QUADRIFILAR HELIX ANTENNA FOR WEATHER SATELLITE RECEPTION

The study designed and implemented quadrifilar helix antenna (QHA) for weather satellite signal reception. The antenna design and optimization were done by varying the element diameter, the radial lengths, the axial lengths, and element materials on adopted model. The simulated QHA has far-field radiation pattern in the upper hemisphere with maximum gain of 4.14dBi at 0 and omnidirectional coverage, half-power beamwidth of 140, bandwidth of 6.5MHz, and VSWR of 1.13. The implemented QHA has major lobe in conformity with the simulated QHA with maximum gain of 10.75dB at 0, and half-power beamwidth of 104.


INTRODUCTION
Polarization of an antenna is related to the orientations of electric fields radiated by the antenna. The Earth station equipped with a fixed receiving antenna would not be able to maintain polarization alignment with the low-Earthorbit satellites (LEOs) if the satellite antenna use linear polarization because of its ever-changing position. The circularly polarized (CP) antenna provides a solution to this problem and hence suitable for space communications [1]. CP antenna can reduce "Faraday rotation" effect due to ionosphere, the Faraday rotation effect causes about 3dB or more signal loss if linearly polarized (LP) signals are employed [2]. The CP antenna is very effective in combating multi-path interference or fading [3,4]. Another advantage of CP antenna is that no strict orientation between transmitting and receiving antenna is required, also the strength of the received signals is fairly constant regardless of the antenna orientation with CP. Quadrifilar Helix Antenna (QHA) provides circular polarization and exhibits radiation pattern in the entire hemisphere. The radiation pattern is omnidirectional in the plane perpendicular to its main axis and thus provides fade-free reception. The basic form of the QHA was developed by Dr. C.C. Kilgus [5][6][7]. It usually consists of four half-turn helices (two bifilar helicals) equally spaced around the circumference of a common cylinder, the two bifilar helicals are arranged in a mutually orthogonal relationship along the common axis as in Figure 1. Kilgus showed that the resonant (element length = λ/2), 1/2-turn, two bifilar helices, antiphased-fed, radiate a cardiodshaped, circularly polarized pattern when the diameter is 0.18λ and the axial length is 0.27λ.
The variables and parameters used in the current and field analysis of the QHA are defined in Figure 1. The fields of the radials and the fields of the helical portions are evaluated independently and the summed. The assumed current distribution is sinusoidal with maxima at the feed and the distal end. (2) For each element of the helix, the current magnitudes are: For elements 1 and 2: For elements 3 and 4: The phase term for element 1 is: ′ cos = ′ ̅̅̅ . ̅ = 0 cos sin cos + 0 sin sin sin + 2 . cos Let: Then for the helical portion of element 1 is given by: ). cos( − ) exp[ ( 0 cos sin cos + 0 sin sin sin + 2 . cos )] Similarly, the field of element 2 is: ). cos( − ) exp[ (− 0 cos sin cos − 0 sin sin sin + 2 . cos )] Elements 3 and 4 (the second bifilar helix) are fed in phase quadrature, to elements 1 and 2, respectively. The fields are:

Fields of the Radials
If the current on the radials is approximated by a uniform distribution, the following simplified solutions result:

ANTENNA DESIGN AND IMPLEMENTATION
There are design factors that determine how well an antenna system will function and thus impact the overall quality of the weather satellite imagery received. The design consists of controlling currents to produce the desired radiation pattern. The three design considerations of primary importance include: -the physical size of the antenna components which is determined by the frequency of the transmissions it is intended to receive. In most Very High Frequency (VHF) antenna designs, the radiating elements are designed for 1/4 or 1/2 wavelengths: -the antenna design should fit the type of RF signal polarization it is to receive; -the antenna needs to provide enough signal gain to produce noise-free reception whenever it is used with an appropriate radio receiver.
The self-phased QHA design method which require only one coaxial cable for the feed, that is, a single excitation, was chosen over the hybrid-phased because the latter would require two feed lines to the bifilar loops, and would add loss and weight to the system. The self-phased is designed with different resonant lengths for the bifilar loops, fed in parallel resulting in 90-degrees current phase shift with respect to one another. This 90-degrees relationship is obtained by making their respective terminal impedances + and − where = , so that the currents in the respective helices are 45-degrees lag and 45-degrees lead. The relative lag and lead currents in the loops differ in phase by 90-degrees does not require any additional components to obtain seperate differential phase excitations [8]. The resonant self-phased QHA, working group satellites was adopted as a model [9]. The diameter, axial length, and loop half-length of the larger bifilar are 0.148 λ, 0.330 λ, and 0.560 λ respectively while the smaller bifilar are 0.141λ, 0.314λ, and 0.528λ respectively.

The design of the QHA
The design was carried out by varying the element diameter, the radial lengths, the axial lengths, and the element material for best performance. Numerical Electromagnetics Code (NEC2) was used for the antenna modeling. The radiating element wavelength is: were: is the speed of propagation of the wave in free space while is the operating frequency of the polar orbiting weather satellite of interest as given in Table 1. where and ℓ are the diameter and axial length of the imaginary cylinder, and ℓ is the loop half-length.
The bifilar loops were segmented into 46 segments each to form straight-line wire segments and the wire segments X, Y, Z coordinates were inputted through the Geometry tab of the NEC editor following the QHA dimensions and the right-hand orientation as shown in Figure 2. Copper AWG0 (8.25246 mm diameter) wire element was selected as the wire-conductor and source excitation of 1volt was applied. The output data for the simulation shows that the QHA exhibits a far-field radiation pattern in the upper hemisphere and has omni-directional coverage sensitive in all directions with maximum gain of 4.37 dBi at 0° as in Figure 3. The radiation pattern shows that the antenna would receive right-hand circular polarized waves from all directions (omnidirectional), thus the antenna can receive signal from the polar orbiting weather satellites. Figure 4 shows the main window with the antenna parameters.

QHA Optimization
NEC2 optimizer and evaluator was used to calculate the values of the element diameter, the radial lengths, and the axial lengths of the QHA that gave the best performance. The element diameter, and the radial lengths were varied at first and thereafter with the axial lengths. The element diameter, the radial lengths, and the axial lengths were replaced by variables with values extracted from the initial geometry inputs ( Figure 5).

The Implementation of the QHA
Local materials were used for the antenna implementation such as: 3/8 inches nominal size (12 mm diameter) copper tubing of about 7.5 m in length, 90-degrees 12 mm copper elbows (8 pieces), BNC RG58 50Ω Cable, lead, and 32 mm PVC pipe with water proof end cap for the antenna support.
The optimized antenna dimension was used for the implementation following the geometry as in Figure 2. The length of the radials and the 1/2-turn elements were cut, the 1/2-turn loops were made by cutting out half-circle papers of radius that produced the circumferences of the loops and the element loops were curved along the circumference-path of the papers. The radials of the large and the small loops were arranged orthogonally, 90degrees to each other on the PVC pipe, the 1/2-turn elements were connected to the radials using the 90-degrees elbows following the right-hand circular polarization geometry. The use of a simple choke balun where the necessary isolating choke is formed from 4-turns winding of the feeder cable round the support mast was employed. Figure 6(a) shows the self-quadrature feeding method used [11] while Figure 6(b) shows the 4-turns winding choke balun [12]. Figure 7 shows the implemented right-hand circular polarized QHA.
a. b. Fig. 6. (a) Self-quadrature feeding method, L for long loop and S for small loop; (b) 4-turs winding choke balun.

The simulated QHA results
The simulated results are provided in the following sections.
The result shows that the diameter of the antenna element has effect on the radiation pattern and the gain(s) of the QHA as shown in Figure 8. Table 2 shows the final antenna optimization obtained from the QHA performance on element diameter, radial lengths, and the axial lengths.  Fig. 8. Radiation pattern(s) and gain(s) of different copper wire guage.

QHA performance on types of element materials
Copper, Aluminum, Brass, Silver, and Phosphor-Bronze wire elements were tested to see their performances on the optimum dimension. The results obtained are presented in Table 3 and Figure 9, Figure 10, and Figure 11. The results showed that copper wire element has the best radiation efficiency, radiated power, minimal loss and best radiation pattern compared to Aluminum, Brass, Silver, and Phosphor-Bronze materials.

The optimized QHA dimensions and properties
Using the "Wire/Segment info" on Geometry tab of NEC2, the geometry of the optimized QHA was calculated by summing up the wire segments as appropriate. Table 4 shows the optimized (new) dimensions and the nonoptimized (old) dimensions. = 0.528 where and ℓ are the diameter and axial length of the imaginary cylinder, and ℓ is the loop half-length.
The optimized QHA has the following properties as shown in Figure 12 (a, b, c): a. Far-field radiation pattern in the upper hemisphere (Elevation) with gains of 4.14dBi at 0°, 2.93 dBi at 45°, 0.69 dBi at 75°, and -0.88 dBi at 90° and has omni-directional coverage sensitive in all directions (Azimuth) with gain of 4.14 dBi; b. Half-power beamwidth of 140°. (It is the angle at which the antenna gain first falls to half power (approx. -3 dB) from the peak. The angular distance between the half power points (-3 dB points) is defined as the beamwidth); c. Impedance of 44.2 + 1.09 (it was 52.6 + 28.5 before optimization); d. VSWR of 1.13 (it was 1.74 before optimization); e. Antenna Efficiency of 99.77% (it was 99.69% before optimization); f. Radiation-efficiency of 88.66% (it was 86.29% before optimization); g. Radiation-power of 99.76W (it was 99.69W before optimization).
Frequency sweep was conducted on the antenna between 135 MHz and 150 MHz, the following results were obtained: voltage standing wave ratio (VSWR) and reflection coefficient graph (Figure 13), gain graph ( Figure  14), and the impedance graph ( Figure 15). The designed frequency of 137.5 MHz is at resonance where the impedance is purely (approximately) resistive.

The Implemented QHA results
Antenna Trainer ST2261 was used to measure the radiation pattern of the implemented antenna. The platform consists of the main unit, transmitting and receiving mast, RF detector and adapter, signal demodulator, accessories case containing antenna types, BNC to BNC 25 inches long cable for antenna connection and others. The main unit has RF generator, modulation generator, directional coupler, antenna rotation (0-360°) and others [13].
The setup for the measurement is shown in Figure 16 and below were the steps taking to plot the radiation pattern of the antenna. The implemented QHA was made the receiving antenna and put in a fix position pointed to the transmitting antenna: • A right-hand helical antenna type was used as the transmitting antenna in order to provide the circular polarized wave which the QHA is designed for the transmitting antenna is attached to the antenna rotation provided; • Both transmitting and receiving antennas faced each other in horizontal plane and distance between them was about 1meter as prescribed by the operating manual; • The RF generator is turned on and the transmitter turned for maximum forward power for optimum radiation of the transmitting antenna; • The FS adjust on the main unit is set at 10 µA (and later 13 µA for the second test) readings for the transmitting unit and the Digital Panel Meter (DPM) at RF detector unit attached to the receiving antenna is monitored for the corresponding output readings; • The readings at intervals of 10 -degrees were taking. The ratio of Iout and Iin, that is, the output readings at the receiving RF detector and the input reading of the transmitting DPM, are computed for the gain of the receiving antenna; • To plot the radiation pattern of the receiving antenna, the gains at each degree are converted to decibels, {20log (gain)}. The degrees versus the gains in dB are tabulated and used to plot the radiation pattern for the receiving antenna.

Comparison between the simulated and the implemented QHA
Comparison was done between the simulated and the implemented antenna. It was done with the parameters of the implemented antenna that could be measured within the available and availability of antenna measuring equipment, Table 5 shows the comparison done. It could be seen that the implemented antenna does not significantly deviate from the properties of the simulated version. The measured parameters are not expected to be same value but must follow same pattern most especially the radiation pattern and the gains. 50 Ω (based on the impedance of the feeding coaxial cable, the balun, and the LNA impedance to be used with the antenna

CONCLUSIONS
The study has provided platform in understanding the quadrifilar helix antenna design, its design optimization, and its implementation using local materials in receiving LEOs signals. The resonant QHA, Working Group Satellites was adopted as a model and the dimensions of the QHA was determined using the frequency of the transmissions the antenna is intended to receive. The design was carried out by varying the element diameter, the radial lengths, the axial lengths, and the element materials for antenna optimization. The simulation was carried out using NEC2, antenna modeling program. } for the smaller bifilar, both with 11.328 wire element diameters at 137.5 (the resonant frequency of the transmissions the antenna is intended to receive). It could be seen from the results obtained that copper wire antenna element has the best radiation efficiency, radiated power, minimal antenna loss and best radiation pattern compared to Aluminum, Brass, Silver, and Phosphor-Bronze material. } for the smaller bifilar, both with 11.328 wire element diameters at 137.5 (the resonant frequency of the transmissions the antenna is intended to receive). It could be seen from the results obtained that copper wire antenna element has the best radiation efficiency, radiated power, minimal structure loss and best radiation pattern compared to Aluminum, Brass, Silver, and Phosphor-Bronze material.
The implemented QHA was measured using antenna trainer ST 2261, it has major lobe as with the simulated QHA, that is, radiation pattern in the upper hemisphere (Elevation) with maximum gain of 10.75 dB at 0°, 8.26 dB at 50°, 0.28 dB at 90°, -1.83 dB at 270°, 7.88 dB at 320°, and 10.51 dB at 360°. The antenna also has back lobe which could be as a result of test environment electromagnetic wave influences, the antenna trainer kit limitations, and possible non-perfect geometry during construction. It could be seen that the results available for the implemented QHA is approximately same as that of the simulated version.