Help with ILS - Glideslope Exam Question

[Rufes]

Hi,

We recently had an exam at TAFE and a question was in it regarding glideslopes. After the exam everyone had different opinions on what the answer was. Some people thought the answer was "a" and others thought it was "b". Anyway the question is below.

Hopefully someone can shed some light on what the correct answer is.

Question:

What causes multiple beams to be produced in a glideslope field pattern at an airport?

a) The carrier and two modulating frequencies
b) The actual vertical radiation pattern of the glideslope antenna
c) The antenna height above ground level

[Clyde Parthangel]

Amswer B. As they radiate, the glidepath aerials are 'reflected' by the ground surface. The interaction between the direct path signals and the reflected signals cause signal addition when they are received 'in-phase' and signal subtraction when they received out of phase. Hence, multiple beams.....

The height of the antennas above ground dictate the angle of the beams. The closer to the ground, the higher the angle.

Just for a little bit of additional information, the modulation that you put onto a signal cannot affect the shape of the radiated signal. That can only be done by the antenna(s)...

[Rufes]

Thank you very much for your in depth response which also answered another one of the questions I wasn't sure about. :ok:

[Gulfstreamaviator]

The height above ground should not affect the primary radiated energy, however when too close to the ground, a small amount is either absorbed thus bending the beam down or reflected to produce a scatter upwards. And thus phase variations.

GS traditionally as a UHF signal did not suffer as much as LOC transmissions on VHF, from reflections.

It could be argued that the answer is C, based on my comments above.

glf

[Gulfstreamaviator]

see also GS dot question.

glf

[ampclamp]

depends on how you read the question.
Are they looking for the source of a defective beam or what constitutes a normal G/S? The english is a bit mutilated (as so many are in aviation exams:sad: ) so it makes it an English test also which is bulldust but...

Is not the G/S beam made of a carrier and 2 modulating frequencies 90 and 150 hz ? Yes so.....
Multiple beams is 'more than one' ( ie 2 ) and it could be argued that what they are looking for is a normal G/S beam and not an irregularity therefore answer A may well be correct and that is what I would chose.