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Transcript
```1.Decibel
2. Transmission
lines, SWR
3. Antenna
properties
4. Some common
antennas
Tiiti Kellomäki, OH3HNY
The decibel
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.
Decibel calculation
• Multiplication of power ratios corresponds
• 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.
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.
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 10·½ = 5.
• 24 dB = 10 dB + 10 dB + 10 dB – 3 dB – 3 dB,
or 10·10·10/2/2 = 250.
Transmission lines
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.
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.
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.
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 = ∞ 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.
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
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 twowire 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.
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-tounbalanced transformer).
Antennas
Antennas
• Antenna is the part of an electromagnetic
system that either transforms energy from
current and voltage into electromagnetic
• An antenna has the same properties
regardless of whether it is used as a
transmitting or a receiving antenna.
• When an antenna is connected to a transmitter,
• 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).
Some useful antenna properties
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).
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).
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
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.
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.
Images of monopoles and dipoles
pattern
folded dipole
pattern
pattern
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.
boom
Yagis
directors
• The Yagi-Uda antenna
is a dipole with some
reflector
• 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.
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.
• A quad is like a yagi but formed of loops
• 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.
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
• The latter is used in UHF and up.
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.
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
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
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
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