ndb

CHAPTER - 1

NON DIRECTIONAL BEACON (NDB)

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1. NON DIRECTIONAL BEACON (NDB) 1.1 NDB AS A NAVIGATIONAL AID Non-Directional Beacon is a radio navigational aid used by the aircraft all over the world for finding directions while flying from one point to other. Discovery of radio and ability of detecting its source of emission, utilizing directional antenna, led to the development of NDB. It is the simplest and oldest system, which has for many years played a vital role in the navigation system for both aeronautical and maritime uses and will probably do so for many years to come. Non-Directional Beacon is a ground station that transmits a low frequency or medium frequency signal, which is radiated Omni-directionally in the horizontal plane (azimuth), with vertical polarization. There is no coded navigation information inside the signal apart from the station identification in Morse code that repeats 7 times per minute. The NDB receiver in the aircraft gives the pilot information of the “bearing  to the NDB transmitter stations, which are located in the air-routes or at ”

the airports. Bearing is the horizontal angular displacement in clockwise direction with respect to North. In addition to the directional information the NDB station also gives indication when the aircraft is passing overhead a station, i.e., the NDB station provides a position fix overhead indicated by a decrease in field strength and an abrupt change of indicator needle by 180 . The NDB is widely used because they are: # Inexpensive # Simple electronics and easy for maintenance # Omni-directional information # Responsibility of accuracy mainly depends upon airborne receiver. 1.2 PRINCIPAL OF OPERATION NDB is simply a radio transmitter that transmits tone modulated RF signal in the LW/MW frequency band with station identification seven times per minute. Volume-1 of ICAO Annex-10 to the convention on International Civil Aviation Organization states that, "The radio frequencies assigned to NDB's shall be selected from those available in the portion of the spectrum between 190 KHz and 1750 KHz. The frequencies being used for NDB can vary from zone to zone. As the frequency band from 525 to 1605 KHz is widely used for Radio Broadcasting, most of the frequencies for NDB's are selected below 525 kHz within 200 to 415 kHz. The signal is amplitude modulated at 95% by a station identification audio tone in Morse code (A2), which repeats 7 times per minute to identify a station. The identification tone consists of two to three letters. The frequency of the modulating tone can be either 400Hz or 1020Hz. Each letter is separated by a dash. For example: The Kathmandu NDB at the Tribhuvan International Airport is coded as KAM, which in Morse code translates as: dash dot dash dash dot dash dash  dash dash K

A

M

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In the aircraft, a receiver called Automatic Direction Finder (ADF) automatically displays the station bearing as soon as it is tuned to a NDB station. The Automatic Direction Finder uses the Loop Aerial that has a specific direction finding property. Depending upon the orientation of the loop aerial, signals in its output varies greatly. A loop aerial possesses the following properties.

1.3 LOOP AERIAL Direction finding maybe carried out in any region of radio spectrum, though certain frequencies are specifically allotted for radio navigation purpose. In aviation only LF/MF and VHF are used for radio direction finding. LF/MF are used for NDB ground stations whereas VHF is used for finding the direction of the aircraft from the ground. The technical features of direction finders operating in various frequencies naturally differ, but the fundamental principles remain the same. In the LF/MF, due to comparatively very large wave length, so called LOOP ANTENNA is extensively used. Loop  Antennas are highly directional in propert y, which could be derived mathemat ically as follows: Consider a rectangular loop antenna of length “a” and width “b” with its plane vertical mounted so that it can be rotated about its vertical axis. Let there be a vertically polarized electromagnetic wave “E” incident on it, coming from a direction making an angle “” with the plane of the loop at its center.  N

B

C b

a

e2

e1

CD b/2

 A

D

AB

½ b Cos 

b/2

Output

The source is assumed to be so far away that the incident wave is a plane wave. Voltages are induced in the vertical members of the loop, but not in the horizontal members as the wave is vertically polarized. The magnitude of the voltage induced in the two vertical members is therefore a.e1 and a.e2, where e1 and e2 are the magnitude of electrical field in rms. The voltages in the two members will not be in phase, as can be seen from the diagram since the arrival times will not be the same. Taking the electrical field at the center of the loop as the reference, the voltage induced in AB lags by an angle , and that induced in CD leads by , where   being the phase difference of the arriving signal with respect to center of the loop. Considering  = 2 and difference in path length is ½ b Cos. Then phase difference equivalent to path length is



= 2. b Cos  2

= .bCos 

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If the electric field at the center e(t)

= E Cos t then voltages induced in two vertical members will

be : e1 = aE Cos ( t - b Cos )  e2 = aE Cos ( t + b Cos )  Therefore resultant voltage at the output of the loop antenna will be e = e1 – e2 = aE Cos (t - b Cos ) - aE Cos (t + b Cos )   Or e = 2 aE. Sin t . Sin b Cos  Since “b” is very small in comparison to  then we could do approximation as Sin b Cos 

= b Cos 

Hence

e = 2 E . ab Sin

t. Cos

From the above formula we could make the following conclusions: a) Output of the loop antenna is dependent of the incident angle antenna is perpendicular to the incident radio signal , i.e. when

“

”. When the plane of the loop ” is 90  the output from the

“

loop is zero and maximum when “ ” is 0 b) Output from the loop antenna will increase when the dimensions “a” and “b” will increase. That is, output is directly proportional to the area of the loop. Accordingly, if there are “N” turns in the loop then output voltage will also increase by “N” times.  Accordingly, a Loop Aerial may have two distinct positions as follows: Null Position If the plane of the loop is at right angle to the direction of the waves coming from the radio beacon, the two sides of the loop will be at the same distance from the station. Thus the signals will arrive at the same time without any phase difference, causing current induced in both sides of the loop to be the same. However, since they are opposite in direction, they will cancel each other producing no rf output from the antenna. This is the null position of the loop aerial.

rf waves

Min. or no signal

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Maximum Position If the plane of the loop aerial becomes parallel to the direction of the waves, signals will reach at both sides with maximum difference in phase. That will produce maximum signal strength. Max. phase difference

rf waves

Max. signal

The Null position is preferred in direction finding because: # It is easy to determine a null than a maximum # It is more accurate and sharper. Sensing There are always two null positions and two maximum positions for a loop antenna. The loop aerial will always receive the same signal by turning it to 180 degrees. This may create confusion about a station and there will be an ambiguity of 180 degrees regarding the direction of the station.

The ambiguity is solved in the modern aircraft receivers by addition of another non-directional antenna for sensing. The ADF receiver uses a rotating loop antenna, which gives the figure of eight pattern, and a fixed sense antenna that gives an Omni-directional pattern. The figure of eight pattern from the loop antenna has positive (+) and negative (-) phase as indicated below. The sensing antenna has omni-directional circular pattern with (+) phase. The composite pattern therefore will be a cardioid as shown below. Circular pattern Cardioid

+ -

+

Figure of eight pattern When pilot tunes to an NDB station the ADF loop antenna automatically turns the indicator towards the direction of the station with reference to magnetic north. This is interpreted in the needle as the Radio Magnetic Bearing Indication.

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1. 4 ADF DISPLAY The Automatic Direction Finders (ADF) are manufactured with either analog or digital display. In either case, in ADF receiver, bearing information is presented on either a Relative Bearing Indicator  (RBI) or the more complex Radio Magnetic Indicator  (RMI).

1.4.1 Relative Bearing Indicator  : This is the simplest type of display, shows the pilot the bearing of the tuned NDB transmitter relative to the axis of the aircraft. The RBI is measured clockwise in degrees (O - 360) from the nose of the aircraft. See Figure above.

1.4.2 Radio Magnetic Indicator : This instrument displays the magnetic bearing of the NDB as well as the heading of the aircraft. Therefore it is more convenient for the pilots. The figure above shows the method of measuring RMI.

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1.5 USE OF NDB By using relative or magnetic bearings, NDB can be utilized for various navigation purposes. Depending upon their use and where they are placed. 5.1 Homing: NDB is installed at the vicinity of the airport. Aircraft find their way to the airport by tracking on to the beacon. 5.2 En-route: NDB is installed in between the airports on the prescribed routes. Sometimes the beacon may be offset from the route. However, by using relative bearing a position fix can be determined. 5.3 Holding : Such an NDB is called Locator Beacon and is placed a few miles away from the airport area. Aircraft circle the beacon at different heights waiting for permission to land. 5.4 Instrument approach: NDB is installed on the center line of the runway. Aircraft make straight-in approach by using the NDB.

1.6 ADVANTAGES OF NDB  Although there are now several more accurate navigational systems available on other radio frequency bands, the NDB is still used in every country in the world, and will continue to do so for many more years to come. The reasons are obvious which can be outlined as follows: # # # # # #

Very simple air-borne and ground equipment Inexpensive to install and maintain Omni-directional information Any number of aircraft can use the same radio beacon Responsibility of accuracy mainly depends on airborne receiver Multi-purpose uses

1.7 LIMITATIONS OF NDB Like any other equipment, NDB also have its own limitations. If an NDB is used under certain condition pilots may get sometime large and potentially dangerous bearing errors. Therefore, NDB cannot be considered as a precision aid and should be used with caution. The principal factors liable to affect the NDB performance are as follows: 1.7.1.Quadrantal Error : Due metallic portions of the aircraft the radio waves get deflected. Error produced by such a phenomenon is called quadrantal error   because it is maximum in all four quadrants. Quadrantal error differs for one aircraft to other, which can be corrected by using the correction curve for that particular aircraft.

Max error

Max error

Max. error

Max. error

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 A typical quadrantal error curve:

+10 +5 0

90

180

270

360

0 -5 -10

1.7.2. Coastal refraction: In coastal areas the differing radio energy absorption properties of land and water result in refraction of NDB transmissions. This causes error, known as coastal refraction. It is most marked when transmission cross the coastline at an angle other than right angle and when the transmitting station is located away from the coast. If the angle is less than 30  the error gets worst. Therefore NDB's in the coastal areas should be used with utmost caution.

True bearing

NDB  Apparent bearing

Land

Sea

1.7.3. Night Effect :  At night, in addition to the interference that can occur due to transmissions from different stations, it is possible to receive the ground wave signals contaminated by the sky wave signals from the same station. This will give rise to bearing errors of varying magnitudes depending on the heights of the ionized layers and the polarization of the signals on arrival at the receiver. Night effect is especially most marked during the twilight hours when skywave contamination can cause fading of signal strength, which will cause wandering of the ADF bearing needle. 1.7.4. Mountain effect :  ADF receivers may be subject to errors caused by the reflection and refraction of the transmitted radio waves in mountainous areas. High ground between the aircraft and the beacon may increase the errors especially at low altitudes. 1.7.5. Static interference: All kinds of precipitation, including falling snow and thunderstorm can cause static interference of varying intensity to the ADF receivers. Precipitation reduces the effective range and accuracy of bearing information. Thunderstorm can produce errors of considerable magnitude including even entirely false indication. Indeed it is often said that in an area affected by thunderstorm activity, the ADF bearing pointer would rather indicate the direction of thunder than the NDB station.

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1.7.6. Lack of failure warning system: Because of lack of failure warning devices on ADF receivers, failure of an NDB station may produce wrong indication which will go unnoticed. Constant monitoring and hearing of identification signal is the only way to detect the failure  of the ground station.

1.8. SITING REQUIREMENTS  An NDB may be located on or adjacent to the airport. If it is used as an a pproach aid then it should be located on the centerline of the runway. In any case, the siting criterion is not very complicated. However, the following should be observed: The NDB site should be smooth, level and well drained. The antenna system should not penetrate the approach or transitional surfaces of the airport. There should be no metal buildings, power lines or heavy metal fences around the NDB station at a distance closer than 100 feet.

1.9. ANTENNA SYSTEM NDB antennas are similar to normal LW/MW antennas. Because of dominating transmission by the ground wave, vertically polarization is necessary. Hence vertical wires or self-supporting structures are the solution. Since, NDB operating frequency is in order of only a few hundred KHz, the practical length of an antenna must be much lower than /4 wave length. For example, for an NDB station working on 250 KHz, its wavelength will be:

or

 = 300/ 0.250 = 1200 meters /4 = 300 meters

To erect an antenna 300 meters tall is not only very expensive but also prohibited near the airport areas due to possible obstruction to the aircraft. In practice much shorter antennas (from 20 to 40 meters) are used. Because the antennas are relatively very short they are always capacitive in nature. Therefore, to resonate a NDB antenna some tuning inductance must be used. As described above, NDB antennas are vertically polarized. Therefore the radiator is kept in vertical position from ground. The earth acts as an image to the radiator. To increase the capacitance of the antenna, a ground radial system has to be provided. A ground radial system, which is also called counterpoise, is a system of copper wires buried approximately 15 cm below the surface of the ground. The size and shape of the counterpoise will vary with the type of antenna system used. Normally the wires are laid at 5  to 10from the center, just below the radiator. Fig. 2.1-M below shows a typical ground counterpoise of an NDB. 1.9.1Radiation pattern The polar diagram of an NDB antenna radiation is shown below. It is Omnidirectional in the horizontal plane (H-plane) and directive in vertical plane (E-plane). Theoretically there is maximum gain along

the earth surface, but in practice we will have maximum field strength at some angle from the surface due to losses in the ground wave component.

Theoretical Practical (H-plane)

(E-plane) Polar diagram of NDB antenna

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1.9.2 Types of antennas  A very simple, effective and widely used NDB antenna is T-antenna, which is illustrated below. The vertical wire is, of course, the actual radiating element and the horizontal wire provides additional antenna capacitance to the ground. To increase the capacitance of the antenna three or more parallel wires are used in the horizontal portion. The normal height of T-antenna is approximately 20 to 30 meters. Sometimes an inverted L-antenna is also used. However, it is more sensitive to unwanted horizontally polarized electric field component compared to a T-antenna. The self-supporting mast or a mast radiator   is also a popular NDB antenna. The normal height of such an antenna is 20 to 40 meters. Top-loaded insulated guy wires increase capacitance. Such an antenna is more efficient than a T-antenna and therefore widely used for long range NDB as well as MW/LW broadcasting. For locator beacons or for the beacons used for approach purposes, since the coverage required is very small, relatively short antennas are used. One of such antennas is Umbrella type. It is a small self supporting mast radiator with several top-loading elements like an umbrella. The top loading increases the capacitance of the antenna, hence it becomes easier to resonate. The normal height of an umbrella antenna is not more than 12 meters.

ILLUSTRATIONS Top loading

Radiator NDB shelter

12.5 m

20 m

30 m

12.5 m

GND

(A TYPICAL T-ANTENNA)

(Fig. 2.1-L)

Copper wire buried 15 cm under ground

5 to 10 degrees

(EARTH RADIALS OF NDB ANTENNA)

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Mast radiator Top capacitance 10m

Top loading

25m

wooden pole

antenna

insulator 12m GND

GND

( A TYPICAL MAST RADIATOR)

(UMBRELLA ANTENNA)

1.10 FACTORS AFFECTING NDB ANTENNA The radiation resistance of an NDB antenna is very low and equals to only a few ohms. If it were possible to match a source of radio frequency energy so that all the power is dissipated into the radiation resistance then these antennas would have been equally efficient as one with much higher radiation resistance. However, in practice the total loss resistance of the antenna is much higher than the radiation resistance. Therefore, most of the energy gets wasted and the efficiency of the antenna becomes too low. There are several factors that affect the efficiency of an NDB antenna. These are briefly described below: 1.10.1 Antenna Reactance : NDB antennas are capacitive in nature. The capacitance of antenna is important to know because it provides the basis for knowing the amount of tuning inductance required for resonance. Since, for resonance : L =1/ C Smaller capacitance will need bigger inductance, causing more loss of energy in the conductor. Thus the capacitance of an antenna should be as higher as possible. This can be done either by increase in height of the antenna, or simply by additional top-loading. The second option is more economical and favourable. The capacitance of an electrically short vertical antenna may be calculated by the use of well known transmission line formula. For a simple vertical radiator (insulated from ground) having a height “H” from the ground and diameter “D”, its capacitance can be roughly calculated from the following formula: C = 5766 X Tan θ Log 2H D Here C in pF, H and D in feet, and θ- electrical length of the radiator in degree.

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The following table gives approximate values of a vertical radiator without top loading in pF. From this it is evident that antenna capacitance is dependent of vertical height and diameter of the radiator element.

 Antenna Height (ft) 50 75 100 150 200 250

0.1 90 131 167 245 322 399

 Antenna diameter in inches 1 120 170 219 314 428 504

12 184 254 308 451 654 795

Where antenna length is desirable to keep short, top loading is used. This greatly increases the capacitance of the antenna thereby reducing the requirement of large antenna tuning inductance.  Additional capacitance generated by top loading in a T -antenna can be calculated as follows: C = 5766 X Tan (0.07315L) Log 4H D Here C in pF, H, L and D in feet. L – Length of top loading wire. 1.10.2 Radiation resistance : The base radiation resistance is another important characteristic. It is a characteristic, which has a direct relationship to the radiated power and consequently to effective range of the NDB. Because the NDB antennas are electrically very short (less than 30 ), the current distribution along the antenna is linear and radiation resistance may be calculated to reasonably close approximation by the formula: 2

R =   /328 , where  is the electric length of the antenna in degree.  = 360 With the above formula it is evident that by increasing the length of the antenna its radiation resistance increases, and hence the efficiency increases. See following table.

 Antenna Height (ft) 50 75 100 150 200 250

200KHz 0.041 0.092 0.163 0.367 0.657 1.020

Radiation resistance in Ohms 300KHz 0.092 0.207 0.367 0.825 1.467 2.295

450KHz 0.206 0.466 0.825 1.857 3.300 5.164

1.10.3 Antenna Q :  Antenna system Q is the ratio of the reactance of the antenna capacitance to the antenna total system resistance. It is always preferable to keep the Q as low as possible to reduce losses in the antenna system. Since

Q = Xc/R ,

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Q can be reduced by increasing capacitance of the aerial. I.e. by addition of top loading or by increasing the height. NDB transmitters are usually required to have a bandwidth of at least 2X1020 Hz. 1020 Hz being the max ident frequency. Bandwidth = f/Q Therefore, at 300 KHz Q = 300,000/2040 = 147 Which means a Q of 147 at 300 KHz NDB station will insure that the ident modulation will be radiated without any distortion. If bandwidth of the antenna is low ( Q is high) then instead of 1020 Hz ident modulation of 400Hz should be used. 1.10.4 Expected range NDB antenna should be designed in such a way that it should radiate reliable signal up to the required coverage area. ICAO has specified that in the coverage areas the field strength should not be less than 70V per meter. Between the latitudes 30N and 30S field strength of 120 V may be required.

1.10.5 Voltage at the antenna base The peak voltage at the antenna base for usual NDB is : V = I A x XC

Where I A - Antenna current and XC - Antenna reactance.

Example: For an NDB with T-antenna top loaded with three wires and 50 ft high and 100 watts transmitter C = 581 pF and antenna Current I A = say 7 Amps at 200KHz XC = 1370 Ohm then V = 9590 Volts For the same antenna without additional top loading, C = 90 pF  XC = 8842  Hence V = 61,894V This amount of voltage is difficult to contain and would probably cause considerable difficulty due to corona, flashover, etc. Therefore the antenna capacitance of an NDB antenna should be kept around 500 pF or more.

C onclusi on : To inc reas e the efficiency and to improve the performance of an NDB antenna its capacitance should be as high as possible and should be more than 500pF at the lower  frequenci es . Thi s can be achieved either by in cr easing the heig ht of the antenna or by  prov idi ng additional top loading .

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1.11 TRANSMITTING EQUIPMENT The NDB transmitter is relatively very simple equipment. The RF carrier is amplitude modulated either by 400Hz or by 1020 Hz tone, which is coded with two to three letters station identification in Morse Code. A simplified block diagram is shown in Fig. 2.1-Q:  Antenna IDENT

TRANSMITT

UNIT

ER UNIT

MONITOR

 A monitor equipment monitors the performance of the radiating signal. Radiation is done in A0/A2 mode. Depending upon the use an NDB could be classifies as one of the following: High Power: usable range extends up to 400 NM. Radio beacons of this type are considered as enroute or homing  radio navigational aids. The transmitter output is normally 100W to 5KW. Low Power : usable range extends from 10 NM to 25 NM. Radio beacons of this type are called locators  and are normally used for approach or holding purposes. The transmitter output power is kept below 100W.

1.12 MONITORING AND CALIBRATION Normally the NDB beacon has two transmitters and two monitors, i.e. dual equipment system. Monitor analyzes the radiated signal and checks the following: # Gives alarm if the transmitted carrier power is reduced more than 3dB. i.e. 50% # Gives alarm if the identification signal is removed or continuous by any reason. # Gives alarm if the monitor itself becomes faulty. When one of the above conditions occurs the monitor unit commands the changeover unit to shut sown the faulty transmitter and to start the standby. The NDB stations are normally unattended, which are monitored for a failure by the technicians through radio. To distinguish main transmitter from standby normally the main is modulated with 1020 Hz and the standby with 400Hz.

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