construction and analysis performance of helical antenna operating at soundofheaven.info achieve this, principle of positive feedback was employed on the frequency. Theory of Helix Antenna Tariq Rahim School of Electronic and information, NWPU, Xian china Review on Helix Antenna 1 Introduction The helical antenna is a. Improving the Helical Antenna Performance by Changing the Pitch Angle and the Shape of Reflector Maja Škiljo1, Zoran Blažević2, Ante Jurišić3, Katarina.
|Language:||English, Spanish, Portuguese|
|Genre:||Children & Youth|
|ePub File Size:||18.63 MB|
|PDF File Size:||16.52 MB|
|Distribution:||Free* [*Regsitration Required]|
❑Monofilar axial-mode helical antennas with wire diameter of λ, λ and λ at center frequency of MHz. ❑Effect of conductor diameter on. PDF | In this overview I discussed various type of helix design and their properties. familiar loop and straight-wire antennas are merely spe- cial cases.2 Thus, a helix of fixed diameter collapses to a loop as the spacing between turns.
As the thickness of wire increases, its inductance decreases and hence the resonance frequency increases. Ilic, and A. Simulated and measured return loss plot at 1. The practical adjustment was made on helical antenna with dielectric cylinder and measured The conventional helical antenna configuration parameters in a laboratory. As noted, the current distribution can be divided into two regions.
When the helix circumference, C , is of the order of about one The charge distribution associated with the T1 mode can be seen in Figure 2b. Higher-order modes can be obtained by increasing of the ratio of circumference to wavelength and varying the pitch angle. There are two radiation modes of important practical applications, the normal mode and the axial mode. Important properties of normal-mode and axial-mode helixes are summarized below.
Fig 2 Instantaneous charge distribution for transmission modes: The maximum radiation occurs in the plane perpendicular to the helix axis, as shown in Figure 3a. In general, the radiation field of this mode is elliptically polarized in all directions. But, under particular conditions, the radiation field can be circularly polarized.
Because of its small size compared to the wavelength, the normal- mode helix has low efficiency and narrow bandwidth. The radiation field of this mode is nearly circularly polarized about the axis. The sense of polarization is related to the sense of the helix winding. The axial-mode helix possesses a number of interesting properties, including wide bandwidth and circularly polarized radiation; it has found many important applications in communication systems. Radiation patterns of helix: There are no exact solutions that describe the behavior of a helix.
However, using experimental methods and approximate analytical or numerical techniques, it is possible to study the radiation properties of this antenna with sufficient accuracy. This section briefly discusses the analysis of normal-mode and axial-mode helices. Furthermore, the helix may be modeled as a series of small loop and short dipole antennas as shown in Figure 4. The length of the short dipole is the same as the spacing between turns of the helix, while the diameter of the loop is the same as the helix diameter.
Since the helix dimensions are much smaller than wavelength, the far-field pattern is independent of the number of turns.
It is possible to calculate the total far-field of the normal-mode helix by combining the fields of a small loop and a short dipole connected in series. It is also seen from the equation that the maximum radiation occurs at 3. Thus, radiation properties and current distributions are obtained using experimental and approximate analytical or numerical methods. The current distribution of a typical axial-mode helix is shown in Figure 5. As noted, the current distribution can be divided into two regions.
Near the feed region, the current attenuates smoothly to a minimum, while the current amplitude over the remaining length of the helix is relatively uniform. Since the near-feed region is small compared to the length of the helix, the current can be approximated as a travelling wave of constant amplitude.
Using this approximation, the far-field pattern of the axial-mode helix can be analytically determined. There are two methods for the analysis of far-field pattern. In the first method, an N-turn helix is considered as an array of N elements with an element spacing equal to S.
The total field pattern is then obtained by multiplying the pattern of one turn of the helix by the array factor. The Hansen-Woodyard condition is satisfied. This condition is necessary in order to achieve agreement between the measured and calculated patterns.
In a second method, the total field is directly calculated by integrating the contributions of the current elements from one end of the helix to another. The current is assumed to be a travelling wave of constant amplitude. Approximating the geometry of normal-mode helix Figure 5. A summary of the empirical formulas for radiation characteristics is presented The terminal impedance of a helix radiating in the axial mode is nearly resistive with values between and ohms.
Smaller values, even near 50 ohms, can be obtained by properly designing the feed. Empirical expressions, based on a large number of measurements, have been derived, and they are used to determine a number of parameters. A summary of these modifications is presented below.
This structure provides significant improvement in the axial ratio over a wide bandwidth. The structure of this helix consists of 4 microstrips printed spirally around a cylindrical surface. A comparative appropriate dimensions of thickness of wire, ground plane study of variation in performance of helical antenna with size, length of wire and height of NMHA for resonance respect to variation in thickness of wire and ground plane size at 1.
By comparing various condition have also been proposed. The geometry of the helical antenna is shown in Fig. It consist of parameters viz. Mode I. Helical antenna is a 3-D structure, which can be considered as combination of linear and loop antennas.
NMHA has advantages of reduced height, ease of installation and good efficiency. The disadvantages are smaller bandwidth . Helical antenna is a resonating structure and many techniques have been reported in literature for construction of NMHA . A helix has greater field concentration than the corresponding monopole due to its physically shorter structure .
In , a pseudo-normal helical antenna has been designed for use with deeply implanted wireless sensors. The biggest drawback of the NMHA is the low impedance, which necessitates the use of impedance matching network. Several methods for impedance matching have been reported. In , a small profile Fig.
Simulations have been performed using different values of this parameters. The values of spacing S , the diameter of the helix D and the number of turns are then fixed to 0. The length of one turn L0 is 0. The ground plane acts as a wave launching device and its shape and size are important parameters on which the performance of a helical antenna, like gain, impedance, beam width, radiation pattern, etc.
The conductor also affects the performance of helical antenna. Effect of variation of ground plane radius on a Return loss and b III. In this section, the design of the proposed NMHA at 1. The simulated return loss and input impedance plots are compared for different values of ground plane radius rg and radius of wire rwire.
Effect of variation of ground plane radius on normal mode helical antenna The simulated return loss and input impedance plots are shown in Figs. The better impedance matching has been obtained for ground plane radius rg value of 5. It shows inductive nature when ground plane radius rg increases and capacitive nature when ground plane radius rg decreases as observed from Fig.
A bandwidth of 8. It is observed from Fig.
Effect of variation of thickness of wire on normal mode helical antenna The simulated return loss and input impedance plots for different values of wire radius rwire are shown in Fig 4. As the thickness of wire increases, its inductance decreases and hence the resonance frequency increases.
Effect of thickness of wire on percentage bandwidth It has been observed that for ground plane radius of 5. Bandwidth of 8. It is also observed from Fig. The antenna is fabricated using a copper wire of 3.