Electromagnetics Laboratory

S. No

Description of Experiment

Photograph

1.

AIM: To determine the characteristic impedance of lumped constant delay line using PSPICE AD.

Brief Procedure: In general, if we examine a transmission line, we will find four parameters, i.e., series resistance (R), series inductance (L), shunt capacitance (C) and shunt conductance (G), distributed along the whole length of the line, then the unit length of the line may be represented by an equivalent circuit. Naturally, a relatively long piece of line would contain several such identical sections.

Such an artificial line is known as lumped constant delay line. Such lines are made to simulate the actual transmission line. The characteristic impedance of a transmission line is given in terms of and as:

Where and are open circuit and short circuit impedances of the transmission line.

2.

AIM: To study voltage distribution along a lumped constant delay line (Transmission line) in the cases when it is (1) Open circuited (2) Short circuited and (3) terminated in and hence determine βusing PSPICE AD software.

Brief Procedure: Any voltage wave travelling down the line is continuously attenuated if the line is terminated in , the charateristic impedence of the line. The voltage at section, , is given by

=

Where is the voltage at the sending end

or =,

Voltages in the preceding equation are the amplitudes or peak values of sinosoidally varying functions,

or =

given

Measuring and n, we can determine, the attenuation per section.

To determine β, one has to plot voltage at each section of line in the cases when the line is open-circuited, short circuited and terminated with . The distance (No. of sections) between first and third minima, third and fifth minima … gives in each case, where is the distance along the corresponding to a phase change of 2 radians. Then β may be computed using

β or

And from =

3.

AIM: To study transient wave phenomena on transmission line using HFSS.

Brief Procedure: Transmission lines are used to transmit electric energy or communication signals from one point to another, especially from a source to load. Also the distances involved often are on the order of or much larger than the wavelength of the wave. Thus it is useful to understand the effect of disturbances or transient phenomena on a transmission line and the resulting voltage changes such as oscillations, reflections etc. transient travelling over assumed homogeneous lossless lengths of transmission line continue to propagate at a uniform speed and are unchanged in shape. However, for a lossy line, the transient decrease in amplitude and are thus attenuated with distance. Also, at points of discontinuity, such as open circuit or other line terminations, part of an incident wave is reflected back along the line and the rest is transmitted into and beyond the discontinuity. The wave impinging on the discontinuity is called an incident wave, and the two waves to which it give rise are normally referred to as reflected and transmitted waves.

4.

AIM: To create, simulate, and analyze a Microstrip transmission line.

Brief Procedure: Microstrip is a type of electrical transmission line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact.

The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is not enclosed, and is therefore susceptible to cross-talk and unintentional radiation. For lowest cost, microstrip devices may be built on an ordinary FR-4 (standard PCB) substrate.

5.

AIM: To study the characteristics of wave propagation in a waveguide by studying standing wave pattern for (a) Short circuit, (b) Open circuit and (c) Horn Antenna (d) Matched termination.

• To verify relationship between guide wavelength λ_g and free-space wavelength λ.

Brief Procedure: In a waveguide there is a single hollow conductor and current-voltage considerations may be difficult to visualize and the line may appear to be short-circuited at each point. However, the wave propagation in waveguides is characterized by varying electromagnetic fields subjected to appropriate boundary conditions such as the tangential component of the electric field and the normal component of the magnetic field being zero at the metal surface. The fields are not necessarily zero, just before the walls of the guide. Consequently, in a waveguide, electromagnetic fields propagate without touching the walls, i.e., with confined flow of electromagnetic energy. Further, these confined, propagating electromagnetic fields may satisfy the boundary conditions in a number of ways called 'modes'. These modes are broadly divided into two classes: (i) transverse electric (TE), the electric field being transverse to the direction of motion (E_z = 0, when waves are travelling in z-direction) and (ii) transverse magnetic (TM), the magnetic field being transverse to the direction of motion (B; or H_z= 0).

Not only this, as desired by the boundary conditions, only half-period variations of the fields are possible (E_t and H must vanish on both the walls). Hence in case of a rectangular waveguide corresponding to 2a = mλ and 2b = nλ (m and n being integers, a and b define waveguide cross-sectional dimensions), the restrictions imposed by the waveguide dimensions determine TE_mn and TM_mn modes. The lowest mode having the lowest frequency and longest wavelength of propagation is called the dominant mode. TE₁₀ and TM₁₁ are dominant modes.

Waveguide propagation is basically a reflection phenomenon. The field configuration existing within a waveguide may physically be regarded as the resultant of a series of plane-waves, all multiply reflected between opposite walls while travelling with a characteristic velocity 1√(𝜇𝜇∈) . These multiple reflections result in a complete transverse standing wave pattern that propagates down a waveguide in z-direction with a velocity far below the Characteristic velocity of the waves and is called as the group velocity. The wavelength corresponding to this group velocity or energy propagation is called as guide wavelength(λg) and is in general different from the free-space wavelength(λ). Further, it is worth noting that the dimensions of waveguide play a dominant role in propagation characteristics and hence dimensions of waveguide are so chosen that only dominant modes travel, thereby eliminating unnecessary power dissipation.

6.

AIM: To study the mode characteristics of a reflex klystron and hence to determine mode number, transit time and electronic tuning sensitivity (ETS).

Brief Procedure: At high frequencies, the performance of a conventional vacuum tube is impaired due to transit time effects, lead inductance and inter-electrode capacitance. Klystron is a microwave vacuum tube employing velocity modulation and transit time in achieving its normal operation. The most common klystron tube, which is used as an amplifier, is two cavity klystron. The other klystron tube is single cavity klystron. It is known a reflex klystron. It has been the most used source of microwave power in laboratory. It consists of an electron gun producing a collimated electron beam. The electron beam is accelerated towards the reflector (repeller) by a DC voltage V0 while passing through the positive resonator grids. The velocity of the electrons in the beam, e and m being electronic charge and mass respectively. The repeller, which is placed at a short distance from the resonator grids, is kept at negative potential with respect to cathode, and consequently it retards and finally reflects the electrons which then turn back through the resonator grids.

7.

AIM: To study Gunn oscillator as a source of microwave power and hence to study I-V characteristics.

Brief Procedure: The Gunn diode is a transferred electron device (TED). In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity, preventing further conduction and current starts to fall. This means a Gunn diode has a region of negative differential resistance by virtue of which a microwave oscillator can be created simply by applying a DC voltage to bias the device into its negative resistance region. In effect, the negative differential resistance of the diode cancels the positive resistance of the load circuit, thus creating a circuit with zero resistance, which will produce spontaneous oscillations. The diode is usually mounted inside a resonant cavity. The diode cancels the loss resistance of the resonator, so it produces oscillations at its resonant frequency. The frequency can be tuned mechanically, by adjusting the size of the cavity.

8.

AIM: To study the E-plane and H-plane radiation pattern of a pyramidal horn antenna and compute: (i) Beam width (ii) Directional gain of the antenna

Brief Procedure: In microwave communications, the transmission and reception of microwave power to/from space, is a primary necessity. The process is effected by an impedance transformer between the space and source, known as antenna. The basic characteristics of an antenna are expressed in terms of field pattern, directivity, bandwidth and gain. A transmission line shall act as an antenna if its output end is well matched to space. Horn antennas or EM horns may have various types of designs. When a circular waveguide is flared in a conical shape we have conical horn. In case of a rectangular waveguide, however, we can either flare broader side to have H-plane or H-sectorial horn, the flaring being parallel to magnetic field, or flare smaller side to have E-plane or E-sectorial horn, the flaring being parallel to the electric field. When both the sides of a waveguide are flared, we shall have a pyramidal horn or E-H horn

9.

AIM: To determine an unknown impedance using VSWR/Smith chart.

Brief Procedure: An unknown load impedance can be found by measuring the VSWR and finding the position of voltage minimum. We know from transmission line theory that the waves incident from the generator on the load get reflected (if the load is not the characteristic impedance) and standing waves are formed between the load and generator. The magnitude, phase and VSWR are the characteristic properties of the load. The input impedance of a transmission line is given by the transmission line equation.

10.

AIM: To study and contrast radiation pattern of various microwave antennas (Pyramidal Horn, Dielectric, Helix and Parabolic Reflector).

Brief Procedure: In microwave communications, the transmission and reception of microwave power to/from space, is a primary necessity. The process is effected by an impedance transformer between the space and source, known as antenna. The basic characteristics of an antenna are expressed in terms of field pattern, directivity, bandwidth and gain. A transmission line shall act as an antenna if its output end is well matched to space.

The angle (in a polar radiation pattern) between the two points on the main lobe where power intensity is half the power intensity at the maximum, is known as 3 dB beam width or simply beam width of an antenna denoted by θ. θH and θE are respectively the beam widths of H and E plane radiation patterns of an antenna.

Gain is defined as the ratio of the power that must be radiated (or received) by an isotropic antenna to deliver a particular field strength in the desired direction to the power that must be radiated by actual antenna to obtain the same field strength in the same direction.