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ANTENNA ARRAY – AN FM GENERATOR
Prof Manilal D Amipara,
Research scholar CMJ University
Head(EC) – Babaria Institute of Technology, Varanama, Vadodara(Gujarat)-391240
E-mail:md_ami@yahoo.co.in, (M)09426763701
ABSTRACT : The paper initially, discussed about Antenna basic & its importance in an
electronic/communication system. It talks about role as an interface between inner world of the system and
external world. It explains how the circularly mounted antenna array, switched one by one, creates situation of
rotating radiating element. The radiated signal suffers the Doppler Effect and outcome is variable frequency.
The variation in frequency is depends on antenna switching frequency. Sinusoidal switching signal results into
frequency modulated signal in a space. Finally, total system block is prepared with FM generator explanation.
Finally, its application as an electronic compass either as a transmitter or as a receiver for military and/or
police department is concluded.
Keywords—Antenna, array, Doppler Effect, Electronic compass, FM generator, Frequency shift, Switching
antenna.
I: INTRODUCTION
Antenna can be defined as “The bidirectional metallic
device (or transducer) which can transmit and/or
receive electromagnetic energy into/from space” [1]
[2]. Antenna is very important part of any
electronic/communication system which is an
interface between inner world of the system and
external world [3].
Its working principle is: An accelerated or
decelerated current results into electromagnetic field
radiation or time varying current results into radiation
of electromagnetic field [1]. The above wording can
be express in a simple mathematics as
[I] * L = [q] * v
(1)
Here the term in bracket [] indicates accelerated or
decelerated value [1][2]. The Maxwell has given first
prediction of radiation. The term L is a dimension of
an antenna and for the optimum value of antenna
length is half wave length for the maximum power
transfer.
Multiple numbers of antennae used to radiate/
receive the electromagnetic signal is called antenna
array.
When antenna used in array, it provided higher gain
and narrow beam width [3]. Width of beam can be
controlled either by increasing number of antenna or
varying phase of feed signal for each antenna [1].
The array of an antenna arranged in a specific fashion
in terms of physical mounting and switched one by
one, results into radiated signal frequency modulated
(FM) signal.
II: HYPOTHESIS
H1) All antennas radiate energy in all directions in
free space. Isotropic antenna (Hypothetical antenna)
radiates equal amount of energy in all the direction.
H2) Antenna elements are kept in circular fashion on
a circumference of a circle and spaced at an equal
angle.
H3) Each antenna in an array are spaced at an equal
distance.
H4) Distance between observer and array is very high
as compare to array aperture.
III: ARRAY
The array of antenna arranged in a circular form and
each element placed on the circumference of a circle
(Marked as 1 to 8), equiangular spaced as shown
below figure 1.
Fig 1 Circular Array Antenna & distant Observer
Let, RF energy source ‘S’ is feeding constant
frequency, ft, to antenna array and each element (1 to
8) are switched sequentially with a constant
switching frequency, fs, (Only one is ON at a time)
and ft >>fs. This creates a situation, as if one antenna
is rotating at a constant number of revolutions per
second around a circle with radius ‘r’. Since a
direction of motion of S is changing, its velocity is
also changing; however the magnitude of its velocity
is constant. The net effect is, it experiences the
Doppler Effect
IV DOPPLER EFFECT
The Doppler Effect can be described as the change in
observed frequency when there is relative motion
between a transmitter and a receiver. This change in
frequency, the Doppler shift, is directly proportional
to the relative speed between the transmitter and the
observer.
If a relative velocity (vr) exists between S and O then,
received frequency
fr = (C +/- Vr) / λ = C / λ +/- Vr / λ (1)
fr = ft +/- Vr / λ (2)
Where, C / λ = ft , & Vr / λ = Doppler shift
Doppler Effect causes the frequency of a received
wave to change when a change in distance occurs
between the transmitting and receiving points for the
wave.
When the frequency source is moving toward the
observer, each successive wave crest is radiated from
a position closer to the observer than the previous
wave. Hence, successive cycle takes slightly less time
to reach the observer than the previous cycle and the
time between the arrival of successive cycle at the
observer is reduced, causing an increase in the
frequency. While they are travelling, the distance
between successive wave fronts is reduced; the waves
experiences "bunching effect". Conversely, if the
source is moving away from the observer, successive
cycle is emitted from a position farther from the
observer than the previous one and the arrival time
between successive cycles is increased, reducing the
frequency. The distance between successive wave
fronts is increased, the waves experiences "spreading
effect".
Now, let us consider a case where RF source rotating
with a constant velocity, on circumference of a circle
having radius ‘r’ as shown in fig 2 below, it is a
combine phenomena of bunching and spreading.
Consider the instant S is at point A. Source S is
moving in the direction as shown by its velocity
vector vs at point A (perpendicular to OA axis). The
distance between S and O is NOT changing. The
frequency received at O at this instant is NOT
modified by Doppler Effect. The same condition
occurs the instant S is at position C.
Fig 2 Circularly rotating source with distant observer
When S is at point B or D, Doppler Effect is most
pronounced because the rate of distance change
between S and O is greatest. At B, source S is
moving away from O in a straight line. At D, source
S is moving towards O in a straight line
In other words, if the source approached the
observer directly, thevelocity would remain
constant (as vs, r is only the radial component).
Because the source passes by the line ‘so’ er, the
radial velocity does not remain constant, but
instead varies as a function of the angle between
itshis line of sight and the source's velocity:
(3)
Where vs is the velocity of the object (source of
waves) with respect to the medium, and θ is the angle
between the object's forward velocity and the line of
sight from the object to the observer.
Consider a general case when S is at a point on the
circle, say point between A & B. Let vs be its velocity
then
vs = r (4)Ѡ
Where, is angular velocity = 2πf (f is orbitalѠ
frequency) & r = radius
While moving path A-B-C (half circumference),
scenario is, source is going away from Observer and
moving path C-D-A, source is going towards the
Observer. Thus, the observed frequency will have a
Doppler shift in a frequency at the point of
observation.
V: FM SYSTEM
The arrangement of an antenna array is made such
that the multiple antenna placed on the circumference
and one antenna placed at the centre of a circle. The
same or synchronized RF source is used as a feeding
signal. The antenna on circumference are switched
one by one. The block diagram of a system is shown
in figure 3 with antenna array
Fig 3 FM System Block Diagram
RF Source: It consists of an oscillator which
generates carrier frequency. RF output is fed to an
Antenna Switching Unit to convert it from one line to
8 lines.
Antenna Switching Unit (ASU): The unit consists
of RF diodes (PIN diodes). It is one line to 8 line
divider. There are eight lines, each connected to RF
input through PIN diode to have 8 output. The PIN
diodes are made ON by control lines, one by one.
Thus, at a time only one antenna is ON (radiates).
Each antenna is switched at the rate of 1/8fs, where fs
is switching frequency.
Control Unit: The unit provides required clock/s and
signal/s. It may be digital or processor based circuit.
System Description:
The antenna (1 to 8 in a fig 6) are placed at the
circumference of a circle and radiating the source
frequency (ft), one by one, as it is switched ON in
that fashion. The observer (point in a space) will
receive the frequency the multiple antenna placed on
the circumference and switched, has change of
frequency. In a space, the received is from the
switched antenna which has frequency of (ft +/- ∆f).
The +/- ∆f, Doppler shift, is directly proportional to
the relative velocity, in turn, diameter of a circle
(D=2r) on which antenna is placed and switching
frequency (fs). For a constant diameter, as switching
frequency is sinusoidal, the +/- ∆f is also sinusoidal.
The received signal in a space at any point, P,
(Unmodulated signal fed to the array) is frequency
modulated signal. Modulation occurs in a space
hence also called space modulation or special
modulation.
VI: APPLICATIONS
Antenna is used in the systems such as radio and
television broadcasting, point to point as well as
point to multi point communication, navigations,
ground to ground or air communication etc.
System can be used as an electronic Compass for
finding a Bearing with reference to North, either as a
transmitter or as a receiver. The antenna switching
signal can be a reference signal if first antenna
switched is in the north. The direction of transmission
or reception can be calculated by analysing
composite signal with reference to switching signal.
In case of reference required at the other end, RF
source can be modified to have two outputs. One for
ASU and other can be fed to additional antenna
which can be mounted at the centre of array. Two
signals, one from centre antenna and other from
circular array. Based on phase difference of the two
signals, bearing can be determined.
VII: CONCLUSION
1) The antenna is a critical part of any
communication system and it is frequency dependant.
2) Frequency of radiated signal from antenna is,
generally, same as the input signal frequency.
However, specific arrangement can be created which
can create variable frequency in a space.
3) System can be (i) FM generator (ii) an electronic
compass (like magnetic compass) transmitter (iii) an
electronic compass as a receiver.
4) If system is used as a receiver, it can detect the
direction of arrival of a signal either unknown signal
(for military – enemy signal location) or unauthorised
radiation (for police department).
REFERENCE
[1] Antennas for all application By John D Krauss
[2] Antenna Theory, Analysis & Design by Balanis
[3] Handbook of airport manual