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Old 22nd Aug 2017, 12:47 pm   #81
trh01uk
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James

another treatment of the "simple" two ray model is in Wikipedia at https://en.wikipedia.org/wiki/Two-ra...flection_model

If you can process the maths then you should get some reasonable answers. If you can't you will probably then see why no-one is rushing to try and answer your questions....


Richard
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Old 22nd Aug 2017, 9:09 pm   #82
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Richard

I've not asked anyone to do calculations for me, I'm just shaking the tree and hopefully provoking some thoughts to bounce off my enquiries. Possibly someone might leap in with new evidence?

Your maths posts have moved me to make the scale drawing that I should have made before. This is based on the things we know which are:

1: The rough length of a typical runway
2: The distance from the start of the runway to the inner marker
3: The distance between markers
4: The hight of the antenna
5: The height of the aircraft at the start of the runway
6: The hieght of the aircraft at the outer marker

Happily there are three points through which to draw our equi-strength field line. See attached

James
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Old 22nd Aug 2017, 9:14 pm   #83
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I'm no pilot but that doesn't look so onerous a flight path to me.

If we assume that the system worked - even badly - the sketch shows pretty much where the field line has to go, or actually does go in fact.

To return to my point about the angle between the antenna and the field line lets zoom in on the aircraft at the start and finish of the glide - see attached
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Old 22nd Aug 2017, 9:24 pm   #84
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

You can see that alpha and beta are different and so even though it is still the same field line there should be less gain at the antenna and the pilot would increase hieght to re-assert what he is being told is the correct line.

Possibly his original line might get to being parallel with the runway in which case its strength would dwindle to zero - but by then the pilot would have climbed up - hence my assertion that the system might not let the pilot hit the ground if he followed the indication correctly.

It's a thought that possibly it is a more subtle system than seen at first sight and deserves perhaps more than just being derided and dismissed out of hand.

Also the reason why I would like to find contemporary texts about the issues it had.

Again - I'm not fighting the Glide paths corner - I'm just thinking about it.


Cheers
James
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Old 22nd Aug 2017, 11:02 pm   #85
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by rambo1152 View Post
Would I be right in saying that the "Standard" in SBA stood for Standard Telephones & Cables?

There were other examples of anglicisation of German radio navigation technology, Sonne became Consol.
Hi Graham

Take a look at this clip - you could well be right! Also there is the usual doubt about the Glide path but no specific reasons given, interesting that it is described as a 'cornerstone'.


Cheers
James
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Old 23rd Aug 2017, 8:52 am   #86
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

thanks for your drawings. Certainly defining the geometry or layout of the situation is critical to making any sort of mathematical analysis of the problem. Let me run down your list of parameters and give you what I think are the values you have stated from the drawings. That will ensure that I am understanding the drawings correctly.

1: The rough length of a typical runway - 3000' or 1000 yards
2: The distance from the start of the runway to the inner marker - 1/4 mile or 1320'
3: The distance between markers - 2 miles or 10,560'
4: The height of the antenna - there are two antennas here: the receiving antenna on the plane is at 1000' at the start of the landing, but clearly falls to 100' at the start of the runway and then to zero at some undefined point (short of the end of the runway!). The transmitting antenna height on the ground is not given.
5: The height of the aircraft at the start of the runway - 100'
6: The height of the aircraft at the outer marker - 1000'

You need to address item 4 above concerning the height of the transmitting antenna. That is critical - and given the large size of the antenna, its critical to understand whether the height stated is at the middle of the dipoles or at the top or the bottom.

I assume the glide path between the outer and inner markers is linear? I make the angle to the horizontal (ignoring curvature of the earth between the markers) 4.9 degrees between the markers - and from that the height at any intermediate point can be calculated assuming we have a steady rate of descent.


Can you define your use of the words "field line"? Despite your drawings, I can't work out what the angles α and β are. A plan drawing as well as an elevation might well help here.

Richard
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Old 23rd Aug 2017, 9:02 am   #87
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by jamesinnewcastl View Post
Take a look at this clip - you could well be right! Also there is the usual doubt about the Glide path but no specific reasons given, interesting that it is described as a 'cornerstone'.
James,

not being an aviation person, I am not sure I understand the words "glide path" in this context? Is it referring to control of azimuth or elevation - or both?

Looking at dictionary definitions leaves me none the wiser. It seems to me that control of azimuth - that is the angle of the planes path in relation to the length of the runway - that is the absolutely critical factor to control here. And SBA appears to do that extremely well. And the piece you quote in the attached file talks about "glide path", but it is quite unclear whether they are referring solely to azimuth control or not.

If we look at control of elevation though - which we have discussed extensively already - what is termed on some sites as "glideslope" - that is far less critical. As the moment I see no way that SBA could control glideslope at all - as others have already stated in previous posts.


Richard
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Old 23rd Aug 2017, 12:06 pm   #88
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi, To land an aircraft successfully both are involved, azimuth control is left / right movement of the rudder to keep the aircraft aligned with the centre line of the runway. Following the glide path /slope is controlling the rate of descent to flare out and touch down.

Ross
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Old 23rd Aug 2017, 1:23 pm   #89
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi

Perhaps I am asssuming too much without stating my assumptions which are:

1. The pilot does not roll the aircraft at all - no reason why he should, he isn't doing aerobatics
2. As others have pointed out the pilot would not dive to land so I assume that the body of his aircraft is horizontal to the ground at all times - so no pitch is involved
3. His side to side movement is completely regulated by the other (independant) feature of the system which are the dots and dashes - so no yaw.

So he is decending only by reducing power and having forward momentum, his descent is then effectively in a single plane which is why a side view drawing is all that is needed to describe the glide path. I think that the drawing is pretty clear. (That is a perfect world situation of course but it's OK for the discussion)

The line taken by his descent is termed the Glide Path I believe - the 1940s pilot switches his control box to 'Glide' when he begins his descent - all the explanations use that expression.

Post #3 in this thread gives a 1936 explanation of how the pilot uses SBA to fly down a suitable glide path and so descend to a point 100ft above and just short of the runway such that he should be able to carry on landing visually.

Post #13 in this thread is a report in FLIGHT by an amateur pilot actually flying repeatedly down the beam using SBA to control his descent, he seems to have used it in practice quite sucessfully.

The indicator in the control panel of virtually all the bombers in the RAF in the 1940s had an SBA indicator including a vertical needle whose original function was to allow the pilot to fly down the glide path.

Testing the Glide Path function to see if it worked did not even need the weather to be bad so the governments and Air Forces who bought it, or thought about buying it would have been able to check it out fully at any time (except they may not have wanted to when the weather was bad!)

To say that the system could not control the glide path is to say that all the people who must have developed and tested the system in the preceeding years had completely missed the fact that the Glide path 'bit' wasn't possible. I'm not saying that they liked it....

As I say, I'm not looking to say that the system was wonderful - I'm just looking at how it worked, what it's features were, perhaps what its subtleties were. The SBA system I am reviving reflects the very first of those fitted so its a warts and all job!

I'll put up the dimensions of the main antenna tonight.


Cheers
James
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Old 23rd Aug 2017, 3:29 pm   #90
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by jamesinnewcastl View Post
Here is a general question for all, again about field patterns. Consider the vertical antenna on the aircraft - as the aircraft moves forward towards the transmitting antenna along the constant strength line, the angle between the transmitted field and the antenna changes - so how does that affect the amplitude of the received signal? Remembering that the pilot is attempting to keep the received amplitude constant then such a variation would cause him to change hieght and so his approach path - for the better or for the worst?
For the better I should think, as any attenuation will tend to slightly negate the much larger increase in signal due to to the inverse square law.

Graham.
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Old 23rd Aug 2017, 3:46 pm   #91
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi,

Not that it affects you argument much but the body of the aircraft will not be exactly parallel to the ground, the normal landing configuration is with the flaps down and with the aircraft trimmed so the nose is slightly elevated.

Ross
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Old 23rd Aug 2017, 7:05 pm   #92
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

even the very simple case of a plane flying in on a steady descent say of 4.5 degrees to the horizontal is far from easy to analyse in radio terms. Forget about rolling, etc - that only further complicates the situation.

You haven't yet answered my query about what exactly you mean by "field line", "angle alpha" and "angle beta".

If you can't improve your drawings, then tell us in words what you are meaning by these words you are using.

Richard
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Old 23rd Aug 2017, 7:15 pm   #93
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

the reason why everyone is having such difficulty believing that a constant signal strength can be achieved as the plane approaches the transmitter is this.

The distance, d, between the plane and transmitter decreases from 3 miles to a few hundred yards - the latter distance depending on exactly where the plane comes to rest on the runway.

The signal strength in the situation we have here is inversely proportional to the fourth power of 4, i.e d^4.

So if d reduces by a factor 10, then the power received (based solely on the distance, d) increases by 10^4 or 10,000 times. Or in dB terms - 40dB.

That is a big change in signal strength. For the signal at the plane to remain constant (as you have set out in earlier posts), some other factor must come into play to counteract the effect of the reducing distance. What could that be?

If we take the situation to be a simple plane earth propagation problem (see references in my posts #80 & #81) then the only possible factor producing a corresponding reduction in signal is the height, h, of the plane. In this case, the signal strength is proportional to height squared or h^2.

The question then is.......can d^4 as d decreases be cancelled out by h^2 as h decreases? I suspect perfect cancellation to give an absolutely constant signal is going to be difficult. I am running some numbers through Excel to see just how good/bad the situation is. I will update this thread if I come up with anything interesting.


Richard

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Old 23rd Aug 2017, 8:08 pm   #94
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Gents,

I scratched my head at first around the constant field strength line and the square law, then realised that horizontal radiation patterns from antennas are generally drawn as the -3db line, which would be a line of constant field strength.

I have been playing with radiation patters from both horizontal and vertical stacks of dipoles at various distances off the ground and with various phase differences at their feed points. So far some experimental dimensions for a vertical stack of 4 dipoles gives the best results (ie straightest and a ~3° angle up from the ground) for an SBA glide slope antenna, looking at the bottom of the primary lobe. What I now need to do is angle the vertical dipoles in the horizontal stack off vertical to see what effect that has. An early photo shows them on a frame which appears to be rotatable in the vertical plane.

James, do you by chance have any dimensions and/or frequencies for the early SBA glide slope antennas? The only info I have is on the WW2 Lorenz azimuth antennas. I believe two overlapping beams were also used with the aircraft following the equi-signal line between the two. Later photos shows them as a horizontally polarised horizontal stack, each with a > shaped reflector.

Thanks

Martin
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Old 23rd Aug 2017, 9:20 pm   #95
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Folks,

I enclose the results of my initial calculations - see enclosed PDF file.

There are two graphs. The first one is with a linear loss of height from 1000' height to 100' (at the start of the runway). Height in graph 1 is shown as the red line. Loss is plotted next to it in blue. Essentially the "loss" is the power loss over the path between the transmitter on the ground and the plane in the air. The absolute figures are unlikely to be reliable - but we are not interested in absolute loss figures. What we are looking for here is the variation in loss while the plane approached the runway.

Note that in graph 1, the change in loss is only about 7dB. Thats just over 2:1 ratio in voltage terms. Quite a large change in terms of the pilots SBA meter - but not huge. A full scale meter reading at 1000' height, would end up just under half scale when at the end of the runway, with height = 100'

The second graph is what you have been waiting for. This is the glide path you have to take if you want a constant signal strength at the receiver. It shows that curved path that several people thought would be near impossible to fly down. Well, you can probably do it if you watch the signal strength meter and keep the meter value steady.

Does all that make sense?

The sharp-eyed will note a problem though with the second graph. You wanted the height at the end of the runway to be 100'. It actually comes out to be about 41' if the signal strength is to be kept constant right up to this point. An alternative way of looking at it is that the 100' point is crossed about 1700' out from the end of the runway.

So the glide path is not strictly correct - but is it usable in aviation terms?

This all comes with lots of caveats of course:

1. That the "simple" plane earth model applies throughout the course the plane follows. Does it? Well, perhaps. I haven't studied the maths enough to be totally sure on that point.

2. The vertical radiation pattern of the transmitter dipoles has no effect with regard to the power output of the transmitter in the direction of the plane *at all plane positions* as its approaches the runway. For simple dipoles this assumption is true.


Richard
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Old 23rd Aug 2017, 11:16 pm   #96
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Sparky67 View Post
I scratched my head at first around the constant field strength line and the square law, then realised that horizontal radiation patterns from antennas are generally drawn as the -3db line, which would be a constant line of field strength.

I have been playing with radiation patters from both horizontal and vertical stacks of dipoles at various distances off the ground and with various phase differences at their feed points. So far some experimental dimensions for a vertical stack of 4 dipoles gives the best results (ie straightest and a ~3° angle up from the ground) for an SBA glide slope antenna, looking at the bottom of the primary lobe. What I now need to do is angle the vertical dipoles in the horizontal stack off vertical to see what effect that has. An early photo shows them on a frame which appears to be rotatable in the vertical plane.

James, do you by chance have any dimensions and/or frequencies for the early SBA glide slope antennas? The only info I have is on the WW2 Lorenz azimuth antennas. I believe two overlapping beams were also used with the aircraft following the equi-signal line between the two. Later photos shows them as a horizontally polarised horizontal stack, each with a > shaped reflector.
Martin,

The frequency was around 33.3MHz - though its usually given as a wavelength of 9 metres. (The vertical marker transmitters were on other frequencies, but we are not considering those).

The antennas are shown in the reference given in posts #1 and #8. Essentially there is one driven dipole in the centre, with two others each side of the driven dipole - and those two are alternately shorted out. That produces a moving lobe pattern - see the reference post #8 and the various figures in there.

The vertical radiation pattern is not affected by the adjacent switched dipoles. Its just a normal vertical dipole pattern. And as far as the plane landing is concerned that radiation pattern does not affect the signal strength it sees at all - because it is never "off beam".

Richard
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Old 24th Aug 2017, 12:01 pm   #97
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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For the better I should think, as any attenuation will tend to slightly negate the much larger increase in signal due to to the inverse square law.
Hi Graham

It's easy to miss the point - remember that the aircraft is flying down a path that results in the same level of signal being displayed on his indicator all the time. That is the whole principle - so no inverse square law effect, and so attenuation does come into play.

He is not flying on a straight line directly to the antenna, rather he is flying on one of the same signal strength surfaces of the distorted donut that is the radiation pattern.

It's surprising at first but imagine he is diving straight down from vertically above the antenna - no field there at all (in theory) so again, no inverse square law applies. (This is the 'cone of silence')


Cheers
James
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Old 24th Aug 2017, 12:21 pm   #98
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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You haven't yet answered my query about what exactly you mean by "field line", "angle alpha" and "angle beta".

If you can't improve your drawings, then tell us in words what you are meaning by these words you are using.
Hi Richard

The field line I refer to is what you see if you cut in half the surface that represents a constant radio field strength in V/m - pretty much one of the lines that you see in standard drawings of radiation patterns, e.g. the distorted donut that I think I posted previously should be what SBA gives.

In theory if you have a radio antenna on that surface and if it is tangential to that surface then you get the full V/m signal. However angle the antenna off the tangent and the level falls or just changes it doesn't matter which way. My point is that this change of angle is what happens to the geometry between the aircrafts antenna the 'surface' as he comes into land. I demonstrate this by alpha and beta - of course the antenna is at a tangent to start with but there is still an angle change which therefore changes his indicated reading.

Because the Pilots indicator is not calibrated it doesn't matter if he starts off at an odd angle to the radiation pattern because he is only looking not to deviate from that choice - the actual level is irrelevant - he picks an 'indicated' level and flies along preserving that level until he sees the runway.

That strikes me as one of the clever parts of the system - with the same resect that I give to J L Bairds crappy Nipkow disc television. Rubbish technically - never lasted but hats off to the man for the concept and pushing it as far as he did.

Cheers
James
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Old 24th Aug 2017, 1:24 pm   #99
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Sparky67 View Post
James, do you by chance have any dimensions and/or frequencies for the early SBA glide slope antennas? The only info I have is on the WW2 Lorenz azimuth antennas. I believe two overlapping beams were also used with the aircraft following the equi-signal line between the two. Later photos shows them as a horizontally polarised horizontal stack, each with a > shaped reflector.
Hi Martin

Sadly all I have is on the SBA as used in WW2. The later systems were different, I believe that the Lorenz team used ground reflections and probably the aerial arrangement you mention to form a real 'beam' that the aircraft flew down.

There was no beam at all in SBA - it was just two distorted donuts produced by shorting out un powered reflectors each side of a single transmitter aerial

Cheers
James
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Old 24th Aug 2017, 1:31 pm   #100
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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I enclose the results of my initial calculations - see enclosed PDF file.
Hi Richard

I'm going to have to read your stuff more carefully so I may take a bit to get back to you!

Cheers
James
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