Non-Resonant and Resonant Antennas

Key Takeaways

• Traveling wave antennas and standing wave antennas are two classifications of long-wire antennas based on the distribution of voltage and current patterns.

• Non-resonant antennas or traveling wave antennas are configurations of antennas whose voltage and current can be represented as one or two traveling waves moving in the same direction.

• When the voltage and current distribution form standing wave patterns, then such types of antennas are called standing wave antennas or resonant antennas.

Dipole antennas are examples of resonant antennas

Theoretically, there are myriads of antenna configurations available to use in electronics and communication systems. However, bizarre types of geometries make it almost impractical to use most of them. The space to accommodate the antenna is a significant factor in determining its type.

Resonant and non-resonant antennas are two classifications of long wire antennas. Depending on the availability of the space, they are chosen for selecting the desired frequency for transmission and reception of electromagnetic signals. In this article, we will discuss long-wire, non-resonant, and resonant antennas.

Long-Wire Antennas

A long-wire antenna is a straight conductor of length equal to the wavelength or its multiples (n) that is used as an antenna. Long-wire antennas are formed by using a number of dipoles with length of wire (L) given by equation:

Note that n is the number of elements and is the wavelength. The directional properties of long-wire antennas increases as the value of n increases.

Long-wire antennas are designed such that they run parallel or horizontal to the earth’s surface. Usually, the input terminals of long-wire antennas are formed by one end of the wire and the ground. Long-wire antennas have traveling waves or uniform patterns in voltage and current and are an example of traveling wave antennas. 

Classifications of Long-Wire Antennas

Traveling wave antennas and standing wave antennas are two classifications of long-wire antennas based on the distribution of voltage and current patterns. In traveling wave antennas, the current and voltage can be represented using traveling waves in the same direction. Traveling wave antennas are non-resonant antennas. Standing wave antennas are bi-directional traveling wave antennas and are otherwise known as resonant antennas. 

Resonant Antennas  

When the voltage and current distribution form standing wave patterns, then such types of antennas are called standing wave antennas or resonant antennas. Resonant antennas exhibit a periodic nature. They have traveling waves propagating in forward and backward directions and are called bi-directional traveling wave antennas. In resonant antennas, radiated waves traveling in opposite directions establish incident and reflected waves.

For developing standing waves in resonant antennas, the sharp peak in the radiated power is intercepted by the antenna at a certain frequency, called resonant frequency. The frequency and length of resonant antennas are directly proportional to each other.

At the resonant frequency, resonant antennas are purely resistive with zero reactance. The pure resistive nature of resonant antennas allows maximum current to flow through them. The resonant antenna ensures a high standing wave current in it. The standing wave ratio of resonant antennas is greater than unity at resonance. At the resonant antenna terminals, the voltage and current waves are in phase. Such phase distribution between the voltage and current makes it easy to impedance match the resonant antennas with receiver and transmission lines. 

Non-Resonant Antennas 

Non-resonant antennas or traveling wave antennas are configurations of antennas whose voltage and current can be represented as one or two traveling waves moving in the same direction. Non-resonant antennas exhibit a non-periodic nature. There is always a progressive phase pattern associated with the voltage and current distributions in non-resonant antennas.

In non-resonant antennas, the waves travel in the forward direction or the same direction and are called unidirectional traveling wave antennas. As the radiated waves are moving in the same direction, there is only an incident wave in non-resonant antennas—there is no reflected wave. Moreover, the wave patterns in non-resonant antennas fail to form standing wave patterns.

In non-resonant antennas, there is no resonant frequency. The frequency of the non-resonant antenna shares an inverse relationship with the length of the antenna. As the frequency increases, the length of the antenna decreases and vice versa.  

Dipole Antennas 

Resonant antennas can be compared to resonant transmission lines. The voltage and current distribution in open-ended resonant wire antennas are similar to the standing wave patterns in open-ended transmission lines.

Dipole antennas are examples of resonant antennas. Dipole antennas are open-ended at the far end, with a resonant length that is equal to the multiples of a quarter wavelength. The current in resonant antennas is directly related to the magnitude of the radiated electromagnetic field. In dipole antennas, the amplitude current distribution can be constant, linear, or sinusoidal depending on the length of the antenna. For infinitesimal dipole antennas, the current distribution is constant. The current distribution is linear for short dipoles and sinusoidal for long dipoles.

Generally, resonant antennas, including dipole antennas, perform well and the antenna efficiency is very high. The antenna efficiency of dipole antennas is dependent on the construction of the antenna. Cadence software can help you in designing dipole antennas.

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