Radiation Resistance Of Dipole Antennas
As I discussed in Chapter 5, the amount of power that an antenna radiates is a function of the electrical length of the antenna. A 1 MHz
(AM radio) electric dipole antenna that is 150 m long radiates the same amount of power as a 100 MHz (FM radio) electric dipole that is 1.5 m long. The equivalence stems from the fact that both antennas have the same electrical length, 1/2. The radiating efficiency is usually expressed in terms of a quantity called radiation resistance. The power emitted by an antenna can be calculated by taking the product of the radiation resistance and the average current on the antenna,
P 1antenna ¥ Rradiation
It is important to keep in mind that the current on antennas is not necessarily the same at different points along the antenna. Just as with long, unmatched transmission lines, the current on antennas can form standing waves. For example, the current on a dipole must go to zero at the ends because the ends are unconnected.
For electrically small antennas, the radiation resistance of an electric dipole is proportional to the second power of the electrical length of the antenna.
The radiating efficiency from magnetic dipole antennas is also a function of electrical size of the loop. For a loop antenna, the radiation resistance depends on the area of the loop, and is therefore proportional to the fourth power of the electrical length of the loop circumference. Figures 11.3 and 11.4 compare the radiation resistance of electric and magnetic dipoles as a function of electrical size.
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