Radio Waves

Radio waves are waves with wavelengths longer than infrared light. Like all other electromagnetic waves, they travel at the speed of light.  The lightning and astronomical objects produce naturally occurring radio waves.  The artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and other applications. Different frequencies of radio waves have different propagation characteristics in the Earth’s atmosphere.

• Different parts of radio spectrum are allocated to the various services.
• The shortest Radio wavelengths are of a few millimeters while the longest radio waves are several kilometers in length.
• Radio waves were first predicted by mathematical work done in 1865 by James Clerk Maxwell.
• In 1887, Heinrich Hertz demonstrated the reality of Maxwell’s electromagnetic waves by experimentally generating radio waves in his laboratory.

Understanding the Radio Spectrum

Part of the electromagnetic spectrum corresponding to radio frequencies is called Radio Spectrum. The radio-frequency spectrum (which is simply referred to as spectrum) is only a comparatively small part of the electromagnetic spectrum, covering the range from 3 Hz to 300 GHz. It includes a range of a certain type of electromagnetic waves, called radio waves, generated by transmitters and received by antennas or aerials. The frequencies in the Radio Spectrum are lower than around 300 GHz, which corresponds to wavelengths longer than about 1 mm.

Band

A small section of the spectrum of radio communication frequencies, in which channels are usually used or set aside, is called a Band.

Classification

To prevent interference and allow for efficient use of the radio spectrum, similar services are allocated in bands. For example, broadcasting, mobile radio, or navigation devices, will be allocated in non-overlapping ranges of frequencies. There are 12 bands as per the provisions of the ITU as follows:

[table id=182 /]

• So, when we move from ELF to THF, the frequency range increases
• When we move from ELF to THF, the wavelength decreases.

However, the above table gets significantly changed when we refer to the Institute of Electrical and Electronics Engineers (IEEE) specifications. The IEEE has divided the Radio Frequency spectrum into 13 bands starting from the lowest frequency HF Band to Highest Frequency mm Band as follows:

[table id=195 /]

But the Radio Society of Great Britain divides these frequencies as follows:

[table id=196 /]

We see that the above classifications of different radio bands have been done on the basis of the wavelength / frequency. On the basis of broadcasting, frequencies the Radio spectrum has been divided into the following parts:

• Long wave AM Radio = 148.5 – 283.5 kHz (LF)
• Medium wave AM Radio = 530 kHz – 1710 kHz (MF)
• Shortwave AM Radio = 3 MHz – 30 MHz (HF)

The radio spectrum has the excellent ability to carry codified information (signals) and that is why it is home of communication technologies such as mobile phones, radio and television broadcasting, two-way radios, broadband services, radar, fixed links, satellite communications, etc. Radio spectrum is a scarce resource, but is relatively cheap to build the infrastructure which can also provide mobility and portability.

As described above, depending on the frequency range, the radio spectrum is divided into frequency bands and sub bands. The following picture tries to simplify the above classifications:

What is sweet spot in Radio Spectrum?

We should know that Low Frequency (LF), Medium Frequency (MF) and High Frequency (HF) broadcasting bands (below 30 MHz) are still used in much the same way as they always have been since the birth of radio broadcasting over 80 years ago for Long Wave (LW), Medium Wave (MW) and Short Wave (SW) analogue broadcasting. Also in the HF band, a growing number of transmissions are being established in digital (DRM) format, primarily for international broadcasting. In the MF band, a limited range of frequencies are available for local analogue Medium Wave (MW) radio services. Please have a look at the following table that shows the general uses of different bands of the Radio spectrum.

[table id=197 /]

The highlighted portion of the above spectrum table shows that the maximum number of the popular services such as FM radio broadcast, television broadcast, PMR, DVB-T, MRI, PMR, television broadcast, microwave oven, GPS, mobile phone communication (GSM, UMTS, 3G, HSDPA), cordless phones (DECT), WLAN (Wi-Fi 802.11 b/g/n), Bluetooth falls in the VHF and UHF Band of the Radio Spectrum. This is evident from the following simple picture taken from BBC:

This portion is called “Sweetspot”. So, please note that sweetspot is the part of the Radio Spectrum, where most modern communication technologies such as DAB Digital Radio, digital television, 3G mobile phones and WiFi wireless Internet access services operate. The sweetspot, in fact, is the upper part of the Very High Frequency (VHF) band and the whole of the Ultra High Frequency (UHF) band, incorporating frequencies from around 200 MHz to 3 GHz as illustrated in this image.

What are Bands and Channels?

In theory, different communication technologies could exist in any part of the radio spectrum, but then we should note that, more information a signal is to carry, the more bandwidth it needs. In simple terms, bandwidth is the range of frequencies that a signal occupies in the spectrum. For example, an FM radio station might broadcast on a frequency of 92.9 MHz but requires a bandwidth of 0.3 MHz (300 kHz) – the spectrum between 92.8 and 93.0 MHz inclusive. Other stations cannot broadcast on these frequencies within the same area without causing or receiving interference. So for the purpose of the management of the services, the spectrum bands are divided into channels. The bandwidth of spectrum channels can vary band by band.

Ground waves

Lower frequencies (between 30 and 3,000 kHz) have the property of following the curvature of the earth via ground wave propagation. This is possible because of the interaction of the radio waves with semi-conductive surface of the earth. The wave “clings” to the surface and thus follows the curvature of the earth.

Why only lower frequencies propagate via ground wave?

Since ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth’s surface. This attenuation is proportional to the frequency and that is why this mode of propagation is useful for only LF and VLF frequencies.

Line of Sight

Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is one of the most common radio propagation modes at VHF and higher frequencies. Because radio signals can travel through many non-metallic objects, radio can be picked up through walls.

How Radio sets work?

The real nice thing about the radio waves is that they will make the electrons in a piece of copper wire move; this means they are capable of generating the electric currents in the wire. It works both the ways, alternating currents in a copper wire generate electromagnetic waves, and the electronic waves generate the alternating currents. The electric currents at “Radio frequencies” (rf) are used by the radio and television transmitters and receivers.

Each Radio system contains a transmitter that consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization). When transmitted, the transceiver “modulates” the RF with an alternating current generated by voice in a microphone. The Modulating frequency is called AF or Audio Frequency. Someone listening uses a receiver which can demodulate the Radio signal. The Receiver removes the RF to leave the AF and thus audio signal is fed to a loudspeaker.

The electromagnetic wave is intercepted by a tuned receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is “tuned” to respond preferentially to the desired signals, and reject undesired signals.

All radio receptions except FM band are affected when cable TV is switched on. Why?

This is a very simple question related to fundamentals of electromagnetic waves. Whenever we switch on electrical equipment, there are electrical currents in different parts of the circuit, which are associated with a magnetic field in its neighbourhood. The strength of this magnetic field reduces as the distance increases. If the current is alternating current (AC), equipment would work as a source of electromagnetic waves. These waves are called EM noise for other equipments. The AC current generates EM waves at such frequencies that can be detected by a radio receiver tuned to an amplitude-modulated band. In the neighbourhood of a noise producing equipment like a TV, the signal received by the antenna of the radio receiver is altered by the picked up EM waves; and it would be taken by the receiver as the total signal to process.

Why transistor radio gives a clear and loud reception when the antenna is held or touched by hand?

We all know that the antenna of a transistor radio is the first stage which collects the radio signal of all frequencies. The clarity and loudness of the audio depends, among many other factors, on the strength of the signal received by the antenna. If we increase the correction area, the signal received is enhanced. A long stretched conducting wire or a spread out conducting object work as signal enhancer when connected to the antenna. Human body has also some conductivity and when we touch the antenna of an operating radio set, we increase the effective signal collection area and thereby the signal strength gets improved and the loudness increases.