A Presentation on TV Antenna System

by AMOD KUMARDy.DIRECTOR(Engg.)STAFF TRAINING INSTITUTE (Technical) DELHI

Presentation on Broadcast AntennaThe presentation has been divided into threeparts: --

Part I - Antenna basics

Part II - Broadcast Antenna structure & Associated components of Antenna system

Part-III Working principles of 3 DB Couplers, BPF, Circulators & Channel- combiners.

PART-I

Antenna Basics

What is Antenna An Antenna is a transducer which transmit or receive electromagnetic waves. or An antenna is a metallic object which used to convert high frequency current into electro- magnetic waves and vice versa.

How Radiation take place?

How Radiation take place ?

Radiation..The forward and reverse traveling waves combine to from a standing Wave pattern on the line, with a voltage anti-node at the open-circuited point but all the forward energy is not reflected by the open circuit.

A small portion of the Electromagnetic energy escapes from the system and is thus radiated.

This occurs because the lines of force traveling toward the open circuit, are required to undergo a complete phase reversal when they reach it.

It must be added that the proportion of waves escaping the system to those remaining is very small, for two reasons.

Radiation1. First, if We consider the surrounding space as the load for the transmission line, we see that A mismatch exists, and thus very little power is dissipated in this load.

2. Since The two wires are close together, it is apparent that the radiation from one will just cancel that from the other. This is because they are of opposite polarities and at a distance apart that is tiny compared to a wavelength. The cure for this problem seems to be an enlargement of the open circuit, i.e., Spreading of the two wires, as in Fig. (a).

..Radiation There is now less likelihood of cancellation of radiation from the two wire tips. The radiating Transmission line is now better coupled to the surrounding space. This means that more power will be dissipated in the surrounding space, i.e., radiated.

Moreover, because of the spreading out, waves traveling along the line find it more difficult to undergo the phase reversal at the end. The radiation efficiency of this system is improved even more when the two wires are bent so as to be in the same line, as in Fig. (b).

RadiationWhen the total length of the two wires is a half-wavelength, the antenna is called a half-wave dipole.

It is shown in Fig. (c), and now even greater radiation occurs.

The reason for this increase is that the half-wave dipole may be regarded as having the same basic properties (for the point of view of impedance particularly) as a similar length of Transmission line.

Radiation Accordingly we have the antenna behaving as a piece of quarter wave transmission line bent out and open-circuited at the far end.

This results in the high impedance at the far ends of the antenna reflected as a low impedance at the end connected to the main transmission line.

This in turn, means that a large current will flow at the input to the half-wave dipole, and efficient radiation will take place.

Radiation Pattern The radiation pattern of an antenna is a graphical representation of the radiation of the antenna a function of direction. When the radiation is expressed as field strength E Volt per meter ,the radiation pattern is a field strength pattern.

If the radiation pattern is expressed in term of power per unit solid angle, the resultant pattern as power pattern. A power pattern is a proportional to the square of the field strength pattern.

Radiation pattern

Calculation of Field StrengthField Strength= 2.85 P ht.hr/d2 mili-volt/meter P - Transmitted Power in KW ht - Height of transmitter antenna in meters hr - Height of the receiving antenna in mtrs d - The distance from transmitting antenna in meters /I - Wave length of signal Field Strength in DBu V / m = 20 log (F.S. in mili volt per meter)

MINIMUM REQUIREMENT OF FIELD STRENGTH VALUES FOR GOOD TV SIGNAL RECEPTIONFrequency VHF VHF UHF UHF Freq. Band (I) (III) (IV) (V)

ITU Rec. 417 48 dBuV/m 55 dBuV/m 65 dBuV/m 70 dBuV/m

ISOTROPIC ANTENNA An Isotropic antenna is a standard reference antenna radiating equally in all direction so that its radiation pattern is spherical. This is very useful property and very easy to visualize but practically such antenna does not exist.

Power density Power density of an antenna is defined as radiated power per unit area Power density =Pt/4r Pt- Transmitted Power r- Distance from isotropic Antenna

Directive gainDirective gain is defined in a particular direction, as the ratio of the power density radiated in that direction by the antenna to the power density that would be radiated by an isotropic antenna. If power densities are measured at the same distance & both antenna radiate the same power.

Directive gain is a ratio of power density and is therefore a power ratio.

Directivity

Directivity is defined as a maximum directive gain i.e. the gain in the direction of one of the major lobes of radiation pattern compare to isotropic radiation.

Power GainIt is the ratio of the power that must be radiated by an isotropic antenna to develop a certain field strength at a certain distance and divided by practical power.The practical power is that power which must be fed to the directive antenna to develop the same field strength at the same distance in its direction of maximum radiation. Ap = nD Ap=Power Gain D = Directivity (maximum directivity) n = Antenna efficiency =1 for loss less antenna

Polarisation

The wave radiated by an antenna consists of an electric field component and a magnetic field component. These two components are orthogonal and perpendicular to the direction of propagation of the wave. By convention the polarisation is defined by the plane of propagation of electrical field component. That means if the electrical field component is travelling in the vertical plane it is called vertically polarised. If the wave contains both vertical and horizontal components it is called circular or elliptical.

The types of polarisation are :

1. Linear polarisation(a) Vertical (V)(b)Horizontal (H)

2. Circular or Elliptical(a) Right hand circular (RHCP)(b) Left half circular (LHCP).

Circular Polarization When an antenna produces vertically and Horizontally polarized fields with equal amplitude and with a phase difference of exactly 90 degrees, the resulting signal is circularly polarized.

BandwidthIt refers to the frequency range over which operation of antenna is satisfactory and is generally taken between the half-power points.

The radiation pattern bandwidth is equal to the difference between the frequencies at which the received power falls to one-half of maximum, in the direction of maximum radiation.

Beamwidth The Beamwidth of an antenna is the angular separation between the two half-power points on the power density radiation pattern. It is also, of course, the angular separation between the two 3-dB down points on the field strength radiation pattern of an antenna and is illustrated in Figure.

BEAMWIDTH

Null Fill There are three methods of introducing null fill in a panel array:

1.Mechanically tilting some panels downward.2.Using a non-linear phase taper between bays.3.Using an unequal power split between bays.

Since some energy is taken from the main beam to fill the null, the maximum gain of the antenna system will be reduced, typically 0.5 to 1.5 dB, when null fill is introduced.

Split Antenna SystemsAn antenna system can usually be divided into upper and lower halves which can be operated separately.

This arrangement allows the use of one half for broadcast operations while the other half is available for painting or maintenance or other work that must be performed in close proximity to the antenna.

Split Antenna SystemsThe signal level will be reduced by 6dB if one half of the antenna is fed with one half of the normal transmitter power.

If the full transmitter power is available, the use of one half of the antenna will reduce the signal level by only 3 dB.

Standing Wave Ratio (SWR) What is Standing Wave Ratio? The SWR is a measure of the mismatch between the load and the line. It is the most important quantity which is to be measured for broadcast Antenna .

Standing Wave Ratio (SWR) The SWR is the ratio of maximum current to minimum current or ratio of maximum voltage to minimum voltage along a transmission line. SWR = Vmax/Vmin Vmax= Vf+Vr Vmin = Vf-Vr Vf= Forward wave Voltage Vr= Reflected wave Voltage

SWR The SWR can be measured in terms of Reflection coefficient. SWR= 1+/1- where is Reflection Coefficient = Vr/Vf Vf= Forward wave Voltage Vr= Reflected wave Voltage

Return Loss It is also measure of reflected waves in comparison with forward waves. or It is the ratio of forward power and reflected power in a transmission line. Return loss in DB = 10 log forward power/reflected power

The SWR is equal to unity when the load is perfectly matched.

When the line is terminated in a purely resistive load, the SWR is defined as SWR = Zo/RL where Zo is character impedance & RL is the load resistance.

The higher the SWR, the greater the mismatch between the line and load, power loss increase with SWR and so a low value of standing Wave-ratio is always sought.

Relation Between VSWR, Return Loss ,Transmission Loss & Transmitted Power

S.IVSWRRETURN LOSS dbTRANSMITTED POWER (%)REF.POWER (%)

1.110002.1.0532.399.90.13.1.1026.4 99.80.24.1.1523.199.50.551.2220.19916.1.3615.997.52.57.1.501496481.581394.95.19.2.09.588.911.110.3.07.47525

Practical implications of SWR. SWR has a number of implications that are directly applicable to broadcast use. 1. SWR is an indicator of reflected waves bouncing back and forth within the transmission line 2. An increase in SWR corresponds to an increase in power in the line beyond the actual transmitted power. 3. This increased power will increase RF losses, as increased voltage increases dielectric losses, and increased current increases resistive losses.

.Practical implications of SWR

4. Higher power in the transmission line also leaks back into the line, which may causes it to heating up in transmission line.5. The higher voltages associated with a sufficiently high SWR could damage the transmitter which have a lower tolerance for high voltages may automatically reduce output power to prevent damage.6. The high voltages may also cause transmission line dielectric to break down and/or burn.

Measurement of VSWR1. VSWR is the measure of mismatch between transmitter and Antenna.

2.VSWR measurements may be taken to ensure that a waveguide is continuous and has no leaks or sharp bends. If such bends or holes are present in the waveguide surface, they may diminish the performance of transmitter and receiver equipment strings. Arcing may occur if there is a hole, if transmitting at high power, usually 200 watts or more.

How can we measure SWR

We measure SWR in the form of VSWR. The VSWR may be measured by Site Master available at all HPTs.

The VSWR of antenna may be measured at 7-port patch panels. VSWR measurement should be done for individual feeder cable and combined feeder cables.

The measurement should be done invariably once in quarter. If reflected power shown on through line power meter is more than 1% of total output power of transmitter than it is a serious concern. VSWR measurement should be taken and reason of high reflected power should be find out.

POWER OUTPUTThe peak power radiated during the sync. Tip (peak) or sometimes the carrier power corresponding to black level is designated as the vision transmitter power.

This power is measured by using a throuline Power meter after isolating the aural carrier. The power read on throuline meter is multiplied by a factor of 1.68 to get the peak power (vision) radiated.

PART-II

Broadcast Antennas structure & Associated components of Antenna system

TV TRANSMITTER ANTENNA SYSTEM.

TV Antenna System is that part of the Broadcasting Network which accepts RF Energy from transmitter and launches electromagnetic waves in space.

The polarization of the radiation as adopted by Doordarshan is linear horizontal.

The system is installed on a supporting tower and consists of antenna panels, power dividers, baluns, branch feeder cable, junction boxes and main feeder cables.

.TV TRANSMITTER ANTENNA SYSTEM

Dipole antenna elements, in one or the other form are common at VHF frequencies

where as slot antennae are mostly used at UHF frequencies.

Omni directional radiation pattern is obtained by arranging the dipoles in the form of turnstile (Fig.15) and exciting the same in quadrature phase.

Desired gain is obtained by stacking the dipoles in vertical plane. As a result of stacking, most of the RF energy is directed in the horizontal plane. Radiation in vertical plane is minimized.

The installed antenna system should fulfil the following requirements :

1. It should have required gain and provide desired field strength at the point of reception.

2. It should have desired horizontal radiation pattern and directivity for serving the planned area of interest.

3. The radiation pattern should be omni directional if the location of the transmitting station is at the center of the service area and directional one, if the location is otherwise.

4. It should offer proper impedance to the main feeder cable and thereby to the transmitter so that optimum RF energy is transferred into space. Impedance mismatch results into reflection of power and formation of standing waves. The standard RF impedance at VHF/UHF is 50 ohms.

Fig. 15 Turnstile Antenna and its Horizontal Pattern

High Power TV Transmitting Antenna System

In the High Power TV Transmitting antenna system, half wave dipole elements are mounted on the four faces of a square tower of suitable dimension for getting an approximate omni directional horizontal radiation pattern.

If radiation in any particular direction is not desired, the panels are left out in that direction. Dipole elements, supported by quarter wave line are backed by screened reflector to keep the radiation out of tower.

High Power TV Transmitting Antenna System

The position of the panels are slightly offset from the center-either clockwise or anti clockwise as shown in fig.16 for achieving wide band impedance match. Required number of panels are stacked vertically at a spacing of nearly half wave length to provide desired gain.

Panels thus stacked are divided into two groups upper half is called the upper bay and the lower half as lower bay. The constitution of antenna panels and feeding arrangements are described in the following paragraphs.

Fig. 16 Mounting of Antenna panels

Constitution of Antenna panels

An antenna panel, BEL make band I, as shown is Fig. 17 consists of a reflector, two half wave dipole elements, a balun, a set of parallel feeders and a variable capacitor. A branch feeder (72 ohms) from one of the output port of the junction box feeds two dipole elements through balun and parallel feeders.

Fig. 17 Antenna Panel - Band I & IIIFig. 17 Antenna Panel - Band I & III

A in mm

B in mm

C in mm

37 cm

57 cm

106 cm

135 cm

73 cm

73 cm

260 cm

242 cm

..Constitution of Antenna panels Band III antenna panel, as shown in fig. 17(b), consists of a reflector, four dipole elements, two baluns, a set of parallel feeders, two variable capacitors, a power divider and two branch feeder cables of 72 ohms impedance each. Variable capacitors are shunted across the parallel feeders to tune out the reactive impedance of the dipole elements. The power divider located on the rear of the reflector has two 72 ohms output points to which one end of the branch feeder cable is connected.

.Constitution of Antenna panelsEach branch feeder cable feeds two dipole elements through balun and parallel feeder as in the case of Band I Panels. Input port of the power divider has 50 ohms impedance and the same is connected to one of the port of the junction box through 50 ohms branch feeder cable. Impedance match between the input and output port of power divider is achieved by tapering method.

Junction Box

Two nos. of junction boxes made from coaxial elements are located at suitable position at the tower. The junction box has one input terminal and a number of output terminals. The number of the later depends upon the no. of antenna panels in each bay. Input port of the junction box for Band I and Band III has impedance of 50 ohms but the output port for band-I has impedance of 72 ohms and that of Band-III is 50 ohms.

Branch feeder cables

Two sets of branch feeder cables, connect the antenna panels. One set has the length L and other set's length is L + quarter wave length. The number of such cables in each set are half the total nos. of antenna panels.

This condition applies when equal no. of panels are mounted on each face of the tower. The impedance of branch feeder cable is 50 ohms for band III and 72 ohms for Band-I.

Fig. 18 Feeding Arrangement 4 Stacks

Pressurization of feeder cables

Semi flexible main cables feeding RF energy to junction boxes and branch feeder cables are required to handle large power. For stability and to prevent the change of characteristic impedance due to moisture absorption, the same are pressurized at suitable pressure by sending dry air from the dehydrator installed in the transmitter building.

The operation of dehydrator is automatic. Connectors are tightened properly and then sealed by a sealing agent to avoid in-different contact and prevent seepage of moisture.

Indifferent contact and moisture would cause reflection resulting into ghost and high VSWR. It may even lead to RF spark, damaging cable and connectors.

Radiation Pattern and Gain The horizontal and vertical radiation pattern are shown in fig. 19 and 20. Fig. 19 Typical Horizontal radiation patternFig. 20 Typical Vertical radiation Pattern

.VHF TV LPT Antenna

The BEL 'V' antenna consists of four quadrant dipoles arranged vertically in two stacks.

A stack contains two quadrant dipoles spaced at half wave lengths on a common balance feeder line made by aluminium tubes.

The tubes are extended a quarter wavelength beyond each stack and short circuited resulting in a quarter wave stub appearing as high impedance at the feed points.

.VHF TV LPT Antenna

Both the stacks are fed with equal amplitude and phase current by connecting the branch feeder cable at the center of the stack as shown in the schematic fig. 17.

The resultant radiation pattern in horizontal plane is almost omni directional as claimed by the manufacturer.

The feed arrangement including branch feeder cables is entirely concealed to prevent entry of moisture.

Fig. 17: Schematic Diagram of BEL LPT Antenna

Fig. 17: Schematic Diagram of BEL LPT Antenna

Fig. 17: Schematic Diagram of BEL LPT Antenna

Fig. 17: Schematic Diagram of BEL LPT Antenna

UHF LPT ANTENNA Slot antenna elements, cut in metal sheets are used as electromagnetic waves radiator at UHF frequencies.

The long sides of the slot carry current of opposite phase and their field cancel out.

The short ends carry currents in phase and radiate efficiently because currents are not only confined to the edges of the slot but spread out over the sheet.

Power is radiated equally from both sides of the sheet, if the slot is horizontal as depicted in the fig. 18.

Fig. 18: Slot Window cut in Metal Sheet

The radiation is normal to the sheet and vertically polarized. The slot antenna can be easily excited with a coaxial transmission line by connecting the outer conductor to the sheet. Inner conductor is connected to the center of the slot. The feed point impedance of such an antenna element is of the order of 50 ohms.

UHF- LPT PARSLOT Antenna.

The UHF LPT paraslot (SL-B) antenna manufactured by SCALA is an array of slot antenna, slot windows are cut in a cylindrical, heavy make aluminium pipe and the same are covered with durable laminated plastic.

The total length of the aluminium pipe is from 24 ft to 30 ft which is mounted on a mast of 30 metre height as shown in fig. 19.

.UHF- LPT PARSLOT Antenna Symmetrical parallel feed system completely housed within the center of the antenna is employed for feeding the slots. The radiation pattern in horizontal plane is off set omni directional, as shown in fig. 20. Maximum radiation occurs in the direction that faces the slot area.

Fig. 19: VHF LPT Antenna (SCALA Paraslot Antenna)

UHF- LPT PARSLOT AntennaSpecification of SCALA - SL - B antenna

Freq. Range:Any UHF channel in Band-IV and Band-VGain:Max 13.6 dB (isotropic) Min. 9.6 dbImpedance:50 ohmPolarization:HorizontalVSWR:1.15 MaxInput power rating:300 WattsBeam tilt:1.75 degree (down tilt)Vertical half powerbeam Width :5.8 degree+Input connector :'N' female

Fig. 20: Horizontal Radiation Pattern of Slot Antenna Array Paraslot SL-8

UHF ANTENNA

Model No. WML-AT-1000V

UHF ANTENNAUHF Band IV/V (Frequency range 470-860 MHz, UHF CH 21st to 68th )

Model No WML-AT-1000V can handle power up to 1 KW.

It is a Omni directional Slot array An with 8 collinear radiating slots to give required Omni directional Polarization & gain .

The slots are in the direction parallel to the axis of cylindrical aluminum pipe of diamete 89/102 mm.

The design dimension of slots are such that the pipe acts as a radiator of H polarized field. It gives substantially Omni directional for selected freq channel

The beam tilt is within 1 degree. But on demand it can be made up to 3 degree.

Precise manufacturing of slots dimension can provide beam width up to 6 degree.

Antenna internal construction uses nickel / silver plated brass / copper parts for divider & matching network.

Parallel feeder transmission lines rods feed the slots. Balloon transformer feeds the feeder , to match with the Tx impedance 50 ohms.

The slots are covered with PTFE sheet to have weather protection.

Antenna is DC grounded against lighting via lightening arrestor & earthing strip.

Body of Ant. is painted with white/ivory RAL( Polyurethane Paint ) against protection from climatic condition.

For R F input 1-5/8 inch flange connector having 50 ohms impedance provided which is protected via shroud which is fixed to bottom of antenna pipe.

Feeder cable with 1 - 5/8 inch flange is connected & flanges are bolted with fasteners.

TABLE FOR WIND-LOAD DATA

Tech Specification For UHF AntennaELECTRICAL PARAMETERFreq of operation : TV UHF Band IV/VPower Handling : 300W & 1KW (Avg)H Radiation Pattern : Omni DirectionalAnt. gain wrt half wave dipole : 10 dB / 12dBBeam Tilt : Not exceeding 1 degreeBeam width : 6 degree (approx)VSWR : 1.15 : 1Input Impedance : 50 OhmsInput connector : 1-5/8 Inch

MECHANICAL PARAMETERTemperature : 0 deg C to 55 deg CHumidity : 95 % at 40 deg CMounting : Top mounting / Slide mountingFinish : Main antenna pipe is painted with polyurethane light gray/white/ivory colorTower height Max : 46 Mtrs.Max Wind speed : 160 Kmpmtr sqWeight of antenna : Max 58 Kg Min 46 KgHeight of antenna : Max 6493 mm Min 5134 mm

UNPACKING Antenna is packed in to wooden boxes. The longer box have main antenna pipe & three spreader arms.

The antenna is covered with bubble sheet to protect against transit damage.The rubber sheet is fixed at slot side. While removing antenna it should be holded from two ends & centre of pipe. No pressure should be exerted on the lightening arrestor.

The PTFE sheet covering the slots should be protected from coming in contact with any sharp edges/ports etc. A slight damage or puncturing of the sheet will nullify the advantages of sealing.The antenna pipe should be kept U shape supports of approximately 200 mm height.

Caution UHF LPT antenna pipe has been packed into wooden box properly for protection against transit damage during transportation

TROUBLE SHOOTING & MAINTENANCECheck the VSWR of the antenna including the feeder cable & check the VSWR cable.If VSWR is high ensure tightness of connections at antenna end & Tx end of cable. Recheck the VSWR.If still VSWR is high disconnect the feeder cable & check the VSWR of the feeder cable with 50 ohms at other end. It should be lower than 1.05 :1

Advantages of slotted antenna

Multi channel operation wide bandwidth/ Low VSWR Power rating up to 160kW Average Top or Side mount Light weight/ Low wind load Single or Dual input Customized Vertical Pattern Multi pattern capacity

UHF SUPERTURNSTILE SLOT ANTENNA HORIZONTAL POLARIZATION BROADBAND 470 860 MHz11 dB GAIN OMNIDIRECTIONAL PATTERNSELF SUPPORTING RADOME ELECTRICAL DATA ANTENNA TYPE UTV-11/4 FREQUENCY RANGE 470860 MHz IMPEDANCE 50 ohm CONNECTOR 1-5/8 EIA MAX POWER 5Kw VSWR 1.05 in operating channel POLARIZATION Horizontal GAIN (referred to half wave dipole) 11 dB LIGHTNING PROTECTION All metal parts DC grounded

Operation Principle of Slot Antenna. The most popular transmitting antenna for UHF television stations is the slotted-cylinder antenna. The need in UHF television is for an antenna which has a gain in excess of 25 and the maximum authorized radiated power is 5,000 kw.

..Operation Principle of Slot Antenna

The antenna is of slotted-tubular-steel construction, as shown in Figure. Each radiating layer consists of three 1-in-wide slots equally spaced around the circumference of the cylinder. The slots are approx. 1.3 wavelengths long and are parallel to the axis of the antenna. Adjacent layers of slots are spaced vertically approximately one wavelength apart, center to center. The adjacent layers of slots are rotated 60o in order to obtain mechanical strength and to provide essentially an omni directional horizontal radiation pattern.

The antenna is made with number of layers of slots varying from 14 to 32 in order to obtain power gains varying form 20 to 55. the energy to feed the slots is distributed by means of a single coaxial-line feeder system within the slotted cylinder.

The slotted cylinder serves as the outer conductor of a coaxial line, the liner conductor being a copper tube within the inner conductor to obtain a method of center feeding the antenna array. Each slot is excited by a tuned loop placed across the face of the slot. The slots are excited with equal in-phase voltages, and the resulting field is assumed to be uniformly distributed across the slot. The radiated field will be essentially horizontally Polarized, with a small amount of vertical polarization

. The power to the slotted cylinder is usually fed equally to the slots in the upper and lower halves of the antenna. In order to obtain null fill-in, power would be fed unequally to various layers of slot . As an example , in an 18-layer antenna the power is normally fed 50:50 to the bottom 8-layers and the top eight layers .

In order to fill the vertical nulls50% of the power could be fed to the top 10 bays and 50% to lower eight bays . More null fill-in could be obtained by dividing the power equally between the upper 11 bays and lower 7 bays. It is possible to obtain electrical beam tilt of the main radiation lobe by varying the phase of the current fed to the upper and lower sections of the antenna.

MECHANICAL DATA DIMENSIONS 6170x970 mm WEIGHT 170 kg WIND SURFACE 2.05 m2 WIND LOAD 1.70 kN (wind speed at 150 km/h) MAX WIND VELOCITY 220 km/h MATERIALS Dipoles (plated brass) Internal Parts (silver plated brass, Polished brass) Radome (Fiberglass) Supporting frame (hot dip galvanized steel) ICING PROTECTION Full radome RADOME COLOUR Grey MOUNTING Directly on top of existing mast by Means of a flange PACKING 5330 x 780 x 850 mm

FOLDED DIPOLE

Folded Dipole and ApplicationsThe folded dipole is a single antenna but it consists of two elements. The first is fed directly while the second is coupled conductively at the ends. The radiation pattern of the folded dipole is the same as that of a straight dipole, but its input impedance is greater. If the total current fed in is 1 and the two arms have equal diameters, then the current in each are is . If this had been a straight dipole, the total would have flowed in the first arm. Now with the same power applied, only half the current flows in the first arm, and thus the input impedance is four times that of the straight dipole. Hence Rr- 4 x 72 = 288 for a half-wave folded dipole with equal diameter arms.

Yagi-Uda Antenna (a) Antenna and pattern (b) Optical equivalent

The Yagi-Uda AntennaA Yagi-Uda antenna is an array consisting of a driven element and one or more parasitic elements. They are arranged collinearly and close together, as shown in Figure. Since it is relatively unidirectional, as the radiation pattern shows and has a moderate gain in the vicinity of 7dB, the Yagi-Uda antenna is used as an HF transmitting antenna. It is also employed at higher frequencies, particularly as a VHF television receiving antenna. The Yagi-Uda antenna does not have high gain, but it is very compact, relatively broadband because of the folded dipole used and has quite a good unidirectional radiation pattern. It has one reflector and several directors which are either of equal length or decreasing slightly away from the driven element.

PART-III

Working principles of 3 DB Couplers, Circulators & Channel combiners .

3 DB COUPLER

3 db Couplers..The 3 db coupler is a four port device consisting of a pair of parallel plates approximately quarter wavelength long supported symmetrically in an outer conductor.

1. A signal applied to port (1) is divided equally between the port (2) & (4) with a phase difference of 90 degree as shown in Fig. No output appears at port (3)

2. If two signals at the same frequency are applied at ports (2) & (4) with phase relation as shown in the figure than the combined output is obtained at port (1) with no power output at port (3).

..3 db Couplers3. When port (3) is terminated with the characteristic impedance Zo and ports (2) & (4) by any other impedance (same magnitude and same phase) the impedance at port (1) equals Zo.

4. When both the ports (2) & (4) are terminated with Zo the impedance at port (1) equals Zo even if port (3) is terminated with any impedance.

CIRCULATOR

CirculatorsA circulator is a ferrite device . It is very often a four-port (i.e. four-terminal) device. It has the property that each terminal is connected only to the next clockwise terminal. Thus port 1 is connected to port 2, but not to 3 or 4 ; 2 is connected to 3, but not to 4 or 1; and so on. The main applications of such circulators are either the isolation of transmitters and receivers connected to the same antenna or isolation of input and output in two-terminal amplifying devices ..

Calculation of coverage area of TransmitterThe approximate coverage area of transmitter is calculated considering the Radio Horizon Distance.d = 4ht +4hrd- Max Radial distance from antennaht- Height of transmitting antenna including hill heighthr-Height of receiving antenna

Approximate Coverage* area for VHF Band III: Transmitters ( Field Strength 55 db) -------------------------------------------------------------------------------------------------------------- Xmitter Antenna Antenna Height in Meters Power Gain 37.5 45 100 150 250 300---------------------------------------------------------------------------------------------------- 100 W 10 16 17.5 24 29 -- -- 500 W 10 23 25 35 40 -- -- 1KW 10 26 28.5 37 46 56 -- 10KW 10 40 -- 57 67 80 84 20KW 10 46 -- 64 75 86 92--------------------------------------------------------------------------------------------------------------* Considering antenna is installed on plane

Approximate Coverage* Area for UHF Band IV Transmitters ( Field Strength 65 dbuv/m)------------------------------------------------------------------------------------------------Xmitter Antenna Antenna Height in metersPower Gain 37.5 100 150 200 250 300-------------------------------------------------------------------------------------300 W 15 13 21 26 -- -- -- 1KW 15 17 26 32 35.5 39.5 44 3KW 15 21.5 32 38 43 47 52 5KW 15 23.5 35 42 45 50 5610KW 15 26.5 38 46 51 56 6020KW 15 30 43 51.5 55 60 66--------------------------------------------------------------------------------------- * Considering Antenna is installed in plane

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