Antennas and Radiation
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Page where the theory and practice of Antennas and Electromagnetic Radiation are discussed.
Contents |
General Theory
One of the most important relations in this theory is c = λf. This relates the wavelength (λ) of a electromagnetic wave to its frequency (f).
Types of Antenna
Half-wave Dipole
This is probably the simplest type of antenna. A half-wave dipole is, predictably, approximately half of a wavelength long at the intended frequency of operation. It is generally center-fed, and has in such a configuration, it has an approximate impedance of 73 ohms.
Other variations of the half wave dipole include the Inverted Vee, which brings the radiating elements of the dipole down to approximately 45 degrees from the feed line. This brings the feedpoint impedance nearer to 50 Ohms, and with the use of a 1:1 balun, allows a half wave dipole to be reliably fed with coaxial cable.
Magnetic Loop
Theory of Operation
A magnetic loop antenna is an antenna that radiates by means of an almost completely magnetic near field, rather than an electric field, or a combination of electric and magnetic fields. The antenna consists of a loop of wire or other conductor, no longer than 1/3 of a wavelength at the frequency of operation, typically shaped in a circle, octagon, or square. Other shapes are possible too, but a circle is the most efficient, as it encloses the most cross-sectional area with a given length of conductor. The loop may consist of one single turn, or multiple turns. Care must be taken when using multiple turns, though, as they are less efficient and have lower Q. This loop of wire will have some inductive reactance, so a resonant value capacitor is used to cancel out any reactances, leaving behind only the ohmic resistance and the radiation resistance of the loop.
The radiation resistance of a loop antenna is expressed as:
Where RRAD is the radiation resistance of the antenna in ohms, A is the total area enclosed by the loop in square meters, and λ is the wavelength of the operating frequency. For most practical loop antennas, the radiation resistance is going to be a very small fraction of an ohm.
To calculate the overall efficiency of the loop antenna, one must also know the ohmic resistance of the conductor used, including the resistance of all connections and joints along the loop, called loss resistance. The formula used is:
Hence, if the radiation resistance and loss resistance are equal, the efficiency of the antenna is .5, or 50%. This represents a signal strength loss of approximately 3dB.
