Applied Research Laboratory, Penn State University
State College, PA 16804-0030 USA
{rlh45, jrf18}@psu.edu
Abstract — This paper describes the design, construction, and testing of an 8 element broadband array with photoconductive attenuators at each broadband monopole element. The photoconductive attenuators are controlled by an IR LED and vary between on and off with a small reflection coefficient and small insertion loss. Test results for the elements and array are presented. Low sidelobe Chebyshev amplitude tapers are demonstrated by controlling the illumination of the photoconductive weights. minimal insertion loss under dark conditions over a frequency band of 50 MHz to 5 GHz.
I.
I NTRODUCTION
Variable weights in an antenna array control the phase and/or amplitude of the signals at the elements in order to generate a desirable far field pattern. Phase shifters are used for electronic beam steering, calibration, and compensation.
Attenuators are used to make small adjustments to the signal amplitude in order to achieve a desired amplitude taper. The weights are often packaged in a T/R module [1] and other times they exist in isolation [2] or as software weights in a digital beamformer [3].
This paper describes an 8 element linear array of broadband monopoles that have photoconductive attenuators.
The attenuators are unique in that the signal at each element can be attenuated by controlling the current sent to an IR LED.
Variable attenuators at each element in the array permit amplitude control of the signals radiated by the elements.
These switches have a low reflection coefficient and insertion from 2.11 to 2.53 GHz. The experimental performance of the elements and integrated photoconductive attenuators are presented in this paper. In addition, the array design with measured far field patterns corresponding to uniform and low sidelobe Chebyshev tapers are presented.
Fig. 1. Photograph of photoconductive switch.
III.
E LEMENT D ESIGN
An optimized coplanar monocone [5] or tab monopole [6] was designed to have a 20% bandwidth (VSWR<2) about a center frequency of 2.2 GHz. Fig. 2 shows a photograph of the element with dimension labels. The element was etched on
FR4 that is 1.5 mm thick with
r
= 4.9
and a loss tangent of
0.025.
II.
P HOTOCONDUCTIVE W EIGHT D ESIGN
Fig. 1 is a photograph of the photoconductive attenuator based upon a design presented in [4]. The attenuator uses a single sided coplanar waveguide. The center conductor of the coplanar waveguide tapers to 0.140 mm and meanders through the center rectangular section without touching the coplanar waveguide sides. The gold conductor lies on top of a high resistivity silicon (HR-Si) substrate. When an IR LED illuminates the rectangular section containing the meandering line, the conductivity of the silicon increases and creates a resistive shunt between the center and outer conductors of the coplanar waveguide. As a result, signal flowing in the coplanar waveguide is attenuated. This device is designed for
Fig. 2. Photograph of the element.
The element in Fig. 2 is modified by etching a rectangular area in the center of the ground plane/center conductor then drilling a hole as shown in Fig. 3. An LED is placed below the element so that it shines through the hole. The attenuator is flipped and soldered to the center conductor and outside

conductors of the coplanar waveguide. Care must be taken to not short the center and outside conductors. Fig. 4 shows the resulting attenuator integrated on the antenna element. high attenuation. This result shows that under high attenuation, the reflection coefficient is still reasonable. The other elements in the array had similar plots.
Fig. 3. Antenna element with hole in feed. The attenuator in will be flipped and soldered over the hole. An IR LED illuminates the attenuator from below.
Fig. 5. Experimental array with element 1 on the far left and element 8 on the far right.
0
-10
-20
-30
Fig. 4. Photograph of the attenuator mounted on the antenna element.
IV.
A RRAY D ESIGN
An 8 element array having broadband monopole elements with photoconductive attenuators is shown in Fig. 5. A broadband 8 to 1 power combiner sums the signals from all the elements and sends the result to a spectrum analyser that serves as the receiver. Fig. 6 is a plot of the uniform array pattern when all the diode currents are set at 0 mA. The measured S
11
of the array with all the diodes off is graphed in
Fig. 7. The array has a VSWR<2 and bandwidth of 18.4% that extends from 2.11 to 2.53 GHz.
S parameters were measured for each element in their positions in the array. The reflection coefficient is important for determining the array bandwidth when the diodes are on and off. Fig. 8 is a plot of the measured S
11
of element 6 when the diode receives 0 mA and 100 mA. This plot shows that the element has a small reflection coefficient for either low or
0 15 30 45
φ
(degrees)
60 75
Fig. 6. Measured uniform array pattern when all diode currents are set at 0 mA.
90
S
21
was measured by transmitting a signal from a broadband monopole placed 8 cm in front of a receive element in the array when the other 7 elements were terminated in 50 ohm loads. This procedure was repeated for each element in the array. The signal received by an element decreases with increasing diode current as shown in Fig. 9. This nearly linear relationship provides excellent control over the power radiated by individual elements. The phase of S
21
at the elements changes less than 5 degrees for a diode current less than 60 mA (Fig. 10). Consequently, these attenuators have excellent phase behaviour over about 10 dB of signal attenuation.

-5 -15
18.4%
-10 -20
1
2
-15 -25
7
8
-20
-25
-30
-20
2 2.11
2.2
2.4
Frequency (GHz)
2.53
Fig. 7. S
11
at the array output with no diode bias current.
2.6
-5
100 mA
-10
-15
0 mA
-30
0 20 40
Diode Current (mA)
Fig. 9. Measured S
21
of the elements as a function of diode current.
60
0
-1
-2
-3
-4
7
2
1
8
-35
2 2.2
2.4
Frequency (GHz)
2.6
Fig. 8. S mA.
11
of element 6 when the diode current is 0 mA and when it is 100
-5
0 10 20 30 40
Diode Current (mA)
50 60
Fig. 10. Measured phase of S
21
of the elements as a function of diode current.
V.
L OW S IDELOBE E XPERIMENT
We used the S
21
plots to convert low sidelobe Chebyshev amplitude weights into diode currents. These currents were applied to the element diodes, then the far field pattern of the array measured in an anechoic chamber. First, a 20 dB Dolph-
Chebyshev amplitude taper was converted into diode currents.
Table 1 shows the Dolph-Chebyshev amplitude weights and corresponding diode currents for one half of the elements in the array. The measured far field pattern appears in Fig. 11. Its main beam decreased by 1.8 dB while the peak relative sidelobe level dropped by 10.5 dB. A second experiment converted the Dolph-Chebyshev amplitude weights 25 dB sidelobes to diode currents. The resulting array pattern appears in Fig. 12. This pattern has a main beam that is 2.5 dB and peak sidelobe level that is 17.7 dB below that of the uniform pattern
Translating the amplitude weights into diode currents works well for the 20 and 25 dB Chebyshev tapers. Sidelobe levels below 25 dB, however are much more susceptible to errors.
For instance, we were unable to achieve a 30 dB Chebyshev far field array pattern. Since the attenuators were put together by hand in a laboratory rather than carefully manufactured to a high tolerance, their associated errors were large. We found little change in S
21
as a function of frequency across the bandwidth when the diodes are off. When the diodes are illuminated at a maximum level, then the diode S
21 decreases by about 1 dB from the lowest to the highest frequency. This frequency dependence does not result in significant sidelobe level changes for a 25 dB taper.

0
-10
Table 1. Amplitude weights and associated diode currents for a 20 dB
Chebyshev taper.
Taper Element 1 2 3 4
Uniform
Amplitude 1 1 1 1
1
2
Amplitude 0.58 0.66 0.88 1.00
Current (mA) 30 27 10 0
Amplitude 0.38 0.58 0.84 1.00
Current (mA) 60 36 10 0
-20
-30
0 15 30 45
φ
(degrees)
60 75 90
Fig. 11. Measured low sidelobe antenna patterns for the 8 element array when the diode currents are set for uniform and 20 dB Dolph-Chebyshev tapers.
VI.
C ONCLUSIONS
A broadband 8 element array with photoconductive attenuators was designed, built, and tested. Results demonstrate that the amplitude of the signals at each element can be controlled with a small variation in phase over the bandwidth. The amplitude weight at each element is proportional to the illumination intensity of an IR LED.
Insertion loss, phase variation, and reflection coefficient are low over 2.11 to 2.53 GHz. The amplitude taper can be switched from a uniform taper to a Chebyshev taper in order to reject interference entering the sidelobes.
0
A CKNOWLEDGMENT
This work was supported Army CECOM under contract
N00024-02-D-6604 DO-295.
-10
-20
-30
0 15 30 45
φ
(degrees)
60 75 90
Fig. 12. Measured low sidelobe antenna patterns for the 8 element array when the diode currents are set for uniform and 25 dB Dolph-Chebyshev tapers.
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