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 article we are going to look at the science behind antennas.
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.
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.
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).
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.
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).
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.
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.
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
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.
Now, let’s see a few practical antennas and how they work.
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 ).
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.
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).
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.
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.
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. To know more about the author check this link