Antennas for transmission or radio waves
- Date: 2007-07-01 - Word Count: 588
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To transmit radio waves we need Antennas. Although there are many different types of transmitting antennas available, the two principle categories are the half wave and quarter wave. Each has its own special characteristics.
In half wave antennas the value of distributed capacitance of a straight wire are such that the antenna is resonant to a frequency whose wavelength is approximately twice the physical length of the antenna. Stated another way a resonant antenna is approximately half wavelength long i.e. one half the length of the wave to which it is resonant and one half the length of the wave that it radiates most efficiently. For this reason the antenna used to transmit radio signals is the half wave antenna.
The half wave antenna is variously referred to as a dipole, a doublet, or a hertz antenna. The half wave dipole is the most commonly used type of antenna. As the name implies it is lambda by two long at its operating frequency. The antenna is centre fed with a seventy five ohms balanced transmission line, giving a good match to the antennas seventy two ohms input impedance.
A dipole can also be fed with a co axial transmission line with the centre conductor connected to one side of the dipole and the shield connected to the other. However some inefficiency will result because the dipole is balanced and co axial line is unbalanced. This inefficiency stems from the current and voltage distributions on the antenna being upset, which allows radio frequency current to flow on the coaxial shield and may cause undesirable radiations from the transmission lines.
To correct the imbalance between a co axial line and a dipole antenna a Balun must be used. A Balun is a balanced to unbalanced radio frequency transformer. Baluns are contracted for specific impedance ratios just like conventional transformers. When fifty ohm or seventy five ohm coaxial cable is used to feed a dipole impedance ratio of one is to one is used.
When vertical polarization is required the antenna must be vertical however because of antenna height considerations vertical antennas are usually used at relatively high frequencies.
The height restrictions at low frequencies can be overcome by constructing a Quarter wave vertical antennas above a perfect ground. The antenna will then have the same characteristics as a half wave vertical dipole, because a perfect ground will produce a mirror image of the quarter wave. If the ground is not a perfect conductor, the vertical quarter wave antenna will loose a considerable amount of power in the resistance of the grounding system. When an antenna is constructed over soil of poor conductivity such as sandy or rocky soil an artificial ground system must be used. The artificial system consists of quarter wave cooper wires called radials extending outward from the base of the antenna in as many directions as possible.
The half wave grounded antenna is known as Marconi antenna. The physical length of the Marconi antenna is normally only lambda by four of the signal to be radiated. However the earth acts on other half of the antenna making the overall effective length a full lambda by two. In other words the antenna consists of a conductor extending approximately lambda by four above the earth, and the earth acts as an additional lambda by four of the antenna. Electrically Marconi behaves as a half wave antenna.
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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect xm radio, xm satellite radio
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In half wave antennas the value of distributed capacitance of a straight wire are such that the antenna is resonant to a frequency whose wavelength is approximately twice the physical length of the antenna. Stated another way a resonant antenna is approximately half wavelength long i.e. one half the length of the wave to which it is resonant and one half the length of the wave that it radiates most efficiently. For this reason the antenna used to transmit radio signals is the half wave antenna.
The half wave antenna is variously referred to as a dipole, a doublet, or a hertz antenna. The half wave dipole is the most commonly used type of antenna. As the name implies it is lambda by two long at its operating frequency. The antenna is centre fed with a seventy five ohms balanced transmission line, giving a good match to the antennas seventy two ohms input impedance.
A dipole can also be fed with a co axial transmission line with the centre conductor connected to one side of the dipole and the shield connected to the other. However some inefficiency will result because the dipole is balanced and co axial line is unbalanced. This inefficiency stems from the current and voltage distributions on the antenna being upset, which allows radio frequency current to flow on the coaxial shield and may cause undesirable radiations from the transmission lines.
To correct the imbalance between a co axial line and a dipole antenna a Balun must be used. A Balun is a balanced to unbalanced radio frequency transformer. Baluns are contracted for specific impedance ratios just like conventional transformers. When fifty ohm or seventy five ohm coaxial cable is used to feed a dipole impedance ratio of one is to one is used.
When vertical polarization is required the antenna must be vertical however because of antenna height considerations vertical antennas are usually used at relatively high frequencies.
The height restrictions at low frequencies can be overcome by constructing a Quarter wave vertical antennas above a perfect ground. The antenna will then have the same characteristics as a half wave vertical dipole, because a perfect ground will produce a mirror image of the quarter wave. If the ground is not a perfect conductor, the vertical quarter wave antenna will loose a considerable amount of power in the resistance of the grounding system. When an antenna is constructed over soil of poor conductivity such as sandy or rocky soil an artificial ground system must be used. The artificial system consists of quarter wave cooper wires called radials extending outward from the base of the antenna in as many directions as possible.
The half wave grounded antenna is known as Marconi antenna. The physical length of the Marconi antenna is normally only lambda by four of the signal to be radiated. However the earth acts on other half of the antenna making the overall effective length a full lambda by two. In other words the antenna consists of a conductor extending approximately lambda by four above the earth, and the earth acts as an additional lambda by four of the antenna. Electrically Marconi behaves as a half wave antenna.
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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect xm radio, xm satellite radio
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