Diffraction par le réflecteur secondaire d’une antenne Cassegrain de révolution

Diffraction by the subreflector of a revolution dual reflector antenna is studied as contribution to the side-lobe radiation of the antenna; a better knowledge of this sidelobe radiation is necessary in order to satisfy more and more difficult specifications. Calculation of the subreflector diffracted field is made with geometrical theory of diffraction and more precisely with the uniform asymptotic theory. The antenna is made of two revolution surfaces generated by two meridian curves which are not necessarily the set hyperbola-parabola of the classical Cassegrainian antenna. Computation results are compared to measurements made on the subreflector of an existing antenna. The agreement is satisfying particularly in the angular zone corresponding to the primary feed spill-over.     

Evaluation des cadences de mesure de champs proches au moyen de sondes modulées

The problems involved in the direct far-field measurements of large antennas have led to the development of the near-field measurement technique. According to this method, the far-field pattern of the antenna is calculated from the near-field measurements close to the antenna. The only inconvenience in this technique is the slow rate of measurements. This slowness is due to the mechanical displacement of the measuring probe or the test antenna. The modulated scattering technique is a method to reduce the measurement time while preserving acceptable levels of accuracy. This article is mainly concerned with estimating the possible measurement rates in typical configurations.     

Gain measurements of vertically polarized antennas over imperfect ground

A technique is described for measuring the absolute gain of vertically polarized HF antennas. A horizontal half-wave dipole, located near the antenna being measured (test antenna), is used as a reference in a comparison type of gain measurement. A rotatable, linearly polarized, transmitting antenna is mounted on an airplane and used to illuminate the test-reference antenna location. With the airborne antenna vertical, the response of the test antenna is measured. The airborne antenna is quickly rotated to a horizontal position and the response of the reference dipole is measured. When the airplane is in the main lobe of the test and reference antennas, these two measurements yield the gain of the test antenna relative to that of the reference dipole. The gain of a horizontal half-wave dipole is presented graphically, versus main lobe elevation angle, for various heights above imperfect ground. Mutual impedance variations were considered in the computations for these graphs. Model antenna measurements were made at 400 MHz to verify the principle of the technique. On-site HF measurements could give gain figures accurate to within a few tenths of a dB of their true value.     

Antenna pattern comparison between an outdoor cylindrical near-field test facility and an indoor spherical near-field antenna test facility

A new spherical near-field probe-positioning device has been designed and constructed, consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility, located at Alenia Marconi Systems, on the Isle of Wight, UK. As part of the near-field qualification, testing was performed on a ground-based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility, also located on the Isle of Wight [F. Steiner et al., Jan. 1994]. These measurements included multiple-frequency measurements and multiple pattern comparisons. This paper summarizes the results obtained as part of the measurement program, and includes discussions on the error budgets for the two ranges, along with a discussion of the mutual error budget between the two ranges.     

Analysis and Simulation of a 1-18-GHz broadband double-ridged horn antenna

A 1-18-GHz broadband double-ridged horn antenna with coaxial input feed section is investigated. For the ridged horn antenna it is found that the radiation pattern, contrary to common belief, does not maintain a single main lobe in the direction of the horn axis over the full frequency range. Instead, at frequencies above 12 GHz, the main lobe in the radiation pattern starts to split into four large side lobes pointing in off-axis directions with a dip of up to 6 dB between them along the main axis. Although this type of horn is the preferred test antenna, which is in common use for over four decades, no explanation for this unwanted behavior was found in the open literature. To investigate this phenomenon in detail, a method of moments approach has been adopted to simulate the complete antenna system. The simulations are in good agreement with the measurements over the 1-18-GHz operational bandwidth and indicate that the use of this type of horn antenna in EMC applications remains questionable.     

Compact antenna test range without reflector edge treatment and RF anechoic chamber

There are several types of CATRs (compact antenna test ranges) used in antenna-pattern measurements. An offset reflector is generally used to generate the quiet zone of a CATR. Serrated edges, rolled edges, or R-cards are generally chosen along the reflector's edge to reduce the edge-diffraction field inside the quiet zone of the CATR. In order to reduce stray signals from the environment, a high-quality RF anechoic chamber is required for a CATR. In this paper, a new type of CATR, without either a reflector edge treatment or an RF anechoic chamber, is developed. A commercially available DBS (direct-broadcast satellite) reflector antenna, without edge treatment, is used as the reflector antenna of the CATR to generate the quiet zone of the antenna test range. In order to improve the quiet zone's performance, the fields due to feed spillover, edge diffractions, and other stray signals are gated out by the ITDAMS (impulse time-domain antenna measurement system). The RF interference in the environment can also be reduced by time synchronization and pulse integration of the impulse time-domain antenna measurement system. In order to verify the capabilities of the proposed CATR, three kinds of antennas (a low-directivity horn antenna, a high-directivity 60 cm direct-broadcast satellite reflector antenna, and a 25 cm Ka-band Cassegrain LMDS - local microwave distribution system - antenna) were measured by the proposed CATR. The antenna-pattern results agreed quite well with those of a near-field range and a far-field range.     

Radiation pattern measurements of electrically large antennas usinga compact antenna test range at 180 GHz

A 3 m-width millimetre-wave compact antenna test range for antenna measurements up to 200 GHz has been constructed at Queen Mary College. Measurements of a 400 mm aperture width 90° offset test antenna operating at 180 GHz are presented, indicating that a 62 dB dynamic range is achievable     

Improved radiometric performance attained by an elliptical microwave antenna with suction

We present a new way to securely mount a medical microwave antenna onto the human body for improved in vivo temperature measurements by microwave radiometry. A low cost and simple vacuum pressure source is used to provide suction (negative pressure) on the aperture of an elliptical antenna with vacuum chamber cavity backing. The concept offers improved electromechanical coupling between the antenna surface and the skin of the body. The proposed solution is evaluated experimentally to test repeatability of radiometric temperature measurements by remounting the antenna many times in one sequence on a given anatomical location. Four representative locations (hand, belly, hip, and chest) were used to test the suction antenna concept against anatomical curvature and load variations. Statistical analysis shows a marked decrease in the standard deviation of measured temperatures with the use of suction compared to conventional manual fixation. At repeated measurements, the vacuum antenna produces less uncertainty and improved estimate of the true lossy load temperature. During body movement, the antenna mounted at bone-filled areas shows greatest potential for improved performance.     

A reconfigurable aperture antenna based on switched links between electrically small metallic patches

A reconfigurable aperture (RECAP) antenna is described in which a planar array of electrically small, metallic patches are interconnected by switches. The antenna can be reconfigured to meet different performance goals by changing the switches that are open and closed. The switch configuration for a particular goal is determined using an optimizer, such as the genetic algorithm. First, the basic concept for the RECAP antenna is verified by comparing theoretical results with measurements for configurations in which the switches are simply wires connecting the patches. Next, details are given for a prototype antenna in which field-effect transistor based electronic switches are used with optical control. Theoretical results for the prototype antenna are then compared with measurements for cases in which electronic reconfiguration is used to change the bandwidth of operation or steer the pattern of the antenna. Finally, an overview of alternate switch/control strategies, some of which were tested, is given along with suggestions for improving the next generation of this antenna.     

Antenna Tests With a Hologram-Based CATR at 650 GHz

A hologram-based compact antenna test range (CATR) is designed, constructed, and used to test a 1.5-m antenna at 650 GHz. The CATR is based on a 3.16-m-diameter hologram as the collimating element. So far, this is the highest frequency at which any CATR has been used for antenna tests. The quiet zone is measured and optimized before the antenna tests. The measured antenna pattern results at 650 GHz are analyzed and compared to the simulated patterns. Feed scanning antenna pattern comparison technique is used to correct the antenna pattern. These tests show the hologram CATR to be promising for antenna measurements up to 650 GHz.     

Performance of a small loop antenna in the 3-10 MHz band

The performance of a typical small loop antenna of 1 m diameter is presented at frequencies in the range 3.6-10.1 MHz. It is argued that the antenna may reasonably describe as electrically small in this frequency range. Measurements of the antenna's bandwidth and, hence, Q factor are presented, along with direct measurements of the radiated field at various distances with a carefully measured RF input power. The radiation efficiency of the loop is derived from the measurements, and is compared with theoretical predictions of the efficiency based on classical electromagnetics. Close agreement is demonstrated between measurements and predictions. Consistency with the Chu bandwidth limit is also demonstrated for this antenna.     

Investigation of the Antenna Factor Behavior of a Dipole Operating Inside a Resonant Cavity

A rigorous analysis of a dipole inside a resonant cavity with wall losses is carried out by solving an electric field integral equation for the current along the antenna. Classic mode expansion is adopted to represent the cavity field, and method of moments is used to achieve the numerical solution of the problem, allowing us to describe the behavior of the antenna parameters in a resonant environment. The theoretical formulation of the problem is applied and validated for a realistic experimental situation, which typically occurs in shielding effectiveness measurements. The analysis focuses on the antenna factor (AF) in order to provide an estimation of the error that is committed when a free-space AF is used for field measurements in resonant structures.     

High-precision measurements on a compact antenna test range

In the letter recent results of antenna measurements on a compact antenna test range (CATR) are compared with those on an outdoor range. Both results are corrected for the influences of the non-plane-wave illumination of the test antenna.     

Optimization of bow-tie antennas for pulse radiation

Resistively loaded bow-tie antennas are considered as radiators for temporally short, broad-bandwidth pulses. Analysis is by the finite-difference time-domain (FDTD) method. The geometrical details of the antenna and the resistive loading along the antenna are chosen to optimize this antenna for pulse radiation. Theoretical results for the reflected voltage in the coaxial feed line and the time-varying radiated electric field are compared throughout with experimental measurements. The optimized, resistive bow-tie antenna is shown to radiate a pulse that more closely resembles that of the excitation than is radiated by a metallic bow tie of comparable size. Issues involving the use of the FDTD method for modeling fully three-dimensional antennas are also discussed. These issues include the use of a simple feed model and the staircasing of the edges of the antenna     

Antenna measurements using a hologram CATR

It becomes increasingly difficult to obtain far-field measurements for large millimetre wave antennas at higher frequencies due to the required large distance between the antennas. A hologram compact antenna test range (CATR) is used to determine the radiation characteristics of a 39 GHz planar antenna in a small facility. The results are compared with those obtained from planar near-field scanning and conventional far-field measurements     

Determination of far-field antenna patterns from near-field measurements

In many cases, it is impractical or impossible to make antenna pattern measurements on a conventional far-field range; the distance to the radiating far field may be too long, it may be impractical to move the antenna from its operating environment to an antenna range, or the desired amount of pattern data may require too much time on a far-field range. For these and other reasons, it is often desirable or necessary to determine far-field antenna patterns from measurements made in the radiating near-field region; three basic techniques for accomplishing this have proven to be successful. In the first technique, the aperture phase and amplitude distributions are sampled by a scanning field probe, and then the measured distributions are transformed to the far field. In the second technique, a plane wave that is approximately uniform in amplitude is created by a feed and large reflector in the immediate vicinity of the test antenna. And in the third technique, the test antenna is focused within the radiating near-field region, patterns are measured at the reduced range, and then the antenna is refocused to infinity. Each of these techniques is discussed, and the various advantages and limitations of each technique are presented.     

Measurements of K-Band Antenna Patterns

Methods and results of on-board antenna pattern measurements of Medium-scale Broadcasting Satellite for Experimental Purpose (BSE) are described in this paper. The measurements were carried out by means of satellite attitude offsets about the pitch and roll axes. Measured antenna patterns at both up-and down-link frequencies (14 and 12GHz) agree with the pre-launch data in the eastwards of the patterns, while they have more gentle gain slopes in the westwards. Further, antenna pattern deformation is observed along with the change in the antenna temperature. The whole appearance of the deformation seems to be complicated, however, in the center regions of the patterns which almost covers Honshu island, the dominant behavior of the deformation looks like antenna beam shift of 0.1°to 0.2°in the east-west direction.     

A microwave anechoic chamber for radar-cross section measurement

A microwave anechoic chamber has been developed at the Department of Electrical Engineering, University of Malaya, for monostatic and bistatic radar-cross-section measurements. The structure of the chamber is a quarter-section geodesic dome, with a 12 foot radius, and raised three feet above the floor. An antenna railing system is installed inside the chamber. The antennas can be moved along the rails in the elevation direction, with the microwave beam pointing at the center of the dome where the target is located. This design enables a very large combination of incidence and scattering angles in bistatic measurements. Four transmitting antennas are fixed at different elevation angles next to one of the antenna rails. By using an azimuth-over-elevation positioner as the pedestal for the target, and by positioning the movable antenna along that rail beside those fixed transmitting antennas, monostatic measurements with incidence angles ranging from 0 to 90 degrees can be accomplished. A vector network analyzer is utilized to measure the amplitude and phase of the radar returns. An IEEE-488.2 interface bus is used to control various hardware components, as well as to perform data acquisition. A computer program was written to automate the measurement system. Data are stored in raw format, and processed later with dedicated software, so that different processing methods and parameters can be applied. This paper highlights the design and construction of the microwave anechoic chamber, as well as the measurement-system configuration for scattering-cross-section measurements     

Angle-of-arrival statistics for low resolution antennas

In a scattering environment, the propagation paths to a receiving antenna arrive from a certain angular spread of directions. For a low-resolution antenna, an apparent direction of arrival may be defined as the azimuth direction of maximum power. A small array is used as the antenna and the spatial derivative of the phase along the array axis is taken as a measure of the sine of the apparent angle of arrival. The sine of the angle is shown to be Student's t-distributed and from this result the PDF and power spectrum of the angle-of-arrival is determined. The distributions are compared with experimental results for indoor as well as for outdoor measurements     

基于矢量网络远场测量的移动通信基站天线测试

介绍了一种基于矢量网络远场测量的移动通信基站天线测试方法,该方法具有扫频测量、有效抑制场地反射信号和场外干扰信号等特点,保证了被测天线的测量准确性和可靠性,同时大幅提高了测试效率,较好地满足了移动通信基站天线的测试需求,并进行了验证。