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1. Why do we need infinite ground plane for monopole antennas ?
Ans.:
Like a dipole antenna, a monopole has an omnidirectional radiation pattern. That is it radiates
equal power in all azimuthal directions perpendicular to the antenna, but the radiated power
varies with elevation angle, with the radiation dropping off to zero at the zenith, on the antenna
axis. It radiates vertically polarized radio waves.
A monopole can be visualized (right) as being formed by replacing the bottom half of a
vertical dipole antenna (c) with a conducting plane (ground plane) at right-angles to the
remaining half. If the ground plane is large enough, the radio waves from the remaining upper
half of the dipole (a) reflected from the ground plane will seem to come from an image
antenna (b) forming the missing half of the dipole, which adds to the direct radiation to form a
dipole radiation pattern. So the pattern of a monopole with a perfectly conducting, infinite
ground plane is identical to the top half of a dipole pattern, with its maximum radiation in the
horizontal direction, perpendicular to the antenna. Because it radiates only into the space above
the ground plane, or half the space of a dipole antenna, a monopole antenna will have a gain of
twice (3 dB greater than) the gain of a similar dipole antenna, and a radiation resistance half that
of a dipole.
2. How do we decide practically the type of polarization we need use? And what kind of
polarization is used in mobile and satellite communication?
Ans.:
Different types of polarization are used in different applications to enable their advantages to be
used. Linear polarization is by far the most widely used for most radio communications
applications. Vertical polarization is often used for mobile radio communications. This is
because many vertically polarized antenna designs have an omni-directional radiation pattern
and it means that the antennas do not have to be re-orientated as positions as always happens for
mobile radio communications as the vehicle moves. For other radio communications applications
the polarization is often determined by the RF antenna considerations. Some large multi-element
antenna arrays can be mounted in a horizontal plane more easily than in the vertical plane. This
is because the RF antenna elements are at right angles to the vertical tower of pole on which they
are mounted and therefore by using an antenna with horizontal elements there is less physical
and electrical interference between the two. This determines the standard polarization in many
cases.
In some applications there are performance differences between horizontal and vertical
polarization. For example medium wave broadcast stations generally use vertical polarization
because ground wave propagation over the earth is considerably better using vertical
polarization, whereas horizontal polarization shows a marginal improvement for long distance
communications using the ionosphere. Circular polarization is sometimes used for satellite radio
communications as there are some advantages in terms of propagation and in overcoming the
fading caused if the satellite is changing its orientation.
3. What is antenna factor?
Ans.:
In electromagnetics, the antenna factor is defined as the ratio of the electric field strength to
the voltage V (units: V or µV) induced across the terminals of an antenna.
For an electric field antenna, the field strength is in units of V/m or µV/m and the resulting
antenna factor AF is in units of 1/m:
4. What kind of receiver would be there on stealth bomber or missile?
Ans.:
The concept of stealth is to operate or hide without giving enemy forces any indications as to the
presence of friendly forces. For this purpose, Radar Warning Receivers (RWR) are used.
Radar warning receiver (RWR) systems detect the radio emissions of radar systems. Their
primary purpose is to issue a warning when a radar signal that might be a threat (such as
police speed detection radar) is detected. The warning can then be used, manually or
automatically, to evade the detected threat. RWR systems can be installed in all kind of airborne,
sea-based, and ground-based assets (such as aircraft, ships, automobiles, military bases).