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Old 27th Aug 2017, 8:26 am   #141
jamesinnewcastl
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Sparky67 View Post
Quote:
Originally Posted by jamesinnewcastl View Post
The early SBA did provide a glide path - the vertical meter is the remnant of that feature - it fell into disuse quite rapidly in the RAF.
Cheers
James
OK thanks James. As I think I mentioned early on my interest is in the Lorenz / Luftwaffe blind landing system in use in 1940, similar in some respects but again somewhat different to your Stirling-related interest. I have downloaded several contemporary handbooks in which my O-level German has so far failed to find any reference to use of the relative signal strength meter. A German friend is going to scan through them this weekend for me. I guess what I really need are the Luftwaffe Fu.Bl pilot's notes...

One advantage of 'following a beam' over the VSI descent would become apparent in a strong headwind, which could increase the time spent between the markers by 25% or more.

Cheers,
Martin
Hi Martin

Take a close look at the picture of the aircraft in post #13. I think that it is the German version of SBA and if you accept the connection between what the pilot is saying and the dial on his dashboard then I guess it is a reference text, at least of someone using it. I have bumped into some German SBA references by Googling, attached are a couple of pics - I guess that you have them though.

The last pic shos a slightly different dial - not sure what this kit is?


Cheers
James
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Old 27th Aug 2017, 8:36 am   #142
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by trh01uk View Post
James,

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
Hi Richard

My drawings are OK - must be you! There are no arbritary lines. I attach the drawing again - this time I have used the colour blue so as to align it with your graph - we have drawn the same line, my x-axis is a few miles long though!

I have used a standard graphical technique to avoid issues with scale, the large circle shows what I have magnified in the small circle - the blue line is your line again and the red line is the half wave dipole of the plane.

Alpha and beta are the angles made at the points indicated.

I also attach another image demonstrating the technique, it's very common.


Cheers
James
James,

when I refer to your angles alpha and beta being related to an "arbitrary line" - the problem is that your diagram shows them as relating to the vertical axis of the plane's antenna and some other "line". And you have not labelled that line. That's what I mean by arbitrary.

The only line that appears to be relevant is the plane's glide path. But if that is what you are meaning, I am at a loss to understand why those particular angles are of any significance at all? The values of alpha and beta will be somewhere in the range of 80 to 90 degrees depending on where you are in the landing sequence.

If you are thinking that the directional characteristics of the plane's antenna might have some effect here on received signal strength, then no, it won't. At least not over the 80 to 90 degree range of alpha/beta. The plane's antenna is a vertical rod of (unknown) length, but clearly a lot shorter than a 1/4 wavelength which is what we would expect for an efficient antenna. A vertical rod of any length is omnidirectional in azimuth - I don't think shortening it makes any difference to the radiation in the vertical plane, and that certainly doesn't vary much over an 80 to 90 degree range.

So at the end of all that......I am left baffled as to what your point is about these alpha and beta angles? I can see no significance in them - even if I have now interpreted your diagrams correctly.

Richard
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Old 27th Aug 2017, 9:12 am   #143
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

John,

I think those papers you quote nail the problem of the simple glide path as envisaged by the Lorenz/SBA system. In particular where you state:

".........a disadvantage was that the shape of the glide path depends on the conductivity and dielectric constants of the earth, and so the descent path characteristics vary with site conditions."

these are key concerns. Plane earth propagation relies on a direct path and a reflected path off the ground combining at the receiver to give a known signal strength. The problem here is that the reflection coefficient of the ground varies, as you say, with conductivity and dielectric constant of the material in the ground. That again varies with how wet it is.

All plane earth model calculations assume that the reflection coefficient is -1. In other words the reflected signal is the same amplitude as the incoming signal, but is reversed in phase by 180 degrees. But when you study real life ground, the amplitude is more typically in the range 0.7 to 0.9 (not 1), and the change of phase is similarly vague in the 150 to 180 degree range.

I haven't attempted to calculate the effect of these non-ideal conditions, but I think we can assume that any lines of constant field strength in the vertical plane would vary depending on how damp the ground was. A further difficulty is that the actual point of reflection moves as the plane comes into land. When the plane is at the outer marker point, the reflection point is about 330' away from the SBA transmitter (both values assume a tx antenna height of 7m). When the plane is at the start of the runway, the reflection point is about 1000' from the transmitter. Is the ground going to present a constant conductivity and dielectric constant over that range? Who knows - but I think it unlikely.


Richard
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Old 27th Aug 2017, 9:32 am   #144
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Folks,

further pursuit of the history of these ILS produces another paper discussing the problem of the glide path. Here's one about Ernst Kramar, the main guy on these systems at Lorenz: https://www.nonstopsystems.com/radio...ell-krmr64.pdf

The interesting bits of relevance to this discussion start on page 83, left hand column 3rd para down.

"The constant-intensity technique, used in prior glide
paths, had two serious drawbacks. First, precision in
following a contour of constant field strength depends on
constancy of sensitivity of the airborne receiving equipment.
Second, the technique results in a curved glide
path, flattening out progressively. This shape does not
suit the normal approach characteristics of aircraft
(constant rate of descent) and it requires a long flat
area ahead of the runway. "


And here is another paper giving history of the development of these systems: http://nvlpubs.nist.gov/nistpubs/sp958-lide/038-042.pdf

On page 41 of that paper (top right) we have:

"After 1933, the U.S. Civil Aeronautics Administration
(CAA) continued to experiment with other systems.
The difficulty remained with the definition of the glide
path. The Lorenz A.G. of Germany in 1934 announced
a blind landing system similar to that of Diamond and
Dunmore, but that system failed to provide a smooth
glide path and hence fell short of adoption there"


I think its pretty clear that the idea of a glide path using simple techniques just didn't work reliably.


Richard
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Old 27th Aug 2017, 9:35 am   #145
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Folks,

and one paper that John quotes in his post #140 by Caradoc Williams, " A Survey of Continuous-Wave Short-Distance Navigation and Landing Aids for Aircraft" is available on line: https://www.nonstopsystems.com/radio...ms-47-full.pdf

Richard
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Old 27th Aug 2017, 9:51 am   #146
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by trh01uk View Post
James,

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
Hi Richard

My drawing wasn't intended to be dimensionally correct in terms of the Glide Path, so you shouldn't evaluate the angles I showed - they were only trying to illustrate my question.

But I can now use your calculations to perhaps put some possible values to the angle between the antenna and the field. To that end I attach your graph with measurements - the important part is that the angle changes by over 31 degrees (your figure, not mine) Now the amplitude vs angle is likely to be sinusoidal and so its possible that the linear angle change effect may be amplified or diminished - I'll need to sit down and work that one out later.

So as the pilot lands, the angle changes - the signal strength decreases - and the pilot 'corrects' by flying higher. Interestingly this may compensate your finding that the landing height is too low (did you calculate 40 feet?)

******************

Another point that I had yet to reach in this thought trail is that the vertical antenna on the Stirling seemed to be poorly thought out as the angle between antenna and field was getting perpendicular at the important part of the landing - very low signal - harder to detect - more noise, etc.

It turns out that the antenna on the Stirling was later changed to be one laid back at a jaunty angle - this would have meant that it was generally more of a tangent to the field over the flight path and so giving a better signal level.

But without knowing the dmensions of the likely field lines I could not come to any sort of theory about why the antenna changed angle - another reason to ask my very original question.


Cheers
James
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Old 27th Aug 2017, 10:05 am   #147
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Richard

Here is a pic of the EZNC settings I think - any good or should I look for another page?


Cheers
James
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Old 27th Aug 2017, 10:13 am   #148
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

I think your problem here may be that you have confused lines of constant field strength (is that what you mean by "field line"?) with angle of propagation of the field from the transmitter.

The latter is what we are interested in. In this system the rays from the transmitter that we are interested in are within a few degrees of horizontal. At no point does it stray more than 5 degrees from horizontal.

This is where you should have started with scaled drawings of the whole set up. I think you have confused yourself by looking at my diagram of constant field strength - and you have probably missed that the vertical and horizontal scales are entirely different. Note that horizontally we are going from 0 - 13,000 feet, while vertically we are going 0 - 1000 feet. And the plot is near square. So any angle you take off that diagram is grossly misleading.


Richard
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Old 27th Aug 2017, 1:32 pm   #149
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

I enclose a sketch of the layout of the plane, runway and markers when the plane is at the outer marker. Its only roughly to scale, but it now gives you some idea of the angles involved - which again are correct in figures but only roughly right on the diagram.

I can see no large angles involved here at all - at least nothing that is pertinent to the radio situation.

The files are in Word docx format (in case someone wants to edit further), and PDF (in case the Word file decides to mess up (they often do).

Richard
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Old 27th Aug 2017, 5:13 pm   #150
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Hi Richard

Here is a pic of the EZNC settings I think - any good or should I look for another page?


Cheers
James
James,

well, I am not an EZNEC expert - or even a frequent user. I have used it sometimes in the past.

I note that you have used 38MHz as the frequency - that's the marker frequency - the correct one for the main beam (originally) was nearer 33MHz (9 metre wavelength). A minor point though - it shouldn' t make a huge difference.

Your values of ground conductivity is 5 mS/m and dielectric constant is 13. That type of ground is described as "Pastoral, medium hills and forestation, heavy clay soil, typical of central VA"

A good starting point to get better figures is the ITU-R document at https://www.itu.int/dms_pub/itu-r/op...1990-PDF-E.pdf

Figure 1 of that document shows variation of both conductivity and dielectric constant with the moisture content of the soil.

At 5% moisture content, conductivity is 2mS/m. At 20% it is 38mS/m. A pretty wide variation. Dielectric constant varies from 6 to 20.

Does it matter we might ask? Well, what we really want to know here is what is the effect on reflection coefficient for the reflected ray from our SBA transmitter to the plane? Is it the idealised -1 value - or something different?

I ran the calcs on reflection coefficient for dry, medium dry and wet soil which has the following characteristics

Type of soil...............Conductivity...........Dielectr ic Constant

Dry.......................... 0.1mS/m..................3
Medium dry..................1 mS/m................15
Wet...........................10 mS/m................30

These figures gave:

Type of soil................Magnitude of reflection.......Angle of reflection

Dry....................................0.75....... ........................180
Medium dry.........................0.57.................. .............179
Wet....................................0.45....... ........................176

So we see that while the angle of the reflection is near to 180 degrees (which means a complete phase reversal), the magnitude varies.

What this does to the signal seen at the plane is to alter the vector sum of the direct and reflected components. That implies change in ground conditions will alter the signal the plane receives.

We might go on to ask whether wet weather has much effect on a tarmaced (or concreted?) runway. The reflection point is almost certainly on some area of the runway, even though it moves about as the plane approaches the airfield. The wetness of the soil will have some effect down to around 1 to 1.5 metres below the surface (according to other docs on penetration of the signal into the ground).

Would a lot of rain alter the wetness of the soil? Possibly. Depends on the water table I suppose. And how well drained the site is. Would 1930s and 1940s (wartime) airfields be well built? I've no idea - no doubt someone on here can quote chapter and verse. Almost certainly they weren't built taking into account any need to try and keep the radio characteristics of the soil and sub-soil constant!



Richard
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Old 27th Aug 2017, 5:47 pm   #151
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

sorry - after all that stuff about reflection coefficients and the rest - I didn't actually answer your question about EZNEC. At least not directly.

You can see the range of conductivity and dielectric constant is wide. You could rerun EZNEC with those parameters changing and see what results you get. I suspect it will vary the antenna pattern quite a bit.

Richard
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Old 27th Aug 2017, 9:14 pm   #152
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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James,

This is where you should have started with scaled drawings of the whole set up. I think you have confused yourself by looking at my diagram of constant field strength - and you have probably missed that the vertical and horizontal scales are entirely different. Note that horizontally we are going from 0 - 13,000 feet, while vertically we are going 0 - 1000 feet. And the plot is near square. So any angle you take off that diagram is grossly misleading.

Richard
Hi Richard

You are quite right of course - there is no more than 5 degrees change when I scale the drawing correctly. This is good because it puts aircraft and approach where they would need to be.

This was the subject of my original question about field size since I could not determine at what point the aircraft would 'hit' the doughnut.

It also means that the 'cone of silence' will be quite abrupt as the field lines are quite tight at that point.


Cheers
James

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Old 27th Aug 2017, 9:16 pm   #153
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi John

Thanks for you input - these contemporary references are exactly the thing I was looking for.

I'm going to have a lot of reading to do!


Cheers
James
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Old 27th Aug 2017, 9:28 pm   #154
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Hi Richard


James,

well, I am not an EZNEC expert - or even a frequent user. I have used it sometimes in the past.

.....

Your values of ground conductivity is 5 mS/m and dielectric constant is 13. That type of ground is described as "Pastoral, medium hills and forestation, heavy clay soil, typical of central VA"

........


We might go on to ask whether wet weather has much effect on a tarmaced (or concreted?) runway. The reflection point is almost certainly on some area of the runway, even though it moves about as the plane approaches the airfield. The wetness of the soil will have some effect down to around 1 to 1.5 metres below the surface (according to other docs on penetration of the signal into the ground).

Would a lot of rain alter the wetness of the soil? Possibly. Depends on the water table I suppose. And how well drained the site is. Would 1930s and 1940s (wartime) airfields be well built? I've no idea - no doubt someone on here can quote chapter and verse. Almost certainly they weren't built taking into account any need to try and keep the radio characteristics of the soil and sub-soil constant!


Richard
Hi Richard

Not knowing very much about the parameters I chose the defaults at first just to get the program to do something - after that I varied them to see if they changed the surface at all - I can't recall that they did much but perhaps I didn't change the right ones! Anyway that's some experimentation to get onto!

Few airfields at the start of the war were tarmac - the first Stirlings (at 30 tons) were used at Oakington on grass runways which got saturated and boggy. The aircraft used to take off without bombs and land at a nearby field to 'bomb-up' - I've not checked to see if that field was tarmac.

The situation isn't straightforward - early in the war you would have to find out which field actually had an SBA transmitter and then what sort of runway they had.

Certainly all the FIDO runways had SBA - but FIDO was much later I think!


Cheers
James
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Old 27th Aug 2017, 9:30 pm   #155
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by trh01uk View Post
Folks,

further pursuit of the history of these ILS produces another paper discussing the problem of the glide path. Here's one about Ernst Kramar, the main guy on these systems at Lorenz: https://www.nonstopsystems.com/radio...ell-krmr64.pdf

The interesting bits of relevance to this discussion start on page 83, left hand column 3rd para down.

"The constant-intensity technique, used in prior glide
paths, had two serious drawbacks. First, precision in
following a contour of constant field strength depends on
constancy of sensitivity of the airborne receiving equipment.
Second, the technique results in a curved glide
path, flattening out progressively. This shape does not
suit the normal approach characteristics of aircraft
(constant rate of descent) and it requires a long flat
area ahead of the runway. "


And here is another paper giving history of the development of these systems: http://nvlpubs.nist.gov/nistpubs/sp958-lide/038-042.pdf

On page 41 of that paper (top right) we have:

"After 1933, the U.S. Civil Aeronautics Administration
(CAA) continued to experiment with other systems.
The difficulty remained with the definition of the glide
path. The Lorenz A.G. of Germany in 1934 announced
a blind landing system similar to that of Diamond and
Dunmore, but that system failed to provide a smooth
glide path and hence fell short of adoption there"


I think its pretty clear that the idea of a glide path using simple techniques just didn't work reliably.


Richard
HI Richard

Again, just the sort of references I was looking for!

Cheers
James
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Old 27th Aug 2017, 9:40 pm   #156
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi All

Thanks to all who have contributed to this thread - it certainly has been a thrash - I need a rest!

I have much to read and digest in order just to catch up with all that has been discussed and referenced but I feel that my basic question has been answered.

Special thanks to Richard who has put in so much of his time and I gained a lot from his input in particular.

No reason to stop posting if you like to keep discussing. Also, I have many SBA related documents if anyone wants a copy you would be welcome

Cheers
James
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Old 27th Aug 2017, 9:46 pm   #157
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

here is an even earlier paper that was referred to by Kramar in 1935. He refers to H.Diamond and F.W Dunmore. The paper they wrote is here: http://nvlpubs.nist.gov/nistpubs/jre...n4p897_A2b.pdf

These are the guys who seem to have come up with the constant signal glide path idea. Only they used a separate transmitter and a separate beam - with a very directional antenna - to provide it - at around 100MHz. Fig.7 shows the antenna. The paper seems to be mainly about control of elevation as the plane comes into land - the azimuth beacon uses totally different technology apparently on 330kHz, with loop antennas.

What this suggests is that the innovation that Lorenz provided was firstly the VHF azimuth localizer system, which looks pretty simple. Then they managed to also dispense with the extra glide path transmitter by using the azimuth beam system instead. Clever.

Richard
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Old 27th Aug 2017, 10:39 pm   #158
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

The last photo in your post #141 is of the blind landing and DF receivers which would have been fitted in a suitable Luftwaffe aircraft around 1940.

Top left is the ~31MHz E.Bl1 receiver for the azimuth beam. Below that is the 38MHz E.Bl2 receiver for the inner and outer markers. This used a dipole antenna under the rear fuselage.

On the right is the LF/MF EZ.2 receiver for direction-finding. This used a loop antenna on top of the cockpit and a switchable sense antenna. The DF sense antenna was also the azimuth beam antenna, shared via a splitter box. This rod antenna was fitted inside the mast supporting the LF/HF comms antenna.

In between the blind landing and DF receivers is the pilot's blind landing AFN.1 instrument, with the marker indicator lamp at the top and the azimuth L/R 'kicker' needle below. On the left of the AFN.1 face is the signal strength meter, which I am hoping my German friend can find some reference to in the Lorenz and Luftwaffe books I have sent him...

Martin
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Old 26th Oct 2017, 5:49 pm   #159
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Still looking through numerous printed pages (in German) from my German friend. One article describes how a Luftwaffe aircraft is directed to a fixed point from which timed legs and turns are made to locate the SBA azimuth beam. Similar to an RAF method at the time, with quite a lot of air-ground WT.

Found the following note in the RAAF 318 Standard Notes for Wireless Maintenance Mechanics (chapter covering the SBA main beam Rx R1124) dated 1944. As it refers to the RAF it may be the RAAF didn't completely re-write the RAF book which became RAAF 318... Would be interesting to read the same paragraph in WW2-dated versions of the equivalent RAF publication, if available.
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Old 26th Oct 2017, 10:31 pm   #160
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Sparky

The RAF had stopped using the vertical 'Glide' needle a long time before 1944 so the clip you attched is correct. AP 1751 SBA Pilot Training is some 50 pages of flying around in various patterns in order to find out where you are and which way round you are - samples attached.


Cheers
James
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