TRANSMISSION LINES

TRANSMISSION LINES

R.F. Energy of a transmitter is guided up to radiator (Mast) by the propagation of Transverse Electro-Magnetic waves along systems of parallel conductors called “Transmission lines or feeder or feeder lines”

• The input energy is stored in the field of conductors and is propagated along the system at some finite velocity.

Antenna

• It is essential to keep the antenna at distance from transmitter due to prevent Radiation hazard.
• Pick up from antenna & consequent problem with transmitter circuit .Normally this distance is either on 50v/m field strength contour or minimum half the wavelength at frequency of operation.

Losses

• The feeder line should carry the power from the transmitter to Antenna with,
• Minimum loss
• Minimum radiation
• The lines are designed in such a way that loss is minimum in the feeder wire & also the radiation from it is minimum.

BASIC TRANSMISSION LINES

• There are three types of transmission lines used at RF. They are:
• i) Open wire feeder lines
• ii) Co- axial feeder lines
• iii) Wave guides

Characteristics Impedance

• Characteristic impedance (Zo) is defined as the input impedance of an infinite line.
• This is determined wholly by the geometry of its cross section. A transmission line can be represented as having R, L, C.
• The inductance, resistance, capacitance & conductance of the line characteristics impedance.

Zo

• The characteristics impedance is given by the following formula
• Zo=√(R+jwL)/ (G+jwC)
• At higher frequencies R & G becomes negligible with respect to reactance’s of L & C, There fore
• Zo is proportional to √ L/C
• The Zo can be lowered or increased depending on some specific requirement by varying the above two parameters.

Lower Zo

• To obtain a lower Zo than designed ,follow as under
• Increase conductor size maintaining the same Conductor to conductor distance.
• Decrease distance between conductors for the same conductor size.
• Increase number of wires in each side

Low & Impedance Feeders

• A)Low Impedance Feeders
• Parallel two or more feeders
• Connect lumped shunt capacitors across the line at a equal distance.
• B) High Impedance Feeders to increase the impedance opposite of above is done.

Types of Feeder Lines

• Balanced Lines: Where there are equal & opposite potential in both wires.
• Unbalanced lines: Here one wire is at high potential & the other side is at low potential.
• Structurally there are two basic forms;
• i) Open wire line
• ii) Enclosed line.

Open Wire feeder Lines

• Zo= 276 log 2S/d
• Where d = Diameter of line & s is the spacing between two lines
• In MW band, normally the feeder lines used are unbalanced & has following characteristics
• 6 wires,230 Ohms
• 16 wires,120 Ohms
• 24 wires,60 Ohms

SW Feeder Lines

• In SW normally the balanced feeder lines are used. The impedance are
• 300 ohms, 4 wire
• 600 ohms, 2 wire

Applications of Feeder Line

• Basic Applications of feeder line are :
• To guide energy from transmitter to Antenna. In this mode energy move along the lines in a single traveling wave.
• For storing energy in excess of that dissipated in load, in form of standing waves.

Reflection Coefficient

• The ratio of the reflected voltage to the incident voltage is called reflection coefficient.
• Γ =Er/Ef
• Where Er= Reflected Voltage
• Ef= Forward (incident ) voltage
• The reflection coefficient is determined by the relationship between the line Zo & the actual load at the terminated end of the line.
• The reflection coefficient can never be greater than 1 nor smaller than zero.
• If the load is purely resistive than reflection coefficient=(R-Zo)/(R+Zo)

Standing-Wave Ratio

• The ratio of the maximum voltage along the line to the minimum voltage is defined as the voltage standing-wave ratio (VSWR). SWR=Emax / Emin
• The ratio of the max current to the min current (Imax/Imin) is same as the VSWR.
• SWR is an index of many of the properties of the line.

Measurement

• It can be measured with fairly simple equipment. If the load contains no reactance, the SWR is numerically equal to the ratio between the load resistance, R & the characteristic impedance of the line. When R is greater than Zo, SWR=R/Zo
• When R is less than Zo, SWR=Zo/R
• (The smaller quantity is always used in the denominator of the fraction so the ratio will be greater than 1).

Flat Lines

• All the power that is transferred along a transmission line is absorbed in the load if that load is a resistance value equal to the Zo of the line.
• In this case the line is said to be perfectly matched. So no standing waves of current or voltage will be developed along the line.
• The VSWR is therefore 1:1 .as the plot of voltage standing wave is a straight line, the transmission line is also said to be “flat”.

Factors Determining Input Impedance

• The magnitude & phase angle of the input impedance depend on the SWR, the line length & the Zo of the line.
• If the SWR is low, the input impedance is principally resistive at all line lengths.
• If the SWR is high, however, the reactive component may be relatively large.

Input Impedance

• The input impedance of the line can be represented by a series circuit of resistance & reactance.
• The series –equivalent impedance is usually denoted as R+jX . By convention a plus sign is assigned to j when the reactance is inductive (R+jX) & a minus sign is used when the reactance is capacitive (R-jX0.

Effect of SWR

• The power lost in a given line is least when the line is terminated in a resistance equal to its characteristic impedance.
• The loss increases with an increase in the SWR. This is because the effective values of both current & voltage become greater.
• The increase in effective current raises the ohmic losses (IxIxR) in the conductors 7 the current raises in effective voltages increases the losses in the dielectric (ExExR)

Losses in the Feeder Lines

• There are four types of losses. They are
• Copper Loss: It is due to the heating of conductor.
• Earth Loss: It arises due to imperfect earth conductivity.
• Insulation Loss: It is due to insulation loss & is minor in a well-designed system.
• Radiation Loss: It is due to irregularity & usually very small for well-designed lines.

Copper Loss

• R.F. Wave travels along the exterior of a conductor due to skin effect. The conductor gets heated up resulting in losses in the feeder line. It can be reduced by increasing the radius of conductor and also by using more number of wires in parallel.
• It is directly proportional to square root of frequency, so root of frequency, more the losses.

Earth Loss

• In unbalanced open wire lines there is division of charges between ground wires & that induced in the earth under feeder lines resulting in part of the return current in the ground.
• The rotation of return current in the grounded wires & to the total current in live wires decides earth losses.

RADIATION Loss

• It can be reduced by laying two nos. of copper wires in the ground throughout the length of feeder wire line & by increasing the height.
• RADIATION Loss
• One cause of radiation from open lines is from the vertical connections at the ends.

Effect of Height

• Decreasing the height can reduce it, but if height is decreased, the ground losses will increase.
• So best way-out is to use better shielding of high potential wires by using greater number of grounded wires.

Choice of Feeder Line Impedance

• When the feeder line impedance is chosen low, feeder current will be more, resulting increase in copper loss & earth loss.
• When feeder line impedance is high, feeder voltage will be high resulting in the use of higher voltage rating insulators.

Feeders in AIR

• IN AIR following types of feeder lines are used.
• 230 ohm 6 wire (open wire) lines-for all old 100kw as well as 10/20 kW.
• 60 ohm quasi coaxial feeder line-megawatt of Chinsuraha, Rajkot & Nagpur
• 120-ohm quasi coaxial feeder-all 300kw & all 100/200 kW new version.

230 Ohms Copper wire

• 230 ohms copper wire Feeder Line is most popular & has been used in all old installations of 10/20/100 kW/MW XTRs.
• There are total 6 wire (8 SWG, app 4.064 mm).
• Two inner are on high potential & four outer are ground conductors.

Quasi Coaxial feeder Line

• In this category of line normally there are two designs:
• In which there are 8 inner wires & 8 outer wires each 8 SWG. This has been used in all 100/200/300kW XTRS.
• In which there are 12 inner conductors of 6mm dia & 16 screen conductors of 8 SWG & this has been used at 1000 kW at Nagpur, Calcutta (Chinsursha) & Rajkot.

Measurement of Zo

• Measurement of Characteristics Impedance Zo of a feeder line is given by the relation Zo= √Zoc.Zsc
• Zoc= Open ckt impedance, measured at input by keeping the feeder line end open
• Zsc= Short ckt impedance, measured at input by keeping the feeder line end short.
• Generally Zoc & Zsc are either capacitive or inductive depending upon the length of feeder line as multiple of lambda/4.
• Zoc & Zsc can be measured with VIM or RF bridge by keeping the line open & shorting high potential wire (inner) with grounded wire (outer) at other end.

Zo

• Another method utilizes the fact that when the feeder line is terminated by its Zo, its impedance as equal to the feeder to the characteristic impedance.
• Input impedance is measured for various termination for which input impedance is same as the termination itself.

Capacity of Feeder Line

• Power handling capacity of a line (120 ohm) is calculated is as below : RF Current carrying capacity of copper conductor X dia of live conductor in inches=76.2 x 0.1574=10 amp
• For 8 wires total current is=10x8=80amps
• Therefore, power handling capacity=IxIxR=80x80x120=768 kW or 760 kW.

Precautions while erecting a Feeder Line

• Bends should be gradual & free of any sharp corners (preferably of 120 degree)
• The exact & equal length of wires should be used at bends. To keep the length same is more important than to maintain the equal spacing as it increases the series inductance of line.

• The poles should be placed at equal distance & symmetrically.
• Splitting joints should be smooth & free of any irregularity.
• The height above ground should be uniform otherwise ground return current will differ, varying the earth losses.