Research on the Scattering Characteristics and the RCS Reduction of Circularly Polarized Microstrip Antenna

Based on the study of the radiation and scattering of the circularly polarized (CP) antenna, a novel radar cross-section (RCS) reduction technique is proposed for CP antenna in this paper. Quasi-fractal slots are applied in the design of the antenna ground plane to reduce the RCS of the CP antenna. Both prototype antenna and array are designed, and their time-, frequency-, and spacedomain characteristics are studied to authenticate the proposed technique. The simulated and measured results show that the RCS of the prototype antenna and array is reduced up to 7.85 dB and 6.95 dB in the band of 1 GHz–10GHz. The proposed technique serves a candidate in the design of low RCS CP antennas and arrays.


Introduction
Circularly polarized (CP) transmitting and receiving antennas are less sensitive to their respective orientations, which leads to that the CP antennas are suitable to various wireless systems such as global positioning system (GPS), radio frequency identification (RFID), wireless local area network (WLAN), and modern radar.Microstrip patch antenna, spiral antenna, dielectric resonator antenna, and slot antenna are the typical types of CP antennas [1][2][3][4][5][6].
The radiation of CP antenna has been analyzed a lot because of its wide usage.Recently, much attention has been paid to antenna scattering since it is the main contribution to the total radar cross-section (RCS) of low-observable platforms.Antenna is a special scatter whose scattering is related with both its loading condition and structure, which embarrasses the design of CP antenna with low RCS and good radiation performances simultaneously [7,8].The low RCS antenna is pivotal to the antenna array on low-observable platforms, which makes the design of antennas with low RCS pressing.
Scattering characteristics of several kinds of antennas have been reported.In [8], the bistatic RCS of a microstrip patch antenna, including the effect of the feed termination impedance, has been calculated.In [9], the bistatic RCS of UWB antenna is studied.Several antennas with low RCS have also been reported.In [10], the insect tentacle structure is proposed to design a low RCS ultrawideband (UWB) antenna.In [11], the fractal slotted patch is applied to reduce the RCS of a microstrip antenna.
Up to now, much more attentions have been paid to the radiation performances of the CP antennas while the RCS reduction is rarely reported.Considering the application of the low RCS CP antennas in the low-observable platforms, the scattering performances of the CP antenna are studied and a novel low RCS L-band CP microstrip antenna is proposed in this paper.

Theoretical Analysis
The configuration of antenna scattering is shown in Figures 1(a) and 1(b).It is well known that an antenna is a special scatter whose scattering is related with its feed termination impedance.The antenna scattering consists of structural mode scattering (in this case a matched load is connected to the antenna feed port, and there is no reflective power form the load), and antenna mode scattering (in this case, part of the received energy would be reflected by the load and reradiated to the space).Therefore, antenna RCS () can  be divided into two categories: structural mode RCS (  ) and antenna mode RCS (  ).Their relationship is given by where  is the phase difference between these two modes [4].
The structural mode RCS depends on the structural characteristics of the target antenna such as the metal surfaces, corners, and edges.While the antenna mode RCS is related to the radiation characteristics of the target antenna.Therefore, the structural mode scattering remains constant, while the antenna mode scattering changes when the impendence of the load of the target antenna changes.
The expression of the scattering field E  (  ) of the target antenna terminated with arbitrary load is given by [12] In (2), the expression between the first square brackets is the structural mode scattering field and that between the second square brackets is the antenna mode scattering field.The scattering fields of the two modes can be expressed by the scattering fields E  (∞) and E  (0) of the antenna that is opencircuited and short-circuited loads terminated respectively.Γ  and Γ  are the reflection coefficients of the load and the antenna.Involving in the polarization, the basic formulation of antenna scattering is given by [12,13] where E  (  ) are the scattering fields of the target antenna with matched load   ;   0 is the amplitude of the wave scattered by the target antenna matched loaded; E  1 is the radiation field of the target antenna excited by a unit amplitude source; u and U are the polarization vectors of the incident plane wave and the target antenna, respectively.
Let k be the polarization of the receiver antenna, then the scattering field from the target antenna can be expressed as follows: Using (3), the V polarized scattering field can be expressed as follow Here the polarizations of the incident plane wave, the receiver antenna, and target antenna are taken into account synchronously.In (5), the expression between the first square brackets is the structural mode scattering field and that between the second square brackets is the antenna mode scattering field.The fact can be concluded that the antenna mode scattering field could be adjusted by changing the parameters Γ  , Γ  , |A(k)|, U, u, and k.
For the monostatic scattering case shown in Figure 1(b), International Journal of Antennas and Propagation  Then, the partial gain of the target antenna can be expressed by [13] where  is the polarization efficiency;  1 (k) is the realized gain of the target antenna when the reflection from the target antenna is involved;  2 (k) is the gain of the target antenna.
Using (7) and ( 9), ( 5) can be rewritten as Here the first and the second parts of (10) represent the  polarized structural mode and antenna mode scattering field.This equation provides us with possibilities to control the scattering field of the target antenna.For instance, if the incident wave is polarization mismatched with the target antenna, that is,  = 0, the antenna mode scattering field is equal to zero.While if the incident wave is polarization matched with the target antenna, that is,  = 1, the antenna mode scattering reaches maximum.If  = 1, the antenna mode scattering will be maximum.The signs of the deduction above are accordant with those of the literature [13].which are printed on the top and bottom surfaces of an FR-4 substrate with relative permittivity of 4.4.GFSA is fed by a coaxial cable through the feed port whose position on the radiator can be calculated by

Antenna Study
where  0 in is the input resistance of the antenna side-fed by microstrip line. in is the input resistance of the antenna probe-fed. is the side length of the radiator. 0 is the distance of the feed port from the radiator edge.
CP radiation can be implemented by two degenerated modes with orthogonal polarizations.Corner-cut rectangular radiator is a commonly used implementation method.As shown in Figure 2, the corner-cut square patch radiator is printed on the top surface of the substrate to implement the left-hand circularly polarized (LHCP) radiation.In order to avoid the edge effect between the radiator and the ground plane, the ground plane should be properly lager than the ground plane.8 quasi-fractal patche is etched on the extra part of the ground plane on the other side of the substrate.Each quasi-fractal patches are connected by the metal strips.The detailed structural parameters are given in Figure 2.
In order to demonstrate the superiority of GFSA in RCS reduction, a reference antenna (REF) which is not slotted is also studied.

Radiation Performances. Measured and simulated
VSWR curves of the two antennas are shown in Figures 3(a) and 3(b), respectively.It can be seen that the curves are well fitted, and GFSA's measured operating band is 1.97 GHz-2.06GHz (about 2.3% relative band width) under the condition of VSWR < 2. Between the measured and The axial ratio curves of the two antennas are shown in Figure 4. GFSA's operating band is 1.958 GHz-1.982GHz and REF's operating band is 1.964 GHz-1.988GHz, under the condition of AR < 3 dB.There is about 6 MHz frequency offset between the two antennas which is caused by the quasi-fractal structure on the ground plane.From Figures 3 and 4, we can see that the two antennas have similar operation band.
Antenna radiation pattern measurement of GFSA has been performed in an anechoic chamber at 1.97 GHz in both x-z plane and y-z plane.LHCP radiation patterns are normalized with respect to the maximum in their crossing points.As seen in Figures 5(a) and 5(b), good agreement between simulations and measurements is obtained, and the maximum radiation is perpendicular to the radiator surface.As shown in Table 1 are the measured LHCP gains of the two antennas.It can be seen that the gains of the two antennas are similar, and the LHCP gains of GFSA and REF have reached 2.96 dB and 2.92 dB, respectively, at 1.97 GHz.
From the analysis above, the fact can be concluded that GFSA and REF have similar radiation performances, and the LHCP gain of GFSA is slightly increased.

Scattering Performances.
Time-domain analysis is an effective method for antenna scattering analysis because it is It can be seen that the curves are almost coincidence between 2 ns and 6.8 ns and then a time difference of 1/2 period appears.From the definitions of the scattering modes in Section 2, the conclusion can be made that the previous waveform is the structural mode scattering wave, and the following waveform is the antenna scattering wave.
As shown in Figures 7(a) and 7(b) are the space-domain RCS curves of GFSA and REF.The incident wave propagates on the negative direction of -axis (perpendicularly) and is x-polarized which means that the direction of the incident electric field is parallel to the -axis.In x-z plane, the RCS of GFSA is lower than that of REF in the angular region of [−107 ∘ , 97 ∘ ] and [−62 ∘ , 62 ∘ ].In y-z plane, the RCS of GFSA is well reduced in the 360 ∘ angular region.From Figures 7(a) and 7(b), we can see that the bistatic RCS of GFSA is well reduced in x-z plane and y-z plane, especially on the direction of  = 0 ∘ (maximum radiation direction of the antenna) by the degree of 1.72 dB.However, the reduction of 1.72 dB is not quite enough which will be discussed together with the array case later.The frequency-domain scattering characteristic of GFSA is shown in Figure 8.The RCS curves of GFSA and REF are basically in coincidence below 1.68 GHz, and the RCS reduction is not impressive, while the distance of the two curves is enlarged soon as the frequency increased.

Array Study
4.1.Radiation Performances.Microstrip antenna is commonly used as a part of the array to realize the patterns of array.In this section, a linearly arranged 4-unit array is designed based on GFSA and REF.The proposed array (PA) is shown in Figure 9.The axial ratio curves of PA and the reference array (RA) are shown in Figure 10.Under the condition of AR < 3 dB, the operating band of PA is 1.974 GHz-1.999GHz and that of RA is 1.974 GHz-2.001GHz.About 2 MHz offset occurred between the arrays which is due to the slots on PA.The measured axial ratio at 1.99 GHz on the radiation direction is 2.56 dB which is 0.63 dB larger than the simulated result 1.93 dB.It is believed that the error of the handmade coaxial feed line leads to the output phase differences between the feed ports of the array in the measurement.
The simulated and measured radiation patterns at 1.99 GHz are shown in Figures 11(a) and 11(b).LHCP radiation patterns are normalized with respect to the maximum in their crossing points.The simulated and measured curves fit well in the angle of [−20 ∘ , 20 ∘ ] in both x-z and y-z planes.The radiation directions of the simulated and measured curves are coincident with each other and are all perpendicular to the radiator surface.As shown in Table 2 is the measured LHCP gains of the two arrays.It can be seen that the gain of the PA is slightly lower than that of PA, and the LHCP gains of PA and RA are 7.4 dB and 7.6 dB, respectively, at 1.99 GHz.

Scattering Performances.
The bistatic RCSs of the arrays are studied when they are lit by the incident wave perpendicularly.The incident wave is x-polarized.The changes of antenna bistatic RCS with incident wave frequency when the observe angles are  = 0 ∘ ,  = 10 ∘ , and  = 20 ∘ , are shown in Figures 12(a)-12(c).As shown in Figure 12(a), the RCS curves of the two arrays are practically coincident in the 1 GHz-1.5 GHz band.This phenomenon reminds us the problem left above.It is caused by the same reason analyzed above that the array is electrically small target to the incident wave in this band, so the Rayleigh scattering characteristic (low frequency scattering characteristic) is dominant which means that the scattering level of the array is determined by the size of the array instead of the detail structure of the array.With the increasing of the frequency of the incident wave, the array is larger and larger comparing to the wavelength of the incident wave, and the detail structure of the array is more and more influential to the scattering.So the RCS of PA is largely reduced in the band of 1.5 GHz-10 GHz.In Figures 12(b) and 12(c), the RCS curve of PA is getting close to that of RA with the observe angle increasing which means that the effect of the proposed technique is getting weaker.However, the RCS curves of the PA are still lower than those of RA.

Conclusion
A novel RCS reduction technique is proposed for CP antenna and array in this paper.In order to suppress the scattering wave of CP microstrip antenna and array, quasi-fractal slots are etched on the ground planes.The prototypes are studied by being compared to the reference ones, and the measured and simulated result shows that the RCS of the prototypes is effectively reduced in the band of 1.5 GHz-10 GHz while the gains and the patterns hold.

Table 1 :
LHCP gains of GFSA and REF.

Table 2 :
LHCP gains of PA and RA.