Test: Signals & Transmission Systems

For a coaxial cable with air dielectric and a shield of a given inner diameter, the attenuation is minimized by choosing the diameter of the inner conductor to give a characteristic impedance of 76.7 Ω. When more common dielectrics are considered, the best-loss impedance drops down to a value between 52–64 Ω.

An analog signal is any continuous signal for which the time-varying feature (variable) of the signal is a representation of some other time-varying quantity, i.e., analogous to another time-varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound waves. It differs from a digital signal, in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values.

Let O be the centre and R is the radius of the earth. PQ is a T.V. antenna of height h. Two tangents QT and QS are drawn from the top Q of the tower. T.V. signal can reach to the point S and T only not at other area:
∠QTO=90[TO⊥QT]
In ΔQTO,
OQ2=OT2+TQ2
OQ=R+h,  QT=d,  OT=R,
(R+h)2=R2+d2
R2+h2+2Rh=R2+d2
d2=h2+2Rh  h⋘R neglect
d2=2Rh
d= √2Rh

Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz (GHz). This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres.

For speech signals (as in telephone) frequency range 300 Hz- 3100 Hz is adequate i.e., a speech signal requires a bandwidth of 2800 Hz.

Optical communication using fibers is performed in the frequency range of 1 THz to 1000 THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of 100 GHz.

Distance of receiving antenna a from transmitting antenna is d = √(2Rh) i.e. d ∝ √h area covered ∝ d2 that is area covered ∝ h. Hence if the area is double, the height of the antenna will also be doubled.

The ground wave is the preferred propagation type for long distance communication using frequencies below 3 MHz (the earth behaves as a conductor for all frequencies below 5 MHz). The ground wave is also used for short distance communications using frequencies between 3 and 30 MHz.

Here , d=100km=100×10³m,
R=6.4×10⁶m
d=√2Rh
or h=d²/2R=(100×10³)²/2×6.4×10⁶=781.25m