A Frequency Agile Semicircular Slot Antenna For Cognitive Radio System

A frequency agile antenna is proposed with its ground plane having a semicircular shaped slot which is capable of switching the frequency to different bands, one at a time, in the wide frequency range of 5.33GHz to 9.90GHz. To achieve frequency agility, switching of six RF PIN diodes which are placed along the slot length is done in various combinations. Frequency tuning ratio of about 1.85 : 1 can be achieved using this design. Results such as return losses, gain, bandwidth, and radiation patterns are presented in this paper.


Introduction
The fundamental design of cognitive radio system generally includes two antennas.One is a wideband antenna that constantly monitors and searches for unoccupied electromagnetic spectrum.The other is a frequency agile antenna that can dynamically adapt its characteristics to perform the essential communication within the unoccupied electromagnetic spectrum.While both these antennas are equally important, clearly the task of designing an agile antenna with switchable multiband functionalities through a wide frequency range is more challenging.Antenna agility in radiation pattern, polarization, and frequency has a capability to fulfill current and future demand for a cognitive radio system, space communication, GPS, MIMO applications, and so forth.The agility function can be either switchable by using RF PIN diode, RF MEMs, and FETs or tunable agility by using varactor diodes.Using single antenna, various researchers have strived to hop between several frequencies subbands having desired bandwidth, radiation patterns, and gain.
Antennas based on different slot shapes have been proposed by various researchers.Slot antennas have been used in number of antenna designs since they offer a number of advantages; for example, by using half U-shape slot [1], they can reduce size of antenna and slots can also provide desired bandwidth adjustments [2,3].Slot design structures based on T-shape [4], L-shaped [5], elliptical/circular [6], slot antenna for multiband operation [7], semicircular [8], C-shaped slots [9], and so forth have been proposed.The method usually followed is to alter the effective length of the radiator so as to permit the current to pass through a larger distance.So the distribution of the current on the radiator length is a simple approach to finding out the radiation characteristics.By varying the length of the slot by the use of switches such as PIN diode [10,11] and RF MEMS [12,13], the distribution of the current on the radiating slot gets perturbed, thus generating multiple frequencies.
Generally, design concept for slot antennas uses either various slots of dissimilar lengths and shapes each resonating at different frequencies [4] or a single slot whose length is changed to produce different frequencies [5][6][7][8].
In this paper, a design of a semicircular agile slot antenna that offers switchable action in the upper frequency band of 5.33-9.90GHz is being proposed.The switching functionality of the proposed antenna, from one frequency subband to another, is electronically done by using six PIN diodes as switches.The proposed antenna can operate one frequency band at a time generating different adjacent frequencies.The proposed antenna can be used with wideband antenna to serve cognitive radio system application.

Antenna Design
The proposed antenna comprises a slot cut on the ground plane of the antenna.The slot is excited by the feedline.The antenna is designed with a 1.6 mm thick FR4 substrate of relative permittivity () of 4.4 and loss tangent of 0.02 and is shown in Figure 1.
The size of the antenna is 50 mm × 46 mm × 1.6 mm.The feedline and ground plane are printed on the two sides of substrate.The proposed antenna is fed by a two-step microstrip feedline, in which the broader lower part of feedline is 7.5 mm × 2 mm and narrower upper part is 8 mm × 1 mm.The sizes of the lower part and upper part of the feedline have been optimized by altering the length/breadth of the feedline for getting the required impedance matching.
In the proposed antenna design, the resonant slot constitutes a semicircular slot of 0.5 mm width whose upper ends are disconnected by a 6 mm width separation as shown in Figure 1.The switches are placed at varying distances from the end, thus varying the effective length of the resonant slot, and are shown in Figure 2.For example, diode D3 is placed at 6 mm from the left hand.Similarly, diode D4 is placed at 8 mm from right hand, respectively.

Current Distribution.
As it can be seen from Figure 3, the length of the path of current distribution on the semicircular slot determines the value of resonant frequency.The increase in the length of the slot causes induced current to travel greater distance, thus making the structure resonate at lower frequency.Similarly, decrease in length of slot causes increase in resonant frequency.The diodes D1-D6 are switched ON or OFF in various combinations to alter the length of the slot.The length of the slot is maximum when all diodes are in OFF state and makes the structure resonate at lowest frequency of 5.33 GHz.Similarly, the length of the slot is minimum when diodes D1 and D6 are in ON state and makes the structure resonate at frequency of 9.90 GHz.For simulation reason, a narrow strip of 1 mm × 1 mm is used to represent the PIN diode.The existence of narrow strip represents the switch in ON state and its nonexistence represents the switch in OFF state.which are placed at varying distances along the slot length, in specified combination.

Return
Since the resonant frequency is inversely proportional to the effective length of the antenna, hence when the effective length is reduced, the resonant frequency is increased.Resonance for any frequency between 5.33 GHz and 9.90 GHz can be achieved by proper adjustment of the length of resonant slot.So the location of the diodes can be suitably adjusted based on required frequency.In the proposed design, the state of switches, their equivalent simulated frequencies, bandwidth, and gain are shown in Table 1.
To achieve frequency agility, the length of the semicircular slot is varied by the use of six PIN diodes.As the total geometric length of the slot is decreased, the length of current distribution also decreases which in turn increases the value of resonant frequency.So as the length of slot is decreased in descending order from State-1 to State-14, the length of current distribution on the semicircular slot is also decreased; thus, resonant frequency increases in ascending order form State-1 to State-14.The frequency tuning ratio of 1.85 : 1 is simulated for this design.

Fabrication and Measurement.
Design of fabricated antenna is shown in Figure 5.As it can be seen from Figure 5, the design also includes additional structures to incorporate the biasing network.In this design, for giving dc biasing voltage to the diodes, the seven slots are etched out from the ground plane in which dc blocking capacitors are inserted.Figure 6 shows fabricated antenna and Figure 7 shows antenna with biasing network.As it can be seen from Figure 7, the design also includes six RF PIN diodes and biasing circuit for desired result.
Figure 8 shows the fabricated picture of antenna with biasing network.The PIN diode implemented in this design is MADP-000907-14020 (Macom) which as per data sheet is having capacitance value of 0.02 pF during reverse bias and resistance value of 5 ohm during forward bias.100 pF dc blocking capacitor is used.A female edge mounted SMA connector has been connected to feed the antenna.Six resistances each of value of 100 ohm are used in the biasing circuit for current limiting purpose and also six RF blocking inductors each of value of 15 nH are used and are shown in Figure 7.When a 3 V battery supply is used, maximum current generated is 10 mA and mini micro push button switches have been used to change the switch state.
The measured return loss and setup for measuring return loss are shown in Figures 9 and 10, respectively.The simulated    show good agreement with each other.The parasitic effect of PIN diodes shifts the measured frequencies to lower frequency region.The variance in measured and simulated results is shown in Table 2.There is tolerable variation in measured values of resonant frequencies.good agreement with each other.The maximum gain of the proposed antenna varies from 2.47 dBi to 6.32 dBi.The comparison of proposed antenna with various reference antennas is shown in Table 3.
The proposed design has shown significant improvement in the number of states and frequency ratio as compared to the reference antennas which have been designed for cognitive radio system application as shown in Table 3.

Conclusion
The proposed antenna can achieve frequency agility for a wide range from 5.33 GHz to 9.90 GHz.To achieve frequency reconfigurability, the effective length of the resonant slot can be altered by switching six diodes placed at different positions in the antenna structure.The frequency ratio of 1.85 : 1 is simulated for this design.

Figure 3 :
Figure 3: Current distribution of proposed antenna at different states.

Figure 4 :
Figure 4: Simulated return loss of proposed antenna (state numbers are represented in rounded boxes).

Figure 10 :
Figure 10: Setup for measuring return loss of State-2.

Table 1 :
Simulated resonant frequencies, bandwidth, and gain at various states of the PIN diodes.

Table 2 :
Measured and simulated resonant frequencies, bandwidth, and gain at various states of the PIN diodes.

Table 3 :
Comparison of various antennas with proposed antenna.