CH5' Resonant Antennas :Wires and Patches

1
CH5. Resonant Antennas Wires and Patches

• Resonant Wire Antennas with
• zero input reactance at resonance
• straight wire dipoles, vee dipoles, folded
  dipoles, Yagi -Uda arrays
• Feeding methods
• Effects of imperfect ground

2
Dipole Antennas

• Dipoles of arbitrary length
• Current distribution
• Sinusoidal distribution (good approximation for
  thin antennas dltlt0.01?)
• Zero at ends

3
Straight Wire Dipoles

• Llt?/2

Standing wave pattern Same direction at the same
time
Opposite direction ? canceling of radiation
4
Various Center Fed Dipoles

• Lgt? ?Currents not in the same direction ?
  canceling effects in the radiation pattern

Standing wave pattern
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Radiation Pattern of Various Center Fed Dipoles
Multiple lobes due to the oppositely directed
currents on the antenna when Lgt?
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Radiation Resistance of Various Center Fed Dipoles
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Triangular approximation for short dipole ?a good
approximation
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resonance
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Radiation Resistance vs. Input Resistance
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Wire Lengths for Resonance

• Input impedance of infinitely thin half wave
  dipole 70j0 ?
• The wire thickness ? ? the length for resonance ?
• As the length is reduced for resonance, the
  input resistance decreases

11
Bandwidth of Resonant Dipoles

• Bandwidth - The range of frequencies within which
  the performance of the antenna conforms to a
  specified standard w.r.t some characteristics

The thicker the dipole, the wider the bandwidth
BW
VSWR21
12
Thin Metal Strip Dipole for Wider BW
Variation of thick cylindrical dipole ?Thin
metal strip dipole with a0.25w a radius of
cylindrical dipole, w metal strip width ?Equal
performance and cost effective
13
Directivity of Half-Wave Dipole Antenna

• D4?Um/Prad
• cf) ideal dipole D1.5
• Length? ? D? until L1.25?
• Further increase of the length ?pattern break up
  and D drops sharply

14
VEE Dipoles for Higher Directivity
Maximum directivity
Input impedance of VEE dipole antenna is less
than that of a straight dipole 106j17? for
L1.5?
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Folded Dipole Antennas
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Folded Dipole Antennas
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Folded Dipole Antennas
Very low
1.47?
0.48?
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Examples of Folded Dipole Antennas

• FM band receiving antenna (?/2)
• Cutting a piece of 300? twin lead transmission
  line about half wave length(1.5m_at_100MHz)
• Short at each end
• Cut one wire in the middle and connect twin lead
  T/L for feeding? direct matching to 300 ? T/L
• Wider bandwidth than an ordinary half wave dipole
• Used as a feed antenna for Yagi-Uda arrays

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Folded Dipole Using Unequal Wire Size
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Feeding Wire Antennas

• Problems of high VSWR
• Dissipative loss on the line
• Hot spots on the line ? cause arching
• Frequency pulling
• Impedance matching between antenna and T/L
• conjugate matching for maximum power transfer
• If ZA ?Zo ? VSWR gt1

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VSWR and Transmitted Power
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Impedance Match

• Impedance matching
• Quarter wave transformer or stub matching at
  microwave frequencies
• Lumped matching at low frequencies
• Disadvantage of matching network
• Narrow band for perfect match
• Imperfect for broadband match
• Changing the antenna input impedance without
  using a matching network
• Displacing the feed point off center

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Input Impedance of the off the center feed
Half wave dipole with displaced feed
Current distribution on full wave dipole for an
off-center feed ?Off center feed is unsymmetrical
feed can lead to undesirable phase reversals in
the antenna
24
Symmetrical Shunt Feed
Shunt Feed Increases the input resistance with
increasing distance from the center point of the
wire antenna

• Tee match model
• ?Shorted T/L a dipole of wide feed gap spacing
• ?Leads to inductive input impedance
• ?For tune out the inductance, shorten the dipole
  length or use shunt capacitor
• ?As D increases, the input impedance increases
  and peaks at D of the half of the dipole length.
• ?Further increase of D reduces the input
  impedance and finally it equals the folded dipole
  antenna

25
Balanced and Unbalanced Operation
Unwanted radiation occurs
radiation pattern changes
unbalanced

• Parallel wires Balanced T/L
• Coaxial cable Unbalanced T/L
• ? a wave traveling down the coax has a balanced
  current mode, but at the symmetric load, a
  current flows back on the outside of the outer
  conductor, which unbalances the antenna and T/L

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Sleeve Balun (Bazooka) 11
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Folded Balun (11)
Forms two lead T/L
Outer conductor currents are balanced by loading
the center conductor current of main coax with a
similar shaped dummy coax
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Tapered Broadband Baluns
Tapering a balanced transmission line to an
unbalanced one over at least several wavelengths
of transmission line length
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Low Frequency Balun
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Balun with Impedance Transformation
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Yagi-Uda Antennas

• Parasitic array antenna
• A few elements of an array are directly fed
• Parasitic array are excited by near-field
  coupling from the driven elements
• Yagi-Uda array
• Simple and high gain
• Uda at Tohoku Univ. 1926
• Yagi was a professof of Uda, who wrote an article
  reviewing Uda's work in English

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Operation of Yagi-Uda Antenna
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Reflector
Lengthening the parasitic element leads to single
main beam in the endfire direction from the
parasite to the driver along the line of
array ?Parasite is called as reflector because it
appears to reflect radiation from the driver
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Director
director

• Director
• ?The parasite is shorter than the driver and
  placed on the other side of the driver
• ?enhance main beam in the same direction
• ? Direct radiation in the direction from the
  driver toward the director
• 3 element Yagi
• ?Maximum directivity about 9dBi

35
Configuration of General Yagi
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Spacing and Size of Reflector and Director

• Spacing
• ?Optimum reflector spacing for maximum
  directivity 0.15-0.25 ?
• ?Director to director spacing 0.2-0.5?
• Typical size
• ? The reflector length is 0.5?
• ? The driver is of resonant length without
  parasitic elements
• ? The director lengths are 10-20 shorter than
  their resonant leneght

Exact director length is sensitive to the number
of director and the interdirector spacing
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Gain vs. ND

• The addition of directors up to about 5 or 6
  provides a significant increase in gain
• The addition of more reflector elements for gain
  is not effective
• Reflector controls the driving point impedance at
  the feed point and the back lobe of the array
• Director controls pattern

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Optimum lengths of Parasitic Dipoles

• Optimum design s for a specified boom length
• For d0.0085 ?
• SR0.2 ?
• For other conductor diameter, refer to
• Fig 5-37

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Design Curves of Yagis
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Effects of Metallic Boom

• Metallic boom is used to force zero voltage
  condition at the element center
• Effects of metallic boom of a finite diameter can
  be compensated by increasing the length of
  parasites
• Alternatively, the parasitic elements may be
  insulated from the boom, in which case no
  compensation is required

41
Corner Reflector Antenna
Analysis using the method of image and array
theory
Invented by Kraus 1938 90 corner reflector Metal
plates at ??45 Gain between 10-12dB over a
half-wave dipole
42
Patterns of Corner Reflector Antenna
Pattern shape, gain and feed point impedance is a
function of the feed to corner spacing
s ?maximum directivity at s 0.5? ?antenna
impedance 70? at s 0.35? for infinite
plates ?finite size of plates leads to the
pattern broadening ?practically, L2s, H1.2?1.5
times of the feed length
43
Wire Antennas above an Imperfect Ground Plane

• Earth is approximated as a infinite and planar
  but poor conductor
• 10-1(rich soil) ?10-3(rocky or sandy soil) S/m
• Fields from a nearby antenna penetrate into the
  earth and excite currents that give rise to ohmic
  losses(?E2)
• Ohmic losses due to the earth increases the input
  ohmic resistance and reduces the radiation
  efficiency of the antenna
• Complex permittivity of ground (relative)

44
Pattern Effects of a Real Earth Ground Plane

• Approximate pattern ? using image theory
• The strength of the images in a real ground is
  reduced from that of the perfect ground plane
  case
• Strength of image can be approximated by
  weighting it with the plane wave reflection
  coefficient for the appropriate polarization of
  the field arriving at the ground plane ? ?V, ?H
• The use of plane wave reflection coefficient is
  only an approximation since antennas near a
  ground plane do not from plane waves incident on
  the ground plane.

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Surface Waves

• Surface wave propagates along the ground plane
  surface
• For HF, VHF frequencies, the surface wave
  attenuates very rapidly.
• Along ground(??90), ?V?-1
• vertical antennas close to a real earth have zero
  radiation
• The surface wave accounts for all propagation as
  in daylight standard broadcast AM
• Horizontal antenna is placed at least 0.2? above
  the ground for image method to be valid

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Vertical Dipole
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Horizontal Dipole
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Magnitude and Phase of Reflection Coef.
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Elevation Pattern of Vertical Short Dipoles

• Perfect conductor ?maximum radiation along the
  ground
• Real earth ground
• ? reduction of intensity due to reduced intensity
• ? beam tilting upward from the ground plane
• ? good ground system is essential for low angle
  radiation, which is important for long distance
  communication links using ionospheric reflection

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Ground Plane Construction

• Ground plane using a metallic sheet larger than
  the antenna extent? impractical at low f
• To reduce the earth ground loss for the monopole,
  highly conductive return path is prepared for the
  displacement current launching at the antenna
  and entering the earth. ? radial ground system
• Typically, number 8 AWG, not deeply buried
• Reduce ohmic losses? increase efficiency
• AM transmitting antenna ?120 radial wires spaced
  equally to a distance of ?/4 (roughly equal to
  the height of the monopole antenna)

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Resistance of Radial Ground System
3MHz
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Resistance of Radial Ground System
antenna
Maximum Radiation
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Small Loop Antennas

• Small loop antennas
• Insensitive to loop shape
• Dependent on the loop area
• Maximum radiation in the plane of the loop and
  zero along the axis normal to the loop
• Amplitude and phase of the current on the loop
  are constant around the loop since the loop
  perimeter length L is electrically small

54
Large Loop Antennas

• The current amplitude and phase vary with
  position around the loop
• Current distribution is close to sinusoidal for
  resonant loops
• Circular or square shape are usually used
• First resonance perimeter length is slightly
  greater than one wavelength
• One-wavelength square loop antenna will be
  analyzed

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One-Wave Length Square Loop Antenna

• Sinusoidal current distribution
• Note the direction of currents at each side
• Finding Electric field is straightforward
• From the current distribution, obtain vector
  potential A
• A has only x, y components without z component
• Calculate E - j?A? E? ,E?

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Calculation of Vector Potential
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Far Field of One ? Square loop Antenna
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Far Field of One ? Square loop Antenna
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Far Field of One ? Square loop Antenna
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Far Field of One ? Square loop Antenna
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Principal Plane Patterns for One ? Square loop
Antenna

• Maximum radiation normal to the plane of the loop
• Polarization in the maximum radiation is parallel
  to the loop side containing the feed
• Null occurs in the direction parallel to the side
  containing the feed
• A lobe exist in a direction perpendicular to the
  side containing the feed
• Pattern is quite different from the small loop
  antenna

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Impedance of a Square loop Antenna

• Resonance for a 1.09? loop perimeter
• Input resistance at resonance about 100?
• Gain 3.09dB

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Microstrip Antennas

• Printed antenna
• Concept 1950s
• Extensive investigation 1970s
• Low profile
• Low cost
• Mass production
• Resonant antenna
• Narrow bandwidth( a few )
• Usually used above 1GHz

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Resonance Half ? Rectangular Patch Antenna
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Operation Principles

• Patch is open at its end ?Standing wave mode
• Fringing fields at the ends are exposed to the
  upper half space (zgt0) and are source of
  radiation
• Electric fields at each end have opposite phase
• From the top, x components of the fringing fields
  are actually in-phase ? broadside radiation
  pattern
• Peak radiation in the z direction
• Fields at the side edges are of odd symmetry? no
  radiation
• The width s of the slots of fringing fields is
  equal to the substrate thickness t

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Pattern of Rectangular Patch Antenna
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Principal Plane Patterns
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Input Impedance and Bandwidth
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Direct Coupled Patch Antennas
Useful for Array
Impedance adjustment through change of inset

• Popular, but has only one degree of freedom to
  adjust impedance
• Feed along a patch centerline in the E plane
  avoids excitation of second resonant mode
  orthogonal to the desired mode
• Narrow BW, For broad BW, increase the substrate
  thickness ?surface waves (bad)
• Coaxial probe feed
• Impedance adjusted by the probe distance from the
  patch edge. ?xp ? ? input resistance ?
• Parasitic inductance due to the coaxial probe ?
  probe radiation, limit on the substrate thickness
  t lt0.1?

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Electromagnetically Coupled Patch Antennas

• No patch contact ? two design parameters
• Probe feed with a gap - partial cancellation of
  probe inductance using gap Cap

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Aperture Coupled Feeding of Patch Antennas
Low dielectric constant material for radiation
High dielectric constant material for tight
binding of fields
Increased bandwidth
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Ex 5-2

• Square half wave patch at 3.03GHz
• t0.114, er2.35? L3.16cm, ZA368?
• Measured results fr 3.01GHz, ZA316?

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Microstrip Arrays
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Helical Antennas (Helix)
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Normal Mode Helix Antenna
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Stub Helix Antenna
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Homework

• 5.1-4
• 5.2-1
• 5.3-1
• 5.3-3
• 5.4-5(read ex 5.1 carefully)
• 5.7-6 use NEC code
• Design project
• Design square microstrip patch antenna operating
  at 3GHz on a t0.8mm thick substrate of er4.5
  for a 50 Ohm match