Shorted Annular Patches as flexible antennas for space applications

2. SAP CHARACTERISTICS
SAP antennas have been extensively studied for their
property to inhibit surface wave radiation. In [2] it has
been demonstrated that an appropriate selection of the
SAP ring external radius inhibits the excitation of the TM0
surface wave mode avoiding the radiation pattern
deterioration due to the surface waves diffraction and
reducing mutual coupling in array applications [3]. The
theory has been extended to elliptical SAP, for dual band
or circularly polarized antennas, in [4].
Another essential feature of the antenna is that the SAP
pattern can be easily controlled varying the antenna
geometry without degrading the radiation characteristics.
In order to clarify this aspect, let us first consider a
conventional circular patch antenna resonating on its
dominant mode TM11. Once the resonant frequency and
the dielectric characteristics are given, the external radius
of the disk it is uniquely determined and the antenna
radiation pattern, strictly connected to the magnetic current
distribution at the external edge of the patch, can not be
varied.
On the contrary, the shorted annular patch, which has the
same magnetic current distribution of a conventional disk,
offers much more flexibility in shaping the radiation
pattern. In fact, with a proper choice of the external and
internal radii, narrower beams that maintain the radiation
characteristics of a circular disk can be obtained.
As example, three shorted annular patch antennas
resonating at the nominal GPS L1 frequency,
1.57542GHz, with an external radius of 35, 45, and 55.7
mm, have been designed considering a substrate with
dielectric constant ε R =2.55 and thickness of 3.2 mm. The
effect of a larger external radius is shown in Fig. 2 where
the co-polar radiation patterns of the three SAP antennas
have been compared with the one of a conventional
circular patch resonating at the same frequency and
designed using the same substrate. As expected, a larger
outer radius of the antenna results in a narrower beam.
Fig. 2 – SAP radiation pattern flexibility.
Another important feature of the proposed antenna is that
the gain of the shorted patch is considerably higher than
other conventional microstrip antennas. For instance, the
circular patch antenna taken as reference in Fig. 2 has a
gain of 6.8dB while the SAP’s gain varies with the outer
radius of the ring following the curve reported in Fig. 3 and
attaining a maximum of 10.67dB.
Fig. 3 – SAP gain vs. outer radius.
In addition to the radiation properties, the SAP antenna
has mechanical features highly beneficial for space
applications. In fact, even if the SAP designs proposed in
this paper have been developed using a conventional
microstrip technology, they can be easily adapted to
solutions employing suspended technology and high
performance materials. This opportunity eliminates the
problems arising from the sensitivity of the electrical
characteristics of the substrates and the distortion
generated by the diffraction of surface waves seems to be
advisable. This configuration is easily accomplished as the
central short provides a supporting element on which
single or stacked SAP’s can be build.
As a further advantage of the shorted ring with respect to
other microstrip geometries, it should be noticed that the
central short provides a path for ESD avoiding the creation
of ad-hoc circuits.
3. APPLICATIONS
So far, the SAP antenna has been successfully applied to
several aerospace applications.
High Directive Antennas
The gain increase combined with the possibility to control
the beam aperture can result highly beneficial in designing
directive antennas of small and medium satellites.
Normally for these applications where beam widths around
50degrees and a gain around 12 dB are needed, small
array of printed antennas are employed. The same
requirements can be easily satisfied in a more efficient way