Combined Single-Layer K-Band Transmitarray and Beamforming S-Band Antenna Array for Satcom

In this paper, a dual-band antenna at 4.5 GHz and 25 GHz is presented. The antenna is a low-frequency phased patch antenna array combined with a high-frequency transmitarray with a fixed beam. The low-frequency patch antenna array and the high-frequency transmitarray feed share the same aperture area. The high-frequency transmitarray surface only uses a single substrate layer and is electrically transparent to the low-frequency phased array with beamforming. The antenna is measured to achieve an impedance bandwidth of 350 MHz and 3 GHz, and a gain of 15.4 dBi and 23.5 dBi, at S- and K-band respectively. With an impressive frequency-ratio of 5.55 and an aperture area of only 12x12cm the antenna achieves an aperture efficiency of 66% and 15% in the two bands respectively. Additionally, the low-frequency beamforming capabilities (with the existence of transmitarray surface) are measured and the antenna is shown to have a 60-degree scanning range with only a 0.22 dB gain drop-off.


I. INTRODUCTION
A NTENNAS can be designed with different features and advantages. Some features and benefits come at the expense of reduced antenna performance in another area. Different use-cases and applications set different requirements for the performance of the antenna [1], [2], [3], [4], [5], [6], [7], [8].
For this work, a very specific use-case is considered. The antenna should utilize commonly used satellite frequency bands. Thus, dual S-and K-band frequency operation is chosen. The antenna should have low-frequency beamforming capabilities while simultaneously maintaining a sufficient high-frequency gain. The purpose of these requirements is for the antenna to be used in a nano-satellite constellation. The high-frequency band is used to establish a high-speed connection to neighboring satellites in the constellation. The low-frequency antenna will utilize beamforming to maintain a communication link with Earth. Reflectarray and transmitarray antennas are often reported in the literature to yield a high gain [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. For space applications, multi-part antennas such as reflectarray and transmitarray antennas often require a deployment mechanism. Between the two options, a transmitarray might be the better option, as the feeding source could be installed on the stationary part of the antenna. The installation of the antenna unto a satellite is a complex and very application-specific task. The antenna is envisioned to be installed on one of then sides of the satellite. No further discussion about antenna installation will be conducted. None of the antennas reported in the state-of-the-art literature have shown the desired antenna performance characteristics of both simultaneously having dual-band operation with a high frequency-ration and low-frequency beamforming capabilities. Some antennas reported in the literature satisfies parts of the requirements, but unlike the antenna presented in this paper none of them covers all the listed requirements simultaneously [16], [18], [22], [24].
In [16] a dual-band transmitarray is presented. However, it is designed for k-and ka-band and could not easily be scaled to a larger frequency-ratio. The antenna presented in [18] is a dual-band antenna that has the same issue. Its frequency-ratio is very low as it is designed for two k-band frequencies. Neither the antenna from [16] nor [18] have any beamforming capabilities. In [22] a dual-band antenna with beam-steering is presented. Ignoring the low frequencyratio the antenna still has some disadvantages. To beam-steer, the transmission surface has to be physically moved. But, since the surface also contains the unit elements for the other frequency band the beam-steering cannot be performed without affecting the opposite band negatively. Paper [24] presents a beam-steering method that used switched to realize a few predefined beam directions. Even with this single band antenna design realizing wide low loss scanning would be troublesome.
In this paper, an antenna design with the described features will be presented. The antenna is a dual S-& K-band antenna with low-frequency beamforming capabilities. Dualband functionality is achieved by combining a low-frequency phased patch antenna array with a high-frequency transmitarray in the same aperture area. Compared with antennas reported in the state-of-the-art, the antenna of this paper is unique. It enables low-frequency beamforming with an dual-band antenna that has a very high frequency-ratio. It achieves this without significantly sacrificing the radiation performance in either of the two bands. To realize good low-frequency beamforming performance the high-frequency transmitarray surface needs to be electrically transparent at the lower frequency. In this paper, good transparency is achieved with the designed single-layer transmitarray unit element.

II. ANTENNA CONFIGURATION
The fabricated antenna prototype is seen in Fig. 1. The proposed antenna has two parts made with a total of four different layers. The bottom part consists of a single 3 mm thick layer of Polypropylene (PP) material placed between two Rogers RO4003C layers both with a thickness of 0.812 mm. The top part is a single layer of RO4003 with a thickness of 1.524 mm. The substrate layers have a permittivity of 3.55, and a loss tangent of 0.027. The PP material has a permittivity of 2.245 and a loss tangent of 0.002. All layers have the same size. The high-frequency unit elements are contained within a 120x120 mm area but the layers have been enlarged by 8 mm on the two opposite sides of the board to allow for six metal mounting beams to be installed. The purpose of the metal beams is to mechanically hold the top surface securely and centered above the lower part. The separation between the two parts is 40 mm.
The bottom part houses the low-frequency patch antenna array and the high-frequency transmitarray feed. The top part houses the high-frequency transmitarray surface. The lowfrequency patch antenna array has eight elements in a 3-by-3 configuration with the center element missing. The center element is removed to accommodate the high-frequency transmitarray feed. The low-frequency patch antenna elements has a center-to-center separation of 35 mm. This  distance is calculated using d = λ 1 + sin(θ ) . The selected distance aims at a scanning range of ±60 • without grating lobes. Fig. 2 shows two cross-section pictures of the bottom part of the proposed antenna. Fig. 2(a) shows the feed of the low-frequency part. The low-frequency patch antenna elements are 17.75 mm. They are fed with SMA type connectors from the bottom of the three layers. The connector pins pass through a 4.15 mm opening in the ground plane and are soldered to the patches 4.80 mm from the patch center Fig. 2(b) shows the feed of the high-frequency part. The high-frequency feed antenna element is 2.75 mm. It is fed with an MMPX type connector from the bottom of the topmost of the three layers. The connector is soldered to a circular pad with a diameter of 0.6 mm. A VIA that passes through a 1.25 mm opening in the ground plane connects the pad to the patches 0.75 mm from the patch center. Around the high-frequency patch, a grounded wall with 16 VIAs is used to shield the patch antenna from surface waves. The bottom two layers have a square cutaway with a size of 18 mm. In the cut-away, an aluminum grounded guard with a 2 mm thickness is inserted to ensure a connected and continuous ground for the high-frequency feed.  The top layer houses a large number of high-frequency unit elements for the high-frequency transmitarray surface. Fig. 3 shows the simulation model of the high-frequency transmitarray unit cell. Each unit element is a copper plus shape that is printed on both sides of the substrate. The pluses on the two opposite sides are connected with four pins. The four pins increase the coupling between the plusshapes which reduces the transmission loss. The pins all have a diameter of 0.3 mm and they are placed symmetrically in the plus shape 0.5 mm from the center of the plus shapes. The desired phase distribution is generated by tuning the length of the plus-shaped unit elements in the range from 1.3 mm to 5.9 mm. The elements are arranged in a square grid with a 6 mm spacing. Fig. 4 shows the simulated phase and magnitude response of the unit element at the two center frequencies for different element sizes and incident angles. The unit element is designed to have a low loss and Fig. 4(a) shows that the unit element has a loss of less than 2.5 dB for both frequency bands. Even for different low-frequency incident angles, the loss remains low. As seen from Fig. 4(b) the unit element achieves a phase range of 330 • at 25 GHz. Additionally, for different incident angles at a frequency of 4.5 GHz the unit element has a maximum phase shift of 7 • . Fig. 1 shows the fabricated prototype antenna. In this section, the measured performance of the antenna prototype will be presented in a comparison with four simulated antennas. The simulated antennas used for the comparison are the antennas seen in Fig. 5.

III. ANTENNA PROTOTYPE MEASUREMENT AND EVALUATION
The eight low-frequency antenna ports are measured individually in an anechoic chamber. While measuring one port the other ports are terminated in a matched load. After the measurement, MATLAB is used to compute a combined lowfrequency radiation pattern. The combined radiation pattern is computed as the phased and weighted superposition of the eight individual radiation pattern measurements.
A comparison between the measured and simulated lowfrequency impedance match of the prototype antenna is seen in Fig. 6. It is seen that the fabricated prototype is shifted in frequency but still maintains a wide impedance bandwidth of 350 MHz. The shift is likely caused by a slight discrepancy in the dialectic constant of the substrate material. Fig. 7 shows the boresight realized gain of the lowfrequency band. Fig. 8 shows the theta slice of the radiation pattern when beamforming with the low-frequency antenna part. Both figures are a comparison between the measured performance of the antenna prototype antenna and  the simulated performance of the reference antennas seen in Fig. 5.
In Fig. 7 it is seen that the reference antenna with a multi-layer transmitarray surface is significantly worse than the proposed antenna. This supports the claim that the designed single-layer unit element is more transparent at lower frequencies such as 4.5 GHz than the multi-layered unit elements typically found in the literature. The frequencydependent gain response seen in Fig. 7 shows that the measurement is well-matched with the simulation. Fig. 8 shows the measured and simulated low-frequency beamforming performance of the prototype antenna in comparison with the reference antenna seen in Fig. 5(a). Because of the good low-frequency transparency of the designed transmitarray unit elements, the beamforming performance of the low-frequency antenna part is not negatively affected by the presence of the high-frequency transmitarray surface. The 60-degree scanning range gain drop-off is measured at only 0.22 dB for the antenna prototype.
Both Fig. 7 and Fig. 8 show that the gain of the measured antenna is very similar to the gain of both the simulated prototype and low-frequency only reference antenna. Thus it is concluded that the presence of the proposed transmitarray surface has no negative impact on the low-frequency gain characteristics.
A comparison between the measured and simulated highfrequency S1,1-Parameter of the prototype antenna is seen in Fig. 9. The impedance match of the fabricated prototype exceeds the expectation from the simulations, as the impedance bandwidth is more than 3 GHz. Fig. 10 shows the boresight realized gain of the highfrequency band. The figure is a comparison between the measured performance of the antenna prototype antenna and the simulated performance of the reference antennas seen in Fig. 5. Since the figure shows that all the simulation curves are almost identical it is concluded that the addition of the low-frequency antenna elements does not negatively affect the performance of the high-frequency antenna part. And, that the designed single-layer unit element type is on par with the multi-layered element type. When comparing the curve of the simulated and the measured antenna prototype it is seen that the measured frequency-dependent gain seems to be slightly shifted to a higher frequency. This is consistent with the observations from the S-parameter measurements. The overall measured gain is observed to be slightly higher in the measurement compared with the simulations.
Both the low-frequency and the high-frequency gain curves show a very similar frequency shift. Additionally, the S-parameters of both the low-/ and high-frequency parts of the prototype antenna also indicate a slight frequency shift. The consistency of the frequency shift further indicates that it might be caused by a slight discrepancy in the dialectic constant of the substrate. Because of the frequency shift, the high-frequency part of the antenna can be observed to achieve a slightly higher overall gain in Fig. 10. This higher gain is caused by the fact that the gain of the feeding horn is slightly increasing with frequency. Fig. 11 and Fig. 12 shows the measured radiation pattern of the prototype antenna. The measured gain and radiation pattern shape are in very good agreement with the expectation from the simulation. Fig. 11(a) and 12(a) shows that low-frequency antenna part is able to achieve a realized gain of 14.31 dBi at 4.5 GHz. Fig. 11(b) and Fig. 12(b) shows that the transmitarray part of the prototype antenna achieves a realized gain of 22.64 dBi at 25 GHz.
The mutual and inner element coupling was not measured. However, Fig. 13 shows how the antenna prototype is simulated to have a low coupling between the two frequency bands of less than −38 dB and a mutual coupling between the low-frequency elements of less than −14 dB. Table 1 shows a compression between the antenna presented in this work and various other antennas reported in the stateof-the-art.

IV. STATE OF THE ART COMPARISON
The proposed prototype antenna achieves dual-band operation with a high-frequency ratio and the antenna also achieves low-frequency beam-forming. The novelty of the antenna is that it achieves these two performance characteristics without significantly sacrificing the radiation performance in either of the two bands. The proposed antenna has a slightly lower high-frequency aperture efficiency, but the high-frequency radiation performance is still deemed to be very competitive.

V. CONCLUSION
A shared aperture dual-band transmitarray and patch antenna array for S-and K-band with beamforming capabilities has been successfully designed. A prototype antenna is measured to have a high similarity to the simulated results. The measurement shows the antenna to achieve impedance bandwidths of 350 MHz and 3 GHz, and a realized gains of 14.31 dBi and 22.64 dBi, at 4.5 GHz and 25. GHz, respectively. Additionally, the prototype antenna is shown to only have a 0.22 dB gain drop-off in a 60-degree beam scanning range. The design and unique features of the presented antenna make it interesting for many new and different use-cases and applications including satellite communication. He has also worked as a Consultant for developments of more than 100 antennas for mobile terminals, including the first internal antenna for mobile phones, in 1994, with lowest SAR, first internal triple-band antenna, in 1998, with low SAR and high TRP and TIS, and lately various multiantenna systems rated as the most efficient on the market. He has worked most of the time with joint university and industry projects and have received more than 21 M$ indirect research funding. Since 1993, he has been with Aalborg University, where he is currently a Full Professor, heading the Antennas, Propagation and Millimeter-Wave Systems Laboratory with 25 researchers. He is also the Head of the Doctoral School on Wireless Communication with some 40 Ph.D. students enrolled. He is currently the Project Leader of the RANGE Project with a total budget of over $8 M investigating high performance centimetre/millimeter-wave antennas for 5G mobile phones. He has been one of the pioneers in establishing over-the-air measurement systems. The measurement technique is now well established for mobile terminals with single antennas and he was chairing the various COST groups with liaison to 3GPP and CTIA for over-the-air test of MIMO terminals. He is currently involved in MIMO OTA measurement. He has published more than 500 peer reviewed papers, six books, 12 book chapters, and holds over 50 patents. His research interests include radio communication for mobile terminals, especially small antennas, diversity systems, propagation, and biological effects.