all india radio document

MEDIUM WAVE TRANSMITTER

CHAPTER 1

INTRODUCTION

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INTRODUCTION

Medium wave (MW) is the part of the medium frequency (MF) radio band used

mainly for AM broadcasting. Medium Wave is the original radio broadcasting band, in use

since the early 1920's. Typically it's used by stations serving a local or regional audience.

However, at night, signals are no longer absorbed by the lower levels of the ionosphere, and

can often be heard hundreds or even thousands of miles away.

The Medium Wave (MW) band better known as the AM Broadcast Band (BCB) is the

band from 530 to 1600 kHz. and currently being extended to 1700. This is the band used for

local stations and the programming is generally intended for a local audience. AM broadcast

stations typically have a daytime range of from 80 to 250 kilometers (50 to 150 miles). But

some stations can be heard much farther away even in the daytime during the winter months.

But at night this range extends considerably farther and you can hear stations 1000 or more

kilometers distant.

Medium wave signals have the property of following the curvature of the earth

(the groundwave) at all times, and also refracting off the ionosphere at night (skywave). This

makes this frequency band ideal for both local and continent-wide service, depending on the

time of day.

Medium wave (MW) transmitter has a frequency of 927 KHz and delivers a power of

10KW/100KW (STAND BY). MW transmitters deliver more power and hence they are

placed in the city outskirts.

For MW transmission any number of antennas can be used. But for MW broadcasting

at Aganmapudi only two antennas are used. Both the antennas act as radiators. MW

transmission uses vertiacal polarisation and propagation type is ground wave.

Through microwave link program is received from the studio at Siripuram to the

transmitter at Aganampudi.

Microwave frequency is approximately 100MHz to 5 GHz. For medium wave

broadcasting mostly 1440MHz is utilized.

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http://en.wikipedia.org/wiki/AM_broadcasting

http://en.wikipedia.org/wiki/Radio_band

http://en.wikipedia.org/wiki/Medium_frequency


At Aganampudi the carrier is modulated with the received studio program and is then

transmitted into air.

MW Band -

Operating frequency - 927 KHz

Purpose – regional transmission

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CHAPTER 2


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10kW HMB 104 MEDIUM WAVE TRANSMITTER

A.M. Transmitter of any power in general will have a separate HF and AF stages. In

the conventional transmitters, vacuum tubes are used right from the first stage to the final

stage and the preliminary stages are solid state devices.

A brief description of RF and AF stages and Power Supply of 10 kW HMB 104 MW

transmitter is given below (Fig. 1).

Fig 2.1 Block Diagram of AM Transmitter (HMB-104)

RF SECTION :

RF section consists of crystal oscillator, buffer, intermediate power Amplifier, Exciter

and power amplifier.

CRYSTAL OSCILLATOR AND BUFFER STAGE :

The crystal oscillator with buffer stage is generally kept together and is shielded by a

metal cover to isolate from other circuits.

This crystal oscillator employs a pentode tube 6 AU 6 or its equivalent, connected as

a triode. The frequency of oscillation is controlled by a quartz crystal and by a variable

trimmer capacitor.

The frequency of the medium wave transmitter should be highly stable. For medium

wave transmitter operating in the range of 540 kHz to 1602 kHz, the variation of a transmitter

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frequency should be within a tolerance of + 10 Hertz. To maintain a high stability of the

transmitter frequency it is necessary that the oscillator should oscillate at a particular

frequency against variations in voltage and ambient temperature. Hence the crystal is kept in

a constant temperature ovens whose temperature is controlled by a thermostat and maintained

at a 75o + 1.5o C.

The oscillator frequency changes considerably under initial transient condition, that is

when power is switched ON. However, it is essential to keep it always ready at a stable

condition. To facilitate this a separate power supply is provided to feed the oven which can

be switched ON and OFF with the help of a snap switch S3 (Oven) located on the AE panel of

the transmitter. Two crystal units X1 and X2 housed separately in different ovens Z1 and Z2

viz. a normal and a stand by unit are provided. Either one of them can be selected by means

of change over switch S2. However, both the ovens Z1 and Z2 are kept ON all the time.

The oscillator output comes to the buffer stage 6AQ5 or its equivalent. It acts as a

buffer between the oscillator and the intermediate power amplifier (IPA). Its output can be

tuned by an adjustable dust iron core of coil L.

IPA STAGE:

This stage employs an indirectly heated beam power tube BEL 25 and it operates as a

class C amplifier.

EXCITER:

This stage is operated as a class - C amplifier, employing air cooled tetrode type BEL

400 and drives P.A. stage. Screen supply is taken from plate supply. The output is a tuned

circuit consists of a fixed capacitor C 29 (Value of C29 depends on the operating frequency)

and coil L3. L3 is having a flipper, through it, fine tuning can be made.

This stage is modulated about 10 to 20%. A small secondary tap from the modulation

transformer supplies the necessary audio and super-imposes on the DC Plate supply. When

the triodes are anode modulated, the grid must be overdriven in the carrier condition in order

that the drive level will be adequate to sustain the peak anode current at 100% modulation.

Alternatively the drive must be modulated. Hence the 10 to 20% modulation. With tetrode the

same effect is achieved by modulating the screen enabling the anode current peaks to be

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attained with the same drive level as that required for the carrier only condition. To some

extent this ceases the grid dissipation limit.

POWER AMPLIFIER STAGE:

This is a class - C power amplifier obtaining the required output by means of three

parallel connected forced air cooled, directly heated triode tubes type BEL 3000. As a triode

tube is used in this stage, neutralization technique is adopted to neutralize, the grid-plate

capacitance. The output circuit is formed by PI () section and 'L' section made up of coils

and condensers. There is a variable coil to tune the output. A second harmonic filter is

connected at the output which attenuates the harmonics. This filter is a simple L C circuit

tuned to the second harmonic frequency. The output circuit also matches the plate impedance

of about 1100 ohms to the feeder impedance of 230 ohms, which is carried out at the time of

installation of the transmitter using Impedance Bridge.

At the time of maintenance, care should be taken that the coil settings are not disturbed.

AF CIRCUITS:

The audio frequency amplifier consists of two voltage amplifiers, a cathode follower

which serves as a driver to the modulator and the modulator is a class B push pull power

Amplifier.

2.1 FIRST AND SECOND AF AMPLIFIER STAGES:

This stage is operated as a class A push pull connected amplifier employing two

indirectly heated pentode type 4P55 or its near equivalent which provides about 30 dB gain.

The output from the first AF stage is coupled to the second stage through the coupling

condensers. Plate supply is obtained from the neutral of the HT. (Plate) Transformer.

Sub Modulator Stage -

This stage employs two 4B 85 (or its equivalent with modifications) in push-pull

mode to excite the modulator. The sub-modulator is a cathode follower. As the grid current

flows in the modulator tube, the input impedance varies widely with different input levels and

hence a cathode follower which possesses low output impedance, very small non linear

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distortion for load impedance variations and good frequency and phase shift characteristics is

used.

The DC potential of the cathodes of sub-modulator and the grid of the modulator

stages are kept nearly at the same negative voltage of about 200 volt.

Modulator Amplifier -

This is the final stage audio frequency power amplifier which supplies the RF power

amplifier, the required modulating power. The HT and the superimposed audio signals are

connected to the plate of the PA valves. It may be noted that the negative feedback Network

is connected in the primary of the modulation transformer.

POWER SUPPLY:

1. Filament Supply - All AC.

2. Low Tension

3. Bias

4. High Tension

FILAMENT SUPPLY:

For PA and modulator valves, there is a separate filament transformers with centre tap

arrangement. The centre tap will be grounded through metering current shunt resistance for

the measurement of a cathode currents and an overload coils in parallel with a resistance.

LOW TENSION:

3 phase 220 V AC is stepped up to 3 phase 520 V AC using a Delta/Star connected

transformer. It is rectified using silicon diodes and filtered using L C components. It gives

DC voltage to the following.

1. Plate and screen of 1st AF, 2nd AF, oscillator and Buffer.

2. Screen grid of sub modulator

3. Sub modulator plate and IPA plate.

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BIAS:

3 phase 400 V AC is stepped up to 3 phase 470 V using Delta/Star connected

transformer and rectified using silicon diodes in two sets SE 2 and SE3 and filtered using L-C

components. SE 2 output supply is connected to the cathode Bias of sub modulator. The out

put of SE 3 is connected to control grid of Exciter and Grid of P.A.

HIGH TENSION:

3 phase 400 V AC is stepped up to 2300 V 3 phase and rectified using silicon diodes

assembly SE4 and filtered using L-C components. Full HT is supplied to plate of modulator

and PA valves. The filtered DC from the star point of the HT transformer is connected to the

plate of 2nd AF and plate and screen grid of Exciter.

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2.2100 kW HMB 140 MEDIUM WAVE TRANSMITTER

RF circuits consists of a crystal oscillator, transistor power amplifier, RF Driver and Power Amplifier of 100 kW HMB 140 MW transmitter are shown in Fig. 2.

Fig.2.2 Block Diagram of RF Chain (HMB-140)

CRYSTAL OSCILLATOR:

To oscillate at a consistent frequency, the crystal is kept in a oven. The temperature of

the oven is maintained between 68 to 72o C and the corresponding indication is available in

the meter panel. Crystal oven is heated by + 12 V. One crystal oscillator with a standby has

been provided. It gives an output of 5 V square wave which is required to drive the Transistor

Power Amplifier. The crystal oscillator works between 3 MHz and 6 MHz for different

carrier frequencies. Different capacitors are used to select different frequency ranges. In

addition, variable capacitor is used for varying the frequency of the crystal within a few

cycles. The oscillator frequency is divided by 2, 4, or 8 which is selected by jumpering the

appropriate terminals. The oscillator Unit gives 3 outputs, one each for RF output, RF

Monitoring and RF output indication.

TRANSISTOR POWER AMPLIFIER:

Oscillator output is fed to the transistor Power amplifier (TRPA). It gives an output of

12 Watt across 75 ohms. It works on + 20 V DC, derived from a separate rectifier and

regulator. For different operating frequencies, different output filters are selected (Low Pass

Filter).

RF DRIVER:

A 4-1000 A tetrode is used as a driver which operates under class AB condition,

without drawing any grid current. About 7 to 10 Watts, of power is fed to the grid of the

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driver through 75: 800 ohms RF Transformer, which provides proper impedance matching to

the TRPA output and also provides the necessary grid voltage swing to the driver tube.

VARIOUS INPUT VOLTAGES:

The cathode of the driver : - 600 V.

Control grid : - 650 V.

Screen grid : - 100 V.

Plate Voltage : + 1900 V

Because the cathode is at -600 V, the effective grid to cathode bias voltage (fixed) is -

50V and the effective plate voltage is 2500 V. The driver develops a peak grid voltage of 800

to 900 V at the grid of PA and PA grid current of about 0.3 A to 0.4 Amps. The required

wave form for operating the PA as class -D operation is also developed at the output of the

driver by mixing about 20% third harmonic with the fundamental which is the operating

frequency of the transmitter.

RF POWER AMPLIFIER:

CQK - 50, condensed vapour cooled tetrode valve is used as a PA stage. High level

anode modulation is used, using a class B Modulator stage. The screen of the PA tube is also

modulated by a separate tap on modulation transformer. Plate load impedance of the PA stage

is about 750 ohms and the output impedance is 120 ohms, and it is matched by L-C

components. Using various combinations of the L-C circuits plate impedance of third

harmonic is created, the Harmonics also are filtered imaginatively at the output side. 11 kV

DC, the HT voltage is connected to the plate of the PA valves through the secondary of the

modulation transformer and RF chokes: hence the AF signal is super imposed on the DC for

the PA plate.

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PA OUTPUT CIRCUIT:

Fig. 2.3 PA Output Circuit (HMB-140)

The L-C combination of the output circuit provides the following:

1. The required load impedance for Class D operation that is there should be third harmonic impedance in addition to the fundamental impedance.

2. Matches the plate impedance of 750 ohms to the feeder impedance of 120 ohms at the operating frequency.

3. Filters all the second and third harmonic before the feeder.

AF STAGE:

Fig. 2.4: AF Stage (HMB-140)

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The AF stage supply the audio power required to amplitude modulate the final RF

stage. The output of the AF stage is superimposed upon the DC voltage to the RF PA tube

via modulation transformer. An Auxiliary winding in the modulation transformer, provides

the AF voltage necessary to modulate the screen of the final stage. The modulator stage

consists of two CQK-25 ceramic tetrode valves working in push pull class B configuration.

The drive stages up to the grid of the modulator are fully transistorized.

HIGH PASS FILTER:

The audio input from the speech rack is fed to active High Pass Filter. It cuts off all

frequencies below 60 Hz. Its main function is to suppress the switching transistors from the

audio input. This also has the audio attenuator and audio muting relay which will not allow

AF to further stage till RF is about 70 kW of power.

AF PRE- AMPLIFIER:

The output of the High Pass Filter is fed to the AF Pre-amplifier, one for each

balanced audio line. Signal from the negative feedback network from the secondary of the

modulation transformer and the signals from the compensator also are fed to this unit.

AF PRE - CORRECTOR:

Pre- amplifier outputs are fed to the AF Pre-correctors. As the final modulator valve

in the AF is operating as Class B, its gain will not be uniform for various levels of AF signal.

That is the gain of the modulator will be low for low level, input, and high for high level AF

input because of the operating characteristics of the Vacuum tubes. Hence to compensate for

the non linear gain of the modulator. The Pre-corrector amplifies the low level signal highly

and high level signal with low gain. Hum compensator is used to have a better signal to noise

ratio.

AF DRIVER:

2 AF drivers are used to drive the two modulator valves. The driver provides the

necessary DC Bias voltage and also AF signal sufficient to modulate 100%.. The output of

AF driver stage is formed by four transistor in series as it works with a high voltage of about

-400 V. The transistors are protected with diodes and Zener diodes against high voltages that

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may result due to internal tube flashovers. There is a potentiometer by which any clipping can

be avoided such that the maximum modulation factor will not exceeded.

AF FINAL STAGE:

AF final stage is equipped with ceramic tetrodes CQK-25. Filament current of this

tube is about 210 Amps. at 10V. The filament transformers are of special leakage reactance

type and their short circuit current is limited to about 2 to 3 times the normal load current.

Hence the filament surge current at the time of switching on will not exceed the maximum

limit.

A varistor at the screen or spark gaps across the grid are to prevent over voltages. As

the modulator valve is condensed vapour cooled tetrodes, deionised water is used for cooling.

The valve required about 11.5 litres/min. of water. Two water flow switches WF1 and WF2

in the water lines of each of the valves protect against low or no water flow. Thermostats

WT1 and WT2 in each water line provide protection against excessive water temperature by

tripping the transmitter up to stand-by if the temperature of the water exceeds 70o C.

Modulation condenser and modulation choke have been dispensed with due to the

special design of the modulation transformer. Special high power varistor is provided across

the secondary winding of the modulation transformer to prevent transformer over voltages.

2.3 POWER SUPPLY IN 100 kW HMB 140 MW TRANSMITTERS:

1. HT -11 kV PA & Modulator : thyristor controlled for smooth variation of HT

2. 800 V Power Supply : Screen voltage to PA valve.

3. 1070 V : Screen voltage to modulate valve.

4. 1900 V : Plate voltage to RF Driver

5. - 650 V : (i) Grid Bias to PA Modulator & RF

Driver

(ii) A tap on -650 V provides -600 V supply to the

cathode of RF Driver

(iii) -100 V for the screen of RF Driver.

6. Thycon Unit : + 12 V DC and - 12 V DC

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7. Audio Unit : + 24 V and + 10V.

8. Reflectometer : + 15 V & - 15 V

9. Control Circuits :

VDDB 15 V - Logic circuits.

VDDC + 12 V - Relays

VDDD + 15 V for indication lamps.

VDDE - 15 V - Comparator.

10. Main supply to transmitter 415 V. 3 Phase 50 Hertz.

Earthing switch operated by a handle from the front of the rack has been provided in

the filter tank. The main HT terminal and also the live ends of the filter condensers C201 to C

210 have been brought to the earthing switch. In addition all the MT voltage (- 650, 800,

1070, 1900) are also brought to the earthing switch.The 11 kV point is discharged initially

through a resistor R - 543 before it is grounded. The earthing switch is interlocked to the

main transmitter by micro switches S 302, S 303 and S 304. In addition, a key interlock

system is provided to prevent accidental contact with high voltages.

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CHAPTER 3

CONTROL & INTERLOCKING

SYSTEMS

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CONTROL AND INTERLOCK SYSTEMS IN TRANSMITTER:

Control and interlocking circuits of the transmitter are to perform four major functions :-

1. Ensure correct switching sequence.

2. Safety of the equipment.

3. Safety of the operating personnel.

4. Indication of the status of the transmitter.

In the following paragraph the details regarding the above aspects are dealt briefly:

1. SWITCHING SEQUENCE TRANSMITTER :

a) Ventilation.

b) Filament

c) Grid Bias/Medium Tension

d) High Tension.

a. Ventilation –

All the transmitters handle large amount of power. Basically the transmitters

convert power from AC main's to Radio Frequency and Audio Frequency energy. The

conversion process always result in some loss. The loss in energy is dissipated in the

form of heat. The dissipated energy has to be carried away by a suitable medium to

keep the raise in temperature of the transmitting equipment within limits. Hence, in

order to ensure that the heat generated by the equipment is carried away as soon as it

is generated the ventilation equipment need to be switched on first. Normally the

cooling provided in a transmitter could be classified on the following lines:

Cooling for the tube filaments.

Cooling for the tube Anodes.

General cooling of the cubic’s.

Cooling for coils, condensers, Resistors etc.

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The cooling equipments comprise of blowers, pumps and heat exchangers.

Another important consideration is that during the switching off sequence the cooling

equipments should run a little longer to carry away the heat generated in the

equipments. This is ensured by providing a time delay for the switch off of the

cooling equipment. Normal time delay is of the order of 3 to 6 Minutes.

The water flow and the air flow provided by the cooling equipments to the

various equipments are monitored by means of air flow and water flow switches. In

case of failure of water or air flow, these switches provide necessary commands for

tripping the transmitter.

b. Filaments –

All the transmitters invariably employ tubes in their drive and final stages of

RF amplifiers and sub modulator and modular stages of AF amplifiers. After

ventilation equipments are switched on and requisite air and water flow established,

the filament of the tubes can be switched on. While switching on filament of the tube,

the control and interlocking circuits have to take care of the following points.

The cold resistance of the filament is very low and hence application of full

filament voltage in one strike would result in enormous filament current and may

damage the tube filament. Hence, it becomes necessary to apply the filament voltage

in steps. Various methods adopted are:

i. Use of step starter resistance: Here the filament voltages of the tubes are

given through a series resistance (called step starter resistance). The series

resistance which limits the initial filament current is shorted and after a time

interval by the use of a timer switch.

ii. Use of special filament transformer which allows slow build up of the filament

voltage.

iii. Application of filament voltage in 3 or 4 steps.

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The emission from the tubes depend upon the temperature of the filament.

Generally it takes some time for the filament to reach a steady temperature after it is

switched on. Hence, it is not desirable to draw any power from the tube till it attains a

stable temperature. This means that the further switching on process has to be

suspended till the filament temperature and hence the emission becomes stable. This

aspect is taken care of by providing a time delay of 3 to 5 minutes between the

filament switching on and the next sequence namely bias switching on.

c. Bias And Medium Tension –

For obvious reasons the control grid of the tube has to be given the necessary

negative bias voltage before its anode voltage can be applied. Hence, after the

application of full filament voltage and after the lapse of necessary delay for the

filament temperature to become stable bias voltage can be switched on. Along with

bias generally anode and screen voltages of intermediate stages and driver stages are

also switched on. Application of bias and medium tension makes available very high

voltages for the various transmitter equipment. Hence, in order to ensure the safety of

the personnel access to these equipment should be forbidden before the application of

bias and medium tension. This is ensured by providing the interlocking so that the

bias and medium tension can be put on only after all the transmitter and other HV

equipment doors are closed to prevent access.

Connection of Load (Antenna/Dummy load):-

After the application of ventilation, filament and bias the anode voltage can be

switched on. But before the anode voltage can be increased the interlocking circuit is

to ensure that the load of the transmitter namely antenna or dummy load is connected

to the transmitter. The tuning process of the various RF stages are complete and none

of the tuning motors are moving.

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Application of Screen Voltage –

In the case of tetrode tubes, the screen voltage to the tube should not be

applied before the application of anode voltage to keep the screen current and screen

dissipation within limits. This is taken care of by an interlocking provision that the

screen voltage is applied only after the anode voltage reach a certain pre-determined

value well above the normal screen voltage.

Release of Audio frequency -

The application of AF signal to the AF stage in the absence of carrier power

would result in the operation of modulation transformer with no load connected. This

is not desirable. Therefore, the AF signal should be applied to the Audio frequency

stages only when the RF power amplifier is delivering the nominal power. Normally

AF frequency signal to the AF stage is released only when the carrier power is

approximately 80% of the normal power.

2. SAFETY OF THE EQUIPMENT :

The various transmitting equipments and auxiliaries are to be safe guarded against

over loads etc. The various safety provisions provided in the transmitter are as follows:

i) All the rotating machinery are provided with switches with magnetic and thermal over

load release.

ii) The air flow and water flow switches and temperature sensors monitors the air flow

and water flow of the cooling medium. If the air and water flow fall below a certain

pre-determined value, it ensures the necessary tripping sequence.

iii) Water levels in the reservoir and water conductivity are monitored constantly.

iv) Momentary release of air flow and water flow switches due to some turbulence for a

short duration will not result in the tripping of transmitters. However, if the fault

persists for a few seconds then the tripping will result.

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v) Sometimes thermal sensors are embedded in the filament transformers to monitor its

temperature.

vi) The filament voltage of various high power tubes is monitored. In case of low or high

filament voltage tripping of the transmitter filament is initiated.

vii) Circuit breakers associated with various rectifiers such as grid bias, screen voltage etc.

protect the rectifiers and associated equipment against over currents.

viii) All the vital currents of the tubes and stages are monitored and indicated by means of

panel meters. This is to monitor abnormality if any on the various operating

conditions.

ix) Also current operated over load relays are provided in the cathode, screen grid and

anode circuits to protect the tubes and the associated rectifiers in case any of these

respective currents exceed a pre-determined value. The operation of over load relays

are indicated by means of flags or latched lamps.

x) The standing wave ratio on the load side is monitored suitably and signal is used to

trip the transmitter anode voltage in case of VSWR is higher than the pre-determined

value.

xi) Spark detectors are provided in various cubicles to ensure the tripping sequence in

case of sparking to prevent damage to the equipments.

xii) Normally the over currents are counted over a period of time and if number of over

currents occur in a short interval the transmitter is tripped up to the filament.

xiii) In addition to the above safety provisions spark gaps and varistors provided at various

high voltage points offer protection to the equipment against high RF voltage.

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xiv) In some of the transmitter a crow bar device is provided to short circuit the stored

energies in the power supply circuit in case of over load. This provision is to protect

the high power tubes

3. SAFETY FOR THE PERSONAL :

Since very high voltages are encountered in transmitters the operating personnel are to

be protected by coming into contact with these high voltages accidentally. The safety

interlocking generally comprises of:

a. An earthing switch which earths all the high voltage supplies before the access to the

cubicles keys are allowed.

b. A key exchange panel from where the key to the transmitter cubicles can be utilised

only after the earthing switch is put on. The earthing switch is interlocked in the bias

circuit and hence the operation of the earth switch automatically switches off up to

bias. This provision ensures that the cubicle doors can be opened only when the bias

and medium voltages are switched off and earthed through the earthing switch.

c. In addition to the above earth hooks are provided at various parts of the cubicle and

high voltage equipment area. The operating personnel are to short through these earth

hooks the high voltage points before any work is undertaken in these equipment.

d. Some of the transmitters are also provided with additional shorting switches in the

cubicles which shorts the supplies in the cubicle as soon as the door is opened.

4. INDICATION LAMPS :

The indication lamps are provided in the transmitter to indicate the status of switching

on of the transmitter as well as to indicate the occurrence of over load etc. These indicating

lamps are provided to help the fault diagnosis.

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CHAPTER 4

MEDIUM WAVE ANTENNAS

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MEDIUM WAVE ANTENNAS

When the electromagnetic waves in the medium wave (MW) range are directed

towards the Ionosphere, they are absorbed by the D-region during the day time and are

reflected from the E layer during the night time, which may travel longer distances to cause

interferences. The wave length of MW signals are very large, of the order of few hundred

metres, and therefore the antenna cannot be mounted a few wavelengths above the earth to

radiate as space waves. MW antenna, therefore, have to exist close to the surface of the earth

and the Radio waves from them have to travel close to the earth as ground waves. If the

electric vector of such MW radiation is horizontal, they will be attenuated very fast with

distance due to the proximity of the earth. MW antenna have to be placed vertically, so that

they radiate vertically polarised signals. It is for this reason, all the MW antenna are installed

vertically close to the ground. However vertical wire antenna, inverted 'L' type antenna, top

loaded antenna and umbrella antenna are at a few All India Radio stations. Directional

antenna systems also exist in many All India Radio stations.

4.1SELF RADIATING MW ANTENNAS:

They are broadly of two types:

Mast isolated from ground and fed at its base.

Grounded mast fed at a suitable point along its height

As most of the All India Radio MW towers are of the first category, only they are

discussed here.(see Fig. 5)

The first consideration of such mast is its height in terms of the wave length. What is the

optimum height ? Obviously the main considerations are economy consistent with maximum

coverage and minimum high angle radiation (sky wave).

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The relative characteristics of mast height 30o to 225o (electrical lengths) are given below :

Electrical height in degrees

Height in wave length ()

Field strength at one mile V/m

Polar pattern

30 1/12 186 Please

60 1/6 189 See

90 1/4 196 Fig. 6

135 3/8 214

180 1/2 242

190 0.53 254

225 5/8 276

From the above analysis, it may be seen that as the height of the MW mast increases, the field strength at one mile increases (range of the transmitter increases) and is maximum for 225o (5/8) of electrical length of the antenna. Examination of the polar

Fig. 4.1 MW Antenna isolated from ground

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pattern shows that as the height increases, the high angle radiation decreases and the

horizontal gain increases. However at 5/8 height, the presence of side lobe will contribute

high angle radiation and therefore sky waves. Therefore electrical length of 190o (0.53)

would look optimum from the points of view of maximum range, high horizontal directivity

and maximum suppression of high angle radiation. 190o antenna is known as 'Antifading'

broadcast antenna as it eliminates the sky wave interference fading beyond the ground wave

range during night.

The height of the MW tower also will have to be coordinated with the civil aviation

authorities from the point of view of nearness of the airport. Should this require reduction in

actual physical height top loading technique can be adopted. This increases the current

distribution in the vertical portion of the radiator, thereby increases the efficiency of

radiators.

However in special cases such as the AIR's National Channel at Nagpur, the stress is

particularly for the night time service, to provide more sky wave average for which two short

antenna of 60m height (0.3) fed suitably are used.

Fig. 4.2 Polar Patterns

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The MW self supporting mast antenna could be excited in 3 different methods.

The first method requires an insulator at the base of the mast. The second method is

called shunt feed and the third top feed. The comparatively low voltage at the base and top of

the mast antenna, simplifies the operating condition of the insulators and enables to

accommodate a larger power into the mast antenna, than the wire antenna.

Shunt feed, earthed mast overcomes the difficulties of installing and maintaining

masts placed on insulators. The feed line is usually connected to the mast at a height equal to

1/5 to 1/10 the height of the mast.

The top fed antenna is fed by means of a coaxial line (or wires vertically forced inside

the body of the mast). The advantages are

More uniform current distribution compared to the base feed.

Absence of supporting insulator.

High radiation resistance and high efficiency.

4.2 TOP LOADED ANTENNA:

It is possible to simulate higher electrical length of the MW antenna for any

physically smaller MW antenna by top loading. A large capacitance disc (insulated from the

mast, and series resonated by an inductance connected across the insulator at the top of short

mast effectively increases the electrical length of the mast.

Another alternative is to use a number of wires in the form of umbrella emanating

from the top of the radiator and secured via insulated rope to the ground (fig. 7). This is

particularly valuable for thin masts. One such umbrella antenna is installed in Nagarcoil and

some other stations of AIR.

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Fig. 4.3 Umbrella Antenna

ECE Department


4.3 'T' AND 'L' ANTENNA :

' T' and 'L' antenna find application in broadcasting. AIR have used such types of MW

antenna in the network. This may perhaps be very handy to rig up one for emergency

arrangements. The antenna is secured on two high (100 to 250m) mast (wood or metal),

spaced 100-250 m apart. (Fig.8 & 9)

The antenna consists of two to sixteen wires spaced 1 to 1.5 m apart. The copper

wires are usually 5 to 8 mm in diameter. The supporting towers may be secured by several

tiers of guys in which insulators are inserted. The antenna down leads directly connect the

radio transmitter. There may not be any need for feeder lines if suitably structured.

The disadvantages are :

Need for two or more masts

Distortion of directional diagram caused by the influence of supporting cables.

The voltage at the base and at the end of wire antenna is very high compared to the mast

antenna,

ECE Department


Fig. 4.4 25M 'T' Antenna

4.4 NEED FOR EARTH RADIALS:

The MW propagates close to the earth as ground waves. The MW mast also is placed

close to the ground. The electric field in the mast extends from the top to the ground. Current

density of typical /2 tower being significant upto 0.37 to 0.4 wave length and current flows

through the ground back to the mast. The electric field passes through the ground. The earth

usually is not a perfect conductor and field may be attenuated. In order to improve the earth

conductivity when it takes off from the mast the conductivity of the earth around the mast is

artificially increased by burring about 120 radial copper wires of about 0.4 long (usually 10

swg) at 4 to 12 inches deep. The radial wires are suitably brazed among them forming a

mesh.

ECE Department


Fig. 4.5 Inverted 'L' Antenna

4.5 MATCHING THE MW ANTENNA:

The MW Power Amplifier output has to be matched to the feeder line which again is

to be matched to the antenna impedance usually by a PI/T/L- network in the Antenna tunning

unit located close to the base of the mast for perfect match. The impedance of the mast at the

feed points can be measured by an impedance bridge VIM. Usually the individual component

values of the PI/T/L-matching networks could be computed using transmitter manufacturer's

information booklet.

ECE Department


GUY SUPPORT FOR MW MAST:

The guy wires are used at a number of levels depending upon the height of the tower,

its cross section, the maximum wind velocity expected in that region etc. The guy wires have

to be insulated from the mast so also the guys are broken into a number of small sections

/10 or /12 separated by low loss, high mechanical strength insulators to minimise distortion

of radiation pattern due to field induced in them. These insulators are shunted by suitable

inductors to provide d.c path for lightning discharges while at the same time blocking the

MW energy from earthing. A high resistance shunt across the insulator is another method of

allowing the static leaks. Some types of insulators have built in thyristors which provide low

resistance to high static charge while presenting high resistance for low voltages. Ultraviolet

detectors which is sensitive to arcs or spark overs may also be used to activate the protective

devices in the transmitter.

Directional MW antenna, using more than one vertical mast exist in a number of

stations like Jullandar, Nagpur (National Channel) in the network. Special care must be taken

to allow for proper bandwidth of the directional antenna system.

They guy tensions are usually given in the completion report. It is necessary to

measure the Guy tensions as per AIR technical manual to ensure the verticality or absence of

twist in the mast. Measurement of verticality and twist of the mast are also required to be

carried out as per AIR Technical Manual. Loss of verticality will affect the range of the

service due to earth's proximity.

ECE Department

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