Optically Switched Frequency for Reconfigurable Dipole Antenna Using Photodiode Switches

The design, fabrication, and measurement of reconfigurable antenna using photodiode switches have been investigated. The antenna had dimension 104 mm x 41 mm x 1.6 mm and FR4 substrate with relative permittivity 4.8. It structure was Coplanar Stripline (CPS) dipole. Two photodiodes had assembled on gap in both dipole arms, which was activated by 650 nm red laser. There was shifting frequency and return loss at under illuminated by laser. The experimentally and theoretically result for the proposed reconfigurable antenna are having a good agreement with large frequency shifting. The fabricated antenna was easily fabricated, simple structure, and possible to realize in many application of Wi-fi communication which using optical switching.


Introduction
Antenna is a basic component in communication system.It is very important and needed in communication system, specifically wireless communication system which has been running remarkably and dynamic.It causes the increase demand of antennas.Communication system which is generally complex needs antennas which have advance capabilities and good performances.So, many researchers have conducted experiments attempting to improve capabilities and performances of antenna in order to meet the need of communication system which grows continuously.
One type of antenna which is able to accommodate the change of communication system necessity is reconfigurable antenna [1].Such an antenna is able to adjust with the necessity of the communication system.In some ways it can convert operating frequency, radiation pattern, impedance, bandwidth via shorting/reactive loading to require resonant mode switching, by moving part (mechanical reconfiguration), optical switching, electronic switching, and feed network switching.Due to the fact that reconfigurable antenna adjustable, this antenna can eliminate the need of multiple antenna for communication systems.
An innovative approach to the synthesis of phase-only reconfigurable, isophoric linear sparse arrays has been investigated in previous research [2].The result shows the abilility to commute their pattern within an arbitrary number of radiation modalities which leads to an effective procedure for the fast antennas design with high performance.In other research, presenting at [3][4][5], mentioned that reconfigurable antenna were fabricated on high-resistivity silicon and equipped with two silicons as material switches.Then the laser was used to control the photoconductive switching material.The technique is conducted to prove that changes of effective length of antenna, it can change antenna resonance frequency.However, using silicon as photoconductive switches will require complicated technique during it insertion process on antenna substrate and silicon has high cost.Other techniques, is done previously by Kiriazi et al. in [6].In this work, open and short of RF-MEMS switches were assembled on antenna which has a high-resistivity silicon substrate.In [7], varactor diode is used as switches.The other switching elements which are PIN diodes [8][9][10][11][12][13], an air gap [14][15], solar cell [16], and photodiode [17] have also been used at previous research.Therefore, in this paper we introduce a new optical switches geometry and technique of reconfigurable antennas by using photodiode as optical switching.

Optical Switches
The switching device is used on this paper is photodiode.Photodiode is one of kind of semiconductor device which has ability to change photon (light) into electric current [18].When the light has certain wavelength on the photodiode, it is absorbed.Then, the photon energy is distributed to valence electron and enhances them to the conduction gap.This process awakes electron-hole pairs which called photo carrier.Photo carrier is produced in depletion region.The high electric field arise on depletion region will develop photocurrent on photodiode.The use of photodiode as optical switching in transmission line is based on the characteristic difference between photodiode that is illuminated by light and unilluminated.When it is illuminated by light, photodiode that is located on gap of transmission line will acts as short circuit.The photodiode act as a bridge is connecting one of the transmission line parts to others.This condition causes an increase of electrical length of transmission line, lowering the impedance of transmission line.On the other hand, on unilluminated photodiode, it acts as open circuit and the transmission line impedance is high [19].The change in electrical length is closely related with the operating frequency of transmission line as is given by Equation ( 1) and ( 2) [20].For operating frequency in unilluminated state (OFF-state) is given by [21][22][23], meanwhile in illuminated state (ON-state) is given by Equation (4).
From Equation ( 3) and (4), we can define shifting of frequency as: Where,

Research Method
Antenna design is a balanced dipole in CPS (Coplanar Stripline) fabricated on fiber substrate with permittivity of 4.8 and 1.6 mm thick.The dimension of the substrate is 104 mm × 41 mm×1.6 mm.The antenna was equipped with two photodiodes PD-1 and PD-2 as photoconductive switching.The input matching impedance (Z 0 ) is 50 Ohm according to impedance from connector.Figure 1 shows the photodiode GaAs type SLD6162RLI.The photodiode characteristics are presented by [24].The antenna is designed in the form of planar structures.The fabricated antenna design is given by Figure 2 with the fixed parameter size on Table 1

Results and Discussion
The characteristic result of antenna is measured by using a Network Analyzer in "OFFstate" and "ON-state".OFF-state is dark or unilluminated condition.ON-state is illuminated by red laser with 650 nm wavelength.The measurements are also conducted in chamber to eliminate noise.Figure 3 shows the return loss responds to frequencies.6) with  = 3.5 which  value obtained from the graph ratio of photocurrent to dark current for GaAs photodiode [24].If the frequency shifting from measurement result is compared with theoretically calculation, it shows nearly equal value.However, if frequency shifting is compared with other researches, it shows significant result.As the research conducted by Kiriazi et al. [6], showed that the antenna that switched by using RF-MEMS operate in dual frequencies and have shifting frequencies 20 MHz for first frequency and 260 MHz for second frequency.In another research [7], where the antenna switched by using varactor diode, achieve dual frequency shifting 357 and 126 MHz.

Conclusion
The design, fabrication, and measurement of reconfigurable CPS dipole antenna using photodiode switches have been presented.The experimentally and theoretically result for the fabricated reconfigurable antenna are having a good agreement with large frequency shifting.The proposed antenna would make it possible to realize in many application of Wi-fi communication which using optical switching.

Figure 3 .
Figure 3. Graph resonant frequency with return loss of fabricated antenna

Table 1 .
. Dimension of Fabricated Antenna