A Light-Weight Inverted Hat Antenna Composed of 12 Blade-Type-Radiators for VHF/UHF Frequency Bands Operation
Abstract— A wideband monopole antenna composed of 12 blade-type planar radiators is proposed which introduce a light-weight Body of Revolution (BOR). The antenna is based on the Inverted Hat Antenna (IHA) structure which results in omnidirectional radiation pattern in azimuth plane. The antenna is based on the Inverted Hat Antenna (IHA) whose profile is defined by 25 elliptical segments. This profile can be used to control the input impedance of the antenna. The present design incorporate the light weight benefit of planar structures and the omnidirectional radiation pattern feature of inverted hat antenna. A prototype of the antenna has been fabricated. An elaborate assembly of the antenna exhibit a 1:130 impedance bandwidth (114 MHz to 540 MHz) with VSWR below 3. Omnidirectional vertically polarized radiation in azimuth plane are also verified with simulations and measurements. A uniform realized gain in a wide frequency range at the angle of 30° below the horizon is obtained. The light weight and wide impedance bandwidth of the proposed antenna make such antenna highly suitable for airframes and ground vehicles platforms.
ideband antennas with omnidirectional radiation pattern in azimuth plane that operate in VHF/UHF frequency bands are required in many industrial and military applications. Especially in developing vehicle or UAV (Unmanned Arial Vehicle) mounted RF signal jammer systems. They provide protection against the thread of Remote Controlled Improvised Explosive Device (RCIED) and prevent the use of unauthorized communication technology. Nevertheless, in designing such antennas for airframes or ground vehicles, aperture size, height and weight of the antenna should be reduced.
Monopole antennas are very common in these applications due to their features such as simple design, low profile regarding to wavelength, proper radiation pattern in azimuth plane, reasonable cost and etc. These antennas are divided into two main categories including planar and 3-D structures. Planar structures encompass a wide range of shapes comprising square, circular and elliptical monopoles-. All of these antennas are light weight and planar structures having a wide impedance bandwidth. However, the lack of omnidirectional pattern in azimuth planes due to their asymmetric geometry shape constraint them from using in some practical applications . The 3-D structures such as discone antennas can provide omnidirectional radiation patterns in azimuth plane. Nevertheless, their height regarding the wavelength in the lowest frequency is relatively high that can limit their use in some applications.
Fig. 1. Geometry of proposed IHA composed of 12 Blade-type radiators with upper mount metal ring, three peripheral Teflon stubs and the Teflon fixture.
Many efforts have been done to improve the geometrical and electrical performance of monopole antennas -. A VHF/UHF antenna based on the conventional discone was proposed in . It was composed of a discone antenna that was flattened with additions of a cavity-backed discone, a short-circuited configuration, a two-plate top structure and a height of only
0.087 QUOTE λ
(that is the free space wavelength at the lowest operating frequency) and had a VSWR bandwidth of 76% from 200 MHz to 447 MHz.
An asymmetric double discone antenna is presented in . The antenna has all-wire structure like octopus legs for covering the lower operating frequency, and the upside-down discone antenna has conical shape playing a role as covering at higher operating frequency. Although the weight of the structure is reduced due to the use of all-wire structure, the height of the antenna would be a limiting factor in some cases.
The Inverted Hat Antenna (IHA) introduced in  is a BOR whose outer surface is composed of multiple ellipses that followed the growth rate of an exponential spiral. The IHA achieves a good vertically polarized gain at lower VHF frequencies while maintaining a desired impedance matching at higher frequencies. The proposed configuration is targeted for Single-Channel Ground-Air Radio Systems (SINGARS) as well as VHF and UHF band.
Some of the proposed structures in the literature do not have a desirable impedance bandwidth and/or radiation pattern. Others have some critical drawbacks in terms of complexity and fabrication cost. Especially in industrial applications which a large number of the products are needed.
In this letter we provide an alternative antenna which incorporate the light weight benefit of planar radiators and the omnidirectional radiation pattern feature of 3-D structures. The result is a low cost and light weight structure with ease of fabrication property. The proposed antenna is shown in Fig. 1. It consists of 12 blade-type radiators whose outer profile is shaped according to the IHA surface. The Blades arranged around a circular path which results a BOR structure. The IHA studied in  is complex, heavy weight and high cost due to the use of Computer-Numerical-Control (CNC) in fabrication process. In the current work, all of the metal components of the antenna comprising 12 blades, the upper loading ring and the ground plane have been manufactured using stacked laser-cut aluminum sheets, resulting in a relatively low cost structure. The profile of the IHA which is composed of elliptical segments is studied considering ellipses’ axes as the control parameters. A parametric analysis will show that the desired return loss and realized gain can be obtained adjusting the outer profile of the IHA and the number of blade-type radiators. A prototype of the antenna with the height of 35 cm has been fabricated. The aperture size and weight of the structure are 40cm and 2 kg, respectively. The antenna has an impedance bandwidth of 130% and omnidirectional radiation pattern. The electrical and aerodynamic properties of the proposed structure make it an appropriate alternative for airframes and ground vehicle mounting applications.
II. antenna design and implementation
The starting point of the proposed antenna is the IHA presented in  and , considering the design requirements over a flat finite ground. The objective of  was to obtain a structure with the lowest height and the smallest aperture size in a way that the gain of the antenna is greater than -10 dBi regardless of the impedance matching of the antenna. The requirement of such a structure is using wideband passive and/or active matching circuits in antenna input port. This is a practical limitation especially in the applications that antenna is derived with a wideband and high power transmitter that it can also introduce some extra losses in the transmitting path.
In this paper, our goal is to design an antenna with predetermined dimensions and inherent impedance matching in the frequency range of 110 MHz to 500 MHz with a vertically polarized omnidirectional radiation pattern. The antenna realized gain must be more than -10 dBi in the angle 30 deg below the horizon. The antenna is excited through a
coaxial cable and full wave simulations have been done with CST microwave studio. The excitation of high order propagating modes in high frequencies deteriorate the monopole like radiation pattern of the antenna to a patch like radiation pattern. This criterion limits the higher end of the operating bandwidth.
According to the given equations and formulas in  and as depicted in Fig. 2 if the outer surface of IHA is comprises from N ellipses (seven ellipses in Fig. 2), so for major radius of elliptical segments:
for n= 1, 2, …, N.
If the total width of IHA is w, by choosing
That resulting in,
For the minor radius of ellipses, the following equation is correct:
Fig. 2. (a) Configuration (cross-sectional diagram) of a seven-ellipse IHA having an aperture size of w=40cm and height of h=35cm. (b) 3-D view of an eleven-ellipse IHA.
As can be seen from the Fig. 2, the values of and simply represent the relative size of the radii to each other. They must be scaled down in order to obtain the exact size of the radii based on the predetermined values of and . So the values of andare given as:
That the scaling parameters are obtained as:
By changing the number of ellipses, the input impedance can be controlled. The parameters of 3, 7, 11, and 25 ellipses IHA structures have been extracted considering
. The simulations have been done on a
diameter ground plane.
Considering the variations of real and imaginary parts of input impedance shown in Fig. 3 through Fig. 4, the IHA with 25 ellipses has been chosen.
Fig. 3. Effect of the number of ellipses on the input impedance (real part) of the IHA.
Fig. 4. Effect of the number of ellipses on the input impedance (imaginary part) of the IHA.
The geometrical parameters of the proposed 25 ellipses IHA, including minor and major radii of each ellipse are given in Table 1.
Table 1: The geometrical parameters of the selected 25 ellipses IHA
A planar 2-D structure named BOR-Projected-Sheet is derived from the projection of 3-D IHA on the x-y plane. A various number of BOR-projected-sheets have been arranged around a circular path to form a 3-D structure. Fig. 9 (a) shows the antenna structure placing on the same ground plane. The simulated VSWR, real and imaginary parts of the input impedance for the structures with 1, 2, 4 and 6 BOR-projected-sheets are shown in Fig. 5 through Fig. 7. As can be seen, by increasing the number of BOR-projected-sheets the VSWR is decreased and for the structure with 6 planes the VSWR<3 is obtained for the frequencies above the 132 MHz. The variations of real and imaginary part of the input impedance are limited in ranges of 100 and 0 ohms, respectively. That means a better impedance matching is resulted.
Fig. 5. Effect of the number of BOR-projected-sheets on the VSWR of the proposed 3-D antenna.
Fig. 6. Effect of the number of BOR-projected-sheets on the input impedance (real part) of the antenna.
Fig. 7. Effect of the number of BOR-projected-sheets on the input impedance (imaginary part) of the antenna.
Fig. 8. The simulated azimuth plane radiation patterns at the frequency of 400 MHz (a) 1 BOR-projected-sheet (b) 2 BOR-projected-sheets (c) 4 BOR-projected-sheets (d) 6 BOR-projected-sheets.
The simulated azimuth plane radiation patterns at the frequency of 400 MHz for 4 latter structures are shown in Fig. 8. It is evident that especially for the structure with one BOR-projected-sheet, the radiation pattern is asymmetric in azimuth plane and completely analogous to the radiation pattern of planar structures. It is also clear that by increasing the number of BOR-projected-sheets, the radiation pattern approaching to omnidirectional pattern in horizontal plane that is common in 3-D structures.
In order to reduce the fabrication cost and weight of the structure, some blade-type radiators are introduced instead of BOR-projected-sheets. One edge of blades is conformed to the outer profile of the IHA. The proposed 3-D structure which composed of blade-type radiators are shown in Fig. 9 (b).
Fig. 9 (a) The Proposed Antenna composed of 6 BOR-projected-sheets. (b) The Proposed Antenna composed of 12 Blade-type radiators.
The simulated VSWR, real and imaginary parts of input impedance of the structures with 2, 4, 8 and 12 blade radiators are shown in Fig. 10 through Fig. 12 respectively. As can be seen from the results, by increasing the number of blades, the VSWR<3 is achieved for the frequencies above the 137 MHz. In addition, the variations of real part of input impedance in the vicinity of 100 ohms are drastically decreased and the variations of imaginary part in the vicinity of zero are reduced. The simulated realized gain versus frequency in the angle of 30 degrees below the horizon for the structures with 2, 4, 8 and 12 blades are given in Fig. 13. It is evident that for the structure with 12 blades, the gain more than -10 dBi is obtained for the frequencies above 75 MHz and the gain variations versus frequency in the range of 110 MHz to 500 MHz are minimized.
Fig. 10. Effect of the number of blade-type radiators on the VSWR of the proposed 3-D antenna.
Fig. 11. Effect of the number of blade-type radiators on the input impedance (real part) of the proposed 3-D antenna.
Fig. 12. Effect of the number of blade-type radiators on the input impedance (imaginary part) of the proposed 3-D antenna.
Fig. 13. Simulated realized gain versus frequency in the angle 30 degrees below the horizon for the structures with 2, 4, 8 and 12 blade-type radiators.
In order to fabricate a prototype of the antenna, one upper mount metal ring and a Teflon fixture are added to keep the blades firmly together. Also three peripheral Teflon stubs are considered to hold the structure tightly over the ground plane. The parameters of each one are optimized to minimize the impact on electrical performance of the antenna. The Computer-Aided-Drafting (CAD) of the designed antenna and the fabricated prototype are shown in Fig. 1 and Fig. 14, respectively.
Fig. 14. Fabricated prototype of the proposed antenna
The measured and simulated results of the VSWR shown in Fig. 15 that confirm each other and indicate the good impedance matching regarding VSWR<3 for frequencies above the 114 MHz. The measured VSWR is slightly lower than the simulated ones especially at lower frequencies. This is a common phenomenon in some low frequency radiators like the results presented in .
Fig. 15. Measured and simulated VSWR of the final designed antenna.
The simulated and measured elevation radiation patterns at frequencies of 200 MHz, 300 MHz, 400 MHz and 500 MHz are shown in Fig. 16 and Fig. 17. The good agreement is resulted between the measurement and simulation. The level of the cross-polarization in both simulated and measured results at all the considered frequencies is almost 26 dB smaller than maximum co-polarization. The important point is that by increasing the frequency, the maximum of radiation pattern moves toward the apex of the structure. In general, this phenomenon can limit the radiation bandwidth of such structures. Although the realized gain of the antenna has not been measured, high radiation efficiency is expected because no lossy materials are used in the fabrication process of the radiation components of the antenna.
Fig. 16. Simulated radiation patterns of the antenna in the elevation plane. Blue dashed line is the simulated co-pol component and red dotted line is the simulated cross-pol component, (a) 200MHz, (b) 300MHz, (c) 400MHz, (d) 500MHz.
Fig. 17. Measured radiation patterns of the antenna in the elevation plane. Blue dashed line is the measurement co-pol component and red dotted line is the measurement cross-pol component, (a) 200MHz, (b) 300MHz, (c) 400MHz, (d) 500MHz.
In this paper a novel light-weight and low-cost monopole antenna has been designed, fabricated and tested. The height and weight of the antenna has been limited to
0.089 QUOTE λ
and 2 kg, respectively. Also, it was shown that the antenna exhibits a monopole-like omnidirectional radiation pattern with low levels of cross polarization. It has been validated with simulation of the antenna installed on a circular ground plane with the same size as the aperture of the antenna. The light weight and wide impedance bandwidth of the proposed antenna make such antenna highly suitable for airframes or ground vehicles platforms. Measured and simulated results confirmed each other.
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