VHF/UHF Wideband Slim Monopole Antenna with Distributed Matching Structures

This paper presents a wideband monopole antenna with distributed matching structures operating on VHF/UHF bands. The antenna consists of a slim rectangular metal strip with a T-type slot etched on the upper half, two rectangular slots on the lower half, and two rectangular plates soldered with the bottom end of the main strip. The antenna is installed on a metal case which serves as ground. To achieve a wide bandwidth, an impedance matching network consisting of the distributed structures, which are composed of two metal plates and a metal thin strip, is devised and integrated with the main radiating strip. The T-type slot near the top tip of the main strip is used for the miniaturization of the antenna. The proposed antenna is fabricated and measured, and the measured results are in good agreement with the simulated results. The sizes of the proposed antenna and the case are 0.19 λ max × 0.014 λ max and 0.17 λ max × 0.1 λ max × 0.034 λ max , respectively. The operating bandwidth is from 203.7 to 516.9MHz (2.54 octaves) with the condition of VSWR ≤ 3:1. This antenna has a slim structure, a wide bandwidth covering partially VHF and UHF bands, and omnidirectional radiation patterns, so it can be the candidate for the applications of backpacked radio stations in military communication scenarios.


Introduction
Whip antennas, with slim shapes and omnidirectional radiation patterns in the horizontal plane, are the traditional antennas for backpacked radio stations [1] operating on VHF and UHF bands with wide applications in military communication, civilian rescue, and other elds [2]. e relative impedance bandwidth of the a-quarter-wavelength whip monopole antennas is narrow [3] and unable to satisfy the need for wideband communication nowadays. Cage antennas, biconical antennas, discone antennas, planar antennas, and sleeve antennas have wide bandwidths but with big sizes that are not suitable for the backpacked radio stations [4]. For example, Souny and Morlaas design a monopole antenna loaded with a cage in [5] achieving the bandwidth of 480-900 MHz but with a diameter of 0.192λ max which is relatively large. Palud et al. present a monopole loaded with a top plane covering the band of 100-3000 MHz in [6], while the diameter is 0.8λ max which is too large. Aten et al. investigate a monopole antenna loaded with a top hat with the bandwidth of 800-2400 MHz but with a diameter of 0.8λ max in [7]. Yadam et al. design an ultrawideband conical monopole antenna that covers the band from 0.5 to 3 GHz to detect partial discharges [8].
e antenna is with a big volume of 0.183λ max × 0.25λ max × 0.25λ max . Omar and Shen present a wideband monopole antenna composed of a cone loaded with two thin and short parasitic posts and a top ring that is shorted to the ground with four meandered lines [9].
is antenna achieves a bandwidth of 4.861 : 1 but with a diameter of 0.115λ max . Dzagbletey et al. present a planar monopole with a cone ground that exhibits a relatively wide bandwidth of 0.56 to 8.2 GHz [10]. With the tridentbranches feed, the monopole antenna is well matched in the low-frequency region but with a diameter of 0.187λ max . Johnson et al. present a monopole antenna loaded with the capacitive top circular patch which is designed to improve matching and pattern roundness for satellite applications on the VHF and UHF band with retractable supporting structures [11]. But the diameter of the proposed antenna is 0.1λ max . Zhang et al. propose a wideband sleeve monopole antenna without the ground plane [12]. But the overall size of the sleeve antenna is large. Zhang et al. present a monopole antenna with two sleeves achieving a bandwidth from 730 to 3880 MHz with a diameter of 0.3λ max in [13] that is too large for a backpacked radio station. Zhao [17] that covers the bandwidth of 30-520 MHz, but the lowest e ciency is 35% due to the loss of resistance. Werner and Werner present a whip monopole loaded with stubs in [18] to achieve miniature, but the bandwidth is narrow. e normal mode helix antenna has a slim size [4], but the bandwidth is narrow.  Kim et al. present a monopole composed of a cylindrical helix and an extendable rod to achieve miniaturization but with the bandwidth of 174-230 MHz that is relatively narrow [19]. Mobile phone antennas have been widely researched, and the techniques of miniaturization and bandwidth broadening are abundant but are not suitable for the applications in the VHF band because the size of mobile phones is small. So, the antennas with whip shapes, wide bandwidths, omnidirectional radiation patterns, and high e ciencies or high gains applied in the VHF/UHF band becomes an urgent need for backpacked radio stations. is paper presents a wideband slim monopole antenna. A middle slim strip in the main radiator and two rectangular plates (near the feed point) together form the matching network to achieve good impedance matching. A T-type slot in the upper half of the main radiator is used to miniaturize the size of the proposed antenna. e antenna is fabricated with copper foil with a thickness of 0.2 mm which makes the monopole antenna occupy little transverse space. e metal case on which the monopole antenna is mounted has a similar shape to an Oxford Dictionary which can not only serve as the ground of the monopole antenna but also can be a container for the relevant circuit boards, batteries, and other devices of the backpacked radio station. e monopole antenna covers the bandwidth of 225-400 MHz with horizontal omnidirectional radiation patterns which can be used in military communication applications. is paper is organized as follows. Section 2 introduces the structures of the proposed antenna. Section 3 presents the operating principle which includes three parts: (1) Performance comparison of the traditional monopole antenna and the proposed antenna with identical outer sizes. (2) e equivalent circuit of the distributed matching network of the proposed antenna. (3) Surface currents and the 3D radiation patterns. Section 4 shows the e ects of the key parameters on the performance of the proposed antenna, and explanations with Smith charts are given below the corresponding VSWR results of every parameter study. Fabrication and the measured results of the   proposed antenna are shown in Section 5. Section 6 gives a brief conclusion of this paper.

Antenna Structure
e proposed antenna is in a slim (the width of 20 mm and the length of 280 mm) and thin (the thickness of 0.2 mm) form that takes up only a little space, which is installed on a metal case served as ground, shown in Figure 1. Loaded with a middle slim rectangular strip (the width of 2 mm and length of 70 mm) and two rectangular plates (the width of 12 mm and the length of 20 mm), the proposed antenna is well matched. e T-slot etched at the upper half of the radiated strip relatively miniaturizes the size of the monopole antenna. e metal box in a form with a shape and size similar to an Oxford Dictionary serves as the ground of the proposed antenna. e speci c dimensions of the wideband monopole antenna are shown in Figure 2. Table 1 lists the values of the variables of the proposed antenna.

Performance Comparison.
To show the improvement of the VSWR bandwidth of the proposed antenna with the middle slim strip, the rectangular plates, and the T-type slot, comparisons between the traditional monopole antenna (without the slim strip, the rectangular plates, and the T-type slot and with other sizes equal to that of the proposed antenna) and the proposed antenna are presented in this part. e traditional monopole antenna is named Ant. 1, and the proposed antenna is named Ant. 2. e 3D structure simulation is by HFSS 2021R1 software. e VSWR curves of the two antennas are depicted in Figure 3. e VSWR bandwidth (VSWR ≤ 3 : 1) of Ant. 1 and Ant. 2 are 215.6-303.2 MHz (33.8%) and 203.7-516.9 MHz (86.9%), respectively. With the overall sizes of the antenna and the ground unchanged, the VSWR bandwidth of the proposed antenna is greatly improved from 33.8% to 86.9%. Ant. 1 is unmatched in the range of 300 to 500 MHz seen in Figure 4 because the part in this frequency region of the blue line is far away from the central matching point of the Smith chart. However, Ant. 2 is matched with 50 Ω in the whole frequency range from 200 to 500 MHz that the red curve in Figure 4 curls around the matching point.

Equivalent Circuit.
e operating principles of the proposed monopole antenna can be well explained with the equivalent circuit model, shown in Figure 5. e middle slim strip loaded near the feeding port serves as an inductor in  Figure 5) are shown in Figure 6. e di erences between the two curves in Figure 6 can be easily seen that the bandwidth of the Ant. 1 loaded with L 36.4 nH and C 1.7 pF is much     capacitor by tuning the size of the slim strip and the rectangular plates. Relatively good results will be obtained by attempting several combinations of di erent variables. e criterion of the optimization is that the curve in the Smith chart is in the circle of VSWR 3 : 1. e following studies of di erent variables are based on a relatively good     Table 1. Although the variables may be independent of each other, the e ects of the di erent values of one variable on the VSWR of the antenna can show that how a relatively good result is found. is part will present how the equivalent inductor, capacitor, the size of the T-type slot, and the size of the ground a ect the performance of the proposed antenna.

Equivalent
Inductor. e length (h 3 ) and the width (w 3 ) of the slim strip determine the values of the equivalent inductor. With h 3 increasing from 30 mm to 90 mm, the VSWR at 450 MHz drops from 5 : 1 to 1.2 : 1 as shown in Figure 9(a), and the points on the curves in the Smith chart shift from the lower part (0.8 − 1.8j) to the middle part (1 + 0j) as shown in Figure 9(b). Meanwhile, the VSWR at  International Journal of Antennas and Propagation 9 200 MHz drops from 5 : 1 to 2.5 : 1 shown in Figure 9, and the points on the curves in the Smith chart shift from (0.5 − 1j) to (0.4 − 0.2j) are shown in Figure 9(b). Curves in the Smith chart get tighter and shift from the lower-left to upper-right part with h 3 increasing, showing that the equivalent inductor increases at the same time.

Equivalent
Capacitor. e values of the equivalent capacitor of the rectangular plates are determined by the gaps (g 1 ) between the plates and the ground and the area of the plates (w 5 × w 6 ). e narrower the gap and the bigger the area of the plates, the higher the values of the equivalent capacitor. As shown in Figures 10-12, with smaller g 1 , bigger w 5, and w 6 (i.e. the bigger equivalent capacitor), the VSWR curves shift to the left end (i.e., the lower band), and the curves in Smith charts get tighter and focus on the matching point ( Figure 11).

T-Type Slot.
e e ects of the loaded position (h 7 ) and the opening of the slot (h 6 ) on the performance of the antenna are studied. By analyzing Figures 13 and 14, the T-type slot mostly a ects the higher frequency range and slightly causes changes in the lower frequency range. e bigger h 7 (i.e., the slot is farther away from the feeding port) and the bigger h 6 (i.e., the farther position of the opening of the T-type slot away from the feeding port) will obtain the

4.4.
Ground. e e ects of the size of the ground (the height h 1 and the width w 1 ) on the performance of the antenna are studied. e thickness w 7 is not studied because the ground is supposed not too thick to a ect the portability. e e ects of w 1 on the VSWR are shown in Figure 15(a). e width of the ground w 1 a ects the middle frequency region that the value of the VSWR is high when w 1 50 mm, while the value stays stable when w 1 is bigger than 150 mm. So, we choose w 1 150 mm. From the Smith chart in Figure 15(b), the middle part of the curve when w 1 50 mm is a little far away from other curves showing not well matched. e e ect of the height of ground h 1 on VSWR is shown in Figure 16(a). e low and middle-frequency region of the VSWR is more a ected. With h 1 getting higher, the values of VSWR are below 3 : 1 when h 1 is bigger than 250 mm. So, h 1 250 mm is chosen.
e Smith chart in Figure 16(b) is obvious that the curves tightly focus on the matching circle when h 1 is greater than 250 mm which means good matching.  Table 1 is fabricated (shown in Figure 17)  Sim.

Validation
Mea. International Journal of Antennas and Propagation shown in Figure 18(a), which is in good agreement with the simulated one. e trivial di erences are caused by manufacturing errors and interferences in the measuring environment. e measured gain curve is shown in Figure 18(b). e di erences between the measured gain and the simulated gain are less than 1 dB which can be accepted as a valid measurement. For the passive antenna with omnidirectional radiation and overall size under 0.25λ, the gain of around 2 dBi is enough for communication applications. e simulated e ciency of the antenna by CST 2018 software is shown in Figure 18(c). e radiation e ciency is almost near 1. e total e ciency is mainly determined by the return loss. e total e ciency is between 50 and 100%.
e measured H-plane radiation patterns at 200, 350, and 500 MHz of the wideband monopole antenna are presented in Figure 19 which are compared with the simulated results simultaneously. e di erences between the measured and simulated results are caused by the measuring environment, but the omnidirectional radiation in the azimuth plane of the wideband monopole can be seen. Table 2 gives the comparisons between this paper and other works. Compared with ref. [11,12], the proposed antenna has a smaller size. Compared with ref. [16,17,19], the proposed antenna has higher e ciencies. Compared with [18], the proposed antenna has wider bandwidth.

Conclusion
is paper presents a wideband monopole antenna with a slim whip-shaped form, omnidirectional radiation patterns, wide bandwidth (204-517 MHz), and high e ciency, which is suitable for the applications of backpacked radio stations. Using distributed matching structure replacing the lumped elements can eliminate the electrical breakdown of the lumped capacitors used in the matching network circuits and provide a method for designing instinctively wideband antennas without extra matching network circuits.

Data Availability
All data included in this study are available upon request to the corresponding author.