Análisis de la interacción genética de EBS con ATXR7 en el control de la dormición

Patch antennas, as seen in Figure 4.24, are based upon printed circuit technology to create flat radiating structures on top of dielectric, ground-plane-backed substrates. The appeal of such structures is in allowing compact antennas with low manufacturing cost and high reliability. It is in practice difficult to achieve this at the same time as acceptably high bandwidth and efficiency. Nevertheless, improvements in the properties of the dielectric materials and in design techniques have led to enormous growth in their popularity and there are now a large number of commercial applications. Many shapes of patch are possible, with varying applications, but the most popular are rectangular (pictured), circular and thin strips (i.e.

printed dipoles).

In the rectangular patch, the length L is typically up to half of the free space wavelength.

The incident wave fed into the feed line sets up a strong resonance within the patch, leading to a specific distribution of fields in the region of the dielectric immediately beneath the patch, in which the electric fields are approximately perpendicular to the patch surface and the magnetic fields are parallel to it. The fields around the edges of the patch create the radiation, with contributions from the edges adding as if they constituted a four-element array. The resultant radiation pattern can thus be varied over a wide range by altering the length L and

Axial mode radiation

Normal mode radiation

Ground plane

Figure 4.23: The helical antenna

Conducting ground plane Feed line

Dielectric substrate

L

W

h

Patch

Figure 4.24: Microstrip patch antenna

84 Antennas and Propagation for Wireless Communication Systems

width W, but a typical pattern is shown in Figure 4.25. In this case the polarisation is approximately linear in the direction, but patches can be created with circular polarisation by altering the patch shape and/or the feed arrangements. A major application of patch antennas is in arrays, where all of the elements, plus the feed and matching networks, can be created in a single printed structure. The necessary dimensions can be calculated approxi-mately by assuming that the fields encounter a relative dielectric constant ofð1 þ"rÞ=2 due to the combination of fields in the air and in the dielectric substrate.

4.10 CONCLUSION

The fundamental parameters of antennas have been described here, together with a descrip-tion of several of the most important antenna types. This should be sufficient to provide an appreciation of the trade-offs inherent in antenna design and the meaning of manufacturers’

specifications. There are many other antenna types, but the set described here should be sufficient to understand the general operation of antennas used in wireless communication systems. Applications of antennas in specific system types will be described in subsequent chapters. For detailed methods for antenna design, consult books such as [Lee, 84] or [Stutzman 81].

REFERENCES

[Balanis, 97] C. A. Balanis, Antenna Theory: analysis and design, 2^ndedn, John Wiley &

Sons, Ltd, Chichester, ISBN 0-471-59268-4, 1997.

[Kraus, 01] J. D. Kraus and R. Marfehka, Antennas, 2nd edn, McGraw Hill education, United States of America, ISBN 0-07-232103-2, 2001.

[Lee, 84] K. F. Lee, Principles of Antenna Theory, John Wiley & Sons Ltd, Chichester, ISBN 0-471-90167-9, 1984.

[Stutzman, 81] W. L. Stutzman and G. A. Thiele, Antenna theory and design, John Wiley &

Sons Ltd, Chichester, ISBN 0-471-04458, 1981.

0.2 0.4

0.6 0.8

1

30 60

90 120

150

180

E

Ground plane Patch

Figure 4.25: Typical patch antenna radiation pattern

Antenna Fundamentals 85

PROBLEMS

4.1 What is the radius of the near-field region for a half-wave dipole?

4.2 Calculate the maximum power density at a distance of 10 m from a Hertzian dipole which is radiating 1 W. Repeat the calculation for a half-wave dipole radiating the same power.

4.3 Calculate the power reflected back to the source by an antenna with a VSWR of 2 and an input power of 10 W.

4.4 Calculate the VSWR for a thin lossless half-wave dipole fed from a 50
source.

4.5 A 900 MHz base station antenna has vertical dimension 2 m and horizontal dimension 35 cm. What is the maximum achievable gain? Given that the antenna is required to have a 3 dB beamwidth in the horizontal direction of 90 , estimate the vertical beamwidth.

4.6 An antenna with a gain of 8 dBi is placed in a region with power density 1 W m^2. Calculate the voltage across the (matched) terminating impedance of the antenna.

4.7 Calculate the polarisation mismatch loss for two linearly polarised antennas in free space at an angle of 20 to each other.

4.8 Calculate the polarisation mismatch loss for a linearly polarised antenna receiving a circularly polarised wave. Show how two linear antennas operated together can be used to reduce this value to unity (i.e. no loss).

4.9 What is the uplink and downlink gain of a 2.7 m diameter parabolic dish antenna at Ku-band (14/12 GHz), if it is to be used in a satellite ground station? Assume an antenna efficiency of 65%. Hint: the frequency bands for satellite communications are often specified as f₁/f₂, where f₁represents the uplink frequency (from Earth to satellite) and f₂ is the downlink frequency (from satellite to Earth).

4.10 Explain why a helical antenna is often used for satellite links. What would the minimum diameter of such an antenna be to make it suitable for L-band satellite applications?

4.11 A satellite communications system is operated in the C-band (6 GHz in the uplink, 4 GHz in the downlink) to provide satellite TV (SATV) services to customers. You have been hired by a SATV company as the telecommunications expert and have been asked to select an antenna which could be used by a customer at home to receive the satellite signal. Determine

(a) The type of antenna required, the operating frequency band and the reasons for choosing this antenna.

(b) Assuming that link budget calculations show that a minimum gain of 38 dBi is required, calculate the dimensions of the antenna.

Antenna A Antenna B

754 m

Figure 4.26: Antennas in free-space at 1800 MHz for Problem 4.12

86 Antennas and Propagation for Wireless Communication Systems

(c) If the customer is unhappy with the dimensions of this antenna (it may not fit in the physical space allocated for the antenna), suggest ways in which this size could be reduced.

4.12 Two directional antennas are aligned facing each other in the boresight direction at 1800 MHz in free-space conditions, as shown in Figure 4.26. Antenna A is a parabolic dish with 65% radiation efficiency, and antenna B is a horn antenna which has a gain of 15 dBi.

Determine

(a) The radiation pattern cuts for azimuth and elevation angles for antenna A, assuming a vertical beamwidth of 45 and a horizontal beamwidth of 55 . You can assume that some side and back lobes for both azimuth and elevation planes are significant.

(b) The directivity for antenna A, in decibels.

(c) The power received at antenna B, given that antenna A has an input power of 1 W. State clearly any assumptions made.

(d) The distance at which antenna B is considered to be in the far-field, if this antenna has a diagonal distance in its mouth of 10 cm.

(e) If antenna B is used as a transmit antenna, what is the power received at antenna A given an input power of 1 W? Explain your answer.

Antenna Fundamentals 87