Antenna 48
Resonant Antennas: Wires and Patches
Dipole Antennas
Current distribution approximation
Un-normalized pattern:
and
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Radiating power:
For half-wave dipole
and
,
resonance. And
, or
at exact
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Antenna 51
Effect of dipole thickness
1. Resonant length: decreases as thickness increases
2. Resonant resistance: decreases as thickness
increases
3. Bandwidth: increases as thickness increases
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Flat Dipole
Vee Dipole
Formula for maximum directivities:
and the directivity
Properties:
1. Greater directivity than dipole
2. Smaller input impedance than dipole
3. Lower sidelobe and one of the main beams.
Antenna 53
Antenna 54
Folded Dipole Antennas
Input impedance for the transmission line mode:
Thus,
Let
be the input impedance of a dipole of length
The input impedance will be
Antenna 55
Note:
Properties:
1. Impedance can be made to be
, the same as the
common two wire transmission line.
2. First and second resonances have similar
impedances.
3. Wider bandwidth than dipoles
For different wire sizes:
where
Antenna 56
Antenna 57
Microstrip Patch Antennas
Resonant half wave patch:
2:1 VSWR Bandwidth:
Antenna 58
Input resistance adjustment factor
, where
the distance from the radiation edges.
Patterns:
, E-plane,
, H-plane,
Properties:
1. Low bandwidth. A few percent for single patch.
2. Low profile, light weight.
3. Easy to construct.
4. Easy to integrate to microwave circuits.
5. Low gain.
6. Increase substrate electric thickness, increase
bandwidth. But, also increase surface wave,
reducing efficiency.
Shorted quarter wavelength patch
Example 5-2
is
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Antenna 60
Gain calculation:
where
is the spacing,
is number of elements.
Antenna 61
Antenna 62
Yagi-Uda Antennas
Sumarry:
1. Reflector
a. Longer than driver.
b. Effect input impedance and the back lobe.
c. Optimum spacing: 0.15 to 0.25 wavelength.
d. Usually only one element. More elements only
increases gain a little bit.
e. Exact optimum length is primarily a function of
the tube diameter and the boom diameter.
2. Driver: usually a half-wavelength dipole.
Antenna 63
3.
4.
Director
a. Shorter than driver typically by 10 to 20%.
b. Optimum spacing: 0.2 to 0.35 wavelength.
c. Effect pattern shape and gain.
d. Gain increases as number of directors increases.
e. Exact optimum length is a function of the tube
diameter, the boom length and the diameter of
the boom.
Design procedure:
a. Look up table 5-4 to get the lengths of the
directors and reflector.
b. Adjust the length by the tube diameter from
Figure 5-37
c. Compensate for the changes in the lengths of
parasitic elements due to the tube diameter by
Fig. 5-37.
d. Compensate for the changes in the lengths of
parasitic elements due to the boom diameter by
Fig. 5-38.
Antenna 64
Example 5-1
Parameters
1. Number of element:12.
2. Wavelength: 1.46m.
3. Tube diameter: 1cm.
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Antenna 66
Corner Reflector Antennas
Summary:
1. Simple and achieve gain of 10 to 12 dB over a halfwave dipole.
2. Good directivity for
.
3. Maximum directivity at
with input impedance
.
4. At
, input impedance is
, negligible
decrease in gain.
5. Effect of finite extent of the plates
a. Broaden the pattern, but have little effect on the
input impedance.
Antenna 67
b.
c.
Usually choose
such that the main beam is
not degraded.
Usually choose to be 1.2 to 1.5 times of the
length of the feed to minimize the direct
radiation by the dipole feed into the back
region.
Large Loop Antennas
Antenna 68
Summary
1. Resonate at integer multiple of wavelength.
2. Input impedance
at one wavelength.
3. Gain 3.09 dB at one wavelength.
Antenna 69
Wire Antennas Above an Imperfect Ground Plane
Vertical oriented dipole
Horizontal oriented dipole
xz-plane:
yz-plane:
Antenna 70
In the united states, typically
and
.
Summary:
1. Plane wave reflection is only an approximation.
2. Surface wave propagates along the ground plane
surface. For HF and VHF frequencies, the surface
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3.
4.
5.
attenuates very rapidly.
For near
,
. Vertical antennas close to a
real earth have zero radiation. Surface wave
accounts for all propagation as in AM broadcasting.
For vertical antennas, surface wave can be neglected
and plane wave reflection can be used..
For horizontal antennas, the antenna should be
above the earth for the plane wave reflection
coefficient method to be valid.
Antenna 72
Feeding Wire Antennas
Impedance matching
1. Matching network
2. Adjust feed point.
For half-wave dipole
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Balun (Balance-unbalance)
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