Antenna 48

Resonant Antennas: Wires and Patches

Dipole Antennas

Current distribution approximation

Un-normalized pattern:

and

Antenna 49

Radiating power:

For half-wave dipole

and, , or at exact

resonance. And

Antenna 50

Antenna 51

Effect of dipole thickness1. Resonant length: decreases as thickness increases2. Resonant resistance: decreases as thickness

increases3. Bandwidth: increases as thickness increases

Antenna 52

Flat Dipole

Vee Dipole

Formula for maximum directivities:

and the directivity

Properties:1. Greater directivity than dipole2. Smaller input impedance than dipole3. 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 is

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 patchExample 5-2

Antenna 59

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. Directora. 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 ofthe boom.

4. 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 5-37c. Compensate for the changes in the lengths of

parasitic elements due to the tube diameter byFig. 5-37.

d. Compensate for the changes in the lengths ofparasitic elements due to the boom diameter byFig. 5-38.

Antenna 64

Example 5-1Parameters1. Number of element:12.2. Wavelength: 1.46m.3. Tube diameter: 1cm.

Antenna 65

Antenna 66

Corner Reflector Antennas

Summary:1. Simple and achieve gain of 10 to 12 dB over a half-

wave 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 theinput impedance.

Antenna 67

b. Usually choose such that the main beam isnot degraded.

c. Usually choose to be 1.2 to 1.5 times of thelength of the feed to minimize the directradiation by the dipole feed into the backregion.

Large Loop Antennas

Antenna 68

Summary1. 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 dipolexz-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

Antenna 71

attenuates very rapidly.3. For near , . Vertical antennas close to a

real earth have zero radiation. Surface waveaccounts for all propagation as in AM broadcasting.

4. For vertical antennas, surface wave can be neglectedand plane wave reflection can be used..

5. For horizontal antennas, the antenna should beabove the earth for the plane wave reflectioncoefficient method to be valid.

Antenna 72

Feeding Wire AntennasImpedance matching1. Matching network2. Adjust feed point.

For half-wave dipole

Antenna 73

Balun (Balance-unbalance)

Antenna 74