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By using loop-type patches, the antenna FSS was successfully designed. Initially, a loop array was printed on one side of the substrate. Then, to increase the stopband bandwidth, different layers of FSS were cascaded to form the final FSS. The FSS is a composite structure with two substrate layers sandwiched between three metallic layers. The 1.6-mm-thick substrate has relative permittivity, εr, of 4.4. The unit element used for all three layers was a loop-type structure, with larger loop for the outermost layer. Figure 4 shows the unit elements used in the different layers of the FSS. The FSS was fabricated with a 6 × 6 element array for two of the layers and 5 × 5 element array for the outermost layer (Fig. 5). Figure 6 shows simulated magnitude of transmission and reflection phase for the FSS. The stopband (transmission less than -20 dB) bandwidth is 8.3 GHz (2.6 to 10.9 GHz). The reflection phase of the FSS is linear which is desirable to increase the gain over a wider bandwidth.

Reflection and transmission characteristics

The FSS is mounted below the antenna to enhance the gain, used as a reflector. Waves radiated by the antenna and propagating towards the FSS are reflected by the FSS. If the waves radiated by the antenna and reflected by the FSS are in phase, antenna gain will be increased. The condition for the reflected waves from the FSS and the radiated waves from the antenna to be in phase is given by

ΦFSS -2βh = 2nπ

where:

n = …-2, -1, 0, 1, 2,…   (3)

and

ΦFSS = the reflection phase of the FSS;

β = the propagation constant in free space; and

h = the height between the FSS and the antenna.

By using Eq. 1, h can be calculated as 26.13 mm.

Fabricated UWB antenna Figure 7 shows the fabricated antenna prototype along with the FSS. The measured impedance bandwidths with and without the FSS are nearly same and equal to 8.7 GHz (from 3.0 to 11.7 GHz). The slight difference between the reflection coefficients (with and without FSS) is due to multiple reflections caused by the FSS.

The radiation patterns of the proposed antenna with and without the FSS were measured in an in-house anechoic chamber, with a double-ridged horn antenna used as the reference antenna. Figure 8 compares measured radiation patterns of the antenna with and without the FSS. The radiation patterns without the FSS show a bidirectional nature in the E-plane and omnidirectional nature in the H-plane.

Measured antenna radiation patterns (top)

Measured antenna radiation patterns (middle)

Measured antenna radiation patterns (bottom)

The radiation pattern becomes more directional with application of the FSS. The backlobes are reduced by around 10 dB (at 7.5 GHz) with the FSS. Figure 9 shows the measured peak gain comparison of the antenna without and with the FSS. It reveals an improvement in the peak gain of around 4 to 5 dBi throughout the band after application of the FSS.

Peak gain

In summary, a compact slot antenna with FSS provides high gain for UWB applications. The slot is asymmetrically placed in the ground plane to improve impedance matching. The antenna design offers a measured impedance bandwidth of 102% at a center frequency of 7.5 GHz. To boost gain, a three-metal-layer FSS is used as a reflector, providing improvement of 4 to 5 dBi in peak gain.

Raj Kumar, Senior Engineer

Tuhina Oli, Engineer

Nagendra, Engineer

R.V.S. Ram Krishna, Engineer

Armament Research and Development Establishment (ARDE), Pashan, Pune-411 021, India

Defense Institute of Advanced Technology (DIAT), Deemed University, Girinagar, Pune-411 025, India

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