A Pin Diode Antenna Switch that Doubles as a Combiner
Greg Adams VIZ Manufacturing Co. Philadelphia, PA
Kong S. Luen AeroComm Ltd. Germantown, MD
Twice a day, at weather stations around the world, a weather balloon is launched and atmospheric soundings record temperature, pressure, relative humidity and wind velocity to an altitude of approximately 100,000 feet. These data are used to predict the weather and study the environment. Many of the weather stations are located in remote areas and the antenna systems must operate in environments ranging from the ice fields of Antarctica to the rain forests of the Amazon to the corrosive salt air of the islands. Mechanical antenna rotors are not sufficiently reliable in these environments, so a 360° steerable antenna array with no moving parts is required.
This article describes a PIN diode switch designed to control such an antenna array. The array consists of six Yagi antennas directed outward radially from a center point at intervals of 60°, and a single quadrifilar helix antenna pointed upward. The Yagi antennas have a 3 dB beamwidth of 60° in the azimuthal plane.
The switch can select either one of the antennas or combine two adjacent Yagi antennas in phase, resulting in a pattern similar to that of a single antenna, but with the direction of maximum radiation midway between the two. The quadrifilar helix antenna is used for omnidirectional operation.
The antenna system is designed to receive signals in the 400 to 406 MHz meteorological band. These signals originate from a radiosonde, which is attached to a weather balloon and sends back meteorological information from a range of up to 100 nautical miles. Since the antenna system is used for receiving only, the switch's antenna ports are referred to as the inputs and the common port is referred to as the output. However, since the switch is a reciprocal device, the antenna system also can be used for transmitting if desired.
When the balloon is launched, the omni antenna is switched on. After launch, data received from the onboard Loran or Global Positioning System (GPS) receiver are used to determine the balloon's velocity and position. At a range of 10 km, the omni antenna is switched off and the Yagi (or pair of Yagis) that points in the optimum direction is switched on.
The switch consists of seven quarter-wavelength-long (l /4) microstrip lines, each with a characteristic impedance of 70 ohm. The input end of each l /4 line is connected to an antenna. The output ends are connected together at a common point.
The simplified three-port switch, shown in Figure 1 , is used to illustrate the principle of operation. A PIN diode is connected between the input end of each l /4 line and ground. The PIN diodes corresponding to the antennas that are not being used are biased on. The short circuits located at the ends of these lines are reflected as open circuits at the output (common) end.
If two adjacent Yagi antennas are used, the 70 ohm l /4 lines transform the 50 ohm antenna impedances to 100 ohm. The two 100 ohm impedances then are connected in parallel at the common port, forming a 50 ohm output impedance. The PIN diodes located at the center of the 70 ohm lines are turned off (open circuit) and have no effect. If only one antenna is used, a PIN diode located at the center of the line is biased on. This second PIN diode is connected in series with a capacitor to ground. The capacitor has an impedance Xc of 100 ohm at the operating frequency. Adding this capacitor causes the impedance reflected to the common port to be transformed back to 50 ohm.
Figure 2 shows the impedance transformation. The Smith chart is normalized to the 70 ohm impedance of the l /4 lines. The 50 ohm antenna impedance is labeled A. Arc 1 represents the first half of the 70 ohm transmission line, which is l /8 long. This first transmission line transforms the antenna impedance to 65 + j25 ohm (point B). Arc 2 represents the 100 ohm parallel capacitive reactance, which transforms the impedance to 65 - j25 ohm (point C). Arc 3 represents the second half of the 70 ohm transmission line, which transforms the impedance back to 50 ohm.
Figure 3 shows the complete seven-input/13-position switch's schematic. In order to minimize the switch's losses, the value of the blocking capacitors that are placed in series with the PIN diodes is adjusted to tune out the series inductance of the diodes. The value of the impedance-matching capacitors (Xc = 100 ohm) also has been adjusted to compensate for the series inductance of the associated diodes. The values of the choke inductors are chosen to resonate with the junction capacitance of the PIN diodes, resulting in a virtual open circuit when the diode is biased off.
A polytetrafluoroethylene substrate with a dielectric constant of 2.2 and a thickness of 0.06" is selected. The diodes chosen are model HSMP-3824 dual PIN diodes. The two diode sections are operated in parallel to achieve a lower on resistance than that of a single diode. When operated at 10 mA, each section has approximately 0.7 ohm RF resistance, resulting in a 0.35 ohm parallel resistance. If a single diode was operated at 20 mA, the RF resistance would be 0.6 ohm. The diode on resistance (0.35 ohm) is transformed to 702/0.35 = 14,000 ohm at resonance. When the switch is in a single-antenna mode, six of these transformed resistances exist in parallel, resulting in 14,000/6 = 2333 ohm. This 2333 ohm impedance appears in parallel with the common port, producing a power loss of 50/2333 = 0.0214 = 0.1 dB. Therefore, the insertion loss due to the RF resistance of the forward-biased PIN diodes is 0.1 dB. The remaining 0.3 dB of loss is due to contributions from the copper loss of the microstrip lines, reverse-biased diodes, choke coils and capacitors.
The logic functions required to decode a serial data stream and apply the correct bias voltages to the 14 diode pairs are implemented in programmable array logic. An earlier attempt to use a microcontroller for this purpose was abandoned because clock frequency harmonics appeared in the switch output.
Performance
Figure 4 shows the insertion loss for the switch assembly in the single-antenna mode and the insertion loss for one antenna in the two-antenna (combiner) mode. In the combiner mode, the signal is 3 dB down because of the power split. Over the 6 MHz design bandwidth, the switch has 0.4 dB insertion loss, 40 dB isolation and 25 dB return loss. This performance is possible because all component values have been optimized at the 403 MHz center frequency. While no attempt was made to improve the broadband performance, over an 80 MHz (20 percent) bandwidth the insertion loss is 1.0 dB, return loss is 10 dB and isolation is 30 dB. Total current consumption is 270 mA.
Conclusion
A PIN diode switch assembly designed to combine and control an antenna array comprising six Yagi antennas and a single quadrifilar helix antenna has been described. The switch was used to select a single antenna or two adjacent antennas in a combiner mode for determination of the antenna pattern direction. The antenna switch/combiner displayed low insertion loss, high isolation and good return loss at a 403 MHz center frequency for acceptable performance over a 20 percent bandwidth and for use in weather station receiver applications.
Acknowledgment
The model HSMP-3824 dual PIN diodes are products of Hewlett-Packard Co.
Greg Adams received his BSEE from Drexel University and has 15 years of experience in the design and development of microwave components and communication systems. He has worked as principal RF engineer at VIZ Manufacturing Co., where he was responsible for the development of telemetry and radio-navigation hardware for balloon-borne sensors. Adams joined Checkpoint Systems Corp., Thorofare, NJ, in September.
Kong S. Luen received his BSEE degree and his MSEE in electrophysics from Howard University and George Washington University, respectively. Currently, he is with AeroComm Ltd., Germantown, MD, as an RF engineer. Luen's research involves the filtering properties of microwave planar networks.