TRANSMISSION LINES

What is transmission line Transmission line is a specialized cable or other structure designed to carry alternating current of radio frequency This lines are consider to be impedance matching circuits design to deliver power from Tx to antenna and maximum signal from antenna to the receiver  Used for, Electricity distribution, trunk lines, cable television signals, , computer network connections and many more…

Fundamental of transmission lines Common types of transmission lines:-

There are two types of commonly used transmission lines:- The parallel-wire (Balanced line) The coaxial (Unbalanced)

It is employed where balance properties are required Parallel-wire Line It is employed where balance properties are required The impedance between each leg above the earth is the same Balanced lines do not have a common. The signal information is carried on both wires. One wire carries the signal called the positive (+ve) signal and the other carries a signal 180 degrees out of phase called the negative (-ve) signal.

Often the wires are twisted together in order to tightly couple the wires electrically The goal is to have any noise that appears on one wire to appear on the other wire. Because the signals are 180 degrees out of phase, the noise will cancel

In connection of folded-dipole antenna to a TV receiver or a rhombic antenna to an HF transmitter These lines are never used for microwaves, since it likely to radiate RF energy Losses are more as frequency increases Dielectric heating also more In parallel lines characteristic impedance is restricted to a range of 100 to 600 ohms

Coaxial Lines It is employed where unbalance properties are required Unbalanced lines consist of two wires. One wire carries the signal and the other is the reference line called the common. The common wire is usually at ground potential. Often the common wire will also be used as a shield for noise immunity

Unbalanced lines have difficulty with noise immunity as any EMI noise will appear on the signal lead. Unbalanced lines are used for short distances because of the inherent problem with noise immunity It is used at UHF and microwave frequencies, to avoid the risk of radiations from transmission lines itself Between 1 and 18 GHz, coaxial lines are used Dielectric heating is less as compare to parallel-wire lines

Different losses in Transmission lines The energy losses that happen in case of transmission lines are shown below: 1. Conductor Heating 2. Dielectric heating 3. Radiation Losses It is observed that the radiation loss in parallel wire lines is much more than that of the coaxial cables

Conductor heating:  The heating rate of a conductor is directly proportional to the square of the current. It is inversely proportional to the characteristic impedance (Zo). Conductor heating will also increase with the increase in frequency. It is also referred as I²R losses It can be minimized in an RF line by plating the line with silver. Since silver is a better conductor than copper, most of the current will flow through the silver layer

Radiation Loss:  The transmission lines act as antennas when the separation distance between the conductors is very small as compared to their wavelength. Then the conductor starts radiating energy. This applies more to parallel-wire lines than the coaxial lines. Radiation losses are difficult to estimate, being normally measured rather than calculated. They increase with frequency for any given transmission line.

Dielectric heating:  It directly depends upon the voltage flowing across the dielectric Similar to conductor heating, it is also inversely proportional to the characteristic impedance of the line In this case the loss also increases with the increase in frequency If we use air as the dielectric medium then the loss will be almost zero Conductor and Dielectric losses are proportional to length and given by manufacturers in charts, expressed in db/100mts.

Single stub is useful at frequencies below microwave range The Double Stub Transmission-line matching device must have variable parameters or degree of freedom, as SW pattern Single stub is useful at frequencies below microwave range At Microwave frequencies to provide second degree of freedom a second stub of adjustable position is added to first stub The resultant is double stub as shown in figure

The two stubs are placed 0. 375 ʎ The two stubs are placed 0.375 ʎ. Two variables are provided and very good matching is possible Such matcher is normally connected between load and main transmission line to ensure shortest possible length

It has a same characteristic impedance as the main line Method for adjustment for matching is trial and error. In this stub nearest to the load is set at a number of points and the further stub is move back and forth until the best possible match has been achieved The SWR is monitored constantly while adjustment is taking place If perfect matching is required, a triple-stub tuner should be used, but the stubs are placed 0.125 ʎ apart

Only forwarded wave in line is measured, not the reflected wave Directional Couplers It is necessary to measure the power being deliver to load or antenna through transmission line Only forwarded wave in line is measured, not the reflected wave The coupling units used for such purpose are known as directional couplers The two hole coupler being among the most popular is shown in figure

It consist of a piece of transmission line to be connected in series with main line together with auxiliary line coupled to main line via two-probes through slots The probes do not touch the inner conductor of auxiliary line. They couple sufficient energy into it simply by being near it

If they touch most of the energy from main to auxiliary line would be coupled but a fraction is all that is needed The energy flow in auxiliary line is mostly same in direction as in the main line and provision is made to deal with energy flowing in “wrong” direction The distance between probe is ʎ/4 (can be any odd number of quarter-wavelength) Auxiliary line is terminated at one end by a resistive load It absorb all energy fed to it and often term as nonreflecting termination, other end goes to a detector probe for measurment

Any wave launched from right to left will be absorbed by load, and not measured The wave entering at auxiliary line at A, proceeding toward detector, will meet at B (another sample of the forward wave), get added and travel toward detector Various mechanical inaccuracies prevent ideal operation of directional coupler The directivity of coupler is standard method of measuring the extent of this unwanted wave

If the ratio of forward and reversed power measured by the detector is 30dB, then the directional coupler is said to have a directivity of 30dB. This value is common in practice Other quantity is directional coupling :- It is the ratio of forward wave in the main line to the forward wave in the auxiliary line. It is measured in decibels, and 20dB (100:1) is a typical value

Baluns It is Balance -to- Unbalance transformer used to connect a balanced line to an unbalanced line or antenna It is used to connect an unbalanced (coaxial) line to balanced antenna such as dipole For high frequencies, several baluns are exist for differing purposes and narrowband and wideband applications The most common balun, a narrow-band one is shown in figure. It is known as the choke, sleeve, or bazooka balun

As shown, a quarter-wavelength sleeve is placed around the outer conductor of coaxial line and connected to it at x The outer conductor of coaxial line no longer has zero impedance to ground at y One of the wires of the balanced line may be connected to it without fear of being short- circuited to ground The other balanced wire is connected to the inner conductor of the coaxial line Any balanced load, such as the simple dipole antenna, may now be placed upon it as shown in figure

Slotted line is a piece of coaxial line with slot in outer conductor The Slotted Line Slotted line simply permits convenient and accurate measurement of the position and size of the first voltage maximum from the load and any subsequent ones as may be described Slotted line is a piece of coaxial line with slot in outer conductor A flat plate is mounted on outer conductor, with a corresponding slot in it to carry a detector probe carriage

It has a rule on the side, with a vernier for microwave frequencies to indicate the exact position of the probe The probe extends into the slot, coming quiet close to the inner conductor of the line but not touching it as shown in figure

In this fashion, loose coupling between line and probe is obtained which is adequate for measurement, but small enough so as not to interfere unduly The slotted line must have the same characteristics impedance as the main line to which it is connected in series It allows the calculation of: Load impedance Standing-wave ratio Frequency of generator being used The practical measurement and calculations methods are normally indicated in instructions that comes with a particular slotted line

Q. 1 Compare parallel and coaxial transmission line. Q Q.1 Compare parallel and coaxial transmission line. Q.2 Define the term standing wave Discuss its pattern for open and short circuited transmission lines. Q.3 Define the characteristic impendence of a transmission line and show that : Zc = underroot R+jwL/G+jwC Q.4 Explain the fundamental of smith chart with application. Why smith chart covers distance of only half wavelength? Q.5 Explain working principle of balance to unbalance transformer with neat sketch. Q.6 what is standing wave ratio? Explain impedance inversion by quarter wavelength line. Explain impedance matching by quarter wave transformer. Q.7 Define term Characteristic impedance of transmission line. determine the char. impedance of coaxial line with following specification : L=0.118 µH/ft, C=21 pf/ft , d=0.025 inch , D= 0.15 inch, Є=2.23 Q.8 Assignment problems of smith chart.