Muhammad Amin

Effect of helix turn angle on the performance of a half wavelength quadrifilar antenna

The impedance and radiation pattern parameters of a /spl lambda//2 quadrifilar helix antenna (QHA) with turn angles in the range 0/spl deg/ to 235/spl deg/ are analyzed. It is shown that by selecting the helix turn angle to satisfy the minimum bandwidth and beamwidth requirements, an improved electrical performance and a reduction in the physical size of the antenna is obtained. This is demonstrated by comparing the performance of a conventional half turn QHA with structures having a smaller pitch length. The computed results are validated by experimental data at L-band.

Mechanically Tunable Multiband Compact Quadrifilar Helix Antenna With Dual Mode Operation

We show that by introducing a gap at the center of the helical sections (where the current is minimum) of a lambda/2 quadrifilar helix antenna (QHA) and varying the axial length and radial gap between the overlapping volutes, the antenna gives a 28% impedance bandwidth which is nine times the bandwidth of a conventional QHA. A 16% bandwidth with a front to back ratio of ges 14 dB is achievable with 5-14% reduction in the size of the QHA. The structure can yield a monopole radiation pattern suitable for terrestrial applications or a hemispherical pattern suitable for satellite use. The simulation results are validated by measurements at L-band.

Single Feed Low Profile Omnidirectional Antenna with Slant 45

It is shown that a side-fed bifilar helix antenna with a single feed, can generate a slant 45 linearly polarized omnidirectional toroidal pattern. The antenna has a low profile and does not require a ground plane. The bifilar helix antenna provides slant 45 polarization over a solid angle of almost steradians as compared to a crossed dipole which generates a tilted 45 linearly polarized pattern only over a solid angle of steradians. The computed results are validated by experimental data.

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A side-fed bifilar is shown to generate a similar radiation pattern as a dipole antenna, but the structure has a significantly reduced axial length. Simulated and measured results show that the helix turn angle can be used to control the ratio of the orthogonal linear field components and the input impedance.

Linear Polarization

In this work, we show a new method which can directly encode an I (in-phase) and Q (quadrature-phase) baseband data stream onto a circularly polarized (CP) microwave signal. Here I and Q components are fed directly into a two port CP antenna in order to yield right hand circular polarized (RHCP) or left hand circular polarized (LHCP) waveforms each encoded with zero or 180deg phase shift in order to effect QPSK modulation. The CP signal from such an antenna will have deterministic unequal amplitude and phase shifts at different far field spatial locations which if not compensated will degrade the bit-error-rate (BER) performance of the receiver. Simulation and experimental results show the BER performance of the receiver when these two orthogonal CP components have spatially induced phase differences or amplitude imbalances between them. The technique described allows a factor two reduction over the normal QPSK requirement by virtue of the polarization diversity inherent in the scheme.

Side-fed bifilar helix antenna

In this paper we present the results of a study to quantify the mutual coupling between the two bifilars of a 1/2 turn 1/2 wavelength quadrifilar helix antenna (QHA). A new circuit model approach is used which is suitable for computing the impedance when the structure is fed by a single voltage source as in the case of a self phasing QHA. The validity of the circuit model is first confirmed by comparisons with simulated results obtained from the literature for various configurations of parallel fed dipoles.

The Effect of Spatial Axial Ratio Variation on QPSK Modulation Encoded Using Orthogonal Circularly Polarized Signals

In this paper mutual coupling effects between coplanar crossed skew dipoles have been investigated both experimentally and theoretically. The crosspolarisation, resonant frequency, bandwidth and mutual impedance are shown to vary with the angle between the dipoles and their relative length. A new approach has been used to calculate the mutual impedance from the measured or simulated value of the driving point impedance of the coupled crossed dipoles and the self-impedance of the individual elements. The results from the computer model have been compared with measurements and are shown to be in good agreement.