Important Short Questions and Answers: High Frequency and Radio Transmission Lines

RADIO FREQUENCY LINE

27. State the assumptions for the analysis of the performance of the radio frequency line.

1.Due to the skin effect ,the currents are assumed to flow on the surface of the conductor. The internal inductance is zero.

2.The resistance R increases with Ö f while inductance L increases with f . Hence wL>>R.

3.The leakage conductance G is zero

28.State the expressions for inductance L of a open wire line and coaxial line.

For open wire line ,

L=9.21*10-7(µ/µr +4ln d/a)=10-7(µr+9.21log d/a) H/m

For coaxial line,

L = 4.60*10-7[log b/a]H/m

29.State the expressions for the capacitance of a open wire line

For open wire line ,

C=(12.07)/(ln d/a)µµf/m

30.What is dissipationless line?

A line for which the effect of resistance R is completely neglected is called dissipationless line .

31.What is the nature and value of Z0 for the dissipation less line?

For the dissipation less line, the Z0 is purley resistive and given by,

Z0=R0 = Ö L/c

32.State the values of a and b for the dissipation less line.

Answer:

a=0 and b=w ÖLC

33.What are nodes and antinodes on a line?

The points along the line where magnitude of voltage or current is zero are

called nodes while the the points along the lines where magnitude of voltage or current first maximum are called antinodes or loops.

34.What is standing wave ratio?

The ratio of the maximum to minimum magnitudes of voltage or current on a line having standing waves called standing waves ratio.

½Emax½ ½Imin½ S = ¾¾ = ¾¾ ½Emin½ ½Imin½

35.What is the range of values of standing wave ratio?

The range of values of standing wave ratio is theoretically 1 to infinity.

36.State the relation between standing wave ratio and reflection coefficient. Ans: S = 1+½K½

¾¾¾

1-½K½

37.What are standing waves?

If the transmission is not terminated in its characteristic impedance ,then there

will be two waves traveling along the line which gives rise to standing waves having fixed maxima and fixed minima.

38.What is called standing wave ratio?

The ratio of the maximum to minimum magnitudes of current or voltage on a line having standing wave is called the standing-wave ratio S. That is,

S= E _max = I _max E_min I _min

39.State the relation between standing were ratio S and reflection co-efficient k.

The relation between standing wave ratio S and reflection co-efficient k is,

1+ k

S = 1- k S-1 Also k = S+1

40. How will you make standing wave measurements on coaxial lines?

For coaxial lines it is necessary to use a length of line in which a longitudinal slot, one half wavelength or more long has been cut. A wire probe is inserted into the air dielectric of the line as a pickup device, a vacuum tube voltmeter or other detector being connected between probe and sheath as an indicator. If the meter provides linear indications, S is readily determined. If the indicator is non linear, corrections must be applied to the readings obtained.

41.Give the input impedance of a dissipationless line.

The input impedance of a dissipationless line is given by,

Z s = Es = R0 1+ k < f -2bs I s 1- k <f -2bs

42.Give the maximum and minimum input impedance of the dissipationless line.

Maximum input impedance, R max = R0 1+ k

1- k = SRo

Minimum input impedance, R min = Ro 1+ k

1- k = Ro

S

43.Give the input impedance of open and short circuited lines.

The input impedance of open aned short circuited lines are given by, Zsc = jRo tan 2 ps

l

44.Why the point of voltage minimum is measured rather than voltage maximum?

The point of a voltage minimum is measured rather than a voltage maximum because it is usually possible to determine the exact point of minimum voltage with greater accuracy.

45. What is the use of eighth wave line?

An eighth wave line is used to transform any resistance to an impendence with a magnitude equal to Roof the line or to obtain a magnitude match between a resistance of any value and a source of Ro internal resistance.

46.            Give the input impendence of eighth wave line terminated in a pure resistance Rr.

The input impendence of eighth wave line terminated in a pure resistance Rr. Is given by

Zs = (ZR+jRo/Ro+jZR)

From the above equation it is seen that ½Zs½ = Ro.

47.            Why is a quarter wave line called as impendence inverter?

A quater wave line may be considered as an impendence inverter because it can transform a low impendence in to ahigh impendence and vice versa.

48. What is the application of the quarter wave matching section ?

An important application of the quarter wave matching sectionis to a couple a transmission line to a resistive load such as an antenna .The quarter –wave matching section then must be designed to have a characteristic impendence Ro so chosen that the antenna resistance Ra is transformed to a value equal to the characteristic impendence Ra of the transmission line.The characteristic impendence Ro of the matching section then should be Ro’ = Ö Ra Ro

49. What do you mean by copper insulators?

An application of the short circuited quarter wave line is an insulator to support

an open wire line or the center conductor of a coaxial line .This application makes se of the fact that the input impendence of a quarter –wave shorted line is very high ,Such lines are sometimes referred to as copper insulators.

50.            Bring out the significance of a half wavelength line.

A half wavelength line may be considered as a one- to – one transformer. It has its greatest utility in connecting load to a source in cases where the load source cannot be made adjacent.

51.            Give some of the impendence –matching devices.

The quarter – wave line or transformer and the tapered line are some of the impendence –matching devices.

52. Explain impendence matching using stub.

In the method of impendence matching using stub ,an open or closed stub line of suitable length is used as a reactance shunted across the transmission line at a designated distance from the load ,to tune the length of the line and the load to resonance with an antiresonant resistance equal to Ro.

53.Give reasons for preferring a short- circuited stub when compared to an open – circuited stub.

A short circuited stub is preferred to an open circuited stub because of greater ease in constructions and because of the inability to maintain high enough insulation resistance at the open –circuit point to ensure that the stub is really opencircuited

.A shorted stub also has a lower loss of energy due to radiation ,since the short – circuit can be definitely established with a large metal plate ,effectively stopping all field propagation.

54.What are the two independent measurements that must be made to find the location and length of the stub.

The standing wave ratio S and the position of a voltage minimum are the independent measurements that must be made to find the location and length of the stub.

55.Give the formula to calculate the distance of the point from the load at which the stub is to be connected.

The formula to calculate the distance of the point from the load at which the stub is to be connected is,

56.            Give the formula to calculate the distance d from the voltage minimum to the point stub be connection.

The formula to calculate the distance d from the voltage minimum to the point of stub be connection is,

d= cos-1|K| / (2β)

57.            Give the formula to calculate the length of the short circuited stub.

The formula to calculate the length of the short circuited stub is, 

This is the length of the short – circuited stub to be placed d meters towards the load from a point at which a voltage minimum existed before attachment of the stub.

58. What is the input impendence equation of a dissipation less line ?

The input impendence equation of a dissipation less line is given by (Zs/Ro)=(1+|K|(ϕ-2βs)/ (1-|K|(ϕ-2 βs)

59.Give the equation for the radius of a circle diagram.

The equation for the radius of a circle diagram is

R=(S2-1)/2S and

C = (S2+1)/2S

Where C is the shift of the center of the circle on the positive Ra axis.

60.What is the use of a circle diagram?

The circle diagram may be used to find the input impendence of a line mof any chosen length.

61.            How is the circle diagram useful to find the input impendence of short and open circuited lines?

An open circuited line has s = a ,the correspondent circle appearing as the vertical axis .The input impendence is then pure reactance , with the value for various electrical lengths determined by the intersections of the corresponding bs circles with the vertical axis.

A short circuited line may be solved by determining its amittance .The S circle is again the vertical axis, and susceptance values may be read off at appropriate intersection of the bs circles with the vertical axis.

62.            List the applications of the smith chart.

The applications of the smith chart are,

(i) It is used to find the input impendence and input admittance of the line.

(ii)The smith chart may also be used for lossy lines and the locus of points on a line then follows a spiral path towards the chart center, due to attenuation.

(iii)           In single stub matching

63. What are the difficulties in single stub matching?

The difficulties of the smith chart are

(i) Single stub impedance matching requires the stub to be located at a definite point on the line. This requirement frequently calls for placement of the stub at an undesirable place from a mechanical view point.

(ii)For a coaxial line, it is not possible to determine the location of a voltage minimum without a slotted line section, so that placement of a stub at the exact required point is difficult.

(iii)In the case of the single stub it was mentioned that two adjustments were required ,these being location and length of the stub.

64. What is double stub matching?

Another possible method of impedance matching is to use two stubs in which the locations of the stub are arbitrary,the two stub lengths furnishing the required adjustments.The spacing is frequently made _/4.This is called double stub matching.

65. Give reason for an open line not frequently employed for impedance matching.

An open line is rarely used for impedance matching because of radiation losses

from the open end,and capacitance effects and the difficulty of a smooth adjustment of length.

66. State the use of half wave line .

The expression for the input impendence of the line is given by Zs = Zr

Thus the line repeats is terminating impedance .Hence it is operated as one to one transformer .Its application is to connect load to a source where they can not be made adjacent.

67.            Why Double stub matching is preferred over single stub matching.

Double stub matching is preferred over single stub due to following disadvantages of single stub.

1.Single stub matching is useful for a fixed frequency . So as frequency changes the location of single stub will have to be changed.

2.The single stub matching system is based on the measurement of voltage minimum .Hence for coxial line it is very difficult to get such voltage minimum, without using slotted line section.