Antennas have come to play a significant role in the reception and transmissi on of electromagnetic radiation or signals which in turn have improved scientific research in the various areas where they have been used. Astronomy is not left out; as a study of everything beyond earth’s atmosphere, useful signals have to travel great distances to reach us. Hence the need for equipment to detect such signals. The Helical Antenna is known to possess high gain (directivity) and can process signals with any type of orientation/polarization which implies high sensitivity hence the desire to use it. Simulation was done using the legacy 4NEC2 software. The purpose of this paper is to design a circularly polarized helical antenna operating in axial mode that is suitable for detection of neutral hydrogen line emission at the frequency of 1420 MHz and to show how it can be used as a feed for an off-set parabolic dish.
An antenna is a metallic structure that converts electric power into (radio waves) electromagnetic waves and vice-versa. It is electrically connected to the receiver or transmitter to receive or transmit radio electromagnetic waves in all horizontal directions equally or in a particular direction of propagation.
Antennas can provide a simple way to transfer signals. Several antenna types with different sizes, shapes, and unique performances have been developed for particular applications. Some types of antennas are Yagi-Uda, Dipole, Quad, Quagi, and Helical Antenna.
In Radio Astronomy, astronomers use radio antennas (also called telescopes) to explore the Universe by detecting radio waves emitted by a wide range of objects in space. Radio astronomers study objects that radiate or absorb energy at frequencies within the radio spectrum: when ground-based, studies are conducted wherever the atmosphere is at all transparent in the range 13 MHz to 2000 GHz. W. Baan [
The Helical antenna is an interesting antenna due of its unique characteristics; it is simple to design, capable of high gain, wide bandwidth (
A principle in the detection of radio signals from space is the Hydrogen atom that makes a transition from its aligned state to an anti-aligned state and emits radio energy at a wavelength of 21 cm or a frequency of 1420 MHz. The line emitted at radio wavelength are many others fainter than optical lines, therefore the detection of new lines in large observable quantity is possible with the use of large antennas or modern day telescopes with sensitive feeds (
The Helical Antenna is chosen because of its high directivity that is the ability to concentrate radiated power in one direction hence the suitability for the H1 emission where neutral Hydrogen gas have to brave the distance between space and earth to reach us.
This paper derives its motivation from designing an antenna to achieve the above purpose.
Antenna design is very specialized as it requires theoretical and largely empirical work and lots of experimentation. It will involve understanding the fundamental antenna theory and operating principles from textbooks and articles, calculations and simulations (using software), and the final construction of an antenna with close to 100% efficiency. Generally, the procedure in terms of materials and method in the design and construction of a helical antenna is as follows:
A directional helical antenna operating in axial mode is designed in this work (
The geometry of a uniform mono-filar helix antenna is shown in
The circumference of the helix is given as
C = π d H (1)
where π is a constant equal to 22/7, d H is the diameter of the helix (in meters). C (in meters) has to be about a wavelength (~1λ) to ensure axial mode operation, as described by Kraus and Kumar as shown in
The wavelength can be gotten from
V = λ f (2)
where V (m/s) is the speed of light in a vacuum, f (Hz) is frequency and λ is the wavelength.
The spacing between the helix turns is given by Kumar [
S = C tan α or 1 / 4 λ (3)
S (meter) and α (degrees) is the pitch angle. From Equation (3), the pitch angle of the windings is obtained as,
α = a r c tan ( S C ) (4)
From
L = S 2 + C 2 (5)
L (meters). Thus, the axial length (i.e. total length) of the helix can be computed from:
A = N S (6)
where N is the no of turns. Also, the Ground Plane diameter is given as;
G p D = 3 / 4 λ , Kumar [
From Equation (1), the diameter is givens as:
d H = C π (8)
The 4NEC2 software developed by Arie Voors was used to model and simulate the antenna. It can be used to calculate and plot Gain, Front to back ratio, Voltage Standing Wave Ratio (VSWR), Radiation pattern (Azimuth and Elevation), Smith chart and other various parameters (Figures 3-5). The software can be found and downloaded free of charge from the Internet, Arie V. [
Using the calculated design values in
Helical antenna is designed from the loop antenna and many small loop antennas joined together forms the helical antenna. It consists of thick copper wire or tubing wound in the shape of helix or screw thread and used as an antenna in conjunction with flat metal plate called a ground plate (
The size of the ground plane is also critical, and it is usually chosen to be between 0.5 - 0.75λ, Juan Pierre Jansen [
Therefore we used the data from (
Two equally measured flat colored Perspex glass sheet (dimensions below) was used as support for the helical wire. Holes of 0.2 cm drilled into them at 5.3 cm spacing this corresponds to Equation (3) for the purpose of holding the loop of each turn as the wire spirals upward. Each sheet has 0.5 cm width hole hewn in the middle to an equal length of 29.2 cm, both sheets are then fused together. See Figures 9-13 for details. A 14 SWG Copper wire was fed through the holes of the fused support structure from the base through to the top in helix fashion.
An SMA connector was soldered to the backside of the reflector with the inner pin passing through the reflector plate and soldered to the edge of the tapered impedance matching (see
| Operating frequency, f | 1420 MHz (Megahertz) |
|---|---|
| Wavelength | 21 cm |
| Circumference of Helix, | 21 cm = λ (λ = lambda, for radiation in Axial mode) |
| Helix diameter dH | 6.68 cm |
| Spacing btw turns S | 5.3 cm |
| Length of one turn L | 21.6 cm |
| No of turns N | 10 |
| Axial Length A | 53 cm |
| Ground Plane diameter GpD | ~16 cm |
| S/No. | Material | Dimension; Properties (cm) |
|---|---|---|
| 1 | Perspex Glass | · Diameter = 6.68, Thickness = 0.4, Length = 58.4, εr = 3.2, Refractive Index = 1.5, Density = 1.18 g/cm3, Melting Point = 160˚. |
| 2 | Aluminum sheet as ground plane reflector | Thickness = 0.02, GpD = 15.75. |
| 3 | Copper Wire | · 14 AWG, Length = 216. |
| 4 | No. of Holes; Hole diameter | · 10, 0.3 |
The support structure with wire in
The support drawings below were done using Autodesk and Revit online tools [
These key parameters constitute/serve as indicators for the determination of the performance of a Helical Antenna: Gain, Radiation Efficiency, Beam width, SWR, and Impedance values. Each of the above when high or low has a significant effect on the overall output/ performance of the antenna structure.
However, the gain is proportional to the no of turns N. From N = 5 and above, the beam width narrows increasing the directivity/ gain. The fewer the turns, the wider the beam width and this significantly reduces the gain as described by Tushar [
Helical antennas are known to possess a characteristically high input impedance as discovered by Kraus [
In
From simulation results, the antenna has 100% efficiency for its size which depicts flawless communication at the designed frequency of 1420 MHz. There is no structural loss in any dielectric, and losses based on coupling to the device as the SWR is 3.6.
The antenna fabricated has 6.16 dB gain at 1.420 GHz center frequency with 45 degrees beam width. It can be used as feed for off-set parabolic dishes (
The subject for future work which we have commenced, intends to complete the fabrication of a Small Radio Telescope backend to detect H1 emission with the designed Antenna; using its results as a demonstration technique for the purpose of Astronomy Outreaches in Nigeria.
Special thanks to Head of Department, Astronomy Section of the Centre for Basic Space Science, Dr. Ikechukwu Anthony Obi for his support, and direction.
I also want to appreciate Mr. Joseph U. Enoch who was a huge source of moral encouragement, we would brain-stormed on ways to get materials, and he assisted in purchase and design of the support structure.
The authors declare no conflicts of interest regarding the publication of this paper.
Musa, E.O., Obi, I.A., Idogbe, E.A., Ali, J.O., Ogbeiwi, C.O., Chukwuka, P.C. and Eze, N.F. (2022) Design and Construction of Axial Mode Mono-Filar Helical Antenna for Hydrogen Line Emission Studies with 4NEC2 Tool. Open Journal of Antennas and Propagation, 10, 29-44. https://doi.org/10.4236/ojapr.2022.103003