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Old 25th Aug 2017, 10:18 am   #121
G4XWDJim
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Richard, Andy,

I have part of AP 1938 Chapter 17 Beam Approach Aircraft Equipment covering the R1124A.

You both probably already have it but just in case you don't you are welcome to a pic of it. 10 foolscap pages. It's part of Standard Notes For Wireless Mechs. 1944

Jim
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Old 25th Aug 2017, 10:22 am   #122
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Sparky67 View Post
Just to clear up any confusion in my mind are we saying the Lorenz system only provided azimuth beams (plus outer and inner markers) and didn't provide a glide slope, with the aircraft descending after the outer marker without reference to an elevation beam?
I believe the later BABS system (~220 MHz using a specialised ground antenna and the Rebecca radio and instruments in the aircraft) was similar, with the American SCS-51 system (introduced in 1944) being the first to provide dedicated elevation information.
Martin,

no we are not saying that. SBA definitely has a signal strength function, and the pilot has a signal strength meter on his dashboard. Its the vertically scaled meter in the standard SBA indicator unit.

Furthermore, the pilot has a gain control which comes into play when he switches to "GLIDE". That adjusts the system so that his signal strength meter has a reading on the scale.

Some early instructions appear to say that the pilot should be flying for constant signal strength. And my calculations show that if you do that - and we assume we have a simple case of plane earth propagation - then you get a somewhat curved path of descent, which I plotted out in my post #96.

Whether RAF pilots ever used such a procedure we don't know. Personally I think I would rather rely on an altimeter or rate of descent meter (if they are available), so that I have some chance of hitting the 100' height required at the start of the runway. My own calculation of constant signal strength, showed that I would get a height of 41' at the start of the runway - probably not ideal! Not much room for error there! But hopefully visual clues of heading rapidly towards the ground would urge the pilot to level out.

Note that there is no "elevation beam" at all in SBA. Any control of elevation is done on signal strength. Theoretically it can be done as I have shown. But my bet is that in real life, "plane earth" calculations are very approximate indeed.


Richard
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Old 25th Aug 2017, 10:39 am   #123
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Richard,
Can you clarify what you are referring to as the 'signal strength meter', please? Is this the pilot's indicator with the vertical 'Glide Path Meter', horizontal 'Course Meter "Kicker"' and Inner and Outer Marker lamps?

By 'gain control' do you mean the knob on the panel labelled 'Volume' or the one labelled 'Glide' - I can't spot any discussion about the latter, as I assume it isn't in use at this time.

Chapter I para 12 says "The problem of the Blind touch-down is largely a matter for the aircraft designer and until radio-electric experiments with the "Glide Path" have been perfected for Service use, height adjustments must continue to be made by means of the sensitive altimeter and Rate of Climb Indicator."

Andy
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Old 25th Aug 2017, 10:44 am   #124
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Jim,
I'm sure I have a copy of that AP somewhere if only I could lay my hands on it.........!
So, yes please a copy of those 10 pages would be of interest.
Andy
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Old 25th Aug 2017, 11:22 am   #125
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by M0FYA Andy View Post
Can you clarify what you are referring to as the 'signal strength meter', please? Is this the pilot's indicator with the vertical 'Glide Path Meter', horizontal 'Course Meter "Kicker"' and Inner and Outer Marker lamps?

By 'gain control' do you mean the knob on the panel labelled 'Volume' or the one labelled 'Glide' - I can't spot any discussion about the latter, as I assume it isn't in use at this time.

Chapter I para 12 says "The problem of the Blind touch-down is largely a matter for the aircraft designer and until radio-electric experiments with the "Glide Path" have been perfected for Service use, height adjustments must continue to be made by means of the sensitive altimeter and Rate of Climb Indicator."
Andy,

yes, sure. The gain control is R2 on the schematics. It is shown as item 8 on AP1186 Fig.22 - the control unit front panel. It only comes into play when the GLIDE/COURSE switch (AP1186 Fig.22 item 7) is set to GLIDE. In this mode the AVC is switched off and you are under manual gain control of R2.

The knob marked "VOLUME" is R7 on the control unit schematic (AP1186 Fig.8). Its a simple pot in line with the pilot's headphones.

The meter in question is the vertical meter - called "Glide path meter" on AP1186 Fig. 4, item 4 in Fig.13. Its wired into the cathode of R1124A V5, and clause 9 explains its function:

"The action of the glide path meter is that of an ordinary D.C. micro-ammeter. It is operated by the D.C. space current of the detector valve of the main beacon receiver. This current is (very nearly) directly proportional to the R/F input to the valve. Thus the glide path meter may be regarded simply as a signal strength meter graduated in arbitrary units."


Richard
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Old 25th Aug 2017, 12:37 pm   #126
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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.
Interestingly, there was quite a bit of technical collaboration between the Germans and the US Standard Electric company - then part of ITT [who also had a controlling shareholding in Standard Telephones & Cables]. 'Standard Elektrizitätsgesellschaft' became a majority-owned subsidiary of ITT Corporation sometime in the early-1930s.

This collaboration included sharing patent-licences and some detailed information on actual circuits/implementations.

When in early WWII the British were faced with HF/low-VHF-band German blind-bombing systems such as Knickebein and X-Geraet, someone apparently remembered this and with the help of ITT/STC looked up all the German patents and technical documentation of the Standard Elektrik Lorenz company who were providing navigation radios to the Luftwaffe, thereby gaining some interesting insights into how the systems worked and how to impede them.
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Old 25th Aug 2017, 12:46 pm   #127
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Andy,

Here are the first five
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Old 25th Aug 2017, 12:48 pm   #128
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

And the final five

Jim,
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Old 25th Aug 2017, 1:22 pm   #129
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Jim, many thanks!
Andy
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Old 25th Aug 2017, 2:09 pm   #130
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

The reason that following a constant RF signal strength doesn't work well is for a couple of reasons (part from being difficult to fly such a path)

Firstly, you end up well on the edge of the radiation pattern. It is not a great idea to be moving out of the main RF beam in order to obtain a constant field reading. Also it will not be so well defined as variations due to nearby objects will start to have a significant effect.

Secondly, the shape of the radiation pattern depends on the earth reflection. This is something that can vary significantly from day to day depending on when it last rained for example.

But I continue to wonder if we are dealing with a documentation error here. It is quite clear in later manuals that the vertical meter was to be set to its low reading as you reach the outer marker, and then it was expected to rise up the scale but not quite reach the top as you reach the inner marker. With that more realistic mode of operating, taking you on a straight glide path, the vertical display would kind of show you how far you had gone along the glide path, and what you would be looking for is a CONSTANT RATE OF CHANGE of the reading, not the reading being constant itself.
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Old 25th Aug 2017, 7:14 pm   #131
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Richard

"I think you need to read my post #96 thoroughly. That gives you the answer you were looking for all along, I think. And that is that yes, you can fly on a constant signal strength, and it gives the curved glide path that was mentioned in historic documents. I have given you the shape of that curved path by calculation - see the attachment in that post."

Yes - excellent!


"The doughnut patterns produced by the dipole has no effect on the signal level as regards the plane height. Its simply irrelevant because over the range of heights any plane is likely to be at, there is no variation of field strength due to the radiation pattern of the transmitter's dipole. Nor does the plane's dipole radiation pattern have any effect."

Not sure I understand this paragraph.


"Angle changes in azimuth of the plane are also irrelevant - except that you get the normal azimuth indication in short or long beeps. It won't change the actual signal strength. If you are struggling with all this, its probably because you are not familiar with the underlying radio principles concerned.

er, I've never struggled with the azimuth, in fact you will see that I hardly ever mention it because it's not relevant to the issue.


"I agree with your 3rd paragraph. The absolute level of signal strength you receive when you are above the outer marker at 1000' elevation doesn't matter. You adjust the gain control to give a sensible reading on the pilot's meter, then you fly the plane trying to maintain two things:

1. The constant tone indicating you are on the correct azimuth line

2. A constant meter reading, corresponding to constant signal strength, and that will give you a curved glide path something like that shown in my lower graph."


Yes - but of course this isn't my design - it's the 1930's engineers solution to the problem. I only keep banging on about it because it is clear that that was the principle and because it worked - the evidence is ovewhelming.



"Given that I now agree with you that the system is, in principle, of giving a sensible glide path based on a constant signal strength, the question remains......"Why did they abandon the constant signal strength procedure?"

One possibility is pilot overload. Perhaps there was just too much to do to try and keep the SBA meter at a constant reading? Aviation experts might like to address that point. Perhaps it was easier just to descend at a constant rate of so many feet/minute descent?"


I have RAF documents that make a definite "No Neons" statement, it doesn't state exactly why but there is anecdotal evidence that the neons were just too bright for the pilots and that they much preferred the audio tones - probably because you can do other things with eyes and hands.

I suspect that as the aircraft came to the start of the runway the closeness of the equi-strength signals would make the needle very twitchy and hard to control at the most nerve wracking part of the landing. The altimeter is much bigger and doesn't twitch nearly so much - I suspect that they would rather descend to 100 ft and just fly at that until the runway appeared.

Possibly then the Altimeter was just a better choice?


Cheers
James
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Old 25th Aug 2017, 7:18 pm   #132
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Sparky67 View Post
I know he did a specific SBA course in 1944 and his log book mentions flying approach exercises without using either the 'kicker' (ie the azimuth indicator needle) or aural signals, but I haven't found any related training notes. Another exercise involved flying a 'figure of 8 without changing height', which could possibly relate to finding and centring on the azimuth beams (accurately rolling into and out of turns in both directions)?

Hi Sparky - possibly he was using a later system - the original pilots manual for the 1930s SBA is choc-a-bloc with turns such as you mention


James
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Old 25th Aug 2017, 7:38 pm   #133
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by trh01uk View Post
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

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
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Old 25th Aug 2017, 7:45 pm   #134
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Sparky67 View Post
Just to clear up any confusion in my mind are we saying the Lorenz system only provided azimuth beams (plus outer and inner markers) and didn't provide a glide slope, with the aircraft descending after the outer marker without reference to an elevation beam?
I believe the later BABS system (~220 MHz using a specialised ground antenna and the Rebecca radio and instruments in the aircraft) was similar, with the American SCS-51 system (introduced in 1944) being the first to provide dedicated elevation information.
Hi Sparky

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
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Old 25th Aug 2017, 7:49 pm   #135
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by M0FYA Andy View Post
Quote:
Originally Posted by trh01uk View Post
OK - but I am not sure the date or issue number are of interest at the moment?
Because James said "I do have AP 1751 (sic) - but it is of a date that does not include the use of the Glide Path - so if anyone has a very early version?"

Does yours have a date, James, if it does then my copy is a different one!

Andy
Hi Andy

Yes it's AP 1751,growing dsylexic in my old age! Bizarrely no date on my copy but no mention of glide path - it's all altimeters and descent rates. Chapter VIII says the same in my copy - which is the RAFM copy Number 023262.


Cheers
James

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Old 25th Aug 2017, 8:04 pm   #136
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi All

I'm sure that I've posted the size and dimensions of the antenna before - can't find the text but to keep you going here are some pictures. You know the wavelengths you could scale the pics but I'll try to find the official dimensions.

Cheers
James
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Old 25th Aug 2017, 8:56 pm   #137
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Attached - let me know if you can't find/read the text!


Cheers
James
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Old 25th Aug 2017, 9:17 pm   #138
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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
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Old 25th Aug 2017, 11:51 pm   #139
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by jamesinnewcastl View Post
Hi All

I'm sure that I've posted the size and dimensions of the antenna before - can't find the text but to keep you going here are some pictures. You know the wavelengths you could scale the pics but I'll try to find the official dimensions.

Cheers
James
James,

yes, the antennas are supposed to be half wave dipoles, and the wavelength is 9 metres. My estimate is that the bottom of the dipole is about two dipoles length above the ground.

That would put the antenna about one wavelength or 9 metres above the ground.

What dimensions did you use in your EZNEC simulation? And what ground parameters?

Richard
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Old 27th Aug 2017, 12:21 am   #140
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I have arrived very late to this fascinating discussion. Here are some contemporary references to S.B.A.

"C.W. Navigational Aids" was one of the areas discussed at the IEE Radiocommunication Convention held at the IEE in March and April 1947. For each area one or more survey papers were presented at the Convention. These were accompanied by more in-depth papers which were published later. The relevant survey paper here was:
Caradoc Williams, " A Survey of Continuous-Wave Short-Distance Navigation and Landing Aids for Aircraft,", Journal IEE, Vol 94, Part IIIA, No. 10, pp 255-266. Section 6.2 on pp 264-265 discusses "Instrument Approach Systems".
Re. S.B.A. this discusses the "kicking pointer" type of presentation and notes that, while it was easy to implement, it was not good and discarded on later systems. Another pointer showed the received signal strength, and could be used to follow the glide path. The paper notes: "This feature has the disadvantage that the descent track is not straight, and low approach angles are experienced in the vicinity of the airfield boundary."

The relevant supporting paper was:
M. Birchall, "C.W. Aids to Approach and Landing", Journal IEE, Vol 94, Part IIIA, No. 15, pp 943-952.
This includes much discussion of S.B.A, starting in Section 3.2 on page 944. Section 3.3 deals with glide path. Re. S.B.A it noted that the procedure was to fly along a line of constant field strength. It goes on to state that 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.
An attempt was made to overcome this problem by using horizontal polarization, but it was then realized that the descent path was changing continuously, and was difficult to hold. In addition "the method resulted in extremely low approaches over the airfield boundary".
This paper is worthy of some serious study.

S.B.A. is also discussed by R.A. Smith, "Radio Aids to Navigation", Cambridge University Press, 1948. pp 90-91. This book was one of the eight in the Modern Radio Technique series published after WW2. The last paragraph states that while S.B.A. was successfully used by one or two highly skilled squadrons, it was not widely used throughout the R.A.F. This due partly due to the unreliability of the equipment but also due to the fact that considerable practice was required to maintain the skills, once learned. Smith also noted that the fundamental weaknesses were its aural display which gave information too slowly, and "the lack of glide path or continuous range information.

I have mentioned the proceedings of Radiocommunication Convention on this forum in the past. They run to just over 1000 pages, and give a fascinating insight into radio developments in WW2. My copy is very near the top of my list of radio books to keep under any circumstances.

HTH and 73

John
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