How does an Antenna work?

October 11, 2019

Antennas are widely used in the field of telecommunications and we know many applications for them. Antennas receive an electromagnetic wave and convert it to an electric signal, or receive an electric signal and radiate it as an electromagnetic wave. In this video we are going to look at the science behind antennas.


A detailed webpage version of the video is given below.


Difference between fluctuating and radiating electromagnetic field

We have an electric signal, so how do we convert it to an electromagnetic wave? You might have a simple answer in your mind. That is to use a closed conductor, and with the help of the principle of electromagnetic induction you will be able to produce a fluctuating magnetic field and an electric field around it as shwon in Fig:1A. However, this fluctuating field around the source is of no use in transmitting signals. The electromagnetic field here does not propagate; instead it just fluctuates around the source. In an antenna, the electromagnetic waves need to be separated from the source and they should propagate (Fig:1B). Before looking at how an antenna is made, let’s understand the physics behind the wave separation.

Fig:1A  Fluctuating electromagnetic field in electromagnetic induction
Fig:1B  Radiating electromagnetic field in a hypothetical antenna

Physics behind the oscillating dipole and radiation

Consider one positive and one negative charge placed a distance apart. This arrangement is known as a dipole, and they obviously produce an electric field as shown in Fig:2A. Now, assume that these charges are oscillating as shown in Fig:2B. At the mid point of their path the velocity will be at the maximum and at the ends of their paths the velocity will be zero. The charged particles undergo continuous acceleration and deceleration due to this velocity variation.

Fig:2A  The electric field lines of an electric dipole are
extended from the positive to the negative charge
Fig:2B  Acceleration and deceleration of charge particles

1. Electric field line at t=0

The challenge now is to find out how the electric field varies due to this movement. Let’s concentrate on only one electric field line (Fig:3).

Fig:3 Electric field is shown at t=0

2. Electric field line at t=T/8

The wavefront formed at time zero expands and is deformed as shown after one eighth of a time period (Fig:4A). This is surprising; you might have expected a simple electric field as shown at this location. Why has the electric field stretched and formed a field like this? as shown in Fig:4B This is because the accelerating or decelerating charges produce an electric field with some memory effects. The old electric field does not easily adjust to the new condition. We need to spend some time to understand this memory effect of the electric field, or kink generation, of accelerating or decelerating charges.

Fig:4A  At t=T/8, the expected shape of the electric field
Fig:4B  At t=T/8, the actual shape of the electric field

3. Electric field line at t=T/4

If we continue our analysis in the same manner, we can see that at one quarter of a time period, the wavefront ends meets at a single point (Fig:5).

Fig:5 At t=T/4, the ends of electric field meets at a single point and the separation and propagation happens

After this, the separation and propagation of the wavefront happens. If you draw electric field intensity variation with the distance, you can see that the wave propagation is sinusoidal in nature (Fig:6). It is interesting to note that the wavelength of the propagation so produced is exactly double that of the length of the dipole. We will come back to this point later. Please note that this varying electric field will automatically generate a varying magnetic field perpendicular to it. This is exactly what we need in an antenna. In short, we can make an antenna, if we can make an arrangement for oscillating the positive and negative charges.

Fig:6 Electromagnetic radiation in a dipole

How do radiation happen in antennas?

In practice, the production of such an oscillating charge is very easy. Take a conducting rod with a bend in its center, and apply a voltage signal at the center (7A). Assume this is the signal you have applied, a time varying voltage signal. Consider the case at time zero. Due to the effect of the voltage, the electrons will be displaced from the right of the dipole and will be accumulated on the left. This means the other end, which has lost electrons, automatically becomes positively charged (7B). This arrangement has created the same effect as the previous dipole charge case, i.e. positive and negative charges at the end of a wire. With the variation of voltage with time, the positive and negative charges will shuttle to and fro.

Fig:7A  A long straight wire with an AC source at its center is a dipole antenna radiating electromagnetic waves
Fig:7B  When a time varying voltage signal is applied, the electrons are
accumulated at one end and created positive charges on other end

The simple dipole antenna also produces the same phenomenon we saw in the previous section and wave propagation occurs. We have now seen how the antenna works as a transmitter. The frequency of the transmitted signal will be the same as the frequency of the applied voltage signal. Since the propagation travels at the speed of light, we can easily calculate the wavelength of the propagation (Fig:8). For perfect transmission, the length of the antenna should be half of the wavelength.

ƒantenna = ƒ input

C = ƒantenna x ƛ antenna

Fig:8 Antenna radiate electromagnetic waves at the speed of light

How do antennas receive signals?

The operation of the antenna is reversible and it can work as a receiver if a propagating electromagnetic field hits it. Let’s see this phenomenon in detail.

Take the same antenna again and apply an electric field. At this instant the electrons will accumulate at one end of the rod. This is the same as an electric dipole. As the applied electric field varies, the positive and negative charges accumulate at the other ends. The varying charge accumulation means a varying electric voltage signal is produced at the center of the antenna. This voltage signal is the output when the antenna works as a receiver as shown in Fig:9. The frequency of the output voltage signal is the same as the frequency of the receiving EM wave. It is clear from the electric field configuration that for perfect reception, the size of the antenna should be half of the wavelength. In all these discussions we have seen that the antenna is an open circuit.

Fig:9 An Antenna can work as a receiver if a propagating electromagnetic field hits it

Construction and working of few antennas

Now, let’s see a few practical antennas and how they work.

1. Yagi Uda antenna

In the past, dipole antennas were used for TV reception. The colored bar acts as a dipole and receives the signal as shown in figure. The dipole is the main driven element of it. A reflector and director are also needed in this kind of antenna to focus the signal on the dipole. The reflector element is always longer and the director element are always shorter than the driven element. This complete structure is known as a Yagi-Uda antenna (Fig:10A). The yagi uda antenna was invented by two japanese scientists Hidetsugu Yagi and Shintaro Uda. It is a directional antenna and used in point to point communication. The driven element or dipole antenna converted the received signal into electrical signals and these electrical signals were carried by coaxial cable to the television unit (Fig:10B ).

Fig:10A  A Yagi uda antenna consist of dipole, directors and reflectors
Fig:10B  The yagi uda antenna converts the received signals into electrical
signals and these signals are carried by coaxial cable to the television unit

2. Satellite dish antenna in detail

Nowadays we have moved to dish TV antennas. These consist of two main components, a parabolic shaped reflector and a low noise block down converter. The parabolic dish receives electromagnetic signals from the satellite and focuses them onto the LNBF as shown in Fig:11. The shape of the parabolic is very specifically and accurately designed.

Fig:11 In satellite dish antenna the incoming signal focused on to LNBF via parabolic reflector

The LNBF is made up of a feed horn, a waveguide, a PCB and a probe (12A). The incoming signals are focused onto the probe via the feed horn and waveguide. At the probe, voltage is induced as we saw in the simple dipole case. The voltage signal so generated is fed to a PCB for signal processing such as filtration, conversion from high to low frequency and amplification. After signal processing, these electrical signals are carried down to the television unit through a coaxial cable (Fig:12B).

Fig:12A  A detailed structure of Low Noise Blockdown Feedhorn (LNBF)
Fig:12B  The satellite dish converts the received electromagnetic signals
into electrical signals which are carried out by coaxial cable to the television unit

If you open up an LNB you will most probably find 2 probes instead of one, the second probe being perpendicular to the first one. The 2-probe arrangement means the available spectrum can be used twice, by sending the waves with either horizontal or vertical polarization. One probe detects the horizontally polarized signal and the other the vertically polarized signal as shown in Fig:13.

Fig:13 The horizontal and vertical probe detects horizontally polarized signal and vertically polarized signal respectively.

3. Microstrip antenna or Patch antenna

The cellphone in your hand uses a completely different type of antenna, called a patch antenna (Fig:14A). These types of antennas are inexpensive and fabricated easily onto a printed circuit board. A patch antenna consists of a metallic patch or strip placed on a ground plane with a piece of dielectric material in-between. Here, the metallic patch acts as a radiating element. The length of the metal patch should be half of the wavelength for proper transmission and reception (Fig:14B). Please note that the description of the patch antenna we explained here is very basic.

Fig:14A  Planar inverted F antenna, a type of patch antenna used in modern cell phones.
Fig:14B  A schematic of simple patch antenna


ABOUT THE AUTHOR



This article is written by Prerna Gupta, a post graduate in Control and Instrumentation. Currently she is working at Imajey consulting engineers pvt. ltd. as a Visual Educator. Her areas of interest are Telecommunication, Semiconductor Material and devices, Embedded systems and design.