A Novel Instrument for Real-Time Measurement of Attenuation of Weather Radar Radome Including Its Outer Surface. Part I: The Concept1. Introduction
With the introduction of dual-polarization systems in operational radar networks, target accuracy for differential reflectivity 
Manz et al. (1999) performed a study evaluating the effects caused by the distribution of joints in the radome on radar performance at C band. Manz et al.’s work suggests the installation of a device to test the transmitted signal and to measure the changes in radome attenuation as a possible solution for wet radome calibration. Bechini et al. (2010) proposed a technique to measure the attenuation under wet conditions for X-band radars. Bechini et al.’s correction based on the disdrometer assumes that water is a film and did not account for rivulet effects that could produce various levels of attenuation in the horizontal (H) and vertical (V) planes. Gorgucci et al. (2013) developed two methods for 
This paper presents a novel technique that enables radio frequency (RF) characterization of any type of radome under any condition. The proposed instrument is a low-cost, low-size solution, and it can be installed in an existing operative radar without significantly affecting the radar signal. One of the novel aspects introduced by the proposed concept is that the radome characterization is based on the reflection coefficient measurement. The new instrument is composed of a reflectometer, time domain gating (TDG) analysis, and a probe. The reflectometer was employed in this research to measure reflections generated at the air–radome interface. The TDG analysis (implemented in the reflectometer) was utilized to remove unwanted reflections coming from the surrounding environment. A suitably designed dielectric rod antenna was employed as a probe. With this instrument it is possible to perform real-time measurement of the reflections because of the radome and water formation on its outer surface and, indirectly, to estimate the transmission loss occurring as a result of the wet radome. The radome is characterized by high-resolution measurements dictated by the probe’s beamwidth. High spatial resolution allows for effects introduced by raindrops accumulated on the surface, the impact of scatterer points caused by imperfections that may be present, and the influence introduced by structural joints to be taken into account. Because a narrow-beam probe is employed, the area of the radome analyzed does not match with the area sampled by the parabolic reflector; a trade-off is discussed later in section 4.
Based on the physical conditions of the outer skin of the radome, different water formations are possible. The possible water distributions are droplets and rivulets or a continuous film. Rivulets, given their vertical geometry, are particularly critical because they produce a higher level of reflection in the vertical than the horizontal plane. When a layer of water is formed on the outer surface of the radome, either as droplets or as a continuous film, part of the incident energy is absorbed by the water. To include the water absorption, the method described here calculates the absorptance of water, as a function of the rain rate, without considering the radome. Then, combining the absorption with the reflectance directly measured, it is possible to estimate transmittance through the wet radome. Since the radome stackup is normally unknown, it is not possible to estimate the related absorptance. However, the energy absorbed by a dry radome is normally very small and can be neglected.
To validate the proposed approach, a laboratory setup was designed to measure the reflectance generated from radome panels at X band.
The paper is organized with an explanation of the proposed technique in section 2. The concept and theory, as well as the algorithm to estimate the water absorption, is provided in section 3. In section 4 the setup, the key tools employed for the proposed instrument, and the TDG principles are introduced. The experimental results obtained from the setup are presented and discussed in section 5. The algorithm to estimate the water absorption will be employed in the second part of this paper (Mancini et al. 2018, hereinafter Part II), where additional experiments for the radome characterization are discussed.
2. Proposed measurement system
a. Two-port system
The system commonly used to measure the attenuation introduced by the radome is shown in Fig. 1a. For this configuration, a probe to transmit and a probe to receive are required. The sample material must be placed exactly midway between the antennas, and the probes must be perfectly aligned facing each other. In such a scenario, the incident electromagnetic field is decomposed into reflected and transmitted fields. This mechanism is schematically shown in Fig. 2. This same process happens inside the radome at the radome–air interface, and it occurs infinite times inside the material. The phenomenon described is shown only three times in Fig. 2 to simplify the diagram. The method represented in Fig. 1a was implemented by Díaz et al. (2015).
A schematic representation of the system to characterize radomes for (a) the free-space transmission coefficient technique conventionally used and (b) the new technique using only one probe. The labels in (a) and (b) indicate the probes and the distance d between the antennas and the material under test (radome). The start and stop times of the TDG (
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
A dielectric slab separating two media made of air. The sketch shows angular incidence in order to make the representation easier. Shown in the illustration are the angles of incidence, reflection, and transmission (labeled i, r, and t), as well as the incident, reflected, and transmitted electric fields (
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
The limitations of the two-port system in characterizing radomes were discussed in Mancini et al. (2017).
b. One-port system
The proposed instrument to measure the reflections generated at the air–radome boundary is schematically shown in Fig. 3. A dielectric rod antenna was employed as a probe. The single-port reflectometer, set in a time domain, reads the reflections directly measured by the probe. By mounting the probe at the antenna feed position, scans in azimuth and elevation with the resolution dictated by the radar system are possible. This configuration allows for full real-time RF characterization of the radome. A probe smaller than the antenna feed is required to minimize attenuation caused by a potential blockage. This requirement is necessary in order to measure the radome reflections without affecting the radar performance. A customized probe with a small transversal area and a narrow beam for high-spatial-resolution tests is required. The probe collects reflections generated from the radome in the same direction that the radar is scanning, because it is mounted in the radar antenna. Although mounted on the radar antenna, the operation principle of the radome characterization device is totally independent of the radar, and it does not affect the radar operating bandwidth. The proposed method allows for measuring the effect of water on the radome outer surface when rain reaches the radar location. Water accumulation is not the same on all the parts of the radome. Multiple factors, including dirt on or damage to the radome and blowing wind, can in fact influence water distribution on the surface.
A schematic representation of the proposed concept, and a detailed illustration of the radome characterization kit.
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
The probe is connected to the single-port reflectometer, and the TDG is set to measure the reflectance at the radome interface, filtering out all the multiple reflections coming from the surrounding environment (section 4). The small-sized instrument, composed of a probe and a reflectometer, allows for ease of transport for infield measurements in the case of mobile radar stations and quick implant in existing operational radars. To simultaneously measure the reflections generated from the radome in the H and V planes, a dual-polarized probe can be suitably designed or two linearly polarized probes, where one is rotated 90° with respect to the other one, can be used. The influence of a dual-polarized probe on the performance of a parabolic reflector antenna was estimated through simulations in Ansys High-Frequency Structural Simulator (HFSS), version 2016. The Ansys HFSS software utilizes a 3D, full-wave, frequency domain electromagnetic field solver based on the finite element method. To evaluate the impact of the instrument on an operational radar, two scenarios were considered. The first case simulated the reflector system of the PX-1000 weather radar (Cheong et al. 2013) with no radome included and without the dielectric probes. Two dielectric antennas were then added for a second simulation, placed right behind the feed of the reflector system, to evaluate their effect on the radiation pattern. An illustration and a photograph of the PX-1000 parabolic reflector are shown in Fig. 4. The measured performance of the PX-1000 radar was executed in an outdoor far-field test range by the manufacturer. Unfortunately, a copy of these measurements is not available, but information such as gain, beamwidth, and the levels of sidelobes are provided. A comparison between the measured and simulated performances of the parabolic reflector, without the probes, is presented in Table 1. The gain and beamwidth achieved through simulation are the same as those measured in actual performance, but in the simulation, a lower sidelobe level (SLL) is obtained. The discrepancy between the measured and simulated sidelobe level is probably due to the fact that, since the measurement was done outdoors, multiple reflections have raised the level of the sidelobe, while this does not occur in the simulation. Since the model well represents the actual radar antenna, in terms of both physical dimensions and electrical performance, the effect of the probes can be now evaluated. The results regarding the influence of the dual-polarized probe on the parabolic reflector performance were presented in Mancini et al. (2017) and are summarized in Table 2. Simulations show that the gain is not affected by the presence of the probe, while a rise of the secondary beams is noticeable. The level of the cross polarization, in the range from −5
An illustration of the PX-1000 antenna showing the feed and the struts arrangement, the focal distance (18 in.; 1 in. = 2.54 cm), and the diameter of the parabolic reflector (48 in.). (a) The side view. (b) The front view showing the struts only. (c) A photograph of the parabolic reflector (the photograph was taken by Arturo Yoshiyuki Umeyama).
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
A comparison between the measured and simulated antenna performance of the PX-1000 at 9.55 GHz.
Effect of the dual-polarized probe on the performance of the PX-1000 parabolic reflector. The comparison is based on simulation.
The probe does not need to sample the radome every time the radar transmits/receives. It is sufficient to take a sample every N times the radar samples. If, for instance, the radar samples N times consecutively, at the N + 1 transmitting period, then the probe samples instead of the radar. The echoes from the radar would be very low in this circumstance, and they would not interfere with the probe. Sacrificing one sample every N samples of the radar would not affect the weather measurement. In practice N is about 100. In this way the probe can sample at the same frequency the radar operates without interference from the radar. To relate the data collected to the actual position, synchronization will be required. In this way it is possible to have information about the radome conditions at the same time and in the same position where the radar is scanning.
3. Concept and theory
The dielectric parameters of water for different temperatures: (left) the real and imaginary parts of the relative dielectric constant and (right) the tangent loss.
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
To prove the model assumptions, simulations in Ansys HFSS were performed to represent the concept shown in Fig. 1b. To achieve this goal, two scenarios were replicated, both employing a dry, multilayer radome operating at X band. The first scenario used two ports to emulate the system shown in Fig. 1a. A second simulation to reproduce the scenario in Fig. 1b used one port to excite the incident wave and set the second port as impedance boundary to simulate the free space. Results are presented in Fig. 1c for the reflection coefficient and Fig. 1d for the transmission coefficient. The curves in Fig. 1c are the results of direct simulation. The transmission coefficient for the two-port case (thick solid curve in Fig. 1d) is also produced from direct simulation. The curve computed for one port in Fig. 1d is obtained by applying Eq. (19), because the materials considered are low loss. Simulations show good agreement between the two-port and one-port scenarios for both transmission and reflection coefficients, confirming the validity of the approach chosen and assuring that the absorption can be neglected in the case of a dry radome. The absorption for the scenario presented can be evaluated by computing the difference between the two curves in Fig. 1d, which is lower than 0.1 dB. The stackup employed for the X-band radome simulated in HFSS is summarized in Table 3.
The radome stackup employed in the HFSS simulations to prove the concept shown in Fig. 1. The radome is designed to operate at 9.55 GHz.
To evaluate the impact of the absorption of a slab of water without the radome, an analytical approach considering water at 20°C, with thicknesses of 0.2, 0.6, and 1 mm, was taken into account. First, 
A comparison between the analytical model and HFSS simulations for different thicknesses of a slab of water (no radome included) at 20
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
Correction algorithm
1) Water film
Once the thickness is calculated, the reflectance and transmittance through a slab of water can be computed by using Eqs. (14) and (15). Then by reordering Eq. (20) for A, the absorptance can be found. The absorption derived is the term Aw to employ in Eq. (22). Since the reflectance for the radome + water film stackup (or wet radome) is directly measured by the reflectometer, and the absorption of the water film (no radome) is known for a specific rain rate, then the transmittance through the wet radome can be estimated by using Eq. (22).
To validate this algorithm, simulations in HFSS were performed. The PX-1000 radome, with a radius of 1.1 m, was used for the simulation. The water thickness was computed for such a radius by using Eq. (23). In Table 4 the thicknesses calculated at different rain rates are shown. The radome stackup is listed in Table 3, and the performance in dry conditions is shown in Figs. 1c and 1d. First, a simulation of this radome was performed by adding a layer of water at 20
The water film thicknesses calculated for different rain rates. The third column shows the maximum relative error between the true value of transmittance, directly measured by simulation in HFSS, and the approximated value computed by Eq. (22) in the frequency band considered. In the fourth column, the relative error (previously described) occurring at 9.55 GHz is shown. In the last two columns, a comparison of the attenuation found in the current study with Frasier et al. (2013) is presented.
A comparison between the true and estimated transmittances for different water film thicknesses. In the legend the equivalent rain rates for each thickness are shown in parentheses.
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
2) Droplets
The DSD calculated at different rain rates, scaled by a factor of 20. Also shown is the relative error, averaged in the 8.5–11-GHz frequency band, and the relative error (before average) at 9.55 GHz, between the true and the approximated values of the transmittance.
(a) The comparison between the true and the estimated transmittance through a radome wetted with droplets. (b) The random droplet distribution corresponding to round 1 of the simulation for 4, 5, and 6.3 mm 
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
To conclude the study of water formation as droplets, a comparison was made between the one-way attenuation obtained by the true value of the transmittance (
Comparison between the one-way attenuation (dB) simulated at 9.55 GHz and the one computed by using Trabal’s method.
4. Experimental setup
This section presents the setup and key components of the proposed instrument. Details about the antenna used as a probe and reflectometer operating in time domain are also discussed.
a. Probe: Dielectric rod antenna
To fully characterize the radome surface using the proposed technique, the antenna employed as a probe requires a low size and a narrow beamwidth. An antenna beamwidth is measured at the half-power (−3 dB) point of the main beam. A narrow beamwidth enables high spatial resolution of the radome measurements and makes it suitable for the investigations proposed in this paper. However, if a narrow-beam probe is used, then the area of the radome sampled by the probe is much smaller than the one sampled by the radar antenna, which operates in the near field at the distance of the radome. For the case proposed, the sampling area of the probe is only 9% of the area sampled by the parabolic reflector. A wider area can be sampled if a probe with a larger beamwidth is designed, but it comes at the expense of the spatial resolution. It is left to the user, based on the primary purpose of the application to define this trade-off. Low weight and small transversal area are key features for easy implementation in operational radars. A small size probe is necessary in order to minimize the signal blockage. The type of antenna selected by the user for conducting this research is the dielectric rod antenna. In the past dielectric rod antennas were used for diverse applications by Mueller and Tyrrell (1947), Watson and Horton (1948), Kobayashi et al. (1982), Studd (1991), Zucker and Croswell (2007), and Stroobandt (1997).
The dielectric rod antenna belongs to the family of surface-wave antennas and is obtained by placing a dielectric rod in the waveguide aperture. The portion of the dielectric located inside the waveguide is called the feed taper. The feed taper has the function of providing impedance matching at the waveguide–dielectric transition, increasing the efficiency of excitation. The body taper, another part of the dielectric, mainly reduces the level of sidelobes and also increases the bandwidth. The terminal taper, located at the tip of the dielectric, improves the impedance matching between the dielectric and the air, decreasing the reflected surface-wave level (Zucker and Croswell 2007). The length of the neck of the dielectric (Fig. 9a) determines the gain and beamwidth. However, if there is not sufficient headroom between the radar feed and the radome, or if a smaller impact of the probe on the radar antenna is desired, then it is possible to decrease the size of the probe by employing miniaturizations techniques (Volakis et al. 2010, 160–180). In general, a smaller version of the dielectric rod can be obtained by choosing a material with higher dielectric constant. If the beam of the probe is narrow, it is possible to use this antenna as an RF probe for characterization of a smaller radome panel. A narrow beam is more easily confined in the radome sample and no fringing effects from the borders occur (Díaz et al. 2015). The far-field distance of this kind of antenna is 2–3λ (λ = 3.2 cm at 9.4 GHz). Considerations about the far-field distance of the probe are important, because the analytical models presented in this paper are based on the assumption of a plane wave propagating, which is valid only in the far-field region of the probe. Thus, all the devices under test must be located in the far field of the probe.
(a) Two illustrations showing the dielectric rod dimensions (mm), at top and side views. The part of the rod that is located inside the waveguide is indicated (dashes). (b) The reflection coefficient of the rectangular waveguide with the dielectric assembled.
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
The dielectric rod antenna employed as a probe needs to be customized in order to operate at the same frequency band as the radar. In the present research, the probe employed for radome characterization was composed of a rectangular waveguide (commercially available) and a dielectric rod optimized to operate at X band (designed in HFSS and in-house 3D printed). The acrylonitrile butadiene styrene (ABS), which is a common thermoplastic polymer, is the material the authors had available for 3D printing the dielectric rod. The probe was then constructed by wedging the dielectric rod inside the waveguide. In Fig. 9a illustrations of the top and side views of the dielectric rod employed in this study are shown. In this paper an H-band rectangular waveguide was employed to operate in the portion of the frequency spectrum that overlaps with X band (8.2–10 GHz). The reason for this was because an X-band waveguide was not available at the time of the experiments. In Fig. 9b the reflection coefficient is plotted. A summary of the electric performance of the antenna is listed in Table 7 and is based on a plot presented in Mancini et al. (2017). The gain of the probe, measured in the far-field chamber, is 17.5 and 19 dB at 8.8 and 9.8 GHz, respectively.
The performance of the RF probe at 9 and 10 GHz, for the H and E planes.
b. Reflectometer
The novelty of this study is that the radome characterization is performed by measuring the reflection coefficient. To accomplish this goal, a vector network analyzer (VNA) reflectometer R140 by Copper Mountain was employed. This device has several applications, including adjustment and testing of antenna–feeder devices, and is used in automated measurement systems. The operative frequency range of the reflectometer is 85 MHz–14 GHz. It operates with the assistance of an external computer that also powers the device. The block diagram of the reflectometer (VNA R140) is shown in Fig. 10. The main components of the VNA are a source oscillator, a local oscillator, a directional coupler, and a digital signal processor (DSP). The output of the reflectometer (test port) is the incident signal. The two-channel receiver filters and digitally encodes the signals. Additional processing of the signals is performed in the DSP. The reflection measurement is performed by comparing the received (reflected) signal with the source signal. An important aspect of the reflectometer is the ability to operate in the time domain using the time domain gating analysis, which will be discussed next. The pulse repetition frequency of the VNA R140 is 30 KHz. The peak power was set to −10 dBm for the measurements subsequently described. The frequency accuracy of the VNA is 
The block diagram of the reflectometer (R140).
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
Measurement accuracy of the VNA R140 reflectometer.
c. TDG
The concept of time gating is to apply a filter in the time domain. Time gating is distance or spatial filtering, which means that only data from a certain range gate are processed. TDG was employed in the past by various researchers for disparate applications. For example, Archambeault et al. (2006) used it to remove discontinuities or reflections in a free-space context and for tuning purposes. Burrell and Jamieson (1973), Wayapattanakorn and Parini (1993), and Fordham (2010) employed the TDG to measure radiation patterns, while Ghodgaonkar et al. (1989) and Zhao et al. (2006) used this filter for calibration in free-space measurements.
1) Time domain transformation
The Fourier transform is the operation necessary to switch from frequency to time domain representation, and it applies to continuous signals. The VNA, or reflectometer, performs a digital Fourier transform, since it works with discrete data, and the measured signal is a sampled representation of the continuous signal. Consequently, a problem of this operation is the presence of aliasing. Aliases are undesired replicas of the same signal distributed periodically in the time axis. It is fundamental to operate in an environment free of aliases in order to make a correct interpretation of the measurements. The R140 is implemented in order to always represent data free from aliases. Another unwanted effect is produced by the truncation generated by the frequency-finite representation of a signal, which in the real world is not limited in frequency, and thus sidelobes are introduced in the time domain. These ringing effects are described by a Sinc(t) function and they cannot be totally eliminated (Agilent 2012). To enhance the time domain response, a window can be applied in the frequency domain to control the sidelobes in the time representation resulting from the truncation process. In the present study, a normal type of Kaiser window with −44 dB of sidelobe level was used.
The gate can be thought of as a filter in the time domain. However, the operation of “gating” is actually applied in the frequency domain by defining the start and stop gate times that determine the dual-frequency gating function. Dunsmore (2008) conducted a study to evaluate errors in the time domain response caused by inaccurate positioning of the gate around the desired region. Other limitations of the gating function are described in Agilent (2012).
2) Range and resolution
In the proposed study, TDG was necessary to more accurately measure the reflections produced from the radome without contamination from the reflections generated by the surrounding environment. The implementation of the TDG to the laboratory setup (Fig. 13a) used to validate the proposed concept is discussed. With the dielectric antenna placed at θ = 0
An example of reflection responses generated by the laboratory setup in the (a) frequency and (c) time domains, obtained with the antenna pointing at θ = 0
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
Analyzing the time domain response without applying the TDG, represented by the thick solid curve in Fig. 11c, it is possible to distinguish the reflections and their related sources, since they are expressed versus the distance in time. The three dominant peaks (1, 2, and 3), located approximately at 0.3, 2.5, and 4.6 ns, respectively, are generated by impedance mismatches. The reference plane, which corresponds to 0 ns, is located at the port of the reflectometer, as shown in Fig. 11b. An open, short, and load calibration is necessary to remove the reflection (peak) resulting from the cable. Peak 1 is created by the cable impedance mismatch (no calibration applied). Peak 2 is due to the impedance mismatch introduced by the waveguide. Peak 3 is produced by the waveguide–dielectric rod junction. Peak 4, located at circa 7.8 ns, represents the reflections generated from the radome placed at 12.5 cm above the antenna. Other minor peaks are visible at time 
The TDG is an important technique for removing undesired reflections. Such reflections are generated by impedance mismatches, and it is fundamental to understand what causes them. Impedance mismatches create loss of energy of the incident signal because part of it is reflected. Furthermore, impedance discontinuities can obscure the response of the subsequent mismatches, because the energy reflected from the first impedance mismatch never reaches the following ones. Such an effect is called masking (Agilent 2012). The factors that contribute to generating discontinuities in the proposed research are the presence of cable, waveguide, and dielectric rod (previously described). Their effects are presented in Fig. 12. In the figure, three cases are considered: the first using an ordinary RF cable for the antenna–reflectometer connection with the open, short, and load calibration applied (dotted curve in top panel; thick solid curve in bottom panel); the second using the cable but without applying the open, short, and load calibration (thick solid curve in top panel; thin curve with open circles in bottom panel); and the last case without using the cable but connecting the VNA directly to the antenna and applying the open, short, and load calibration (thin curve with asterisks). These three cases are presented in both frequency and time domain. In the frequency plot, the cases were compared with the reflection coefficient of the antenna measured with a Keysight performance network analyzer (PNA; N5225A) for the purpose of reference. A PNA is a very accurate device for performing measurements of linear characteristics of RF components and devices. The oscillations produced by the RF cable (thick solid curve in top panel; thin curve with open circles in bottom panel) are clearly noticeable. When applying the open, short, and load calibration, the plane of reference is shifted to the output of the cable, and then the cable reflection (peak 1) is moved to the negative axis of the time. By removing peak 1, the curve that was obtained by using the cable with an open, short, and load calibration applied (dotted curve in top panel; thick solid curve in bottom panel) matches with the thin curve with asterisks, where no cable was used, and it also matches with the reflection coefficient, in the frequency domain, of the antenna measured with a Keysight PNA.
The open, short, and load calibration effects using a 3-GHz bandwidth in the (top) frequency and (bottom) time domains. Peak 1 corresponds to the cable mismatch, which is present only in the thick solid curve in the top panel and the thin line with open circles in the bottom panel. Once the calibration is applied, peak 1 moves to the negative axis of the time. As consequence of the calibration, peaks 2–4 in the thin curve with open circles (bottom panel) are shifted to the left on the plot (thick curve and thin curve with asterisks).
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
The effects caused by the impedance mismatches generated from the cable, waveguide, and dielectric rod have been discussed and shown in the frequency and time domains. Benefits resulting from the open, short, and load calibration have also been mentioned.
5. Laboratory experimental results
To validate the concept discussed in section 3 (Fig. 3), a laboratory setup was built to enable the testing that provided preliminary results. This section is divided into two parts. The first part describes the measurements obtained from a setup composed of flat radome panels (Fig. 13). The second part presents the results obtained from the modified setup (Fig. 14).
The setup for the laboratory experiment employing the proposed technique using four metal strips as reference. (a) A graphic representation of the setup, including the probe and reflectometer. (b) A photograph of the setup, with the probe and reflectometer. (c) The transmittance without TDG applied. (d) The transmittance with TDG applied.
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
The modified laboratory setup employing a Ku-band radome: (a) graphical representation, including the probe and reflectometer, and (b) a photograph of the setup, with the probe and reflectometer. (c) The reflectance without applying the TDG. (d) The reflectance with the TDG applied. (e) The transmittance without applying the TDG. (f) The transmittance with the TDG applied.
Citation: Journal of Atmospheric and Oceanic Technology 35, 5; 10.1175/JTECH-D-17-0083.1
In Figs. 13a and 13b, a schematic representation and a photograph of the setup are shown. The laboratory setup is composed of five radome panels arranged to approximate a quarter of the circumference. The rotary motor with the probe mounted on it is located at the origin of the circumference. The panels are placed abutting each other, leaving small air gaps between each other. These air gaps are located at θ = 17°, 37°, 57°, and 77° with respect the initial position of the rotary motor, as shown in Figs. 13a and 13b. To keep the radome panels in a stable position above the antenna and to secure the rotary motor to a fixture a wooden support was used. Because of imperfections in the setup, the distance between the antenna and each panel is not identical.
The radome panel is made of foam as an inner layer (6.62 mm) and teflon as a skin layer (0.53 mm). In Fig. 13a the antenna is placed at the start position (broadside; θ = 0°) of the rotary motor, the probe beamwidth is shown in green, the TDG is displayed in red and centered at the radome location, and the predicted results for the reflection measurement are shown. Both a flat response of the reflection coefficient from the radome panel and a strong reflection coefficient peak generated from each metal strip located at the junction between two consecutive panels are expected.
The setup was modified for the second part of this study. The reason for this modification was to characterize an operational radome that is not flat but presents a curvature. Three of the radome panels were removed and a Ku-band radome (also applicable at X band) replaced them, leaving a large air gap between the radome and the remaining two panels. The Ku-band radome has a circular shape with a 60-cm diameter and presents a small angle curvature from the center to the peripheral area. However, at the borders the termination is cornered, so it can be mechanically fixed to the reflector. In Figs. 14a and 14b, a schematic view and a photograph of the modified setup are presented, respectively, showing the predicted behavior for the reflection coefficient as in the previous setup.
The TDG width is defined by the start and stop times. For the following experiments, they were set to 7.5 and 8 ns, respectively. Considering that the distance between the probe and the panel was slightly different because of fabrication imperfections of the setup, the TDG width was set in order to capture only the reflections from each single panel, making sure that all radome panels fell inside the TDG width. This was done by performing initial tests with larger time spans for the gating. These experiments produced consistent results; they were not reported here for the sake of brevity.
A LabVIEW interface was created to have a fully automated system. The measurement criterion is identical for both of the setups employed. First, the rotary motor rotates the RF probe to an angle with increments (or resolution) dictated by the user (1° for this study). Then a sample is taken. This is repeated until the final designed angle (i.e., 80°) is reached. During the tests the probe did not move while the sample was taken. The measurements were repeated three times for each scenario and compared with the average.
a. Case 1: Five-section uniform radome
This section describes the results obtained from the laboratory setup shown in Fig. 13. Measurements of the reflections are plotted at 9.4 GHz, using a 3-GHz bandwidth (7–10 GHz) in order to have good resolution in the time domain. The experiments were performed three times under the same conditions to assure the reproducibility of the experiment. The measurements were then compared with the average. In Figs. 13c and 13d, m1, m2, m3, and avg represent the three measurements and the related average, respectively. The purpose of these experiments was mainly to validate the dielectric antenna and the reflectometer, with TDG applied, as a system to evaluate the effect introduced by the radome in terms of reflections. In particular, the goal was to prove that the TDG is a fundamental analysis to remove undesired reflections and to improve the measurement quality (Fig. 11c). In addition, while conducting experiments when varying transitions existed between radome panels, the measured results show the effect of the discontinuities, mainly produced by diffracted fields. Diffracted fields are difficult to see with a continuous interface. Metal strips were used to cover the air gaps between the consecutive panels with the purpose of providing reference in the tests, as shown in Fig. 13a. In the first experiment, the TDG was not employed in order to determine whether a reflectance measurement of good quality could be achieved without using the gating in the time domain. Strong reflections (peaks) from the metal strips were expected and therefore could be visualized without TDG. Mancini et al. (2017) showed results of the reflectance normalized with respect to the strongest of the peaks generated from the metal strip. In the current manuscript, the same results are shown in terms of transmittance by applying Eq. (19) and are shown in Fig. 13c. The absorption of the dry radome was neglected. The results presented in Fig. 13c show that the multiple paths generated from other surfaces of the setup affected the measurements considerably by canceling some of the peaks. From Fig. 13c, only the first strip (M1), located at 17
b. Case 2: Ku- and X-band radomes
In this paragraph measurements performed using the setup presented in Fig. 14 are shown at the frequency of 9.4 GHz. To have a complete scan of the Ku-band radome, it was necessary to start the measurements at θ = −25
6. Conclusions
A novel instrument to characterize the effect of a radome in dry and wet conditions was presented. The proposed instrument enables the RF characterization of a radome surface in any condition, employing a single RF probe, in contrast to conventional methods that require two RF probes. The concept, instrument description, formulation, and initial results were discussed in this manuscript. Additional experiments using the radome of an operational radar at X band, tested under the influence of artificial and natural rain, are described in Part II.
The results discussed in this paper demonstrate the validity of the proposed instrument, highlighting that this technique can provide accurate results when the absorption is negligible (section 3; Fig. 1d). In case of water accumulation on the radome surface, the absorption can be estimated by modeling the water formation as a film or as droplets, as function of the rain rate. To obtain an accurate characterization of the radome, it is necessary to combine the TDG technique with a narrow-beam RF probe. This manuscript provides the most important concepts to better quantify the absorption and transmission loss of a wet radome, and it can be used for real-time measurements of a radar system under the influence of precipitation.
Sufficient headroom between the radar feed and the radome must exist in order to employ this technique in an existing radar. This is one of the main limitations of the proposed method. Such a limitation, however, does not apply to future radar systems, where the radome can be designed to accommodate the probe. Another limitation involves the trade-off between the probe beamwidth to achieve high space resolution and the probe size, which could have an impact on the radiation pattern of the radar, where the narrower the beam is, the higher is the mismatch between the area of the radome sampled by the probe and the radar antenna. These trade-offs need to be taken into account during the probe’s design.
The authors thank Simon Duthoit, Damon Schmidt, Danny Feland, Valery Melnikov, and the ARRC, NOAA, and SENSR for help and the immediate assistance they provided anytime it was necessary.
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