EXPERIMENT NO.1
AIM:-To study the variation of field strength of radiated wave with distance from transmitting antenna. Equipment Required:•
•
Yagi (4el) & Yagi (3el) antenna Antenn Antenna a Tripo Tripod d & gradua graduated ted dis disc/s c/step tepper per motor motor tripo tripod d with with connecting cables, measuring tape.
Theory:Inverse square law, radiation:- As one of the fields which obey the general inverse square law, a point radiation source can be characterized by the relationship below whether you are talking about Roentgens, rads or rems. All measures of exposure will drop off by inverse square law. Inverse square law, electric: - As one of the fields which obey the general inverse square law, the electric field of charge can be put in the form shown below where point charge Q is the source of field. The electric force in Coulomb’s law follows the inverse square law. Inverse square law, general: - Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. The intensity of the influence at any given point r is the source strength divided by the area of sphere. Being strictly geometric in its origin, the inverse square law applies to diverse diverse phenomenon phenomenon point source of gravitation gravitational al force, force, electric electric field light, light, sound or radiations obey the inverse square law. Procedure:1. Keep both tripods at a minimal distance of 0.5 from each other, centre to centre
using measuring tape. 2. The minimal distance ensures that we are testing the antennas in far field
region. 3. Transmitted RF signal from Yagi(3el) antennas is intercepted by Yagi(4el)
send to receiver. 4. Note down the level reading at 0.5 meter distance.
5. Take the reading at 1m distance. Ensure that no scattering objects are in the
vicinity of antenna. 6. Take further readings at 1.4 & 2m. 7. Readings can be distorted if the Yagi(4el) captures signal from its behind due
to wall or ceiling. 8. Plot these readings on Cartesian plane with distance between antennas on X-
axis & signal level in dB on Y-axis. 9. Repeat the measurement once again to confirm the readings.
Result: - The field strength reduces as the distance between
transmitting & receiving antennas is increased. The power intercepted by the receiving antennas from the transmitting antenna is inversely proportional to the square of the distance between them. Hence every time the distance between the antennas is doubled, the received power gets reduced by one fourth. Parabolic plot of received signal strength versus distance between antennas provides the inverse square law for ideal conditions.
EXPERIMENT NO.8 Study of log-periodic antenna Objective:(a) T o study the radiation pattern of log-periodic antenna in Azimuth & Elevation planes on log & linear on polar & Cartesian plane. (b) To measure the beam width(-3dB),front to back ratio ,side lobe level & its angular position ,plane of polarization, directivity & gain of log-periodic antenna. (c) To study the antenna resonance & measure VSWR, impedance,
impedance bandwidth using RLB & adjust the antenna dimensions for resonance.
Equipment Required:•
Antenna Transmitter, receiver & stepper motor controller
•
Dipole antenna, Log-periodic antenna
•
Antenna Tripod & stepper pod with connecting cables, Polarization connector
Theory: - Log-periodic antennas are broadband antenna. Bandwidth
of 10:1 is easily obtained & even 100:1 is feasible if the theoretical design closely approximated. The broadband characteristics of logperiodic antennas include both impedance & pattern. The logperiodic principle can be understood by the “log-periodic dipole array”. It is observed that all the dimensions increase in proportion to the distance from the origin. It has a number of dipoles of different lengths & spacing & is fed by a balanced two wire transmission line that is transposed between each adjacent pairs of dipoles. It is fed at narrow end & the maximum beam radiation is shown. Dipole length increase along the antenna such that included angle alpha is constant. Lengths & spacing are graduated in such a manner that certain dimensions of adjacent elements bear a constant ratio to each other.
These dimensions are length L & spacing R or S. Scale factor or design ratio is represented by τ. Value of τ is less than 1.Hence dipole lengths & spacing are related asR1/R2=R2/R3=R3/R4………….Rn/Rn+1 = τ = L1/L2=L2/L3=…………=Ln/Ln+1
Or R n/R n+1=Ln/Ln+1= τ Procedure:-
T o plot the radiation pattern of log-periodic antenna in Azimuth & Elevation planes on log & linear on polar & Cartesian plane.
1. Connect the dipole antenna to the tripod & set the RX frequency to
600MHz.Keep the antenna in horizontal direction. 2. Now connect the log-periodic antenna to the stepper tripod. 3. Adjust log-periodic elements as per fig.
Result: - The log-periodic antenna is polarized in the direction of its
elements. A log periodic antenna with elements ends horizontal is horizontally polarized & with vertical elements is vertically polarized. Log periodic antenna is basically frequency independent type of antenna.
EXPERIMENT NO.3 Study of directional antenna Objective: -
To plot the radiation pattern of directional antenna.
Equipment Required:•
•
Dipole antenna & Yagi antenna Antenna Tripod & connecting cables
graduated
disc/stepper
motor
with
Theory:-
An isotropic radiator is a fictitious radiator & is defined as a radiator which radiates uniformly in all directions. It is also called isotropic source or omnidirectional radiator or simply unipole. An isotropic radiator is a hypothetical lossless radiator or antenna with which the practical antennas are compared. Thus, an isotropic radiator or antenna is used as a reference antenna. Since all the practical antennas have at least same directional properties i.e. directivity & hence there is no such thing an isotropic or electron energy. However in acoustic we have & a point source of sound is an example of isotropic radiator. Radiation pattern of antenna is nothing but a graph which shows the variation in actual field strength of EM field at all points which are at equal distance from the antenna. Obviously the graph of radiation pattern will be 3-D & hence cannot completely be represented on plain paper. In order to draw the radiation pattern of antenna field strength are measured at every point which lies on the surface of an imaginary sphere of fixed radius treating antenna as centre & then a 3-D solid figure is constructed from the reading so obtained. In this case distance from the centre to the surface of sphere represents the field strength in that direction. In order that the radiation pattern of an antenna is represented on plain paper, the 3-D solid figure so obtained is cut by a plane passing through the fixed point & the figure now obtained are used to represent the radiation pattern in a usual way. The radiation patterns are different for different antennas & are affected by the location of antenna w.r.t ground. The gain of an antenna is a basic property which is frequently used as figure of merit. Gain is closely associated with directivity & directivity itself dependent entirely upon the shape of radiation patterns of an antenna. The ability of an antenna or antenna system to concentrate the radiated power in a given direction or conversely to absorb effectively the incident power from that direction is specified by various
antenna terms e.g. antenna gain or simply gain or directive gain or power gain or directivity. In case of an antenna, Gain is a relative term in which the actual antenna is compared with a reference antenna. The reference antenna normally used in hypothetical lossless isotropic radiator or antenna. The gain of the antenna may be defined in any of the following ways:(1) Gain of the antenna without involving the antenna efficiency is defined as”the
ratio of maximum radiation intensity in gain direction to the maximum radiation intensity from a reference antenna produced in the same direction with same power input. Gain (G) = Maximum radiation intensity from test antenna/ Maximum radiation intensity from reference antenna with same power input (2) When reference antenna is taken as isotropic antenna, the gain of subjective
antenna is G0 G0 = Maximum radiation intensity from test antenna/ radiation intensity from isotropic antenna with same power input ’
G0 = Φm /Φ0 ’
Where, Φm = Maximum radiation from test antenna
Φ0= radiation intensity from lossless isotropic antenna Since gain denotes concentration of energy, the high values of gains are associated with narrow beam-width. Further gain of an antenna is closely related to directivity (D). Gain is equal to directivity are same otherwise not. When efficiency is cent percent, then gain & directivity are interchangeably used. Procedure:1. Connect the dipole antenna to the transmitter tripod, set the length of the
antenna elements to λ/4 or 12cm each from centre of the beam. Keep the antenna in horizontal direction. 2. Now connect the yagi antenna to the receiver graduated disc/stepper motor tripod & set the receiver to 600MHz.
3. Set the distance between antennas to be around1m. Remove any stray object
from around the antennas, especially in the line of sight. Avoid any unnecessary moment while taking the readings.
4. Now rotate the yagi antenna around its axis in steps & note down. 5. Plot the readings in polar or Cartesian plane. 6. Now replace the yagi (4el) or Cartesian plane & find whether they are
directional antennas or not. Result: -
The plot shall form the major lobe in bore sight direction & smaller side lobes in other direction on polar plane in azimuth. On Cartesian plane the response shall form a big hump & several smaller ones around it. Similar to the one shown in fig. below. Thus a yagi antenna is highly directional antenna.
EXPERIMENT NO.4 Objective: -To demonstrate the transmitting & receiving antennas radiation patterns of an antenna are equal & hence confirm the reciprocity theorem of antennas. Equipment Required:•
•
Dipole antenna & Yagi antenna Antenna Tripod & connecting cables
graduated
disc/stepper
motor
with
Theory:-
The reciprocity theorem is the most powerful theorem in circuit & field theories both.
Reciprocity theorem states that if an emf is applied to the terminals of an antenna no.1 & the current is measured at the terminals of another antenna no.2, then an equal current both in amplitude & phase will be obtained at the terminals of antenna no.1 if the same emf is applied to the terminals of antenna no.2. (OR) If a current I 1 at the terminal of antenna no.1 induces an emf E 21 at the open terminal of an antenna no.2 & the current I 2 at the antenna no.2 induces an emf E 12 at the open terminal of an antenna no.1, then E12 = E21 provided that I1 = I2. Assumptions:-
It is assumed that:(1) Emf’s are of same frequency (2) Medium between two antennas are linear, passive & isotropic. (3) Generator producing emf & the ammeter for measuring the current have zero impedance & or if not, then both generator & ammeter impedance are equal. Explanation:1. A transmitter of frequency f & zero impedance is connected to the terminals of
antenna no.2, which is generating a current I 2 & inducing an emf E 12 at
the open terminal of an antenna no.1.
2. Now the same transmitter is transferred to antenna no.1 which is
generating a current I2 & inducing a voltage E21 at the open terminal of an antenna no.2. Thus, according to the statement on reciprocity theorem, I1 = I2 provided E12 = E21 Since, ratio of emf to current is impedance. Therefore the ratio of E12/ I2 is given the name transfer impedance Z 12 as in case 1, & so also the ratio E21/ I1 as transfer impedance Z21 as in case 2. The ratio of voltage,E 1 to the current,I 2 is defined as Transfer Impedance ZT or Z12 i.e, ZT = Z12 = E1/ I2 Thus, from the reciprocity, it folllows that the two ratios i.e two
impedances are equal i.e Z12 = Z21 This, ofcorse is nothing but mutual impedance(Z m) between two antennas. Therefore, Zm = Z12 = Z21 = E12/ I2 = E21/ I1
E12/ I2 = E21/ I1
Limitations:(1) Although the rayleigh-carson theorem is applicable to radio communication
through ionosphere but it fails to be true only when the propagation of radio work is appreciably effected by the presence of the earth’s magnetic field. (2) It holds good for all practical radio work but for long distance communication
through ionosphere however still it is expected to apply results overaged over a reasonable interval of time in which case it cannot be expected to be exactly correct at every given time. Procedure:-
(1)Connect the yagi antenna to receiver graduated disc/stepper motor
