_{1}

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From the actual perspective of working principle of localizer beacon subsystem of Instrument Landing System (ILS), consideration of the distance information from localizer antenna to field monitor antenna and wide aperture effect of localizer antenna, broke through the limitation of signals synthesized only far-field (FF), established the near-field (NF) model. The three-dimensional mathematical model of localizer beacon was designed, and the signals at both near-field and far-field were analyzed qualitatively. At the environment of Antenna Fault as well as Antenna Distribution Unit (ADU) phase shifter simulation, the characteristics of near-field and far-field were also compared. The simulation results showed that the model met the requirement of theory of localizer beacon, and the gap between two models was so evident, which resulted from the broken geometric symmetry in NF area. The model could provide valuable theoretical basis for performance evaluation and maintenance of the ILS, and meanwhile, provide reference for the further analysis of localizer beacon.

The Instrument Landing System (ILS) had its beginnings in the United States and England during the years 1939 to 1945 [

Many series of Localizer Beacons were produced by antenna manufacturing companies. For example, series- 3500: 3523B [

The guidance function of ILS depends on spatial signals radiated from ground equipment, and the quality of the signals is up to antenna array and electronic equipment, such as Antenna Distribution Network (ADU). So it is of importance to understand and analyze the field signal in the air, which is radiated by antenna array. Also, it is of advisability to simulate the spatial overlying signal, through which method the spatial electromagnetic environment would be forecast before the flight check. Furthermore, it is of use to make simulation of antenna error or signal distortion; next it can be convenient to find the faulty from which antenna. Based on the signal simulation, operation condition of ILS equipment would be mastered well and more scientific; meanwhile, the maintenance of electronic device would be more appropriate and reliable.

In the paper, the simulated software Matrix Laboratory (MATLAB) was used to analyze the 7220A Localizer Beacon Subsystem of ILS. This software could be used to mathematical modeling, simulate and analyze the dynamical system. Not only could it provide intuitive drawing but also afford integrity and accuracy of the data. To provide valuable reference of performance evaluation and operation maintenance of ILS, the theoretically analysis and mathematical modeling of Localizer Beacon were given.

The ADU feeds the antenna elements with the proper amplitude and phase of the CSB (carrier and sideband, a signal amplitude modulated to equal depths by the guidance tones, 150 Hz and 90 Hz) and SBO (sideband only, takes the form of a double sideband, suppressed carrier with the two guidance tones modulated in opposite audio phases) signals. By changing the amplitude of the SBO feeding the ADU, the Course Sector Width is adjusted. The Course Line is determined of mainly the mechanical alignment, but could be adjusted by using a phase shifter inserted at the output of the ADU to one of the antennas. For adjusting Course Line effectively, this antenna feeding the maximum SBO amplitude.

It can be found at _{0} more than D 100 times, to the receive point “dot P”, the R_{1} and R_{2} can be seen as paralleled. The vectors E_{1} and E_{2} are E_{1} = E_{0}/β_{1}, E_{2} = E_{0}/β_{2}, E_{0} is its amplitude and β is its phase. To the “dot P”, the vectors E_{1} and E_{2 }become E_{1} = E_{0}/Ф_{1} + αsinθ, E_{2} = E_{0}/Ф_{2}-αsinθ. Where Ф is the initial phase, αsinθ is the phase difference comparative to the original point.

The frequency range of localizer beacon is from 108.1 to 111.975 MHz, works in very high frequency (VHF). Choicing the frequency 111.1 MHz for instance, then the corresponding wavelength is 2.7 m.

The electrical length

Two antenna elements spaced apart:

Two antenna elements fed in phase (0˚) spaced 2D apart:

Antenna No. | Dist from CL (m) | COU CSB ampl. (V) | COU CSB phase (˚) | COU SBO ampl. (V) | COU SBO phase (˚) | CLR CSB ampl. (V) | CLR CSB phase (˚) | CLR SBO ampl. (V) | CLR SBO phase (˚) |
---|---|---|---|---|---|---|---|---|---|

1 | −26.03 | 11 | 0 | 3.29 | −90 | ||||

2 | −22.54 | 15 | 0 | 4.20 | −90 | 0.32 | 90 | ||

3 | −19.23 | 29 | 0 | 5.75 | −90 | 5.65 | 180 | 0.32 | 90 |

4 | −16.09 | 45 | 0 | 6.59 | −90 | 5.65 | 180 | ||

5 | −13.13 | 66 | Ф + 0 | 6.66 | Ф − 90 | 11.72 | Ф + 180 | 0.54 | Ф − 90 |

6 | −10.34 | 82 | 0 | 6.00 | −90 | 11.72 | 180 | 0.32 | −90 |

7 | −7.73 | 93 | 0 | 4.82 | −90 | 32.17 | 180 | 4.32 | −90 |

8 | −5.30 | 100 | 0 | 3.38 | −90 | 12.86 | 0 | 4.32 | −90 |

9 | −3.04 | 100 | 0 | 1.89 | −90 | 94.60 | 180 | 9.25 | −90 |

10 | −0.95 | 93 | 0 | 0.55 | −90 | 204.83 | 0 | 37.52 | −90 |

11 | 0.95 | 93 | 0 | 0.55 | 90 | 204.83 | 0 | 37.52 | 90 |

12 | 3.04 | 100 | 0 | 1.89 | 90 | 94.60 | 180 | 9.25 | 90 |

13 | 5.30 | 100 | 0 | 3.38 | 90 | 12.86 | 0 | 4.32 | 90 |

14 | 7.73 | 93 | 0 | 4.82 | 90 | 32.17 | 180 | 4.32 | 90 |

15 | 10.34 | 82 | 0 | 6.00 | 90 | 11.72 | 180 | 0.32 | 90 |

16 | 13.13 | 66 | 0 | 6.66 | 90 | 11.72 | 180 | 0.54 | 90 |

17 | 16.09 | 45 | 0 | 6.59 | 90 | 5.65 | 180 | ||

18 | 19.23 | 29 | 0 | 5.75 | 90 | 5.65 | 180 | 0.32 | −90 |

19 | 22.54 | 15 | 0 | 4.20 | 90 | 0.32 | −90 | ||

20 | 26.03 | 11 | 0 | 3.29 | 90 |

Note: The voltage amplitudes are relative to the CSB Course amplitude at antenna 8. The Ф at antenna 5 is the phase simulated the PH449 with ADU, making it possible to fine-tune the CL DDM.

Two antenna elements fed out of phase (180˚) spaced 2D apart:

From

The DDM is definite as:

Consider the vertical section, the phase difference consists of two parts: horizontal part

Among them, the distribution function of LPDA is

is in accordance with the information supported by antenna designed company. (NORMARC 7000B ILS, 2013) [

_{H} increasing, the value of CSB slides sharply but SBO shots up. This design makes DDM increasing linear in low azimuth region. The range of the first lobe of SBO is just 0˚ - 5˚, which reflects the narrow beam. It can avoid multipath reflection causing for obstacle near the both side of the runway. For Course CSB, the peak value of first lobe is greater than the second 40dB, demonstrating good effect of sidelobe suppression. Besides the main lobe, the coverage task is undertaken by CLR CSB. Again, it is a good design for two frequency capture effect: Each transmitting antenna had its own RF carrier, closely spaced, so that both would be in the pass- band. The stronger of the two would tend to “capture” the receiver detector [

The capture effect is showed in _{H}.

Besides the transmit power, distribution relationship of ADU, the coverage strength also affected by the height of the transmit antennas. _{V} below 5˚. Obviously, field strength enhanced with increasing of antennas height, almost linear increase below 4 m. Because the radiation lobe made up of antenna and its image was drove down by the antenna height raised, when the height reaches λ/(4sinθ_{v}), the maximum value of lobe was achieved. The red dot line in _{v}, not the constant value. So, in case

of low field strength coverage, or coverage not enough, elevate the height of antenna is a advisable choice, which can strengthen the signal coverage effectively. For amplitude of CSB, there is no obvious difference between NF and FF, because the height of antenna is far less than the distance from antenna to receive point. Comparatively the longitudinal distance of antenna is approximate to the distance of NF monitor. So the horizontal differences should be paid more attention to.

The space pattern of CSB coverage is exhibited in

From the vector distribution of CSB and SBO, the DDM and SDM pattern are calculated, shown in

The low angle is dominant by course signal, which has a good linear characteristic below 2˚. The DDM starts to transition to clearance signal near cross point, where the course and clearance powers are equal. Different to course DDM, the curve of clearance DDM is comparatively smoothly, the value keeps on about 0.3 from 5˚ - 30˚, but steeply rising in the later region. This perfectly satisfies the requirement that from the angle where the DDM is 0.18 to 10 degrees of the course line, the DDM should not be less than 0.18. From 10 degrees to 35 degrees the DDM shall not be less than 0.155, which defined by ICAO Annex 10 [

The position of the FF monitor has been performed at

7220A Instruction manual, 2012) [

It can be found at _{0} < 100D, R_{1} and R_{2} could not be seen as paralleled, the specific size should be considered. The value of R_{1} and R_{2} has been given at the bottom of the

L is the distance alone the y-axis, D is the distance alone the x-axis from original point to the antenna, which can found is

The amplitude of R_{1} and R_{2} is not identical, which is inversely proportional to the radiation distance. So, under the NF circumstance, all the calculation of CSB, SBO, ddm and so on is made by vector synthesis. The synthesis amplitude and phase is:

Comparing to the pattern of FF, there are several differences. The theoretical Course and Clearance radiation patterns 120 m far from the original point have been shown in

The DDM pattern of NF has been showed in

There are four representative kind of DDM pattern listed in

_{H}. The DDM value fluctuates below 100 m, and shots up sharply until 1000 m, and then increases mildly, keeps about 0.155 in FF at last. The enlarged area with four curves of half DS sector up to 4˚, seen in the inset, illustrates that the DDM value is enhanced with the DS rising. It is similar to the FF behavior, DDM value enhanced linearly with the increasing of horizontal azimuth, seen in

To understand the characteristic of the two models entirely and systemically, the calculation of signal coverage, cross point, DDM pattern as well as simulation of antenna failed and phase shifter have been further researched.

The strength of the receive signal decided by two part, radiation loss and antenna gain. The concrete calculation of signal strength at NF and FF monitor position given in following discussion.

where the G_{t} is the gain of single transmitting antenna, for LPDA, G_{t} is about 10 dBi.

G_{r} is the gain of monitor antennas, G_{t} of NF and FFM is about 6.5 dBi.

FFM: −7 dB, θ_{V} = 0.069˚ for h = 6.0 m, D = 5 Km.

NF: 20 dB, θ_{V} = 0.716˚ for h = 1.5 m, D = 120 m.

The difference of loss between FFM and NF is distance, height of antenna and G_{lobing}.

The G_{lobing} of NF is −21.2 dB, and of FFM is −29.5 dB (supposing E = 1). The Loss of NF is 54.9 dB, and of FFM is 87.3 dB. So the strength difference between NF and FFM is (87.3 − 54.9) + (29.5 − 21.2) = 40.7 dB in all.

Through the coverage for FF in

components have been received at the transition region. The discussion has also been mentioned above, described about

The signal failed of some antenna would affect the signal synthesis of both CSB and SBO, thereby changing the DDM distribution. The DDM patterns of FF (12,000 m) and NF (120 m) under the simulation circumstance of Ant. 10 failed has been showed in _{H} = ±2˚. It can be seen the dotted line is the normal DDM curve, the curve of FF reflects the better linear characteristics. Under the Ant. 10 failed circumstance, both the DS of two models has changed. The FF one shortens a little but the NF one decreases to about 3˚. Moreover the CL position of NF one has also deviated, seen in the inset. To conclusion, there are two obvious differences under the circumstance of antenna failed.

At the flight check situation, sometimes the CL is deviated from the center line, which should be straightened. The phase calibration of shifter simulated with 360˚ rotation, set in Ant. 5 with the maximum level of SBO, found in

perfect odd symmetry, the maximum variation (ΔCL) appears at 90˚ and 270˚, which is about 10 μA, and returns to zero at 180˚. The latter one, less than 100D (2600 m), has the almost the same maximum variation, but doesn’t embody the good symmetry, the value deviates zero at 180˚. The bottom one shows the NF result from 60 m to 120 m, the symmetry disappeared completely. The curve of 60 m has a smaller variation, other three have the similar perform, exhibit negative value from 0˚ - 300˚, the variation monotone increasing in this region, and exceeds 200 μA at 300˚. This variation is much more than that of FF. However, the situation in both NF and FF, the variation is not is not so evident in the range from ±30˚, especially in NF.

Two models of Normarc localizer beacon 7220 A, far field model and near field models, were established and extensively studied by means of MATLAB modeling, theoretical analysis and experimental measuring. Far field model reflects infinite signal synthesis characteristic. The results are identical to that described in Normarc Training Manual. Near field model exhibits the situation that the receive distance is less than 100 times of the half aperture of antenna array. The distribution of signal coverage and DDM is extremely sensitive to the distance. So the NF model is more complex than FF one, and adds the order parameter: longitudinal dimension. This originates from the broken geometric symmetry in NF area. Accordingly, the NF plot pattern is significantly different from the plot of FF. The further simulation of antenna failed and phase shifter also confirmed this argument.

C. Q.Qu, (2015) Research on Signal of Field Monitor of 7220A Localizer Beacon Subsystem of ILS. Open Journal of Antennas and Propagation,03,37-50. doi: 10.4236/ojapr.2015.34005