Transmission lines, wave guidesAnd Antenna fundamental

Subject :- Basic Electronics

Transmission lines• The electrical signal is taken from transmitter output

to the transmitting antenna by special conductors , called TRANSMISSION LINES.

• The receiving antenna receives the EM waves transmitted by the antenna and convert it into electrical signals.

Types of Transmission lines

Coaxial (unbalanced) Parallel-wire(balanced)

Equivalent circuit representation• Where R & L are represent the power

loss.• C is formed between two conductors.• G represents the leakage resistance of

the dielectric material.

Primary line constant

Name Symbol Units Unit symbol

loop resistance R ohms per metre Ω/m

loop inductance L henries per metre H/m

insulator capacitance C farads per metre F/m

insulator conductance G siemens per metre S/m

Secondary line constant

• The propagation constant of a lossy transmission line is

Characteristic impedance

Where ω =2πf The value of Z is depends on the frequency.

Advantages:

• Simple construction • Flexibility• High mechanical strength• Not very expensive

Disadvantages:• It can be used for only point to point communication.• Power loss increases with increases in frequency.

Waveguides At frequencies higher than 3 GHz, transmission of electromagnetic energy becomes difficult.

This is due to the losses that occur both in the solid dielectric needed to support the conductor and in the conductors themselves.

A Hollow metallic tube of uniform cross section for transmitting electromagnetic waves by successive reflections from the inner walls of the tube is called waveguide.

Waveguides are used at microwave frequencies to guide the EM waves .

Frequency range :

• The cross sectional dimension of a waveguides should be of the same order as those of working wavelength.

• Therefore waveguides can be used at higher than 1 GHz.

Angle of incidence(A) Angle of reflection (B)(A = B)

When a probe launches energy into the waveguide, the electromagnetic fields bounce off the side walls of the waveguide as shown in the diagram.

Due to zigzag motion the velocity of propagation inside the guide is less than that in the free space.

The angles of incidence and reflection depend upon the operating frequency.

Wave propagation

Advantages :- • Small in size• Reduced losses as compared to a transmission line.• Operation at very high frequencies is possible (upto 325

GHz)• Large power handling capacity• There is no possibilities of flashover or sparking due to

absence of conductors

Disadvantages :-• Absolute efficiency is low• Low frequency operation is not possible as they

becomes bulky at low frequency

Antenna fundamental

An antenna is a way of converting the guided waves present in a waveguide, feeder cable or transmission line into radiating waves travelling in free space, or vice versa.

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver.

Directivity The directive gain can be define in any direction . However Directivity means the maximum directive gain which is obtained in only one direction in which the radiation in which the radiation is maximum.

Directivity = Maximum Directive gain

Antenna efficiency and losses

The resistive part of the antenna impedance is split into two parts, a radiation resistance Rr and a loss resistance Rd. The power dissipated in the radiation resistance is the power actually radiated by the antenna, and the loss resistance is power lost within the antenna itself

This may be due to losses in either the conducting or the dielectric parts of the antenna.

ŋ = Rr / ( Rr + Rd )Radiation efficiency ŋ of the antenna is

Power gain The Power gain of antenna is define as ratio of power fed to an isotropic antenna to the power fed to a directional antenna ,to develop the same field strength at the same distance, in the direction of maximum radiation.

Power gain = Power fed to the isotropic antenna

Power fed to the Directional antenna

Bandwidth Bandwidth of antenna is defined as frequency differencebetween the half power points.There is 2 types One is related to radiation patternOther is related to its input impedance.

TYPES OF ANTENNAS

Dipole antennaHorn antenna

Parabolic antennaYagi antenna

The Dipole antenna

Halfwave Dipole antennaA more practical antenna is the half-wave dipoleA dipole does not have to be one-half wavelength, but that length is handy for impedance matchingA half-wave dipole is sometimes referred to as a Hertz antenna

Folded Dipole antennaFolded antenna is a single antenna but it consists of two elements.First element is fed directly while second one is coupled inductively at its end.Radiation pattern of folded dipole is same as that of dipole antenna .

AdvantagesHigher input resistanceHigher bandwidth

Ease of construction

Yagi-Uda antennaIt is a directional antenna consisting of a driven element dipole or folded dipole) and additional parasitic elements (usually a so-called reflector and one or more directors). All the elements are arranged collinearly and close together.The reflector element is slightly longer than the driven dipole, whereas the so-called directors are a little bit shorter. The design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole.

AdvantagesIt is directional antennaModerate gain about 7 Db.Very compact

Large bandwidthAdjustable front - to - back ratio Used at high freq.

Application Used as transmitting antenna. Also used at high frequencies at VHF as TV

receiving antenna Stack of antenna can be used as a super gain

antenna.

Microwave antennaThe Frequency band from 1 to 100 GHz is called as the microwave frequency region. The antenna operating in this frequency band are called as the microwave antenna.The microwave antennas are expected to be highly directionalTypes of microwave antenna.1. Dish antenna2. Horn Antenna

Horn antennas

A horn antenna is used to transmit radio waves from waveguides out into space, or collect radio waves into a waveguide for reception.

Horn are widely used as antennas at UHF and microwaves frequencies , above 300 MHz.

It is used as feeders for longer antenna structures such as parabolic antennas

The three most commonly used horn configuration are :a) Sectoral b) pyramid c) circular

Application

As feed antennasSatellite antennasAntennas on the spacecrafts

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