Page 207 - Satellite Communications, Fourth Edition

Antennas 187

3 3
is 2000 10 0.15 10 300 V. The characteristic impedance of
a microstrip line is given by Z > 2e r , where Z is the impedance of the
0
0
same line with an air dielectric. The characteristic impedance increases
as a function of h/W, where W is the width of the line and h the thick-
ness of the dielectric. Thus thinner dielectrics lead to lower charac-
teristic impedance, and this combined with the high dielectric constant
means that the line width W has to be narrow. Values given in De
Flaviis et al. (1997) for the ferroelectric material barium modified
strontium titanium oxide (Ba 1 x Sr TiO ) show a dielectric constant
x
3
in the region of 600, dielectric thickness between 0.1 and 0.15 mm, and
line width of 50 m, for a characteristic impedance of 50 Ω. The bias
voltage is 250 V.
The ferroelectric dielectric is used in a number of different ways. In
the paper by De Flaviis et al., the material was used simply as the
dielectric for a microstrip delay line. It has also been used as a lens to
produce scanning by deflecting an antenna beam (see Ferroelectric Lens
Phased Array at http://radar-www.nrl.navy.mil/Areas/Ferro). The lens
is shown in Fig. 6.37. The ferroelectric dielectric in each column is biased
to provide a progressive phase shift so that an incident plane wave
normal to the edges of the dielectric columns will emerge from the oppo-
site edges in a direction determined by the phasing in the lens. A single
lens produces one dimensional scanning. Ferroelectrics are also
employed in reflectarrays described next.

6.20 Reflectarrays
A reflectarray, as the name suggests is an array of antenna elements
that acts as a reflector. A reflector array incorporates a planar array as
a reflector, as shown in Fig. 6.38. The planar array basically replaces
the parabolic reflector shown in Fig. 6.13. Reflected waves from each
of the elements in the array can be phased to produce beam scanning;
Fig. 6.38b shows the construction. The reflected wave is actually a com-
bination of reflections from the antenna elements and the substrate.
Figure 6.38c shows the polar diagram for a 784 element array. Further
details of this array will be found in Pozar (2004).
A number of methods of producing beam scanning have been pro-
posed. Fig. 6.39 shows a 2832 element, 19-GHz reflectarray which
employs ferroelectric phase shifters for the elements. Further details will
be found at www.ctsystemes.com/zeland/publi/TM-2000-210063.pdf.
Varactor diodes have also been used to provide a phase shift that is con-
trolled by an applied bias. A varactor is in effect a voltage controlled
capacitor, and changes in the capacitance introduce a corresponding
phase shift. Figure 6.40 shows one arrangement for a five-element array.