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Radio telescope is an astronomical instrument consisting of a radio receiver and an antenna system that is used to detect radio-frequency radiation reflected by meteors and aero planes .Because radio wavelengths are much longer than those of visible light, radio telescopes must be very large in order to attain the resolution of optical telescopes.
Radio telescopes vary widely, but they all have two basic components: a large radio antenna and a radiometer or radio receiver. The sensitivity of a radio telescope depends on the area and efficiency of the antenna.
Since using optical telescope requires atmosphere isn't cloudy or rainy in addition you can't use it during daylight, we designed a telescope to detect the Meteors without any constraints depending on radio wave rather than the optical wave as used in optical telescope.
The general principle of meteor observation by forward scattering of radio waves off their trails is easy to understand. It is illustrated in FigureÂ 1. A lower VHF radio receiver (30-100Â MHz) is located at a large distance (about 500-2000Â km) from a transmitter at the same frequency. Direct radio contact is impossible due to the curvature of the Earth. When a meteor enters the atmosphere, its trail may reflect the radio waves from the transmitter to the receiver. At the receiver, where the signal of the transmitter is normally not received, the transmission can then be received for a moment, as long as the meteor trail is present. Such reflections can last from a tenth of a second to a few minutes. The received signal characteristics are related to physical parameters of the meteoric event.
To distinguish the signals reflected from meteors rather than others such as airplanes, the signals reflected from meteors are very sudden, are mostly loud and clear, and fade out gradually.
1.3 What is the Meteor
Meteors or shooting stars as they are more commonly known, are the streaks of light produced when a meteoroid burns up in the Earth's atmosphere. It looks like a star falling towards us as it momentarily flashes above us. The meteoroids, which produce the meteors, are dust and rocks in space.
1.4 system structure
The system we built consists of an FM radio receiver, Yagi antenna and software for data collection developed in MATLABÂ®, we expect the telescope to be capable to identify the reflected signal that is above certain threshold level which is 10 percent higher than the noise level.
TheÂ electromagnetic spectrumÂ is theÂ rangeÂ of all possible frequencies ofÂ electromagnetic radiation. Radio frequency radiation is a specific type of electromagnetic (EM) wave.
2.2 Radio Waves
Radio wavesÂ are a type ofÂ electromagnetic radiationÂ withÂ wavelengthsÂ in theÂ electromagnetic spectrumÂ longer thanÂ infraredÂ light. Radio waves haveÂ frequenciesÂ from 300Â GHzÂ to as low as 3Â kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers, electromagnetic radiationÂ travelsÂ by means of oscillatingÂ electromagnetic fieldsÂ that pass through the air and theÂ vacuumÂ of space.
The table below gives detailed information about the frequency range, the length of waves for each band and some typical uses of that portion of the radio spectrum.
3 - 30 kHz
100 km - 10 km
Long range navigation and marine radio
30 - 300 kHz
10 km - 1 km
Aeronautical and marine navigation
300 kHz - 3 MHz
1 km - 100 m
AM radioÂ and radio telecommunication
3 - 30 MHz
100 m - 10 m
Amateur radio bands, NRC time signal
30 - 300 MHz
10 m - 1 m
TV, FM, cordless phones, air traffic control
300 MHz - 3 GHz
1 m - 10 cm
UHF TV, satellite, air traffic radar, etc
3 - 30 GHz
10 cm - 1 cm
Mostly satellite TV and other satellites
30 - 300 GHz
1 cm - 1 mm
Remote sensing and other satellites
receive radio wave from far away FM station is almost impossible due to the curvature of the Earth but when a meteor streaks through Earth's atmosphere, about 10% of its energy is released as light, with the remainder is dispersed in a trail of ionized air or ionization train and this electrified air will reflect the radio waves which coming from commercial FM station.
Because of this we can use a commercial radio as a receiver. If you are tuned to an FM radio station over the horizon from your home, and you hear music or voices, a meteor has probably passed between you and the radio station.
Radio Telescope hardware
3.1 Radio Telescope Hardware
The easiest way to observe meteors by radio is to use commercial FM radio and attach it to an FM Yagi antenna, then try to find an empty frequency from 87.5-108.0 MHz, where you do not hear any kind of music or talking, and is being used by a distant transmitter mostly a distant FM radio station about 300-2000 km away from your location of observation. It is better to choose a station that transmits over 30 kilowatts, and point the antenna towards the station
3.2FM Radio Receiver
AÂ radio receiverÂ is an electronic device that receivesÂ radio wavesÂ and converts the information carried by them to a usable form
It is highly recommended to use a commercial radio, since this types of radio can be tuned to a specific frequency of a distant transmitter that you don't hear anything but a continuous static "Noise", and if a meteor passes in the right place in the sky, you will briefly observe the signal from a distant FM station for a while. The period of the signal varies from 0.1 second to several seconds or even minutes
After we selected the best FM Radio receiver; we take the voice signal of radio as analog input to the MIC input of sound card in the computer using 3.5mm Jack cable in order to convert it to digital signal.
Antenna is an electromagnetic device that collects or emits radio waves. It consists of material that conducts electricity arranged in such a way that it is in tune with the frequency of a radio signal, the antenna tuned to a particular frequency will resonate to a radio signal of the same frequency. When properly tuned an antenna will collect this energy and make it available to drive the amplifiers in a radio receiver.
For the detection of meteors and planes, we constructed and attached an antenna to our project so we can easily collect the radio waves that cannot be heard by the human ear and they are also so weak.
In our project we built a 3 element yagi antenna referred to the Japanese man who design it. It is a particularly useful form of RF antenna design and is widely used in applications where an RF antenna design is required to provide gain and directivity, in this way the optimum transmission and reception conditions can be obtained. It will efficiently collect the radiated energy of a distant FM radio station whose waves have been reflected by a meteor trail.
The Yagi-Uda RF antenna as shown in figure: 2.2 is one of the most popular and widely used antennas because of its simplicity, low cost, directional radiation and relatively high gain, design has a dipole as the main radiating or driven element. Further "parasitic" elements are added which are not directly connected to the driven element. Instead they pick up power from the dipole and re-radiate it such a manner that it affects the properties of the RF antenna as a whole.
Figure â€Ž02.â€Ž0.: The 3-Element Yagi-Uda Antenna
4.2 Yagi Antenna history
The Yagi-Uda antenna was invented in 1926 byÂ Shintaro UdaÂ ofÂ Tohoku Imperial University,Â Sendai,Â Japan, with the collaboration ofHidetsugu Yagi, also of Tohoku Imperial University. Yagi published the first English-language reference on the antenna in a 1928 survey article on short wave research in Japan and it came to be associated with his name.
The Yagi was first widely used duringÂ World War IIÂ for airborneÂ radarÂ sets, because of its simplicity and directionality.Â Despite its being invented in Japan, many Japanese radar engineers were unaware of the design until very late in the war, partly due to rivalry between the Army and Navy.
4.3 Operation of Yagi-Uda Antenna
A basic dipole is cut to resonance at the center of the frequency band and is utilized as the driven element. High gain is attained by the addition of parasitic elements positioned either in front or behind the driven element. These parasitic elements are called directors and reflectors depending on their length and positioning with respect to the drive element.
The parasitic elements of the Yagi antenna operate by re-radiating their signals in a slightly different phase to that of the driven element. In this way the signal is reinforced in some directions and cancelled out in others. It is found that the amplitude and phase of the current that is induced in the parasitic elements is dependent upon their length and the spacing between them and the dipole or driven element.
Using a parasitic element it is not possible to have complete control over both the amplitude and phase of the currents in all the elements. This means that it is not possible to obtain complete cancellation in one direction. Nevertheless it is still possible to obtain a high degree of reinforcement in one direction and have a high level of gain, and also have a high degree of cancellation in another to provide a good front to back ratio.
To obtain the required phase shift an element can be made either inductive or capacitive. If the parasitic element is made inductive it is found that the induced currents are in such a phase that they reflect the power away from the parasitic element. This causes the RF antenna to radiate more power away from it. An element that does this is called a reflector. It can be made inductive by tuning it below resonance. This can be done by physically adding some inductance to the element in the form of a coil, or more commonly by making it longer than the resonant length. Generally it is made about 5% longer than the driven element.
If the parasitic element is made capacitive it will be found that the induced currents are in such a phase that they direct the power radiated by the whole antenna in the direction of the parasitic element. An element which does this is called a director. It can be made capacitive tuning it above resonance. This can be done by physically adding some capacitance to the element in the form of a capacitor, or more commonly by making it about 5% shorter than
the driven element.
TheÂ bandwidthÂ of a Yagi-Uda antenna refers to the frequency range over which its directional gain and impedance match are preserved to within a stated criterion. The Yagi-Uda array in its basic form is very narrowband, with its performance already compromised at frequencies just a few percent above or below its design frequency.
It is found that the addition of further directors increases the directivity of the antenna, increasing the gain and reducing the beam width. The addition of further reflectors makes no noticeable difference.
4.4 Our Yadi-Uda Antenna design
As we talked earlier, this radio telescope will operate at very high frequency band and more specific at FM radio frequencies band, this band starts at 88 MHz and ends at 108 MHz to make our antenna operate effectively at any frequency within this band we will choose the center frequency and designed the antenna based on it.
The Yagi-Uda antenna consists of two main parts:
The antenna elements
The Driven element
The axis of the antenna that is called the antenna boom in our case it is in wood.
Yagi Antenna Construction
The Driven element
It's the most important element in the antenna typically made from a metal which is electrically connected to theÂ receiver its collect the incoming radio waves for reception, and converts them to tiny oscillating electric currents, which are applied to theÂ receiver and connected to the receiver through aÂ feed line ,A driven element will be "resonant" when its electrical lengthÂ is 1/2 of the wavelength of the frequency.
Wavelength is the distance of a radio wave that will travel during one cycle
The reflector is the element that is placed behind of the driven element and its resonant frequency is lower than the resonant frequency of driven element, its length is approximately 5% longer than the driven element, the spacing between the reflector and the driven element will 0.15 of wavelength
The director is the shortest of the parasitic elements and this end of the Yagi is aimed at the receiving station. It is resonant slightly higher in frequency than the driven element, and its length will be about 5% shorter than driven element and the spacing of the director from driven is equal to 0.15 of wavelength.
Yagi antenna simulation
After we calculate the dimensions of antenna we need simulate our design using YagiCAD software and 4NCE2 antenna simulator to consider some important specifications
The radiation pattern describes the relative strength of the radiated field in various directions from the antenna. The radiation pattern is a "reception pattern" as well, since it also describes the receiving properties of the antenna, the radiation pattern is three-dimensional, but it is difficult to display the three-dimensional radiation pattern in a meaningful manner. Often radiation patterns are measure that are a slice of the three-dimensional pattern, which is of course a two-dimensional radiation pattern which can be displayed easily on a screen.
Figure 2.3: Radiation pattern of Yagi Antenna in horizontal plan
In figure 2.3 we see the theoretical model of the antenna and the most of received power comes from specific direction almost angle with the horizon, that's what we need to detect the meteors above some region.
Figure 2.4: Radiation pattern of Yagi Antenna in vertical plan
Figure 2.4: 3D view for Radiation pattern of Yagi Antenna
We notice if we need reduce the angle of received power must increase the element number of antenna as illustrated in figure 2.4 this will also effect on the beam width so the bandwidth will reduced.
Figure â€Ž02.3: Effect of increasing the number of antenna element
Antenna gainÂ is a key of performance which combines theÂ antenna'sÂ directivityÂ and electricalÂ efficiency, As a receiving antenna, the gain describes how well the antenna converts radio waves arriving from a specified direction into electrical power.
in our design the gain at center frequency (98MHz) equal to 7.81 dBi as shown in figure 2.4 this graph show the effect of sweep the frequency on the gain of antenna.
Figure â€Ž0. Sweep frequency gain response
We can control the gain using the following parameters:
Number of elements one of the main factors affecting the Yagi gain, is the number of elements in the design.
Element spacingÂ the spacing can have an impact on the Yagi gain, although not as much as the number of elements
Number of elements
Approximate Anticipated gain(dBi)
Table :Relation between number of element and the gain of antenna
For efficient received of energy the impedance of the radio, the antenna and the transmission line connecting radio to the antenna must be the same. Radios typically are designed for 75 ohms impedance and the coaxial cables used with them also have a 75 ohm impedance.
Efficient antenna configurations have an impedance more than 75 ohms, some sort of impedance matching circuit is then required to transform the antenna impedance to 75 ohms.
Due to the difficulty of the testing antenna in our university because the lack of necessary equipment, we will depend on the average between the theoretical impedance value of the antenna and the simulated value to design the matching circuit and reduce the impedance of antenna to some value near to 75 ohm.
When we stimulate our design using 4NCE2 software gives the following result as shown in figure
As shown in above figure the impedance of antenna at 98 MHz equal to
Then we need to design matching circuit to reduce the resistance from 268.2 â„¦ to 75 â„¦
Coaxial feeder is used in many applications where it is necessary to transfer radio frequency energy from one point to another. Possibly the most obvious use of coax cable is for domestic television down-leads, but it is widely used in many other areas as well. While it is sued for domestic connections between receivers and antennas
Coaxial feeder is normally seen as a thick electrical cable. The cable is made from a number of different elements that when together enable the coax cable to carry the radio frequency signals with a low level of loss from one location to another. The main elements within a coax cable are:
The center conductor of the coax is almost universally made of copper. Sometimes it may be a single conductor whilst in other RF cables it may consist of several strands.
Insulating dielectricÂ Â Â
Between the two conductors of the coax cable there is an insulating dielectric. This holds the two conductors apart and in an ideal world would not introduce any loss, although it is one of the chief causes of loss in reality. This coax cable dielectric may be solid or as in the case of many low loss cables it may be semi-airspaced because it is the dielectric that introduces most of the loss.
Outer conductorÂ Â Â
The outer conductor of the coaxial cable is normally made from a copper mesh. This enables the coax cable to be flexible which would not be the case if the outer conductor was solid, although to protect the signal from any outside noise
Outer protecting jacket
Finally there is a final cover to the coax cable. This serves little electrical function, but can prevent earth loops forming. It also gives a vital protection needed to prevent dirt and moisture attacking the cable, and prevent the coax cable from being damaged by other mechanical means.
How it is work?
How to Observe
Connect the receiver to the antenna. Connect your data logging equipment to the earphone jack of the receiver. Start your software. Start the receiver. And away you go. These are the basic steps required. More detail on how to do each step is described on other pages on this website. See theÂ Site MapÂ for an overview of what is available.
It is better to observe overnight when human activity is at a low ebb. Also, the number of meteors observable increases at around 6 a.m. when the leading edge of the earth travels into the stream of sporadic or shower meteors. These meteors tend to enter the atmosphere at a greater speed because of the Earth's velocity through space of 28.8 km per sec. This higher energy translates into higher levels of ionization. As a result of these two factors, many radio meteor observers conduct their observations between midnight and 7 a.m.
why we prefer to test in night ??
Identifying Radio Interference
Radio interference from lightning, car ignitions, computers, light switches and a host of other terrestrial sources can cause signal strength bursts in meteor observations. Interference often looks like meteor reflections but aren't.
A way to determine if interference is affecting observations is to find out what interference "looks like" in your data. For example, test the effect of turning the computer monitor on and off while you are running a data session on Radio Sky Pipe. Take a close look at the trace and write down the characteristics of the trace.Â
Does it rise immediately or take some time to rise to the maximum?Â
Does the spike remain for a long time or does it immediately drop off?Â
How does interference caused by turning the monitor onÂ differ from turning it off?Â
Does flipping a light switch cause interference?Â
Does it make a difference if it is a lightswitch connected to a light bulb or a fluorescent light?