衛星地面站之小型化S頻段單脈衝追蹤天線饋入器設計

Design of a Miniaturized S-Band Monopulse Tracking Antenna Feed for a Satellite Ground Station

李昀叡(Yun-Jui Lee)

指導教授 ： 唐震寰 ； 鍾世忠

國立交通大學/電機學院/電信工程系所/博士(2019年)

摘要

本論文主旨在設計一個衛星地面站的小型化S 頻段單脈衝追蹤天線饋入器，用於追蹤低軌道衛星，並進行衛星通信。本文所提出的天線饋入器，體積小而且可以減輕重量，適合用於Cassegrain 的天線系統。此外，文中也提出追蹤電路與頻率選擇次反射面之設計，以及說明單脈衝追蹤之原理。
首先，介紹及討論一些常用在衛星方面的追蹤方法，及說明如何實現電路。單脈衝技術最簡易的形式是振幅比較法，傳統產生差場型的作法類似雷達追蹤，使用2 × 2 的號角天線陣列做相位的組合。這樣的方式通常需要處理複雜的相位電路以及控制四個號角天線的相位才能形成所需要的場型。然而，若一個圓形波導或是同軸線能夠激發出高階模態，則也可用來產生差場型。因此，多個號角天線的設計可以簡化成單一孔徑天線的設計，這樣的設計較為簡易而且輕巧。我們提出一個S 頻段單脈衝追蹤天線饋入器，包含了號角天線、TE21 追蹤耦合器、錐形波導、隔片極化器及訊號合成電路。此饋入器用於3.7 米的Cassegrain 天線系統，發射的頻段為2.025–2.12 GHz，接收/追蹤的頻段為2.2–2.3 GHz。
再者，由於雙孔的TE21 追蹤耦合器尺寸仍然太大，為了縮小尺寸及減輕重量，我們提出了小型化之S 頻段單脈衝追蹤天線饋入器，利用同軸線之結構來實現TE21 追蹤耦合器，可以省去號角天線與錐形波導之設計，使其體積不會太大且易於置放在FSS 次反射面的背後。同軸線的內導體為一圓形波導管，只允許TE11 模態通過而TE21 模態無法通過，而且同時亦能當作饋入器的天線來使用。而且此TE21 追蹤耦合器又比雙孔的TE21追蹤耦合器還要來的小，因此，整體之S 頻段單脈衝追蹤天線饋入器可以變得輕巧。同軸線的結構則可以用來接收差場型的訊號，利用兩個模態的傳播常數的不同，在外導體適當的位置饋入訊號，使TE21 同軸模態的訊號強度大於TE11 同軸模態的訊號。接近隔片極化器的一端設計為金屬牆，為了使TE11 模態的訊號不易進到同軸線裡，我們設計長度為TE11 模態的一個波長，使天線輻射的這一端等效成一個虛擬的金屬牆擋住TE11模態的訊號。接著，我們將設計之天線饋入器放置在直徑為6 m 碟型天線的焦點處進行模擬分析，得到碟型天線天線增益約為39.1 dBic，低1 dB level 的角度約為1 度，而且axial ratio 在±0.5 度內皆小於1.1 dB。
除了設計單脈衝追蹤饋入器，我們也推導了目標追蹤的原理。藉由分析TE11 和TE21波導模態在遠場的圓極化電場，可以偵測出追蹤目標的偏移角度，並修正碟型天線的方位，使其能持續追蹤LEO 衛星。並且設計目標追蹤電路，把從和及差場型得到的訊號做結合，利用類比偵測器可以得到電壓並且由ADC 做取樣，再由MCU 做數位訊號的處理及控制相移器之不同的相位。
文末，在FSS 次反射面的設計上，利用將等效的集總電感與電容分別放在基板的兩面，滿足傳輸線上並聯一個並聯形式的LC 電路，即可在特定頻率範圍內形成band-pass的特性，並且設計兩個方形loop 在out-band 的部分造成一個傳輸零點，使其特性更好。在設計模擬上，皆有符合需求的特性結果。為了要驗證模擬的正確性，我們將板材改成FR4，並且重新設計尺寸進行量測，但由於收發天線的天線增益較小、波束寬較胖，易
受到反射訊號的影響，造成量測的結果抖動劇烈，但趨勢上與模擬的結果大致吻合，間接證明了設計的準確性。

關鍵字

單脈衝追蹤天線系統 ； 開路端同軸線 ； 衛星通訊 ； TE21同軸模態 ； TE21追蹤耦合器

並列摘要

This dissertation is focused on the design of a miniaturized S-band monopulse tracking antenna feed for tracking low earth orbit satellites and communicating. The proposed antenna feed is compact and light, which is suitable for using in the Cassegrain antenna system. Moreover, designs of monopulse tracking circuits and frequency-selective subreflector are also proposed, and the principle of monopluse tracking is discussed.
In the first stage of this dissertation, several techniques to track targets are introduced and discussed. The simplest type of monopulse technique is the simultaneous lobing system, which utilizes amplitude comparison. The conventional method for creating difference patterns was similar to radar tracking through the phase combination of multiple horns, such as a 2 × 2 horn array. Such methods generally require complex phasing network arrangements and four-aperture antenna arrangements. However, a circular waveguide or a coaxial line also can create difference patterns if higher order modes can be generated. Hence, multiple horns can be reduced to only one aperture to ensure that the design is simple and compact. We propose a compact S-band monopusle tracking antenna feed using two-hole Bethe hole coupler, which is composed of a horn antenna, a TE21 tracking coupler, a waveguide taper, a septum polarizer, and signal combining circuits. The antenna feed is used in a 3.7-m Cassegrain antenna system. The uplink band of the antenna feed ranges 2.025–2.12 GHz, and the downlink/tracking band ranges 2.2–2.3 GHz.
Secondly, considering the weight of the S-band monopulse tracking antenna feed and the need for minimizing the blockage of the antenna systems, a miniaturized S-band tracking antenna feed must be designed with acceptable antenna performance. We fabricate the TE21 tracking coupler by using the structure of a coaxial line. The inner metal of the coaxial line is a circular waveguide. With an appropriate size of the aperture, the TE11 modes can exist in the circular waveguide while the TE21 waveguide mode becomes an evanescent mode. A metal cylinder is added as an outer shield to form a coaxial line for tracking targets. Because the wavelengths of the TE11 and TE21 modes in a coaxial line depend on the inside diameter of the shield, the shield is designed to be small as possible so that the difference between the wavelengths of the TE11 and TE21 modes is as large as possible. At the end of the coaxial line is a metal plate connected to the circular waveguide and the cylindrical metal shield as a short-circuited wall, the length of the coaxial line is designed to be one wavelength of the TE11 coaxial mode. It seems that there is a virtual short-circuited wall at the open end of the coaxial line to prevent the leakage of the TE11 waveguide mode into the coaxial line. To reduce the coupling of the electric fields of the TE11 coaxial mode in the probe feeds, the distance between the metal plate and probe feeds is set to be equal to a half-wavelength of the TE11 coaxial mode. Hence, there is a virtual short-circuited wall near the probe feeds for the TE11 coaxial mode but not for the TE21 coaxial mode. Then, the antenna feed is used in a 6-m Cassegrain antenna system. The simulated antenna gain of the sum patterns is about 39.1 dBic. The 1-dB-beamwidth is 1 degree and the simulated axial ratios are lower than 1.1 dB within 0.5 degree range.
Moreover, the principle of monopulse tracking is proposed to analyzing the characteristic of the CP electric fields of the TE11 waveguide mode and the TE21 coaxial mode in the far field. According to monopulse theory, the changes of the angles of tracking targets can be detected and the orientation angle of the 6-m dish antenna can be modified to align the antenna boresight with the LEO satellites. The input signals are received from sum and difference patterns and combined in a power combiner. The voltage value of the combined signal is obtained by an analog detector and then sampled by the ADC. The MCU is acted like a digital signal processor and control the phase shifter in different phases.
In the last part of the dissertation, when an equivalent inductor and a capacitor of lumped circuits printed on each side of a substrate, it is equivalent to a band-pass filter at a certain frequency. Using two square loops printed on one side of a substrate, a transmission zero is created in the out-band which improves the performance. The simulated results fit the requirements. To prove the right of our design, we change the substrate as FR4 and redesign the dimensions of the structure. Because the antenna gain of the transmitted/received antenna is small with wide antenna beamwidth, the transmitted wave is easily affected by the reflected waves in the environment. Hence, the measured results are vibrated seriously but still fit the simulated results.

並列關鍵字

monopulse tracking antenna system ； open-ended coaxial line ； satellite communication ； TE21 coaxial mode ； TE21 tracking coupler

參考文獻

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art_stallings5_wirelss5/elementLinks/stallingswireless05fig01.gif

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[3] J. S. Cook and R. Lowell, “The autotrack system,” Bell Syst. Tech. J., vol. 42, no. 4, pp. 1283–1307, July 1963.

[4] J. V. Anders, E. F. Higgins, J. L. Murray, and F. J. Schaefer, “The precision tracker,” Bell Syst. Tech. J., vol. 42, no. 4, pp. 1309–1356, July 1963.