Investigation of a Multi-Waveguide Fed Horn Antenna

A horn antenna fed by a linear array of 4 H-plane coupled rectangular waveguides is presented. The antenna was simulated, constructed and measured. The gain of this horn is higher by more than 3 dB than the gain of conventional horn with the same height. Good agreement between simulation and measurement was achieved.


Introduction
High gain, high power, flat antennas are needed for many applications, especially as airborne radar antennas. Slot antennas [1] can be used for these applications, however their bandwidth is small. Another alternative is the box-horn array [2][3], where the grating lobe is suppressed by the null of the box-horn. A linear array of pyramidal horns with a slant polarization is proposed by [4]. Here we propose a simple, relatively flat antenna, which is a compromise between a conventional horn antenna and a waveguide array, which can be designed for high gain, high power, low sidelobe level, and scan ability.
The structure of the paper is as follows: in chapter 2 we describe the geometry of the antenna. Chapter 3 presents simulation results, and a gain comparison between the proposed antenna and a conventional horn antenna, In chapter 4 measured results are shown. Finally chapter 5 is devoted to conclusions and discussions. The inner dimensions of the waveguides are 60 x 30 mm, and the aperture of the horn is 530 x 160 mm. The height of the horn surface is 100 mm and the length of the input waveguides is 50 mm.

Simulation of the Proposed Antenna
The simulated antenna scheme is shown in fig. 2    It is shown that the return losses at the ports are low, as well as the mutual coupling between neighbor ports, where port 1 is a side port and port 2 is next to port 1. A 3-dimensional radiation plot at 3.75 GHz is shown in figure 4, where the relative voltage weights are (0.7:1:1:0.7) with equal phases, for getting approximately 20 dB sidelobe level. E-plane and H-plane patterns at 3.75 GHz are shown in fig. 5. A summary of the radiation pattern results is given in table 1.

Simulated Gain Comparison
A comparison between the gains of the proposed antenna, optimized to maximal gain at 3.75 GHz, with aperture dimensions a 1 = 406 mm, b 1 = 156 mm, and the gain of a conventional horn, optimized to maximal gain at 3.75 GHz, with aperture dimensions a 1 = 163 mm, b 1 = 160 mm, is shown in figure 6. In both cases the height of the horn surfaces is 100 mm. It is seen that the gain of the proposed antenna is higher by more than 3 dB than that of the conventional horn as was expected. The aperture efficiency defined as H ap = D O 2 / 4 S A p [1], where D is the directivity, Othe wavelength, and A p is the aperture area, are 62% for the conventional horn, and 56% for the proposed optimized antenna, checked at 3.75 GHz.

Measurement
A picture of the proposed antenna is shown in fig. 7. The antenna is fed by an external 1:4 divider, where the simulated U-shaped transitions inside each waveguide are shown in fig.  8.
The measured return loss of the antenna is shown in fig. 9.  The measured H-plane radiation pattern of the antenna is shown in fig. 11 and the measured E-plane radiation pattern of the antenna is shown in fig. 12. The measured gain of the antenna was 16.5 dBi at 3.75 GHz. A comparison between simulated and measured results is presented in table 2

Conclusions
A horn antenna fed by 4 H-plane coupled rectangular waveguides was simulated and measured. It is seen that the agreement between simulations and measurements is good. This type of antennas can be used as high gain, high power, flat antennas, and can be used also for calibration. The waveguide array can be chosen as a linear or two-dimen-sional array, hence a great advantage here is the flexibility in the dimensions of the antenna. However, an efficient and compact build-in divider is required.