a logarithmic scale,
  the decibel scale.
• A(dB) 10 log Po/Pi 10 log 100000 50 dB
• B(dB) 10 log Po/Pi 10 log 0.0002 37 dB
• Positive decibel readings indicate gain and
  negative values attenuation.

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Decibel calculation

• Multiplication of power ratios corresponds to
  addition of decibel values.
• If the first block amplifies by 3 dB and the
  second one attenuates by 10 dB, the whole circuit
  has an effect of 7 dB, or the output is one
  fifth of the input.
• Po/Pi Po/Pa Pa/Pi 2 0.1 0.2, or 7
  dB.

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Decibel units of measure

• Decibel readings always mean power related to
  some known level, e.g. input power, carrier
  power, or noise level.
• A commonly used dB unit is dBm "decibels over one
  milliwatt". One watt in dBm is 10 log 1 W / 1 mW
  10 log 1000 30 dBm.
• One milliwatt is no more than one milliwatt,
  hence the ratio is one and 1 mW in dBm is 10 log
  1 0 dBm.

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Rules of thumb

• Negative decibels are for attenuation, positive
  for amplification.
• Adding 0 dB is the same as multiplying by one.
• Adding 10 dB is multiplication by 10.
• Adding 3 dB is multiplication by 2.
• 7 dB 10 dB 3 dB, or 10½ 5.
• 24 dB 10 dB 10 dB 10 dB 3 dB 3 dB, or
  101010/2/2 250.

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Transmission lines
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Transmission lines

• Because the wavelength of a RF signal is short
  (say, 80 m to 23 cm in your normal frequency
  range, or millimeters), cable lengths are large
  in terms of wavelengths.
• Signal voltage level varies rapidly in time and
  space.
• Wires must be thought of as transmission lines.

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Characteristic impedance

• Characteristic impedance tells the ratio of
  voltage to current (or electric to magnetic
  field) on the line.
• On a 50-ohm line, a 1-volt signal will be a
  20-milliampere signal.
• This impedance is not related to loss.
• 50 ohms is most often used in amateur radio.
  75-ohm coaxial cable is used in tv networks.

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Mismatch

• If the characteristic impedance of the
  transmitter, the cable, and the antenna are not
  the same, power is not delivered properly.
• Power is reflected in every impedance
  discontinuity.
• At some points, the forward and reflected
  voltages will add, and at some points cancel
  each other. Hot and cold spots are formed on
  the cable.

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Standing wave ratio

• Standing wave ratio is the radio of the maximum
  and minumum voltage on the line.
• SWR 1 means that there is no standing wave,
  thus no reflections, and all the power is
  delivered properly.
• SWR 2 means that 10 of the power is reflected
  at the end.
• SWR 3 means that 25 of the power is reflected
  at the end.
• SWR 8 means that no power is delivered to the
  load.
• To avoid reflections, impedance can be matched so
  that the transmitter sees a 50-ohm load.

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Coaxial cable

• Coaxial cable is formed of two coaxial tubes.
• Between the outer and the inner conductor there
  is some dielectric material.
• All electric and magnetic fields are inside the
  cable.
• Coaxial cable is not affected by nearby metallic
  objects or slight bending.
• 50-ohm coax is the most common transmission line
  used by radio amateurs.

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Two-wire lines

• The electric and magnetic fields of a two-wire
  transmission line spread into the surroundings of
  the line.
• Two-wire lines radiate and cannot be used above
  HF.
• Nearby metallic objects affect the performance of
  two-wire lines.
• In twinlead, there is insulating material between
  the wires. Its characteristic impedance is
  often 240 or 300 ohms.
• Open wire is essentially air-insulated and of
  450 ohms.

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Symmetry in transmission lines

• If one of the conductors on a line can be thought
  of as a ground, the line is unbalanced.
• Coaxial cable is unbalanced.
• If the lines are identical, the line is balanced.
• Two-wire lines are balanced.
• Symmetrical antennas need to be connected to
  symmetrical lines or else a balun
  (balanced-to-unbalanced transformer).

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Antennas
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Antennas

• Antenna is the part of an electromagnetic system
  that either transforms energy from current and
  voltage into electromagnetic radiation or vice
  versa.
• An antenna has the same properties regardless of
  whether it is used as a transmitting or a
  receiving antenna.

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Antenna as a load

• When an antenna is connected to a transmitter,
  the radio sees it as a load, an impedance.
• The antenna input impedance is strongly dependent
  on frequency.
• At resonance the impedance is real (resistance).
• Antenna bandwidth may be defined as the bandwidth
  where the reflected power due to impedance
  discontinuities is sufficiently small, or in
  other words, the SWR is small enough (say, 2 or
  3).

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Some useful antenna properties

• Radiation pattern is a plot of radiated power in
  different directions.
• An isotropic antenna radiates equally in all
  directions.
• Directivity tells how efficiently the antenna
  radiates in the maximum direction, compared to
  the isotropic antenna (dBi).
• Directivity can also be expressed in dB compared
  to a half-wave dipole (0 dBd 2 dBi).

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Some useful antenna properties

• Gain is the same thing but multiplied by
  efficiency. It is the property of a real antenna.
• Gain is also expressed in dBi or dBd.
• Efficiency tells the ratio of radiated power to
  power delivered to the antenna. If the efficiency
  is 73 , 27 of the delivered power is lost as
  heat because the antenna is made of lossy
  materials.
• If the directivity is 10 dBd and efficiency 50 ,
  the gain is 10 dBd 3 dB 7 dBd (one half of
  the original).

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Dipoles

• A half-wave dipole is the simplest antenna.
• The dipole is commonly used in HF.
• You need a balun to feed it properly.
• A half-wave dipole has an input impedance of 73
  ohms, so you can connect it directly to a 50-ohm
  radio.
• The bandwidth covers any one HF band.
• The half-wave dipole gain is 2 dBi 0 dBd
  (small).
• A dipole can be of any length, e.g. l, 2l
• Dipoles are balanced.

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Monopoles

• A monopole is half a dipole used with a ground
  plane.
• The monopole and its mirror image form a dipole.
• Common sizes are l/4 (gain 0 dBd), 5l/8 (gain 2
  dBd), ½l and one l.
• The longer the monopole, the higher the gain.
• Monopoles are commonly used in HF, VHF, UHF, and
  in handheld devices.
• Monopoles are unbalanced.

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Images of monopoles and dipoles
5l/8 radiation pattern
l/4 radiation pattern
folded dipole
dipole radiation pattern
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Loops

• The most common loop size is one wavelength.
• The loop radiates in the direction of the hole.
• Loops are used in HF.
• The exact shape of the loop is not important,
  just set up some wire in trees.
• The loop has a small gain, 1 dBd.
• Loops are balanced.

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Yagis
radiator
boom
directors

• The Yagi-Uda antenna is a dipole with some
  additional elements.
• The (half-wave) dipole part radiates, behind it
  is a long reflector element, and in front are one
  or more directors.
• The yagi is a directive antenna with a gain of 2
  to 20 dBd.
• To achieve more gain, the number of elements must
  be increased.

reflector
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Yagis

• Yagis are used in HF above 14 MHz, and up to
  1200 MHz.
• Yagis need to be carefully designed and fed.
• Two or more yagis can be stacked in order to
  achieve gain
• if you place them side by side, the horisontal
  lobe will narrow,
• one upon the other narrows the vertical lobe.

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Quads

• A quad is like a yagi but formed of loops instead
  of dipoles.
• You can place loops of different sizes within
  each other without problems.
• Quads are used in HF and VHF.
• A quad may have a gain of 4 to 10 dBd.

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Helices

• A helix can be used in two ways
• if the diameter is small compared to a
  wavelength, the helix will radiate like a dipole,
• if the diameter is a large fraction of a
  wavelength, the helix radiates axially and its
  polarisation is circular.
• The former type is used in handheld radios.
• The latter is used in UHF and up.

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Reflector antennas

• To achieve large gains, reflector antennas can be
  used.
• The diameter of the reflector must be several
  wavelengths.

• Reflector antennas are used in microwave
  frequencies, SHF.
• A gain of 30 dBi may be achieved.

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Exam questions

• You are constructing a 432 MHz transmitting
  antenna. Which one is correct?
• ? 43 cm half-wave dipole
• ? 34 cm half-wave dipole
• ? 43 cm 5/8-wavelength monopole
• ? 34 cm monopole and ground plane

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Exam questions

• You are constructing a 432 MHz transmitting
  antenna. Which one is correct?
• 43 cm half-wave dipole
• 34 cm half-wave dipole
• 43 cm 5/8-wavelength monopole
• 34 cm monopole and ground plane

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More exam questions

• Coaxial cable has the property
• ? not to radiate because the electromagnetic
  field stays inside the cable
• ? its characteristic impedance is affected by the
  distance between the conductors
• ? it can be mounted on a metallic roof
• ? it can be bent with a sharp radius (minimum
  radius 5D)

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More exam questions

• Coaxial cable has the property
• not to radiate because the electromagnetic
  field stays inside the cable
• its characteristic impedance is affected by the
  distance between the conductors
• it can be mounted on a metallic roof
• it can be bent with a sharp radius (minimum
  radius 5D)

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Questions?

• Feel free to ask further questions.
• Check the nice antenna books at the club room
• ARRL Antenna Book
• Antennisuunnittelu-kurssin pruju
• Simple and Fun Antennas for Hams