This paper demonstrates the use of materials with variable conductivity in antenna design. These materials vary their conductivity in response to an electrical stimulus. Making part of an antenna from a variable conductive material provides some control over the resonant behavior of the antenna. This capability is useful for dynamically changing the operating frequency of the antenna or turning elements on and off for an adaptive array.
rlh45@psu.edu
jrf18@.psu.edu
variable conducting material. The variable conducting material changes the resonant frequency of the patch by increasing or decreasing the size of the patch. The patch can be made to frequency hop or turn on and off at a set frequency.
Table 1 lists the conductivity and dielectric constant of some materials mentioned in this paper. More information on polypyrroles and polyanilines can be found in reference [9].
Table 1. Electrical properties of some materials used in the design of some microstrip antennas. Dynamically changing the shape and/or material properties of parts of an antenna can significantly alter the antenna performance. It is possible to change the conductive properties of some materials through an electric current or optical signal. If part of an antenna changes from conductive to resistive then the frequency behavior of the antenna changes. Only a few materials are capable of changing their conductivity in response to a signal. A smart material is defined as material that can sense and adapt to external stimuli [1]. A few papers have appeared in the literature demonstrating the use of smart materials in electromagnetic applications. Dynamically adaptive radar absorbing material was made from one or more impedance sheets with variable impedance above a conducting surface [2,3]. The impedance sheet was made from the conducting polymer polypyrrole. A second paper showed that a reflector antenna can adapt its pattern to reject interference using small Salisbury screens made with resistive sheets having variable resistivity [4]. A variety of reconfigurable antennas using optical switches have been reported [5]. A bowtie antenna made from silicon is activated using a light tank delivery system for a semiconductor laser [6]. In addition, reconfigurable antennas have been designed that rely on making silicon conductive through an optical signal [7] or a dc bias [8].
This paper starts by explaining how to alter the conductivity of semiconductors at microwave frequencies using light from an LED. Next, it explains how to reconfigure a rectangular patch antenna to change its frequency response by making use of strategically placed
σ min
(S/cm) σ max
(S/cm) ε r
Copper NA
Quartz 10
-15
5.76×10 5 NA
Polypyrroles 10 -7 10 3 NA
Polyanilines 10 -7 10 3
3.9
Silicon <10 -5 >10 3 11.9
Silicon is an example of the variable conductive material that could be used in the antenna. The conductivity can be changed by shining light from an
LED on the silicon. A semiconductor has an electrical conductivity that can be varied between that of a good conductor and a good dielectric, depending on the bandgap energy and level of doping and defects in the material. Illuminating an intrinsic (i.e. undoped) semiconductor with light having a photon energy greater than the bandgap significantly increases the number of free electron and hole charge carriers and thereby increases its conductivity. This effect is exploited in photodectector and solar cells, and has been reported to a small degree in the design of silicon based reconfigurable antennas [10]. Optoelectronic responses of silicon have been shown to be reliable, reproducible and can occur in time scales ranging from nanoseconds to milliseconds.
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Alternatively, Controllable conductivity has been demonstrated in polymer electrolyte-silver-polyaniline composites whereby the application of an electric field can reversibly alter the material from a state of low to high conductivity. It has been shown in related polymer composites that with controllable resistance at microwave frequencies that switching is stable and reproducible over more than 1000 cycles tested, although longer cycling and switching speeds have not been reported [11].
Another class of materials showing large and reversible changes in conductivity are phase-change chalcogenides, such as Ge
2
Sb
2
Te
5
, recently proposed for non-volatile memory applications [12]. These materials are relatively resistive in the amorphous state and become conducting upon recrystallization. In this case rapid changes of nearly three orders of magnitude are possible in small structures by methods involving laser heating and controlled cooling.
To be useful in adaptive antennas, the materials employed require fast switching times and repeatable characteristics over time.
Materials that can change their conductivity due to an electrical stimulus, can be useful in reconfiguring an antenna to change the pattern and locations of the flow of currents. Changing the conductivity of parts of an antenna will change the input impedance as a function of frequency. Consequently, an antenna can be made to change its resonant frequency. Also, tuning an antenna element in an array will turn it on or off at a given frequency. When the element is resonant at the array frequency, then it is on and the element weight is equal to one. When the element is detuned, then it is off and the element weight is equal to zero.
Our approach is to illuminate a small section of a variable conducting material using a light emitting diode
(LED) in order to convert a mostly dielectric material into a conducting material. Our first idea is illustrated in
Figure 1. The top picture shows the LEDs off and the large section of the patch is not conductive. The bottom picture shows the LEDs turned on and the large section of the patch is conductive. In the on state, the patch is resonant at the desired frequency.
Placement of the variable conducting material is crucial in determining the antenna response. We have chosen to use a narrow band inset fed rectangular patch to demonstrate frequency tuning with variable conductive material. An inset patch is shown in Figure 2. The light colored area is made from copper and is a narrow band patch resonant at a frequency, f
1
. Additional variable conductive patches can be placed in locations A through
D. A, B, and C change the dimensions of the patch, hence its resonant frequency. In particular, when these areas are conductive, the resonant frequency will decrease.
Switching area D between resistive and conductive will change the resonant frequency and possibly the polarization, depending upon where D is placed. Multiple areas similar to D can be placed in the patch as well.
Figure 1. LEDs used to turn on a piece of semiconductor .
Figure 2. Inset fed rectangular patch antenna with copper (light) and potential locations for variable conducting material (dark).
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The example is a rectangular microstrip patch antenna having a lossless quartz substrate that is 3 mm thick with
ε r
= 3.78. The antenna operates at three distinct frequencies: 1.8 GHz, 2.0 GHz, and 2.2 GHz. Switching the conductivity of the B and F sections of the patch
(Figure 3) causes a change in the operating frequency.
When the patch is resonant at one frequency, it is not resonant at the other frequencies. The ground plane has holes for the LEDs to shine through the quartz substrate to the variable conductive material on top. The light gray area in Figure 3 is made of copper while the dark gray areas are the material with variable conductivity. These areas switch between being highly conductive when the
LEDs are on to highly resistive when the LEDs are off.
The patch is modeled using the transient solver in CST
Microwave Studio [13] over a frequency range of 1.5 to
2.5 GHz.
There are four possible antenna arrangements with sections B and F being either conductive or resistive
(Table 2). A Nelder Mead downhill simplex algorithm optimized the length and width of P as well as the length of the feed inset at 2.0 GHz. The best
11
value is -42 dB and resulted for a patch dimension of 32.73 mm 40.25 mm with an inset of 7.05 mm. Section B is highly conductive while section F is highly resistive. The length of section F was found by minimizing
11
at 1.8 GHz.
Next, the length of B was found by finding its length that minimizes
11
at 2.2 GHz. The only combination that was not optimized consisted of B being resistive and F being conductive. It turns out, that under these conditions, the antenna is resonant at 1.96 GHz. Table 2 summarizes the results and includes the bandwidth associated with each possible combination of sections B and F being resistive or conductive.
Table 2. The B and F sections of the patch antenna in
Figure 3 can be switched between highly conductive and highly resistive. The narrow band operation occurs at the frequency in column 3 and having the return loss in column 4.
B F Frequency s
11
Bandwidth
Figure 3. Diagram of adaptive patch with dimensions in mm. The dark gray areas can be switched between conductive and not conductive.
Figure 4. Graph of s
11
for the resonant frequency at
2.0 GHz. The patch is shown with light gray being copper and dark gray being material with variable conductivity. res con 1.96 GHz -11.39 dB 1.02% con res 2 GHz -42.50 dB 1.65% con con 1.8 GHz -16.17 dB 1.41% Figure 5. Graph of s
11
for the resonant frequency at
1.8 GHz. The patch is shown with light gray being copper and dark gray being material with variable conductivity.
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Figure 6. Graph of copper. s
11
for the resonant frequency at
2.2 GHz. The patch is shown with light gray being
Figure 7. Graph of s
11
for the resonant frequency at
1.96 GHz. The patch is shown with light gray being copper and dark gray being material with variable conductivity.
The frequency of operation of a patch antenna can be changed by making part of the patch from a material that has variable conductivity. This allows the antenna to either change its frequency of operation, or to serve as an adaptive element in an array antenna. We are currently investigating the best methods to bond dissimilar materials. We have experimentally created an ohmic contact between metal and silicon by doping the semiconductor under the metal using ion implantation.
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