Antenna Theory - Long-Wire
We have gone through different types of short wire antennas. Now, lets us look at the long wire antennas. The long wire antennas are formed by using a number of dipoles. The length of the wire in these type of antennas is n times λ/2
$$L = n \ \lambda/2$$
Where,
L is the length of the antenna,
n is the number of elements,
λ is the wavelength
As ‘n’ increases, the directional properties also increase.
Types of Long-wire Antennas
Long wire antennas are divided into two types namely − Resonant Antennas and Non-resonant Antennas.
Resonant Antennas
Resonant Antennas are those for which a sharp peak in the radiated power is intercepted by the antenna at certain frequency, to form a standing wave. The radiation pattern of the radiated wave is not matched with the load impedance in this type of antenna.
The resonant antennas are periodic in nature. They are also called as bi-directional travelling wave antennas, as the radiated wave moves in two directions, which means both incident and reflected waves occur here. In these antennas, the length of the antenna and frequency are proportional to each other.
Non-resonant Antennas
Non-resonant Antennas are those for which resonant frequency does not occur. The wave moves in forward direction and hence do not form a standing wave. The radiation pattern of the radiated wave matches with the load impedance in the non-resonant antennas.
These non-resonant antennas are non-periodic in nature. They are also called as Unidirectional travelling wave antennas, as the radiated wave moves in forward direction only, which means that only incident wave is present. As the frequency increases, the length of the antenna decreases and vice versa. Hence, the frequency and length are inversely proportional to each other.
These long-wire antennas are the basic elements for the construction of V-shaped antennas or the Rhombic antennas.
