Chungbuk.ac.kr



Antenna Design Lab. 05 – Experiments with a Half-Wave Dipole Antenna

5.1 Purpose

- Learn how to use a network analyzer.

- Get a feel on an antenna using a dipole.

5.2 Theory

1) Network Analyzer

- A network analyzer measures the reflection and transmission coefficients versus the frequency of a two-port device:

[pic]

S11 : voltage reflection coefficient at port 1

S21: voltage transmission coefficient from port 1 to port 2

S12: voltage transmission coefficient from port 2 to port 1

S22 : voltage reflection coefficient at port 2

- Some network analyzers have the capability of inverse-transforming the frequency domain response and displaying the time domain response of the device under test.

[pic] [pic]

Fig. 5.1 A network analyzer and its block diagram.

- An example of a network analyzer measurement:

[pic]

Fig. 5.2 Measurement of a device using the network analyzer.

- Network analyzer calibration: a process that removes the effect of imperfect components inside the network analyzer from measured results.

- De-embedding: a process that removes the effect of transmission lines connecting the device to the network analyzer from measured results.

[pic]

Fig. 5.3 Hewlett-Packard HP85047A network analyzer.

2) Dipole Antenna

- A dipole is a two-piece straight metal antenna fed at the center.

- When a half-wave dipole is placed along the z-axis, at far fields we have [pic].

- The radiation pattern of a half-wave dipole resembles the number 8 with a maximum occurs in the direction normal to the dipole axis while a null occurs in the direction of the dipole axis.

- Half-wave beamwidth and gain of a half-wave dipole are 78º and 2.1dB respectively.

[pic]

Fig. 5.4 A dipole and its radiation pattern.

[pic]

Fig. 5.5 A 2-m dipole antenna.

- The dipole's impedance changes with the frequency. Its first resonance is at the frequency where its total length is about a half wavelength (to be more exact, L = 0.468λ). At resonance the dipole's input impeance is about 70+j0 (Ω).

[pic]

Fig. 5.6 The dipole input impedance versus the length.

- A dipole is often fed by a coaxial cable. In this case there is an imbalance in the magnitude of currents on two halves. Even with this current imbalance, one can still use a coaxial cable feeding for the sake of simplicity.

[pic]

Fig. 5.6 A coax-fed dipole and the current imbalance.

3. Measurements

3.1 Fabrication of Two Dipoles

1) Make two half-wave dipoles using a 0.4-mm diameter wire for breadboard wiring. Prepare four 0.4-mm diameter wires of length 50mm.

- Solder one to the center conductor of a coaxial cable and the other to the outer conductor of a coaxial cable.

- Bend two soldered wires to make the dipole axis perpendicular to the coaxial cable axis.

- Cut the end of each wire so that the length of each wire is 44mm. In this case the dipole resonates at about 1575MHz (GPS frequency). A fabricated dipole is shown in Fig. 5.7.

[pic]

Fig. 5.7 A fabricated dipole antenna operating at 1575MHz.

2) Repeat the above steps to make one more dipole.

3.2 Network Analyzer Calibration

1) Carry out 2-port calibration of the network analyzer over 1000-2000MHz.

3.3 Measurements

1) Label two dipoles 'Dipole 1' and 'Dipole 2'.

- Draw the shape of dimensions of each dipole. (report)

2) Connect 'Dipole 1' and 'Dipole 2' to ports 1 and 2 respectively.

3) Keep any object away from two dipoles. Measure S11 and S22. Plot the magnitude of S11 and S22 in log scale. (report)

4) Plot S11 on the Smith chart. (report)

5) List Rin and Xin at f0(center), f1(left -10dB reflection), and f2(right -10dB reflection). (report)

6) Place two dipoles in parallel with the separation d. Measure S21 at 1575MHz at various values of d. Fill in the following table. (report)

|d/λ |Magnitude of S21(dB) |Phase of S21 (deg.) |

| |Measured |Friis Transmission |Measured |Formula e-jkd0e-jk(d-d0) |

| | |Formula | | |

|d0=0.25 | | | |Use measured value |

|0.5 | | | | |

|1.0 | | | | |

|1.5 | | | | |

|2.0 | | | | |

|3.0 | | | | |

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download