Microwave Communication

Microwave refer to high frequencies (above 300MHz) and short wave lengths, at the microwave components depends on the changing electromagnetic fields instead of current in the conductor or voltage across the 2 points a microwave propagated through the line of sight , therefore it is necessary to install repeater station at about 50km interval.

Microwave Frequencies


Frequency Range


225----390 MHz


390 MHz----1.5 GHz


1.5------5.2 GHz

















Microwave Tubes

It is not possible to generate the microwave with conventional tubes because of the constructional limitations. Special microwave generators are used for this purpose; these are Magnetron, Klystron and Traveling Wave Tube (TWT). These tubes are used for high power microwave amplifiers and oscillators. Microwave system requires power levels of a few watts to hundreds of watts. So, microwave tubes are selected to meet this requirement. The microwave tubes mentioned above are described below.


The Magnetron is a high power microwave oscillator, uses the interaction of electric and magnetic fields in a cavity to produce oscillations of very high power. It was invented by Randall and Boot. The construction of cavity magnetron is shown in the figure. The magnetron is a diode of cylindrical construction. It has an anode with permanent cavities and a heated cathode. The cavity dimensions determine the frequency of oscillation. It uses a radial electric field and an axial magnetic field. The electric field is provided by the potential difference between the anode and cathode by the dc supply voltage. The magnetic field is provided by the permanent magnet, as shown in the figure No.7.1. The cathode produces electrons by thermionic emission and is attracted by the anode.

The electric field produces straight line motion from cathode to anode while the magnetic field produces circular motion. Because of the interaction of the electric and magnetic fields, the path of electrons accelerated toward the anode is not straight but cycloidal. The electrons are alternately accelerated and decelerated because of cycloidal motion. When electrons are decelerating, they release some of their energy. The released energy is pumped into the cavity.

The two fields are so adjusted to make the length of the cycloidal loops equal to twice the distance between the cavity openings. Each cavity acts like a resonator. The spacing between adjacent cavities makes them to have out-of-phase oscillations. The entire process is regenerative, i.e., positive feedback reinforces the oscillations.

The release of microwave energy from the magnetron tube is taken from one of the cavities, by means of a coaxial line or through a waveguide depending on the power and frequency.

The velocity of the electrons is alternately increased and decreased; this process of accelerating and decelerating the electrons is called velocity modulation. The accelerating and decelerating period of the electrons is comparable with the total transit time. This is the general principle of microwave tubes.



The Klystron can be used as an amplifier as well as oscillator at microwave frequencies. The constructional detail of Klystron is shown in the figure. The principle of operation of a two cavity Klystron is shown here.

The cathode at one end of the device emits a beam of electrons. These electrons are focused (external magnetic focusing is not shown in the figure for simplicity) and attracted by a positive electrode at the other end of the device. In the two cavities Klystron, the beam of electrons passes through two cavities. One of these cavities is called a buncher, the RF input signal is applied here and the other is called a catcher, the amplified output signal is taken from here.

The velocity of the electrons beam is modulated by the input RF signal which is applied to the buncher cavity. The RF signal produces oscillation within the buncher, due this oscillation the electrons bunch and expand along the drift tube. The bunched electrons move toward the collector, while passing through the gap of the catcher cavity, electrons are decelerated thus giving some of their energy to the catcher. As a result of this, the RF output taken from the catcher is an amplified version of the input applied at the buncher.

The Klystron being used as a microwave amplifier, but can also be used as an oscillator if the amplified output signal at the catcher is feedback to the input at the buncher.

Multi-cavity Klystron with four or more cavities, produce several Kilowatts or RF power over bandwidths up to a few hundred MHz. Klystron using one cavity can also be used as an oscillator, and is called Reflex-Klystron. The Reflex-Klystron is given below:


The Reflex-Klystron is shown in the figure .No.7.3. The Reflex-Klystron is used as an oscillator with only one cavity. There is no external feedback because the reflex-Klystron provides its own internal feedback. A repelled is used to repel back the velocity modulated electrons thus giving its energy to the cavity. The reflex-Klystron is used in microwave equipment at moderate signal levels.

Reflex Klystron

Traveling Wave Tube (TWT)

The Traveling-wave tube can be used as a medium or high power microwave, amplifier. The TWT, because of its construction and working principle has enormous bandwidths and low noise. The heated cathode at one end of the tube produces a beam of electrons and is attracted to the collector at the other end of the tube. The input signal is fed at one end of the tube and an amplified version of the input signal is taken from the other end.

The constructional detail of the tube is shown in the figure. As the input signal travels along the helix inside the tube so there is a continuous interaction between the signal field and the electron. Thus the process of velocity modulation, bunching, and continuous interaction results in increased amplitude of the signal. The amplified signal is taken from the output. Hence the TWT acts as a microwave amplifier; it can also be used as a microwave oscillator by returning some of the output signal to the input.

Microwave Transmission Lines

The transmission lines are a means of carrying signals or power from one point to another. In microwave communication systems, it is necessary to interconnect points which are some distance apart from each other. For this purpose usually two types of transmission lines are used in microwaves. These transmission lines are: Coaxial cables and Waveguides. The Coaxial cables and Waveguides are used to carry output power from a/microwave transmitter to the antenna and the signals received by the antenna to the in-door unit.

Coaxial Cable

It is a transmission line in which two concentric conductors are separated by a uniform loss-less dielectric. The coaxial cable consists of two concentric conductors, a solid conductor inside with a tubular outer conductor. The two conductors are insulated from each other. The construction of the coaxial cable may be in rigid or flexible forms. In the rigid form, the dielectric used is air and the central conductor is located inside the outer hollow conductor by means of loss-less dielectric insulating supports, called spacers or beads. In the flexible cables, the central conductor is surrounded throughout by the flexible dielectric material such as polyethylene. The outer conductor is perfectly shielded. The construction of the coaxial cable is shown in the figure.

Coaxial Cable

In the coaxial cable the electromagnetic field propagates along the dielectric, while the current flows along the surfaces of the inner and outer conductors because of "skin effect" due to high frequencies.

The coaxial cable has very low radiation losses and low susceptibility to external interference.

Wave Guides

Heavy losses in the conductors of twin lead, open wire lines and coaxial cables occur due to "skin effect' at high microwave frequencies. Therefore, they are generally not used as transmission lines at such high frequencies. The skin effect causes current to flow on or near the surface of the conductor, i.e., the center of the conductor does not carry any current. At such high frequencies, waveguides are used" as transmission medium. Because waveguide is a hollow metal structure and has no inner conductor. Thus at microwave frequencies, waveguides are used for coupling the energy. The propagation of energy in the waveguides depends on the changing electric and magnetic fields. The waveguide can be of any shape. It may be rectangular, circular, or elliptical in cross section. The size of the waveguide depends on frequency, i.e., higher the frequency, smaller will be the size. The waveguides are generally made of brass or aluminum to avoid rusting. To minimize the losses at higher frequencies, the waveguides are silver plated from inside. The most popular shape is the rectangular waveguide. For comparison rectangular and circular waveguides are shown in the figure.

Rectangular Waveguide
Rectangular Waveguide
Circular Waveguide
Circular Waveguide

The signals travel through the waveguides from the transmitter to the antenna or from an antenna to the receiver. The waves are guided in the waveguides. The walls of the waveguides are conductors; therefore reflection from them takes place. The conduction of energy does not take place through the walls but its function is to confine or guide the energy. The conduction of energy takes place through the dielectric filling the space which is usually air. The waveguides are used at giga-hertz or frequencies or higher. There are no radiation losses in waveguides, attenuation is less and the power capacity is greater than that of a coaxial line of the same size and at the same frequency.

Interfacing Microwave Station with a Telephone Exchange

Interfacing Microwave Station with a Telephone Exchange

The telephone, exchanges can be linked with each other through microwave stations by locating the transmitting and receiving antennas on towers or hilltops, because the microwaves propagates in the line-of-sight. Thus the transmitting and receiving antennas must be situated in the line-of-sight. Repeater stations are used, if the distance between the transmitting and receiving antennas is greater than 50 km. The microwave communication across oceans is achieved by the use of Satellites. The block diagram in the figure shows the connection of two telephone exchanges through microwaves.

The output of the telephone exchange is applied to a MUX (multiplexing network). The multiplexed signals are then sent to the microwave station by using cables or any other wireless media. At the microwave station, the signals are modulated onto microwave carrier. It is then amplified by high power microwave amplifiers and transmitted towards another microwave station. At the receiving end the whole process is reversed and the signal is applied to another telephone exchange. Thus, in this way two or more telephone exchanges can be linked together through microwaves.


The merits and demerits of microwave communication system are listed below:


  1. Capital cost is generally lower.
  2. Installation is quicker and easier.
  3. Additional service may be provided quickly and cheaply.
  4. Irregular ground difficulties are overcome.
  5. Equalization need only be applied for the equipment as the frequency characteristics of the transmission path are essentially constant over the transmission bandwidths.
  6. Repeater spacing may be increased by increasing tower heights.


  1. Restriction to line-of-sight operation on conventional links.
  2. The problem of suitable access to repeater stations from main highways and provision of accommodation for maintenance, 'c. The provision of power supplies for the repeaters.
  3. It is difficult to provide short distance branch circuits to intermediate exchanges or subscribers.
  4. Adverse weather conditions can cause severe fading and beam bending.
  5. The high level of linearity required in the repeaters poses a severe design problem.