_{1}

^{*}

A new kind of image type glide path beacon subsystem of Instrument Landing System (ILS) was reported. Capture effect is designed to overcome far-field (FF) reflectors and rising terrain. Low angle CSB signal was highly depressed, was ?41 dB at 1? comparative to the max value. Both Carrier and Side Bands (CSB) and Side and Bands Only (SBO) signals were radiated by three antennas. Difference in Depth of Modulation (DDM) provided good linear variation around glide path angle (
*θ*). At near field area, the site of near-field (NF) monitor was just about 57 m in front of the system, needed less length of reflection plane than that of M-array system, result from the system had only 90% of the M-Array antenna height.

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

A series of glide path antennas were produced. Such as image type: null reference glide path antenna [

In the paper, a new kind of image type glide path antenna system is introduced. It is similar to the M-array glide path antenna system, radiated by three antennas with capture effect. Since it uses separate transmitters to provide the course and clearance, it is tolerant to far-field reflectors and rising terrain. Moreover, this new type could tolerant to more difficult sites and needs less size of smooth ground and reflection plane. Because this system looks as a shrunk M-Array system, called S-Array glide path antenna. The discussion below introduces the structure of the S-Array as well as its signal distribution. And then, the 3D pattern of the space signal would be exhibited. Meanwhile the near field characteristic should be analyzed detailed. At last, the several parameter of S-Array is compared with other kind of image type glide path beacon and proved the best performance.

The S-Array glide path antenna system consists of three antennas, upper, middle and lower antenna. h_{1} = 1.3h, h_{2} = 2.0h and h_{3} = 2.7h respectively. The structure of the antenna and the position of the near field monitor antenna can be found in _{1} = 5.6 m, h_{2} = 8.6 m, h_{3} = 11.6 m. Meanwhile the near field monitor (NF) is placedat a distance (D) about 57 m in front of the antenna system. The phase error between h_{1} and h_{3} to NF is 360˚. And the height is little higher than the value D ⋅ tan θ , mainly depends on tested value on the spot after the flight check.

The relative amplitude and phase for standard CSB, SBO, and Clearance (CLR) feeding of the three antennas are showed in

The coverage range requirement can be found in

The lobbing diagram of three kinds of signal: CSB, SBO and CLR listed from 0 to 2θ are showed in

The space pattern of kathrein antenna is showed in

The theoretical course and clearance radiation patterns are described in

ddm = 2 E SBO E CSB cos ( φ SBO − φ CSB ) .

The value of ddm is a constant value 0.4 for clearance signal predominant, while for course signal predominant, the value is twice the quotient of vector E_{SBO} divided by E_{CSB}. The boundary, the red cross-point, of them is marked in the top of the figure.

There are three requirements for ddm distribution around the glide angle θ.

Height | CSB | SBO | CLR | |
---|---|---|---|---|

Upper Ant | 2.7h | 0.14 ∠ 0˚ | 90 Hz 0.093 ∠ 0˚ 150 Hz 0.093 ∠ 180˚ | 90Hz 0.125 ∠ 0˚ 150Hz 0.375 ∠ 0˚ |

Middle Ant | 2.0h | 0.88 ∠ 180˚ | 90 Hz 0.173 ∠ 180˚ 150 Hz 0.173 ∠ 0˚ | NA |

Lower Ant | 1.3h | 1.0 ∠ 0˚ | 90 Hz 0.093 ∠ 0˚ 150 Hz 0.093 ∠ 180˚ | 90Hz 0.125 ∠ 0˚ 150Hz 0.375 ∠ 0˚ |

Good linearity characteristic, symmetry and 75 μA (ddm = 0.0875) at 0.88θ. To get 75 μA, ddm = 0.0875 for lower half sector at 0.88θ, the coefficient k_{1} for SBO relative level is:

7 160 ⋅ sin ( 1.3 h sin ( 0.88 θ − F S L ) ) − 0.88 sin ( 2 h sin ( 0.88 θ − F S L ) ) + 0.14 sin ( 2.7 h sin ( 0.88 θ − F S L ) ) − 0.535 sin ( 1.3 h sin ( 0.88 θ − F S L ) ) + sin ( 2 h sin ( 0.88 θ − F S L ) ) − 0.535 sin ( 2.7 h sin ( 0.88 θ − F S L ) )

where the h is electric length: h = π 2 sin ( θ − F S L )

ddm = −0.0875 for upper half sector at 1.22θ, the coefficient k_{2} is:

− 7 160 ⋅ sin ( 1.3 h sin ( 1.22 θ − F S L ) ) − 0.88 sin ( 2 h sin ( 1.22 θ − F S L ) ) + 0.14 sin ( 2.7 h sin ( 1.22 θ − F S L ) ) − 0.535 sin ( 1.3 h sin ( 1.22 θ − F S L ) ) + sin ( 2 h sin ( 1.22 θ − F S L ) ) − 0.535 sin ( 2.7 h sin ( 1.22 θ − F S L ) )

Approximately, k_{1} = k_{2} = 0.173. The further calculation results with different FSL from −0.5˚to 0.5˚ are listed in _{1} is decreased with the increasing of FSL. And the error of k_{1} and k_{2} is negligible, which demonstrates the good symmetry. Discussion about ddm curves below would further demonstrate the linear variation around θ.

Three curves of ddm distribution are exampled in

The antenna elements should be aligned along a straight line, it shall be perpendicular to the average forward slope (FSL). If the forward slope is rising, the lower antenna shall be forward compared to the middle one.

The sideways offset of the antenna elements shall be accurately adjusted. Orientation is such that the upper antenna is closer to the runway than the middle one. And the middle antenna shall be closer to the runway than the lower one.

Detailed, for the formula below, FSL positive if ground rising toward threshold, SSL positive if ground plane rising from antenna mast toward runway.

The formula of the alignment parameters are listed below:

The longitudinal offset lower to upper (sft_{d}) is: 1.4 h ⋅ sin ( F S L ) , middle to upper (sft_{m}) is 0.7 h ⋅ sin ( F S L ) . And lateral offset upper to middle (off_{u}) is:

FSL (˚) | k_{1} (%) | k_{2} (%) | Δk (10^{−6}) |
---|---|---|---|

−0.5 | 20.21 | 20.23 | −19 |

−0.3 | 19.06 | 19.07 | −15 |

−0.1 | 17.90 | 17.92 | −12 |

0 | 17.33 | 17.34 | −93 |

0.1 | 16.75 | 16.76 | −69 |

0.3 | 15.60 | 15.60 | −14 |

0.5 | 14.45 | 14.45 | 52 |

k_{1} for 0.88θ, k_{2} for 1.12θ and Δk = k_{1} − k_{2}.

( 2 2 − 2.7 2 ) ⋅ h 2 2 ⋅ D + h ⋅ sin ( S S L ) ,

lower to middle (off_{d}) is:

( 2 2 − 1.3 2 ) ⋅ h 2 2 ⋅ D + h ⋅ sin ( S S L ) .

where h is: h = λ 4 sin ( θ − F S L ) .

It is worth noting that the Side Slope (SSL) just affects sft only, but FSL would affect the height of antenna, off and sft.

The case of FLS = 0 and D = 120 m: h_{1} = 5.58 m, h_{2} =8.59 m, h_{3} = 11.60 m, off_{u} = −25.3 cm, off_{d} = 17.7 cm.

The signal synthesis in near field (NF) region is different to that in far field. The boundary of them is the horizontal distance (1160 m) that 100 times of the h_{3}. Outside the range of this distance, the radio signals for the direct and its image one are considered as parallel. But within this distance, they are not regarded as parallel any more. The phase error appears and should be revised. The follow discussion introduces the location of NF monitor and the NF behavior for ddm distribution.

The phase of the total received signal at the monitor point M will be as shown in _{M} = (Φ_{r} + Φ_{d})/2, and Φ_{r} is the electrical distance from radiated antenna to the receive point of the direct signal, similarly Φ_{d} works as the electrical distance of the reflected signal.

To monitor the glide path angle, the phase error should be 0˚ or 360˚. For a nearly 0˚ phase error, the distance must be more than one kilometer. This distance corresponds to a very high monitor antenna. But for 360˚ phase error, the distance is approximately 57 m. The phase error Φ_{MU} − Φ_{ML} for θ = 3˚ as a function of the distance L in front of the antenna system is shown in

distance is approximately 62 m to capture the zero point of ddm for NR. In particular, the monitor antenna located at about 41 m and 2θ (6˚) to obtain the zero point of ddm for SR. Similar to S-Array, the distance is approximately 82 m for M-Array. Above all, besides the distance for SR (~41 m), the S-Array one be possessed of a shortest distance of the currentimage type glide path antenna. It means the height of monitor antenna is also the lowest among four types. These characteristic is advantage to install and maintain, at the same time, reduce the cost.

For near field monitor, the ddm curve is essential to study. This is the monitoring basis from which the far field space signal distribution could be speculated. So the ddm distribution at 57 m (Φ_{MU} − Φ_{ML} = 360˚) from the antenna system is given in

Despite the alignment of lateral offset, the ddm curve would remain disrupted in NF, especially near the threshold. The direct result is the deviation of the zero point of ddm. In approaching range, the ddm distribution from 600 m to the threshold is presented in

To research the NF condition further, the characteristic of ddm not only on glide path, but also in front of the NF are also studied. The angle (ddm = 0) of two cases versus corresponding distance are showed in

Among the former three types of image glide path antenna, the most comparable one is M-Array with three antennas as well as capture effect. Introduction below discusses the characteristic of offset, ddm distribution in NF, anti-interference capability, environment needs.

After the glide path iron tower has been erected, it is not easy to move the position

of each antenna, especially the old one which has been used for years. During the flight check, sometimes the position of the antenna should be aligned, such as mending glide angle by align the height of the middle antenna, optimizing the structure of III zone by adjusting offset, and improving TCH by twisting the antenna and so on. It is essential to change to the antenna position as soon as possible. In other words, the less extent, the quicker finished. The extent of several offsets of two types is listed in

Theoretical calculation demonstrates that it is more easily to improving TCH for S-type than M-type by rotating the upper antenna. Obviously the S-type has the less changing extent. The parameter h_{3} is the symbol of the size of the antenna system, the larger it is, the more expensive and inconvenient to maintain. The parameter h_{3} − h_{1} represents the working range to maintain or repairmen. For example, if the transmitter cable is damaged, the ADU should be calibrated again after replaced the new cable, and operator would connect the cable up and down in this working region, the lager this region the poorer efficiency. And so do lateral offset adjustment.

The ddm distribution for near field monitor has been listed in

For the monitor antenna, S-type is lower and has a shorter distance to glide path tower. This means the lower cost for manufacture and convenient to adjust monitor to the position of ddm = 0.

The beam bend potential (BBP) represents the anti-interference ability, the less bbp value the better it works. S-Array has the lowest radiation in lower angles to

Type | h_{3} | h_{3} − h_{1 } | off_{d} | off_{u} | stf_{d} | stf_{m} |
---|---|---|---|---|---|---|

M-array | 14.32 | 9.55 | 0.285 | −0.475 | 0.050 | 0.025 |

S-array | 12.89 | 6.69 | 0.219 | −0.312 | 0.035 | 0.017 |

ΔM | 0.99 | 0.66 | 0.038 | −0.064 | 0.034 | 0.017 |

ΔS | 0.89 | 0.47 | 0.029 | −0.041 | 0.024 | 0.012 |

Note: The case of θ = 3˚, f = 333.35 MHz, FLS = 0.3 and D =120 m. ΔM means the variation from FLS = 0.3 to FLS = 0.1, the other parameters reserved. All the units of the data above are meter.

Type | BBP (1˚) | BBP (μA) | CSB (1˚/Max) | Frequency |
---|---|---|---|---|

Null Ref | 20.2% | 173 | 50% | Single |

SB Ref | 14.8% | 127 | 26% | Single |

M-Array | 2.7% | 23 | 5.2% | Double |

S-Array | 0.04% | 0.34 | 0.5% | Double |

reduce illumination of terrain objects in order to reduce bends. Strictly speaking, the radiation of M-type is a little lower than S-type below 0.7˚, because the S-type appears negative value, but after that, it owns the lower radiation up to 3˚. As seen in

A new kind of image type glide path antenna was presented. To depress low angle radiation, the capture effect was also designed. And all of upper, middle, lower antennas radiated CSB and SBO signals. The calculation showed the CSB signal is −41 dB at 1˚ elevation angle comparative to the maximum value of CSB. DDM provided good linear variation around glide path angle.

To achieve a less length of reflection plane, the height of the system was reduced, which had only 90% of the M-array size, and needed 10% less length of reflection plane. Meanwhile, it achieved a shorter distance of NF, approximately 57 m. This could save the cost of facility installation and maintenance. The shortened antennas gap results less size of offset, which could improve work efficiency in flight check adjusting.

The author declares no conflicts of interest regarding the publication of this paper.

Qu, C.Q. (2018) Brief Introduction of a New Kind of Glide Path Antenna. Open Journal of Antennas and Propagation, 6, 60-72. https://doi.org/10.4236/ojapr.2018.63006