Power lines can be hazardous to low-flying aircraft. Fortunately, it may be possible to use microwave and millimeter-wave radar systems to accurately detect power lines, and prevent accidents. Even under conditions of fog, rain, and snow, it should be possible to measure the polarimetric radar backscattering from a power line using microwave or millimeter-wave radar systems. A main concern, addressed in this study, is the effect of thick ice on these measurements.

Studies have been performed on the use of electro-optical laser radar for collision warning.¹ Unfortunately, the laser radar has numerous shortcomings, including limited range, significant atmospheric attenuation under inclement weather conditions, and difficulty in automating a detection system to warn a pilot of oncoming power lines which make it impractical for power-line collision-warning systems.

Millimeter-wave radar systems, on the other hand, can be used to detect thin objects like power lines in foggy, cloudy, snowy and rainy conditions. While all microwave and millimeter-wave radars are not suitable for this application, some have been used effectively.² In one such case, the millimeter-wave radar used linearly polarized waveforms and modeled transmission lines as long, perfectly conducting cylinders.³ Since power lines are conducting cylindrical wires, they represent difficult targets for many radars. But because a high-voltage power line is made up of strands of wires in a helical arrangement, backscattering detection can be used to detect the power-line cables.

Due to the radar resolution possible at millimeter-wave frequencies, the helical geometry of the power lines is an important factor in influencing the scattering behavior of the electromagnetic (EM) waves, and this can be used in detecting power-line field emissions in off-specular directions. The surface of the power-line cables is periodic along the axis of the cables and usually the period is only a fraction of the helical pitch.

High-voltage power-line cables are usually constructed from a number of aluminium strands twisted helically around a central core of one or more steel strands. The current-carrying capacity of the cable depends on the number of layers and diameter of aluminium/steel strands. However, in an electrical power distribution network, low-tension and high-current cables are used, made of either aluminium or copper strands.

A typical power-line cable geometry can handle current loads to 420 A (Fig. 1). Although research has been performed on polarimetric radar backscattering measurements of a variety of power line cables,^4-6 no known literature has reported on the effects of thick ice layer on backscattering measurements. For the current research, measurements were made of the radar cross section versus angle of rotation of a 2-cm-diameter power-line cable at 5, 10, 40, and 82 GHz. The scatterometer systems are based on 8753 vector network analyzers from Agilent Technologies (model 8753A for the 5- and 10-GHz bands and model 8753C for the 40- and 82-GHz bands). The measurement systems feature phase and amplitude measurement capabilities and 100-dB dynamic range. The scatterometers used at microwave frequencies (5 and 10 GHz) are slightly different than those used at the millimeter-wave bands (40 and 82 GHz). The microwave system employs a single radar antenna; the millimeter-wave system has dual antennas.

Figure 2 shows a microwave system capable of measuring the backscatter of electrical cables with a good signal-to-background-noise ratio for all incident angles. The electrical cable is mounted on a Roha-cell foam pedestal in an anechoic chamber. For the radar cross-section measurements, the position of the electrical cable with respect to the antenna coordinate system is accomplished by an azimuth-over-elevation positioner. The azimuth turntable is driven by a computer-controlled stepper motor with accuracy of a fraction of a tenth of a degree; the elevation controller is a precise analog positioner.

A transmit/receive antenna for the microwave system consists of an orthomode transducer and a dual-polarized square horn. The scatterometer provides receive and transmit signals to a switch and a microwave amplifier assembly. The microwave receive and transmit signals pass to the 8753A network analyzer and a computer. The computer controls the turntable stepper motor and sends data to the printer.

Figure 3 shows the block diagram of the millimeter-wave radar cross-section measurement system. The electrical cable is mounted on a turntable with Roha-cell foam on one side of the anchoic chamber. The turntable operates in both azimuth and elevation planes with rotational information controlled by the computer. The scatterometer, which is installed at one end of the chamber, operates in coherent mode. The scatterometer, contains two corrugated horns (for transmit and receive), two isolators, a polarizer and a otrthomode transducer. A Faraday rotator is used to achieve polarization in the transmitter while the orthomode transducer is used to establish receiver polarization. Two separate millimeter-wave oscillators are employed, operating at 40 and 82 GHz.

Radar cross-section measurements on the cable were set up in a 7-m-long tapered anechoic chamber; the cable was set a distance of 5.5 m from the receive and transmit antennas. The power-line cable, which was 25 cm long and 2 cm in diameter, was mounted on a Roha-cell foam structure, with the foam structure rigidly attached to the turntable platform. The permittivity of the Roha cell s 1.07 at microwave frequencies and 1.1 at millimeter-wave frequencies. The loss tangent of the Roha-cell structure is less than 0.001 at both microwave and millimeter-wave frequencies. Its radar cross section is below −35 dBm.