Download Chapter 19: Antennas

Chapter 19: Antennas
By: James VE3BUX
Definition
The Modern Dictionary of Electronics defines an
antenna as:
• That portion, usually wires or rods, of a radio
transmitter or receiver station used for
radiating waves into or receiving them from
space. It changes electrical currents into
electromagnetic waves, and vice versa.
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Basic Electrical Properties
Antennas:
• Behave differently based on design
• Are (ideally) purely resistive at resonance
• Exhibit polarization
• Radiate in a predictable pattern
• Have an input impedance & exhibit inductive
or capacitive character out of resonance
• Have bandwidth
3
Antennas: How important?
• Antennas are the “portal to the ether”
• Design and build of antennas are often one of
the most critical aspects of radio
communications
• A really well designed and built antenna can
work wonders whereas a “wet noodle” or
“rubber resistor” antenna are almost entirely
useless
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Antennas: Major design types
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Antennas: Vertical “Whip”
• Until recently, these were the
most prevalent antennas
• Generally used in a vertical
arrangement
• Can be multi-band (ie. Cover
multiple frequencies without
a tuner)
• Omnidirectional
6
Antennas: Dipole (“Doublet”)
• Extremely simple to make &
costs very little
• Exhibits some directionality
• Can be stacked / arrayed to
increase “gain”
• Very common first project
• Effectively omnidirectional
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Antennas: Yagi-Uda
• Directional antenna
– Design parameters determine
F/B ratio
• Excellent for basic radio
direction finding
• Three basic elements:
1
2
3
1. Reflector(s)
2. Driven element
3. Director(s)
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Antennas: Cubic Quad
• Directional antenna
– Design parameters determine
F/B ratio
• Very similar to Yagi-Uda, very
slightly better
• Same three basic elements:
– Reflector(s)
– Driven element
– Director(s)
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Antennas: Magnetic Loop
• Omnidirectional antenna
– Does have some directionality
• Reasonably immune to local
noise due to principle of
operation
• Can generate very significant
voltages!
• Very small size
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Antennas: Dish
• Very directional antenna
– Design parameters determine
F/B ratio
• Significant “gain” as frequency
increases
• Great for E-M-E
• Polarized!
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Antennas: Horn
Holmdel Horn
• Very directional antenna
• Generally not used below
10GHz
• Polarized!
• Extremely efficient (low
radiation resistance)
• Can achieve incredible gain
figures
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Properties: Resistive at Fr
• Antennas are said to behave in a purely
resistive manner when they are driven with
energy near their resonant point
• This resistive state indicates a good “match” of
impedances between the radio and the
antenna system
• Recall that when Rload = Rinternal the maximum
power transfer occurs
– (Table 7-1, p65)
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Properties: Polarization
• Electromagnetic waves are orthogonal fields which
are composed of lines of force which are:
– Electric
– Magnetic
• Direction of electric field determines polarization
– Generally the same direction as the most significant
radiating element
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Polarization: Importance
• Above HF frequencies, matching polarization
becomes extremely important
– Loss can be as much as -40dB
– Loss = 10log10(cos Θ)2 where Θ = difference angle
• HF propagation makes polarization far less
important
• Special types of polarization exist to solve
particular challenges
– LH or RH Circular
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Properties: Have radiation pattern
• Antennas radiate their energy differently
based on their design
• Radiation pattern is effectively the “shape” of
an RF field being generated by a transmitting
antenna
• Need a bit more information to discuss
patterns…
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Field Strength
• The strength of a radio signal is defined by the
amount of voltage induced onto a second
antenna at some distance
• Due to varying types of antennas and their
differences in reception, a standard antenna with
a length of 1m is used
• Field strength is thus defined to have the unit
volts per meter, or v/m
– Generally μv/m due to the weak nature of radio
signals
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S-units: An aside
• S-units on a radio are based on 6dB difference
– Recall field strength is in volts and therefore the
log math requires 20log10(V1/V2) so 6dB = 2x
• A signal which is said to be “S9” should mean
that the receiving antenna would measure
50μv across a 50Ω load (antenna)
– Also defined as:
• -73dBm for HF
• -93dBm for VHF
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Radiation Pattern: Basics
• To discuss radiation pattern, we need to
establish how to present field strength data
•Antenna is placed at the
center and where
necessary, “elements” are
shown on the graph
•Concentric rings indicate
field strength, usually in a
relative sense
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Radiation Pattern
• The most basic pattern is a so-called isotropic
radiator
– The antenna radiates energy in all directions “equally
poorly”
• Imagine a small sphere at the center of a beach
ball
– Small sphere at the center is the antenna
– Beach ball is the shape that the radiated energy will
take in ideal conditions
– The surface of the ball is EM energy - as the radius
increases, the “thickness” of the ball decreases; think
of the EM field magnitude as the thickness
• Inverse square law
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Radiation Patterns: Dipole
• Dipole antennas are the first to exhibit
directivity
• Imagine a wire antenna which is vertical, the
energy radiates as shown:
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Radiation Patterns: Vertical
• Simple ground mounted 1/4λ monopole
(vertical) antenna
• Good “take-off” angle provides DX
opportunities
• Ground conductance becomes important and
will affect pattern
– Antennas of this nature generally rely on (or are
significantly enhanced with) the presence of
“ground radials” which serves as a distributed
ground plane
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Radiation Pattern: Yagi-Uda
• Great “directional” antenna
• Focuses energy well and is thus said to have
“gain”
• Antennas on towers
are very often of the
Yagi-Uda style
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Input Impedance: Design & Location
• Antenna impedance depends on a few factors:
– Design
– Height above ground
– Resonant frequency
• Matching 50Ω output of radio to the antenna’s
impedance is important
– Ideal power transfer
– Reduce wasted energy coming back as reflected
power (SWR)
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Impedance: Design
Antenna Type
Free-space impedance (Ω)
Yagi (direct feed)
20-25
1/4λ vertical (whip)
37
Dipole (Inverted V)
~50
Dipole (Basic Design)
73
Cubic Quad
115
Dipole (Folded Design)
300
Windom (Off-center fed dipole)
600+
• Plenty of ways to match the radio to the antenna
• Matching networks (“tuners”) are often the solution
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Properties: Bandwidth
• Antennas have a resonant point due to electrical length of elements
• As you move away from the resonant point, the impedance changes
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Properties: Bandwidth
• Some parameters which affect bandwidth are:
– Physical design of antenna (dipole vs Yagi, etc)
– Antenna element diameter
– Environment around antenna
– Feedline losses
– Shortening / “Loading” of antenna
• Bandwidth edges are generally defined by 2:1
SWR points (similar to -3dB concept)
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• Questions?
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