Fig. 1. Geometry of the designed antenna. (a) First three layers from top, for
radiating patches. (b) Stack-up illustration. (c) Last two layers from the bottom,
for feeding network.
TABLE I
DIMENSIONS FOR THE ANTENNA
As the main radiator, the patch and the slots are optimized to
resonate at 1600 MHz. A capacitive feeding structure is utilized
for increasing the matching bandwidth. The position of the feed
is also optimized in the simulation for achieving the best input
impedance matching.
For the sake of CP, a quadrature feed configuration is
adopted. A hybrid is implemented in a circular shape
and placed under the ground plane. It is composed of four
quarter-wavelength transmission lines with characteristic
impedance of 50 and 35.4 [10]. The circuit model of the
hybrid, the designed microstrip structure, and its simulated
performance are presented in Fig. 2, which indicates perfect
matching for the input port of the hybrid (i.e., P1 and P4) and
phase difference between the two outputs that connected to
the antenna (i.e., ). It thus can be seen that when the
feeding network is excited at P1 (port-1). P2 (antenna feed-1)
and P3 (antenna feed-2) have the relative phase of and ,
Fig. 2. Designed hybrid. (a) Circuit model. (b) Designed microstrip layout.
(c) Dimulated performance.
Fig. 3. Simulated port isolation with and without the parasitic patch and its
parametric study.
respectively, which results in LHCP. By contrast, RHCP can
be obtained by exciting the hybrid at P4 (port-2).
Since the high-permittivity substrate ( ) is involved
and due to the fact that the size of the antenna is extremely
confined in the limited space, in particular, due to the thin substrate
thickness, which is only 6 mm, about , the radiating
patch (top layer) itself does not provide satisfactory bandwidth
[7]–[9]. Consequently, with the hybrid, such imperfect
matching results in undesirable isolation of the two input ports
(i.e., P1 and P4). Therefore, a parasitic patch is added for increasing
the impedance-matching bandwidth of the top patch,
so that the isolation can be improved. The parametric study on
the patch size ( ) is presented in Fig. 3. It can be seen that when