Antenna 77

Broadband Antennas

Definition

Percentage of the center frequency:

Ratio of upper and lower frequencies of operation:

Usually, if the impedance and the pattern do not change significantly

over about an octave ( ) or more, we will classify it as a

broadband antenna.

Traveling-wave long wire antennas ( )

Current distribution: . Ground effect is ignored.

Radiation pattern:

Angle of maximum radiation:

Impedance 200 to 300 .

: n forward lobes in .

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Antenna 78

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Antenna 79

Traveling-wave Vee Antenna

Good vee pattern:

Rhombic antenna

For horizontal rhombic antennas meters above a ground, to achieve

an elevation angle , the following must meet.

Impedance: 600 to 800

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Antenna 80

Beverage Antennas ( long wire antenna with ground effect)

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Antenna 81

Helical Antennas

Normal Mode

Dipole:

Loop:

To achieve circular polarization, . Solving for S, we

have

or

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Antenna 82

Normal Mode Helix Antenna (NMHA) or resonant (quarter-wave)

stub helix antenna: (monopole on a ground plane). Vertical

polarization dominates.

Radiation resistance

Example 6-1: A Stub Helix Antenna for Handheld Cellular Radios

Axial Mode ( )

1. Circular polarized near axis.

2. Gain~15 dB.

3. Single End fire.

4. Main beam narrows as number of turns increases.

5. Traveling wave modes.

6. Usually . Bandwidth ratio .

7. Optimal

Pattern

for both and

For , ,

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Antenna 83

Maximum Gain: .

Axial ratio:

Input resistance:

Example 6-2

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Antenna 84

Biconical Antennas

Assume , from , we have

therefore,

and

Pattern:

Impedance:

For ,

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Antenna 85

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Antenna 86

Finite Biconical Antenna ( )

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Antenna 87

Bow-tie Antenna

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Antenna 88

Discone Antenna

Typical value:

Optimum values:

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Antenna 89

Principles of Frequency-Independent

Antennas (bandwidth>10:1)

1. Emphasis on angles rather than lengths.

2. Self-complementary structures.

a. Only for planar structures.

b.

3. Thick metal.

4. Self-scaling: for resonant structures.

Spiral Antennas

Self-complementary, bandwidth 40:1

Equiangular Spiral Antenna

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Antenna 90

Self-complementary:

Expansion ratio :

Typically

Upper and lower bound of bandwidth:

or ,

or

Impedance

Pattern: bidirectional, two width beams broadside,

circular polarized up to

Number of turns: one-half to three turns. Optimum .

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Antenna 91

Archimedean Spiral Antenna

and

Cavity-backed Archimedean spiral:

over 10:1 bandwidth.

Pattern:

Example: Fig. 6-29

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Antenna 92

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Antenna 93

Conical Equiangular Spiral Antenna

Pattern: single mainbeam at -z direction

For , maximum at and ,

circularly polarized.

Impedance:

Lower and upper bound of frequencies

and

Square Archimedean: offer size reduction of 22%

because the increase of perimeter from to .

Sinuous Antenna: Two opposite

arm pairs produce orthogonal

linear polarizations.

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Antenna 94

Log-Periodic Antennas

Log-periodic toothed planar antenna

Define:

scale factor

width ratio

Log period

Self-complementary

Pattern characteristics:

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Antenna 95

at

at

Linear polarized

Broadside

Log-periodic toothed wedge antenna

Single main beam in +z direction.

Polarization in y direction.

Frequency independent for

Cross-polarization ratio: ~18 dB

Impedance: ( ), ( ).

Other variants

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Antenna 96

Log-Periodic Dipole Array (LPDA)

Define spacing factor

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Antenna 97

Since , then

Also

Lower bound and upper bound freq.

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Antenna 98

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Antenna 99

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Antenna 100

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