Finding the Best Antenna Length
Why antenna length are the way it is.
Antennas are all around us, from broadcasting radio and TV signals to wifi routers and even our mobile phones. Yet, despite their difference in length, they are still able communicate with each other. This article aims to examine the significance (if any) of the length of an antenna.
Resonant frequency
Before we answer why antenna lengths are the way it is, it is imperative that we grasp the concept of resonant frequency first. In classical mechanics, resonant frequency occurs when the system is oscillating at its’ natural frequency. This causes a spike in the amplitude of the oscillating system. It is at this point that the system is able to absorb or dissipate the maximum amount of energy.
In the context of signals, resonant frequency occurs when the reactance1 of the antenna is zero. This means that any resistance experienced at this frequency is purely due to the material of the antenna. As such, the transmissibility at this frequency is at its’ maximum.
Multiples of 1/4 of wavelength
How then to achieve resonant frequency? The short answer is that any antenna length would be able to achieve maximum transmissibility so long as it is in multiples of 1/4 of the wavelength.
The longer answer is that since all signal typically transmitted and received by modern antennas are electromagnetic waves, it also means that they all travel at the speed of light regardless of medium.2 Recalling the formula v=fλ from high school, and with v (which in this case is the speed of light) begin constant, one can conclude that wavelength is inversely related to frequency. Hence, resonant frequency also depends upon the wavelength.
The more observant readers might have noticed the word multiple in the header. This is because for maximum transmissibility, the antenna need not be exactly 1/4 of wavelength, but rather a multiple of 1/4 of it. In other words, if a signal has a wavelength of 0.4m, the ideal antenna length could be 0.1m, 0.05m and so on. This is why different devices with different antennas length are still able to communicate on the same frequency.
Furthermore, it is also important to recognise that thus far this article has only talked about maximum transmissibility. This refers to when the transmission and reception of signals is at it’s best. Antennas are still able to receive signals even if its length is not best suited for the signal’s frequency, with the exception that the signals may be distorted or weaker. Nonetheless, such handicap can be overcome by the use of amplifiers, to simply make the signal strength much stronger in order to offset any reactance encountered.
Reactance is the opposition to the flow of current due to a circuit’s inductance and capacitance.
Visible light is also a EM wave.