Basic Consideration
The study of antennas must include a quick review of impedance matching and rasonant circuits.Maximum Power transfer could be achieved when source matches the load.This is maximum power transfer Theorm.For eg : In radio jove the transmission cable used is of 75 Ohms in order to match the impedance of 73 ohms at feed.I will be talking of calculation of impedance at feed later.
Material of antenna
An antenna is a structure that is generally a metallic object often a wire used to convert high frequency current into electromagnetic waves or vice versa.
RADIO JOVE uses a dipole antenna which is made up of copper wire.Even a single piece of wire acts as an antenna.We can simply try to listening disturbances at SW receiver using a long piece of wire wrap with aluminium foil.
DIPOLE ANTENNA FOR RADIO TELESCOPE
1) Theoretical Length of Antenna
2)Exact Length
3)Height of Antenna
4)Transmission Line
5)Impedance
6)Velocity factor
7)Dual Dipole Configrations
8)Components in Antenna System
I will be discussing all the above mentioned points for a dipole antenna one by one.
THEORETICAL LENGTH OF ANTENNA
One of the simplest antenna is called a dipole antenna. It can be made from two pieces of copper wire and three insulators. A radio wave is an electromagnetic wave travelling in the space at the speed of light. The frequency of the wave is the number of cycles that occurred each second, and the wavelength is the distance that the wave travels during one cycle. The frequency (f), wavelength and speed of light are related by the simple equation:
Wavelength = speed of light/frequency
For Radio Jove we chose 20.1mhz as our antenna operating frequency after considering the fact that the bursts occurring in the outer celestial bodies( eg all planets) emits radio waves broadly on all frequencies but till 15mhz all radio waves get absorbed by the ionosphere layer in our atmosphere and it start coming on earth from 18mhz but its maximum strength is inbetween 20 to 20.4mhz, so we decided to have 20.1mhz as our centered operating frequency. The free space wavelength is therefore:-
Wavelength=3x108/20.1x106= 14.925meters
Wavelength= 14.925meters
Now, For dipole antenna
Length of antenna is given by
Length= Wavelength/2= 14.925/2= 7.46meters= 24.46feet
Length of dipole antenna to be used at 20.1 Mhz comes out to be 24.46 feets.
"Diagrammatic Representation of Theoretical Antenna Length"
Dipole Antenna(Theoretical Length)
EXACT LENGTH OF ANTENNA
By above analysis we find the length of dipole antenna to be 24.46 feets.But antenna length is taken to be 95% of Theoretical length.
Why antenna length is taken to be 95% of theoretical length?
Energy travels slowly in solid than in air.Current at the end of rod does not have space to move freely as center of antenna.So ends exhibit capacitive effect.This effect is offset by making antenna size smaller than normal size.
Exact antenna length=95% of Theoretical Length
Exact length=24.46*0.95=23.28 feet.
Length of copper wire to be used for antenna construction is 23.28 feet.
Dipole antenna(Exact length)