This study presents a new, simple method for reducing the back-lobe radiation of a microstrip antenna (MSA) by a partially removed ground plane of the antenna. The effect of the partial ground plane removal in different configurations on the radiation characteristics of a MSA are investigated numerically. The partial ground plane removal reduces the backlobe radiation of the MSA by suppressing the surface wave diffraction from the edges of the antenna ground plane. For further improving the front-to-back (F/B) ratio of the MSA, a new soft-surface configuration consisting of an array of stand-up split ring resonators (SRRs) are placed on a bare dielectric substrate near the two ground plane edges. Compared to the F/B ratio of a conventional MSA with a full ground plane of the same size, an improved F/B ratio of 9.7 dB has been achieved experimentally for our proposed MSA.
Microstrip antennas (MSAs) are used in modern communication systems due to its low cost, lightweight, and planar structure. One of the major concerns in practical MSA design is surface wave excitation. When an MSA is fabricated on a substrate, it shows significant performance degradation by surface waves. On a finite ground plane, surface waves propagate until they reach an edge where they reflected back and diffracted by the edges. Particularly when the patch antenna is printed on high dielectric substrates, its back radiation pattern increases owing to surface wave diffraction from the edges of the antenna ground plane. Numerous efforts have been made earlier to reduce the surface waves on an MSA. One approach is to construct an artificial periodic structure such as an electromagnetic band-gap (EBG) [
To investigate the effect of the ground plane size on the radiation characteristics of an MSA, a conventional MSA with a full metallic ground plane that operates in the dominant mode (TM10) at 2.5 GHz is designed.
The simulated peak broadside gain curve of the probe feed MSA with a different size of the ground plane (S mm × S mm) is plotted in
Next, in order to investigate the effect of partial ground plane removal on the F/B ratio of the MSA, some metallic parts of the MSA ground plane with a width of GL is removed, as shown in
In the back-lobe region radiation pattern of the MSA, the E-plane edge diffraction of surface waves has a much larger contribution to the magnitude than the H-plane edge diffraction [
As a result, the partial ground-plane removal method can reduce the back radiation of the MSA owing to the suppression of surface wave diffraction from the edges of the conventional MSA ground plane. On the other hand, in the case of a MSA with a partial ground plane removal, there are field minima at the ends of the removed ground plane, as shown in
For further improving the F/B ratio of the MSA with a simple soft-surface structure consist of split ring resonators (SRRs), we investigate the field excited near the MSA.
For reducing the ground plane edge diffraction along the two H-plane edges, which account for back radiation of the MSA, we prepared a soft-surface structure consisting of an array of stand-up SRRs. By placing a stand-up SRR near the side of the partially removed ground plane edge, where the magnitudes of the tangential magnetic field component (Hy) are strongly distributed, some parts of the complex power density can be reduced. A schematic of the simulation setup for an optimum stand-up SRR unit cell within a dielectric substrate and the simulated transmission coefficient are shown in
A single SRR unit cell is placed inside a waveguide, and a vertically polarized transverse electromagnetic (TEM) wave is incident normally on the front side of port 1, as shown in
It is well known that incident electromagnetic wave can resonantly couple to the LC resonance of an SRR through either the electric or magnetic field. This occurs either when the electric field is parallel to the side containing the SRR gap or when the magnetic field has a component normal to the plane of the SRR ring. When the incident wave is polarized as indicated in
The optimized physical dimensions (in millimeters) of the MSA and the stand-up SRR are as follows: S = 120, G = 75, P = 18, d = 15.1, a = 13.8, w = 0.4, h = 1.27 and g = 0.7. In total, six SRRs are placed 0.5 mm off the ground plane along the H-plane edge direction (x-axis) of the MSA. Initially, the effect of varying the number of periods N in terms of the F/B ratio of the MSA was studied.
The distance l between the ground plane edge and the arrayed SRRs is another determinant parameter. The effect of introducing the soft surface made with two periods of SRRs (N = 2) near ground plane was studied as a function of the distance l from the ground plane edge to the SRRs.
could be expected. From
For the sake of comparison, the simulated Eand Hplane radiation patterns of the different MSAs at 2.445 GHz are shown in
In order to verify the simulations, a prototype MSA loaded with the soft surface in a partially removed ground plane is fabricated on a Rogers R3210 substrate (having a relative dielectric constant of 10.2, a thickness of 1.27 mm, and a loss tangent of 0.0025); a 50 Ω SMA coaxial probe connecter is installed for feeding the MSA. The photograph of the fabricated antenna is shown in
In both the E-and the H-planes, the MSA shows a smooth symmetric omni-directional pattern with slight backward radiation. The measured broadside (at θ = 0˚) and back-lobe gains (at θ = 180˚) are around 2.8 and –21.4 dBi, respectively. As a result, an F/B ratio of 24.2 dB was achieved experimentally.
We have investigated the effects of partial metallic ground plane removal on the radiation characteristics of a MSA. The partial ground plane removal method has been shown to improve the F/B ratio of the MSA by at least 9.7 dB as compared with that of a conventional MSA having a full ground plane of the same size. The structure of the proposed MSA is very simple and can be implemented with ease. When this structure is used between two MSAs, a mutual coupling reduction effect can also be generated. This structure can be applied for reducing back radiation in the MSA antenna design.