Modeling and Practical Implementation of a Broadband Double-Ridged Horn Antenna with an Operating Range More Than an Octave and a High Level of Cross-Polarization Discrimination

Introduction. The resolution of the problem of radio polarimetry in multiposition microwave screening systems (MMSS) with aperture synthesis requires the use of antennas with a high level of cross-polarization discrimination (XPD) in a wide spatial angle. The radio images are reconstructed in MMSS at distances commensurate with the aperture of the antenna structures. Therefore, the value of the spatial angle, at which high XPD is required, can reach 30°. This leads to a new problem of creating an antenna configuration of the X and Ku band, the application of which in MMSS will resolve the problem of constructing a radio image of depolarized microwave radiation scattered on the human body in the form of hidden dangerous objects.Aim. To develop a double-ridged receiving antenna for long-term operation in MMSS with an XPD level of 28 dB at a spatial angle of 30° and operating frequencies of 8…20 GHz.Materials and methods. The requirements for the receiving antenna in MMSS were determined. Theoretical justifications were proposed for the choice of antenna design. Aperture synthesis was used to construct microwave images in MMSS. The stages and results of modelling broadband double-ridge antennas were presented using the CST Studio software broadly applied for three-dimensional electro-magnetic field modelling. The results of modelling pyramidal and conical double-ridged antennas, as well as those in circular and elliptical waveguides, were analyzed. The designed antenna was tested in an anechoic chamber. The measurement results were compared with those obtained during simulation.Results. An elliptical double-ridged horn antenna with a VSWR of no more than 2 and cross-polarization discrimination in a spatial angle of 30° of no less than 28 dB for the frequency range that covers an octave was designed and constructed.Conclusion. The developed antenna can be used in MMSS for the purpose of detecting the effect of micro-wave radiation depolarization as hidden dangerous objects on a human body. Such characteristics of the antenna as its high XPD value in a wide spatial angle will allow the future introduction of microwave polarimetry in MMSS.

Introduction.Currently, multiposition microwave screening systems (MMSSs) often use crosspolarisation methods to detect metal objects on the human body [1].The MMSS shown in Fig. 1 at the place of operation is designed to continuously scan the flow of people for hidden explosive devices and automatic weapons on the human body and in backpacks.It uses the radio polarimetry method [2,3], in which broadband double-ridge antennas are used to obtain complete information about the scattered field from the target [4,5].The necessity of ensuring a high level of cross-polarisation discrimination (XPD) [6,7] in the antenna structures used to obtain reliable results comprises a complex technical task for the frequency range having more than an octave of overlap [8,9].XPD (Cross Polar Discrimination -XPD [10]) refers to the minimum ratio of amplitudes of linear components of the electromagnetic field of main and cross-polarisation defined in the Ludwig-3 coordinate system [11] for a given spatial angle.
In the MMSS developed by the authors of this paper, aperture synthesis is used to build a microwave image.A significant limitation for the construction of radio images in cross-polarisation [1] is the unsatisfactory XPD value of the used antenna.
When carrying out the experiment described in [1], the minimum XPD value for the task of classifying dangerous objects on the human body is established: 5.0 in main polarisation and 0.2 in crosspolarisation, which is 28 dB.For the developed MMSS [12], it is necessary to maintain this value during the detection in the area of reflected radiation analysis (Fig. 2, 3).
The multiposition system (Fig. 2) consists of two antenna arrays located at an angle of 45° to the Xaxis.The inspection area includes the area for the analysis of past radiation 2 and the analysis area of reflected radiation 3 [13].From the location and size of the reflected emission analysis area 3, it follows that the spatial angle at which the specified XPD value is to be maintained must be at least 30°.
In the first MMSS version [12], Vivaldi printed antennas were used (Fig. 3); however, in the process of developing a system for detecting hidden objects by means of cross-polarisation scattering analysis, it was found that the XPD of these antennas failed to meet the requirements, since its level in the angle of 30º was only 4 dB at linearly polarised radiation.In a spatial angle of more than 30º at frequencies above 14 GHz, this antenna begins to dominate the cross-component, while the discrimination value drops to -10 dB.These parameters prohibit the use of this antenna to detect hidden dangerous objects described in [1].
Other Vivaldi antenna parameters in the operating range of 8…18 GHz: voltage standing wave ratio (VSWR) less than 1.8; the average value of the directional factor is 9 dBi; the width of the main lobe at the level of 3 dB in the dielectric substrate x0z (E-plane) 70…60º, in the cross-polarised plane y0z (H-plane) 85…30º are generally unsatisfactory for systems with aperture synthesis within the spatial angle of 30º.The considered receiving antenna is additionally distinguished by its structural fragility and instability, as well as the insufficient repeatability of its electrodynamic parameters from sample to sample.
Work objective.To develop a receiving antenna with a high XPD level, operating in the X and Ku frequency ranges, by means of which to reliably solve the problem of restoration of radio images in the developed MMSS for microwave radiation, depolarised by the scattering of dangerous objects concealed on a human body.
In the developed receiving antenna, we set out to achieve the described parameters and eliminate the noted design flaws.
Research methods.Broadband structures developed on the basis of double-ridge antennas are well-known and widely used [4,5].In [4], the authors consider a double-ridge pyramidal horn powered by a coaxial cable; here the size of the structure and obtained electrodynamic parameters are given, but the XPD level is not indicated.In order to estimate this parameter, an electrodynamic simulation of the double-ridge pyramidal horn antenna was carried out using the CST STUDIO SUITE.Additionally, in order to determine XPD within the spatial angle of 30°, a double-ridge conical horn antenna and elliptical waveguide were modelled.
For antenna coordination, a double-ridge structure of exponential shape was used (Fig. 4), given by the expression: where 1 , y 2 y are the coordinates y of the beginning and end of the exponential part of the double-ridge structure; αthe exponent coefficient; hhorn length.Double-ridge structures of the considered antennas were fed by a coaxial cable with a wave resistance of 50 Ohm.
Let us estimate the aperture size for the pyramidal horn.According to [14], the H-shaped waveguide formed by two ridges in a rectangular waveguide has a lower critical frequency for the main wave type where с is the diameter of the horn aperture; λ is the wavelength of electromagnetic radiation.Thus, for the average frequency range of 13 GHz and DF =10 dBi, it is necessary to have 25 mm.c  Modelling.Fig. 4 presents the analysed model of the double-ridge pyramidal horn antenna.The shape of the ridge 2 is defined (1), the dimensions 22 mm a  and 30 mm b  are selected to obtain the required DF according to (3).
For this model, the frequency dependencies of VSWR (Fig. 5, b) and XPD within the spatial angle of 30° were obtained (Fig. 5, a) for different values of the α coefficient.The above graphs show that the coefficient  significantly influences the XPD level, while VSWR for all the above α values does not exceed 2. The optimal value for this task is the value 0.1,    however, the obtained XPD value 18 dB min does not meet the stated requirement of 28 dB, while production errors will further degrade this parameter.
Therefore, we decided to study the conical horn.Fig. 6 shows the model of the conical double-ridge horn antenna under study, consisting of two parts: the horn itself with an aperture of the diameter c and a circular waveguide of the radius 8 mm.The ridge structure has an exponential edge shape   , dz given by (1).The ridges are beveled on the short-circuit wall side for better coordination.
According to the limits set (3), the values of VSWR and XPD within the spatial angle of 30° for the diameter of the horn 16, 20, 25 and 30 mm are calculated (Fig. 7).
According to the results of modelling, the model of the double-ridge conical antenna has a VSWR no greater than 2 in the upper frequency range and XPD no less than 25 dB in the frequency range with overlapping in the range equal to 1.8.The DF at 13 GHz for the aperture 25 мм с  corresponds to a value of 10 dBi calculated per (3).At the same time, the XPD frequency dependency (Fig. 7, a) shows that with an aperture diameter of more than 20 mm, it deteriorates significantly.This is due to the out-phase of the conical horn: for larger aperture diameters, the phase distribution in the antenna aperture will be different from the uniform, i.e. non-incoherent, distribution.Based on the results shown in Fig. 7, the required level of XPD within the spatial angle of 30° is achieved by reducing the aperture size to 16 mm.It should be noted that it is impossible for the aperture to approach the size of the round waveguide because, in this case, the minimum operating frequency of the antenna would increase unacceptably.
The required electrodynamic and design parameters can also be obtained in a double-ridge antenna developed based on an elliptical section waveguide.The elliptical shape of the waveguide eliminates the degeneration of the wave type 11 , H which is characteristic of a circular waveguide, as well as providing fixation of the polarisation plane [17].
We assumed that the location of the ridges along the small axis of the elliptical waveguide would im-prove the XPD value within the spatial angle of 30°.Fig. 8 shows a 3D model of an elliptical double-ridge antenna (1 -50-Ohm coaxial cable with a fluoroplastic insulator, terminated with SMA connector).ridge thickness 3.4 mm; slit width at the power source 1 mm; antenna length 68 mm.The conical external shape of the antenna reduces the reception of foreign radiation by its aperture compared to a cylindrical antenna.Increasing the size on the right end of the antenna added rigidity and stability to the entire structure.
In the antenna aperture plane, there is a small fluoroplastic ring, which, according to the results of the simulation, significantly reduces the level of cross-polarised radiation in the lower part of the frequency range.
Modelling of the elliptical double-ridge antenna gave the following parameters: -VSWR 2;  -XPD in a spatial angle of 30° not less than 30 dB in the overlapping frequency range 2.2; the width of the main lobe and DF meet the requirements to ensure the reception of the signal from the target in the analysis area of the reflected microwave radiation (see Figs. 2, 3).
Results.Fig. 11, a shows the layout of the manufactured double-ridge elliptical antenna.The results of the study of its electrodynamic parameters in the plane set by the angle φ (see Fig. 8) are shown in Fig. 12. XPD distribution is based on the deviation angle θ from the antenna axis symmetry (see Fig. 8).
In order to preserve the XPD level during the formation of an ensemble of receiving elliptical double-ridge antennas, it is necessary to use an absorbing material, for example, ECCOSORB VHP-2-NRL.Fig. 11 ridge antenna shows that the experimental data and the results of modelling are well correlated.The XPD value of the manufactured antenna at a frequency of 18 GHz within the spatial angle of 30° is better than 30 dB; with a decrease in the frequency, the level of discrimination increases.This is entirely consistent with the results of the simulation that allows the use of antennas in MMSSs.

Conclusion.
A double-ridge elliptical antenna was developed and manufactured, with VSWR 2  and XPD in a spatial angle of 30° not less than 28 dB in the frequency range overlapping octave.The antenna can be used in MMSSs to detect the effect of microwave radiation depolarisation by hidden dangerous objects on the human body.Structurally, the double-ridge antenna is assembled in three partsthe upper and lower halves of the elliptical waveguide and the double-ridge plate, which are joined together in one piece with screws.Although this design meets the tight design tolerances set by the simulation, it requires high production standards.

Figs. 9 ,b
10 show the frequency dependencies of VSWR, XPD within the spatial angle of 30, DP width in E-and H-plane, E BW and Н BW and DF.These results were obtained at the following elliptical antenna sizes:transverse dimensions of an elliptical waveare the major and minor semiaxes of an ellipse, respectively;