Parametric Study of UWB Biconical Antenna for Wide-band Applications

Th is paper presents Ansoft HFSS based parametric analysis and design of UWB b icone antenna. The design frequency of proposed antenna is 2.4 GHz. Th is antenna covers the 1.3 ~ 17.5 GHz frequency band with Return loss less than -10 dB. The bandwidth has been achieved by making a simple and easy modification in the design parameter of the bicone antenna. The voltage standing wave ratio (VSW R) less than 2 within the frequency range of 1.3 GHz ~ 17.5 GHz of this antenna is noticed. This antenna can be used in S (2 GHz to 4 GHz), C (4 GHz to 8 GHz), ext. C (3.4 GHz to 6.725 GHz) and X (8 GHz to 12 GHz) band frequencies and all three frequency band of industrial, scientific and medical (ISM), which are reserved internationally for the use of radio frequency (RF) energy for purposes other than communications. Examples of applications in these bands include radio-frequency process heating, microwave ovens, and medical diathermy machines.


Introduction
Wireless commun ication is the fastest growing field of communicat ion technology which has captured the attention of social life in the present scenario. For the successful implementation of any wireless communication system, the antenna technology plays one of the important roles. A proper design of the antennas can reduce the system requirements and improve the overall system performance. An overview of the frequency band allocated for modern wireless communication standards are summarized in Table  1.
Ultra Wideband (UWB) is an emerging technology for future short-range wireless communications with high data rates as well as radar and geolocation. A land mark patent in UWB co mmun icat ions was submitted by Ross in 1973. However, it was in 1989 that the term "Ultra Wideband" appeared in a publication of depart ment of defence in the United States (U.S.) and the first patent with the exact phrase "UWB antenna" was filed on behalf of Hughes in 1993. To guarantee the coexistence of UW B with other communicat ion standards, the authorized transmitted power is always very lo w wh ich limits the develop ment of UWB communicat ion systems with very high data rates and/or the coverage of larger distances. The association of Multiple Input Multip le Outputs (MIMO) systems (wh ich explo it rich scattering environ ments by the use of mu ltiple antennas) with UWB technology is more and more studied.
It seems to be a very potential approach for enhancing capacity, increasing range, raising lin k reliab ility and improving interference cancellation. There are two criteria available to identify an antenna to be considered as an Ultra Wide Band (UWB) antenna. One defin ition requires a UWB antenna to have a fractional BW greater than 0.25 [1]. An alternate and more recent defin ition p laces the limit at 0.2 [2]. That is given by equation 1: Where ℎ is the upper end frequency and is lower end frequency of the antenna operational band.
There are three classes of UWB antennas based on their applications. a. First is the "DC-to-Daylight" class having the maximal bandwidth.
b. Second is the "multinarrow band" class-receiving or detecting narrowband signals across broad swaths of frequencies.
c. Third the "modern" UWB antennas-having 3: 1 bandwidth. A modern UW B antenna must be well behaved and consistent across the operation bandwidth of antenna [3].

Biconical Antenna
A biconical antenna consists of an arrangement of two conical conductors, which is driven by potential, charge, or an alternating magnetic field at the vertex. The conductors have a common axis and vertex. The two cones face in opposite directions. Biconical antennas are broadband dipole antennas, typically exh ibit ing a bandwidth of 3 octaves or more .
Further, when the wireless systems that are potential candidates for cognitive rad io are considered, UWB seems to be one of the tempting choices because it has an inherent potential to fulfil some of the key requirements of cognitive radio. These requirements include no spurious interference to licensed systems, adjustable pulse shape, bandwidth, transmitted power, support of various throughputs, provision of adaptive multip le access, and security of informat ion. However, it is not claimed that a cognitive wireless system using only the UWB technology can handle all the requirements of an ideal cognit ive radio. Advances in reconfigurability of RF front-ends, particularly reconfigurable antennas, afford a new "hardware" dimension for optimizing the performance o f wireless communication systems. Recent development in co mmun ication, surveillance and Electronics Support Measurer (ESM) demands antennas with very wide bandwidth and omni radiation pattern. Broadband antenna has received much attention for application in wireless communicat ion systems, listed in Table 1. Several Antennas have already been reported [4], to suit the above mentioned requirement. Among these antennas, one of the most useful omnid irectional antennas is the biconical horn [5]- [6].
Antenna engineers discovered that, starting fro m a dipole or monopole antenna, thickening the arms results in an increased bandwidth. Thus, for a thick dipole o r monopole antenna, the current distribution is no longer sinusoidal and where this phenomenon hardly affects the radiation pattern of the antenna, therefore this strongly influences the input impedance too. This band widening effect is even mo re severe if the thick dipole takes the shape of a biconical antenna. The biconical antenna evolved towards a single cone which presents a limited well matched impedance bandwidth but a stable phase centre within the bandwidth. Another evolution of the biconical antenna is the bow-t ie antenna, the flat version of the biconical antenna.
Several techniques have also been proposed to improve the antenna bandwidth, for examp le, the use of a bevelling plate, a double feed or an asymmetrical feed arrangement, a trident-shaped feeding strip etc. Ho wever, they are not p lanar structures because their ground planes are perpendicular to the radiators. This drawback limits their practical applications due to their large area.
Biconical antenna was first introduced by Schelkunoff [7]. In theory, both the input impedance and radiation pattern of infinite biconical antenna have nothing to do with the frequency characteristics, As regards to finite biconical antenna, both the input impedance and radiation pattern change as the frequency changes. A biconical antenna is used in a system that requires 360 degree coverage in the azimuthal p lane with a part icular coverage in the elevation plane [8]. Due to the frequency independent nature of its construction, the biconical antenna is well-suited for use in ultra -wide band systems. The biconical antenna, consisting of two linear cones, excited symmetrically at the apices [9].
In Biconical (or " Bicon") antenna is a broadband radiator, however its poor efficiency at low frequencies, results in low field strengths when compared to the input power. It may behave either an omnid irectional or d irectional antenna if it is kept vertical or horizontal respectively.
The impedance of the feeding point of b iconical antenna is usually chosen to be 50 Ω, however when the vertical angle of the conical geometry is enlarged, impedance becomes small. And it beco mes a dipole aerial if vert ical angle reduces it to the utmost limit, and the impedance range is found to be about 73Ω to 50Ω. Hence the Biconical antennas are often used in electro magnetic interference (EM I) testing either for immunity testing, or emissions testing from 25-200 MHz (Co mmercial specifications) or 30-200 M Hz (Military applications).

The Proposed Bicone Antenna
Biconical antennas were originally intended for use over the frequency range 20 MHz to 200 MHz. The first designs had a poor return loss, typically less than 4 d B over most of the frequency range. The antenna shown in Figure 1 is composed of two symmetric cone-like structures. The design parameter like outer and inner radius of the cone are 10cm and 0.51cm respectively, while the height of the cone is 6.5cm.There is a port between the two cone whose gap is 0.5cm and width is also 0.5cm. The return loss versus frequency curve corresponding to this antenna are plotted in Figure 2.The antenna is able to direct the beam in horizontal direction.
In present research a biconical antenna is developed with symmetric conical d iscs. The antenna is simulated with Ansoft HFSS simu lator. The antenna is matched over a very large bandwidth (1.3GHz to 17.5GHz). Antenna performances are generally altered by the change in design structures. We optimize the performances by changing the design parameters. Figure 3, Figure 4 and Figure 5 shows the optimization considering cone inner radius, while Figure 6 to Figure 9 shows optimization with cone outer radius. Optimization with cone height is shown in Figure 10, Figure  11 and Figure 12. Port gap and port width optimizat ion are shown in Figure 13 to Figure 15. The results of optimized biconical antenna are shown in Figure 16, Figure 17 and Figure 18. Table 2 shows the optimized parameter of a bicone antenna.

Conclusions
In this paper, a novel broadband biconical antenna with symmetric conical discs suitable for wireless communication systems is designed. The results of this antenna are impressive due to its improved bandwidth (1.3 GHz-17.5 GHz) having Return loss < -10 d B and VSW R < 2. Ho wever available literatures indicate that no such bicone antennas having this much BW was proposed earlier [10][11][12][13][14][15]. If a bicone transmits to a receiving TEM horn, then the induced current in the horn will be exact ly identical to the driving-point voltage of the bicone. Such observations can have very important ramificat ions in broadband high-speed informat ion transmission. The broadband characteristics of the biconical antenna make it a good choice for making sweep measurements and for automated measurement systems. It is also used in electro magnetic interference (EMI) testing either for immunity testing, or emissions testing.