In the radio-frequency spectrum, the various frequency ranges possess different propagation characteristics thus each part of the spectrum is used for specific types of applications. The propagation characteristics of radio waves show high dependency on frequency.
In the previous content on the radio frequency spectrum, we have discussed that a part of the electromagnetic spectrum with the lowest frequency range holds radio waves. More simply, the waves acquiring the lowest frequency portion in the electromagnetic spectrum are known as radio waves.
Before discussing the characteristics of the radio waves let us first discuss how the radio frequency band is divided and their typical radio services.
1. Very Low Frequency (VLF): Its range is between 3 to 30 kHz. It is used in worldwide telegraphy.
2. Low Frequency (LF): The frequency lies within 30 to 300 kHz. Generally, navigational aids, point to point long-distance communication uses this frequency range.
3. Medium Frequency (MF): It ranges between 300 to 3000 kHz and is used for the purpose of navigation.
4. High Frequency (HF): Its range is between 3 to 30 MHz and finds applications in general communication between two points that are separated at a moderate distance and short-wave broadcasting to distant places.
5. Very High Frequency (VHF): VHF holds a frequency between 30 to 3000 MHz and is suitable for short-distance communication in case of radar, telephony, and frequency modulation.
6. Ultra-High Frequency (UHF): It ranges between 300 to 3000 MHz and used in television and in radar relay systems for short-distance communication.
7. Super High Frequency (SHF): The frequency between 3000 to 30,000 MHz is referred to as SHF and is used in satellite communication as well as radar, radio television relay links.
8. Extremely High Frequency (EHF): Its range is between 30,000 to 300000 MHz and used in fields of experiments and amateur.
Also, there exists another frequency band known as extremely low-frequency band abbreviated as ELF that has a frequency range between 30 to 300 Hz, and the wavelength ranges between 10,000 to 100 km. This frequency range is used in all such applications where penetration of the waves is required at great depths into the ground and the oceans. Thus, shows suitability in communication to submarines.
In order to study wave characteristics, the above-discussed frequency bands are subdivided which are given as:
Let us now proceed further and understand the propagation characteristic of each type of wave according to their respective frequency band which is shown in the figure above.
Characteristics of VLF waves – Range between 3 to 30 kHz
- It is primarily used in ground wave propagation.
- As these are very low-frequency waves, thus, possess long wavelengths and hence holds the ability to get traveled to thousands of km along the surface of the earth.
- These are not generally used for the purpose of conventional communication as its bandwidth is limited by the lower carrier frequency of the waves.
- Waves in this frequency range are propagated completely getting reflected by the lower portion of the ionosphere and the earth. This means like the wave is guided within the waveguide, in a similar way this frequency waves are guided within the region of ionosphere and earth.
- The approximate height possessed by these waves lies between 70 to 80 km.
- These waves offer low attenuation at frequencies around 20 kHz and propagate to long distances but need antennas that possess huge physical sizes.
- On a general basis, the attenuation offered in VLF propagation is 30 dB/ 1000 km.
- The transmission method involved in VLF propagation is quite inefficient but it is occasionally useful as it eliminates the surface wave transmission variations.
Characteristics of radio waves which are a combination of VLF and LF – Range is 20 to 100 kHz
- This frequency range uses ground wave as well as skywave propagation. Generally, up to a distance of 1000 km ground wave propagation is used while for more than 1000 km sky wave propagation is used.
- The ground wave propagation provides relatively low attenuation. However, sometimes received signals suffer some seasonal or yearly changes.
- Normally the propagated signals in this range are not subjected to fading.
- The signals transmitted in the night hours are stronger than during day time.
Characteristics of radio waves which are a combination of LF and MF – Range is 100 to 535 kHz
- The frequency range majorly opts for sky wave propagation when the distance is moderate. As with an increase in frequency, attenuation increases in the case of ground waves.
- During the day hours, there exist high absorption in sky wave propagation at such high frequencies thus is not suitable for long-distance communication. However, shows reliability in communication during night hours.
- With the increase in frequency, the range of ground wave propagation is reduced.
Characteristics of radio waves which is some parts of MF – Range is 535 to 1600 kHz
- The major application of these radio waves is radio broadcasting.
- As sky wave propagation in daytime leads to complete absorption of the signal thus during the day hours broadcasting is facilitated by ground wave propagation. However, in the ground wave propagation, during the daytime, the signal strength shows rapid reduction at these frequencies.
Characteristics of radio waves which are a combination of MF and complete HF – Range is 1600 kHz to 30 MHz
- When the frequency range exceeds 1600 kHz then only in case of short distances ground wave propagation is used. This is so because, at this frequency range, a high attenuation of the ground wave is noticed. So, as far as long-distance communication is concerned, sky wave propagation is used.
- The maximum usable frequency i.e., the upper limit for communication shows dependency on distance, height along with electron density at the points of reflection in the ionospheric region.
- The lower frequency limit depends on the noise level and absorption over the path.
- There is an optimum frequency level which is considered at around 15% below the upper limit as at this frequency the short-term fluctuations are considerable.
- For communication beyond 1000 km, the determination of optimum frequency is done by the F2 layer. While distance ranging between 200 to 1000 km, the determination of maximum usable frequency is done by E layer.
Characteristics of HF waves – Range between 3 to 30 MHz
- The long-distance communication with such high frequency is facilitated by sky wave propagation.
- At these frequencies the ionospheric absorptions are quite low, hence, even miles of propagation distance is achievable.
- When short-distance communication is required then low frequencies are needed to be used. While at lower frequencies the D layer introduces appreciable attenuation.
Characteristics of waves with a frequency above 30 MHz
- Generally, above 30 MHz, space wave propagation is practically used. In very rare cases only, the frequencies above 30 MHz undergo reflections by the ionosphere towards the earth.
- The transmission range offered is usually less than 100 miles.
- Here the major cause of unwanted interference is reflections due to buildings or other structures.
- At this frequency, the major applications involve broadcasting of television signals, in radar, in aircraft, in radio-relay systems. For short-distance radio communication, these include point to point and mobile walkie-talkie, etc.
So, from this discussion, it is clear that the spectrum distribution of radio waves categorizes its various properties.