Antennas: from Theory to Practice 5. Popular Antennas

1. Antennas: from Theory to Practice5. Popular Antennas Yi HUANG Department of Electrical Engineering & Electronics The University of Liverpool Liverpool L69 3GJ Email: Yi.Huang@liv.ac.uk

2. Objectives of this Chapter To examine and analyse some of the most popular antennas using relevant antenna theories, to see why they have become popular, what their major features and properties (including advantages and disadvantages) are, and how they should be designed.

3. Classification of Antennas Wire-Type AntennasAperture-Type Antennas Dipoles Horn and open waveguide Monopoles Reflector antennas Biconical antennas Slot antennas Loop antennas Microstrip antennas Helical antennas Linearly polarised antennas Circularly polarised antennas Element antennas Antenna array Narrow-band Broad-band Transmitting Receiving

4. 5.1 Wire Type Antennas Dipole Antennas Evolution of a dipole of total length 2l and diameter d

5. Current distribution of dipoles Current distribution along an open transmission line is: Thus the current distribution on the dipole is

6. Radiation pattern of dipoles In the far field:

8. Input impedance of dipoles

9. Electrically short dipoles • When the dipole length is much shorter than a wavelength (< l/10), it can be called an electrically short dipole • The input impedance can be approximated as • Radiation pattern is E() = sin • The directivity is D = 1.5 (1.76dBi)

10. Half-wavelength dipole • The most popular dipole • Radiation pattern: E() = cos[(/2)cos]/sin • Radiation resistance: 73  • Directivity: 1.64 (2.15 dBi) • The input impedance is not sensitive to the radius and is about 73 Ω which is well matched with a standard transmission line of characteristic impedance 75 Ω or 50 Ω (with a VSWR < 2). • Its size and radiation pattern are suitable for many applications

11. Example 5.1 A dipole of the length 2l = 3 cm and diameter d = 2 mm is made of copper wire (s = 5.7  107 S/m) for mobile communications. If the operational frequency is 1 GHz, a). obtain its radiation pattern and directivity; b). calculate its input impedance, radiation resistance and radiation efficiency; c). if this antenna is also used as a field probe at 100 MHz for EMC applications, find its radiation efficiency again, and express it in dB. Solution on pages 135 - 137

12. Some popular forms of dipole antennas

13. Monopole Antennas  l • Half of a dipole antenna mounted above the earth or a ground plane • Normally one-quarter wavelength long • almost the same feature as a dipole, except the 37 radiation resistance, higher gain, a shorter length, and easier to feed! • Based on the Image Theory Ground

15. Effects of the ground plane Its size and material property of can change the radiation pattern (hence the directivity) and input impedance.

16. An example

17. Some popular forms of monopole antennas

18. Duality Principle Duality means the state of combing two different things which are closely linked. In antennas, the duality theory means that it is possible to write the fields of one antenna from the field expressions of the other antenna by interchanging parameters: System 1 System 2

20. Loop Antennas • For a short dipole • Thus for a small loop

21. Directivity of a loop

22. Current distribution of a resonant loop

23. Radiation pattern of a one wavelength loop – this is very different from that of a short loop!

24. Radiation patterns of loops with various circumferences

25. Input impedance of loops

26. Helical Antennas It may be viewed as a derivative of the dipole or monopole, but it can also be considered a derivative of a loop.

27. Normal mode Helix • It may be treated as the superposition of n elements, each consisting of a small loop of diameter D and a short dipole of length s, thus the far fields are • They are orthogonal and 90 degrees out of phase; • The combination of them gives a circularly or elliptically polarised wave. • The axial ratio:

28. When the circumference is equal to the axial ratio becomes unity and the radiation is circularly polarised.

29. Axial Mode Helix • The axial (end-fire) mode occurs when the circumference of the helix is comparable with the wavelength (C = pD ≈ l) and the total length is much greater than the wavelength. • This has made the helix an extremely popular circularly-polarised broadband antenna at the VHF and UHF band frequencies • The recommended parameters for an optimum design to achieve circular polarisation are:

30. The normalised radiation pattern: Half power beamwidth: 1st null beamwidth:

31. The directivity: • The axial ratio • Radiation resistance

32. Example 5.2 Design a circularly polarised helix antenna of an end-fire radiation pattern with a directivity of 13 dBi. Find out its radiation resistance, HPBW, AR and radiation pattern. Solution on pages 150 - 152

33. Radiation patterns Which is better?

34. Yagi-Uda Antennas

35. The driven element (feeder) is the very heart of the antenna. It determines the polarisation and centre frequency. For a dipole, the recommended length is about 0.47l to ensuring a good input impedance to a 50 Ω feed line. • The reflector is longer than the feeder to force the radiated energy towards the front. The optimum spacing between the reflector and the feeder is between 0.15 to 0.25 wavelengths. • The directorsare usually 10 to 20% shorter than the feeder and appear to direct the radiation towards the front. The director to director spacing is typically 0.25 to 0.35 wavelengths, • The number of directors determines the maximum achievable directivity and gain.

36. Log-periodic Antennas

37. The antenna is divided into the so called active region and inactive regions. • The role of a specific dipole element is linked to the operating frequency: if its length, L, is around half of the wavelength, it is an active dipole and within the active region; Otherwise it is in an inactive region and acts as a director or reflector as in Yagi-Uda antenna • The driven element shifts with the frequency – this is why this antenna can offer a much wider bandwidth than the Yagi-Uda. A travelling wave can also be formed in the antenna. • The highest frequency is basically determined by the shortest dipole length while the lowest frequency is determined by the longest dipole length (L1).

38. Antenna design • This seems to have too many variables. In fact, there are only three independent variables for log-periodic antenna design. the scaling factor: the spacing factor: the apex angle:

40. In practice, the most likely scenario is that the frequency range is given from fmin to fmax, the following equations may be employed for design Another parameter (such as the directivity or the length of the antenna) is required to produce an optimised design.

41. Example 5.3 Design a log-periodic dipole antenna to cover all UHF TV channels, which is from 470 MHz for channel 14 to 890 MHz for channel 83. Each channel has a bandwidth of 6 MHz. The desired directivity is 8 dBi. Solution on page 160

42. 5.2 Aperture-Type Antennas • They are often used for higher frequency applications (> 1GHz) than wire-type antennas.

43. Fourier Transform and Radiated Field

44. How to link the aperture E field to the radiated field Directivity:

45. Near field and far field

46. Example 5.4 An open waveguide aperture of dimensions a long x and b along y located in the z = 0 plane. The field in the aperture is TE10 mode and given by Find i). the radiated far field and plot the radiation pattern in both the E and H planes; ii). the directivity. Solution on pages 166 - 168

48. Horn Antennas • Horn antennas are the simplest and one of the most widely used microwave antennas – the antenna is nicely integrated with the feed line (waveguide) and the performance can be easily controlled. • They are mainly used for standard antenna gain and field measurements, feed element for reflector antennas, and microwave communications.

50. Pyramidal Horn Design To make this horn, we must have i.e.