Page 215 - Satellite Communications, Fourth Edition

Antennas 195

6.25. Determine the depth of the reflector specified in Prob. 6.23.

6.26. A 3-m paraboloidal dish has a depth of 1 m. Determine the focal length.
6.27. A 5-m paraboloidal reflector works with an illumination efficiency of
65 percent. Determine its effective aperture and gain at a frequency of 6 GHz.

6.28. Determine the half-power beamwidth for the reflector antenna of Prob.
6.27. What is the beamwidth between the first nulls?

6.29. Describe briefly the offset feed used with paraboloidal reflector antennas,
stating its main advantages and disadvantages.

6.30. Explain why double-reflector antennas are often used with large earth
stations.

6.31. Describe briefly the main advantages to be gained in using an antenna array.

6.32. A basic dipole array consists of five equispaced dipole elements configured
as shown in Fig. 6.26. The spacing between elements is 0.3l. Determine the
current phasing needed to produce an end-fire pattern. Provide a polar plot of
the AF.

6.33. What current phasing would be required for the array in Prob. 6.32 to
produce a broadside pattern?

6.34. A four-element dipole array, configured as shown in Fig. 6.26, is required to
produce maximum radiation in a direction 0 15°. The elements are spaced by
0.2l. Determine the current phasing required, and provide a polar plot of the AF.
6.35. A rectangular patch antenna element has sides a 9 mm, b 6 mm.
The operating frequency is 10 GHz. Plot the radiation patterns for the 0
and 90° planes.

6.36. For microstrip line, where the thickness t of the line is negligible
compared to the dielectric thickness h, and the line width W ≥ h the effective
dielectric constant is given by

e r 1 e r 1
e e >
2 h
2 1 12
Å W
e r is the dielectric constant of the dielectric material. The characteristic
impedance is given by

1
120 W W
Z 0 c 1.393 0.667 ln a 1.444bd
h h
2e e