Frequency in UHF range propagated by means of
Space wave
Each propagation mechanism owns a band, and UHF (300 MHz to 3 GHz) belongs to the space wave — option 2.
| Mode | Band | Mechanism | Range |
|---|---|---|---|
| Ground / surface wave | VLF – MF, below ~2 MHz | Follows the earth's curvature, guided by the conducting ground | Up to ~100 km |
| Sky wave | HF, 3 – 30 MHz | Refracted back by the ionosphere | Thousands of km |
| Space wave | VHF and above, > 30 MHz | Direct line of sight plus ground reflection | Radio horizon |
Why the ionosphere stops working at UHF. A wave is returned only if its frequency is below the critical frequency for the angle of incidence,
\(f_{MUF}=\dfrac{f_{c}}{\cos\theta}\)
and the maximum usable frequency rarely exceeds about 30 MHz. A UHF wave passes straight through the ionosphere and into space — a defect for terrestrial links but exactly what makes UHF and above the bands used for satellite communication.
Why the ground wave also fails. Surface-wave attenuation rises steeply with frequency, because the wave induces currents in the ground and the loss grows as the skin depth shrinks. Below 2 MHz the ground guides the wave for hundreds of kilometres; at UHF it is absorbed within metres.
What is left is the space wave, which travels in a straight line from transmitting to receiving antenna, together with a ray reflected from the ground. Its range is set by the radio horizon:
\(d\ \text{(km)}=4.12\left(\sqrt{h_{t}}+\sqrt{h_{r}}\right)\qquad h\ \text{in metres}\)
the factor 4.12 rather than 3.57 reflecting the slight downward refraction of the atmosphere, which extends the radio horizon about a third beyond the optical one.
The practical consequences are familiar: UHF television and mobile services need tall masts and a dense network of sites, coverage stops at the horizon, and the same line-of-sight property allows the same frequency to be reused a few tens of kilometres away — the basis of cellular planning.
Hence, UHF propagates as a space wave.
Similar Questions
If the axial ratio of an electromagnetic wave is zero dB. Then, the wave is:
A plane wave is propagating in a uniform medium with propagation constant \((0.6\pi + j\,0.4\pi)\) its wavelength is
Given below are two statements :
Statement I : Phase velocity of an electromagnetic wave in a bounded medium can be greater than the speed of light in an unbounded medium.
Statement II : Frequency of electromagnetic wave is affected by the permittivity of the medium.
In the light of the above statements, choose the correct answer from the options given below :
Assertion (A) : The ionosphere is that region of the earth’s atmosphere in which the constituent gases are ionised by radiations from outer space.
Reason (R) : Throughout the ionosphere, there are several layers in which ionization density either reaches maximum or remains almost constant. It depend on the intensity of sun. It also depends upon the atmospheric pressure.
Ultra High Frequency (UHF) spectrum is defined as :
Assertion (A) : Light is electromagnetic wave in nature, these may be x-rays, radio waves, microwaves etc.
Reason (R) : The amount of energy depends on the intensity of the light rays. The energy associated with each photon is proportional to the frequency.
Select your answer using the codes given below :
The free space wave number ‘k0’ is defined as
Important Questions from Electromagnetic Wave Propagation
If the Polarization vector is given as N and the Direction of propagation is given as K then which one of the following relations is correct?
Ez ≠ 0 and Hz ≠ 0 represent which type of mode of propagation of microwaves?
A lossless transmission line has a characteristic impedance of Z0 and capacitance per unit length of C. The velocity of propagation of the travelling wave on the line is
The phenomenon of microwave signals following the curvature of earth is
The relationship between wavelength and frequency of an electromagnetic wave
