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Old 2nd Feb 2017, 10:21 pm   #41
jamesinnewcastl
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Merlin

William Woolard - he's a blast from the past! Enjoyed watching the video.

I hope you meant the section in the 3rd video, if you did then although the general principle is the same, SBA was used for landings and not for long distance bombing/guidance.

I think, though I've not looked into it, that the system William was discussing used some sort of aerial array to get the directionality and distance. Considerably more complex that the SBA system I would suspect. Also I wasn't too convinced that the audio signal was a real one, especially as it is unlikely that any transmitters are still working!

Cheers
James
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Old 5th Feb 2017, 12:34 pm   #42
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

My recent research on the Fu.Bl blind-landing aircraft equipment confirms your thoughts - the pilot would need considerable training, familiarisation and skill to fly centrally down the azimuth beam and be at the correct height over the marker beacons, particularly in some of the weather conditions the system was designed for.

My understanding is the 1930s/40s Lorenz system used a vertical transmitting dipole with reflectors spaced either side of that by about a quarter wavelength. At the centre of each reflector was a relay which opened and shorted the element. I am not clear whether one or both sides of the unused 'reflector dipole' were then grounded... When the left field was being transmitted the left relay was closed and right relay was open, causing the major lobe-type radiation pattern to be off to the left. Same for the right element, with the radiation pattern off to the right. The relays would be synchronised to the length of the dot and dash transmission, 0.125 and 0.875 second respectively. The side-lobes overlapped at +/-90° to the main lobes, which was where the steady carrier and continuous tone would be heard. I have tried modelling this with MMANA (based on the portable ‘Antenne AFFA 2’ dimensions) but although the beam elevation looks good I’m not getting the expected sharp azimuth results shown in the Lorenz diagram. Possibly due to the unknown element grounding and inductive loading.

Not that it helps much with beam patterns, but you can now see and hear the early Lorenz blind-landing system working. Search for 'FuBl Knickebein' on Youtube. Also, there is quite a lot of documentation on the Lorenz system, in technical German, in the cdvandt.org manuals archive.

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Old 5th Feb 2017, 6:01 pm   #43
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Sparky

Amazing videos! Nice to hear a more realistic audio and see the 'kicker' needle working - I had thought that the first time I would see that would be on my own revamped SBA system

Those videos are very new, who put them up? Where is the system?

I strongly suspect that there isn't a great deal of difference between the system you have researched and the SBA apart from the physical implementation. I'll put up some AP pages about the system which should answer your transmitter questions (from the British system anyway).

Cheers
James
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Old 5th Feb 2017, 6:55 pm   #44
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Sparky

Attached are some salient pages detailing the British SBA system.

Also the figure in my first post showing the antenna switching is actually annotated in German so I assume that it may relate to your system of interest.



James
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Old 5th Feb 2017, 10:28 pm   #45
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Yes James, the idea of switching reflectors is exactly the same as the Lorenz system. The Luftwaffe portable system mentioned feeds the relay power via a single RF coax and uses a series of chokes to separate it and the RF at the elements. From photos, the fixed system appears to have individual feeds. I suspect, with effectively two isolated quarter-wavelengths in the position of the first director, the radiation pattern shown is ideal and is shown for explanation purposes. I haven't been able to replicate it that cleanly using antenna modelling software. If the split element is removed, yes, a clean pattern. There again, I am no expert using this software...!!
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Old 6th Feb 2017, 12:40 am   #46
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Sparky

Your radiation pattern is interesting as I note is that the sofware predictions so far all show a pattern which is only just distorted while all the graphics used to explain the operation show a 'kidney' shaped pattern that only just falls over the centre line on it's low strength side.

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?


Cheers
James
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Old 6th Feb 2017, 10:40 am   #47
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
...show a pattern which is only just distorted
You have to bear in mind that the plots are invariably using a logarithmic scale and are plotting the RF field strength.

If you made a plot which takes into account what the received dots+dashes signal sounds like, i.e. how well merged they are after the receiver has decoded the signal, then it may well look more like the textbook descriptions.

What is called for is an actual experiment!
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Old 6th Feb 2017, 12:06 pm   #48
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by GMB View Post
You have to bear in mind that the plots are invariably using a logarithmic scale and are plotting the RF field strength.
Yes agreed with regard to the fall off in amplitude with distance, but I am reasoning that the shape of the calculated field is that of equal field strength, what ever its amplitude happens to be. The diagrams presented I believe are intended to be read as on/off, present/not present for simplicity, however that disguises the actual nature of the system in that you only get a true continuous tone along the centre line (with my previous caveats). However I am more than happy to believe that the real shape is different from that calculated.

Quote:
If you made a plot which takes into account what the received dots+dashes signal sounds like, i.e. how well merged they are after the receiver has decoded the signal, then it may well look more like the textbook descriptions.
Yes, I think I covered this in my previous caveats, while the dots/dashes in theory can only be equal precisely along the centre line the ability of the electronics/ears to make a large enough distinction when the aircraft is deviating off course will give the impression of a 'beam' with some physical width.

Quote:
What is called for is an actual experiment!
Indeed, that is why I was interested in who had made the You Tube videos of the German system - the sounds seem quite realistic but you have to assume that they are simulated in some way, or are they? It would be nice to know how they were generated.

I suspect that you would need to be some way up in the air to do such experimentation in order to negate any ground effect. If more convenient radios/frequencies were used the equipment would be lighter. A drone would be an ideal test bed I should think.

I suspect that there are some written reports about the performance of the system which may be from experiments made at the time. I'm starting at The National Archives for this (though the training film lounging at the NFI may be much better). I already have some RAF papers complaining about aspects of the system and even showing changes to individual capacitor values and valve selection.


Cheers
James
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Old 6th Feb 2017, 12:44 pm   #49
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
A drone would be an ideal test bed I should think.
...or a kite.
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Old 28th Jul 2017, 10:43 pm   #50
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Default Standard Beam Approach - More Research

Hi

I've been re-reading info on SBA and have some feedback for G4YVM David who was certain that following the field lines would be too steep - the attached clips show that he was right but it also shows how this was compensated for.

The technique for following the field lines seems to have been popular in 1936 and fading out just before the war.

I received an audio file of an WW2 SBA training course today - and got to hear the various tones for the first time. While all that was nice it only added another mystery. According to the field patterns the 'cone of silence' should be just that, like the top of an apple - but for some reason the centre of the cone seems to have a 'stalk'! So as the pilot flies over the top of the main transmitter he hears the tone fall away but directly above it appears to reappear for a short period, then falls away, then reappears. Strange?

Cheers
James
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Old 29th Jul 2017, 8:41 pm   #51
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Default Re: Standard Beam Approach - More Research

Interesting. Thanks for that feedback

D
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Old 30th Jul 2017, 7:31 am   #52
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Default Re: Standard Beam Approach - More Research

James,

I still find the whole idea of having any sort of defined pattern in the vertical plane a pretty unlikely one. As noted somewhere in the references, the main azimuth patterns could be distorted just by having the odd hangar in the wrong place (I think a civilian system at Croydon was mentioned).

When you consider the vertical patterns of dipoles, they are heavily influenced by the height of the antennas above the electrical ground plane. And that electrical ground plane is not the physical surface of the ground necessarily - it can be some way below ground level depending on the water table and the dampness of the soil. Clearly those are factors that are going to vary with time of year, and also along the glide path as well depending on ground conditions and soil types.

So for all those reasons I think any attempt to formalise a glide path in the vertical plane would be pretty well hopeless. That isn't to say that some expert pilots at specific runways couldn't get to know what the pattern normally was, and then make use of the field strength as a guide of some sort. But that would only apply to those pilots expert in that specific runway and who had built up experience over many landings to see just how the system behaved. No use at all for a pilot trying to use the system on that runway for the first time (or probably the 2nd, 3rd, 4th etc times!).

Richard
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Old 30th Jul 2017, 7:37 am   #53
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Default Re: Standard Beam Approach - More Research

I might add that this particular topic is of some personal interest to me as I had an uncle who flew hurricanes out of an airfield in Northumberland in 1941. Late that year, he was coming into land in the dark, and apparently stalled the plane at some 2 miles out, crashed and was killed.

The little we now about the crash reports say that there was some "instrument failure" involved. Another uncle of mine (thankfully still alive) who flew spitfires in WWII, says he thinks it far more likely that there was a lack of training on specific planes, and that my uncle who crashed just failed to keep his airspeed high enough thus leading to a stall which he could not recover from. He had been training on some other aircraft in Canada prior to a posting in Northumberland, so its certainly plausible that lack of training was a problem. I just don't have enough facts about his experience to check that one.

Its interesting that the crash was 2 miles out from the runway, which is about the start of the SBA glide path. I don't know whether SBA was actually fitted to his hurricane either. My uncle still alive says that no SBA was available in fighters at that date. Again I haven't checked out detailed facts on that.

Richard
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Old 30th Jul 2017, 7:48 am   #54
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Default Re: Standard Beam Approach - More Research

James,

just one further comment on your cone of silence and "stalk". It would need detailed study of the antennas to be sure, but you in general you are talking about flying directly over a 500 watt transmitter - presumably at some low elevation?

I would say, based on long experience of RF engineering, that you get RF leakage in all directions from random bits of the system - even a leaky coax cable is going to give you strange effects with that much power when you are close in. And then you add in odd reflections from the ground, bits of metal (even the screws holding the wooden supports together!) and you have a pretty random RF pattern, which is impossible to predict in detail. Even slight defects in the manufacture of the dipoles could produce strange effects and slightly distorted patterns.

With so many variables, its a wonder the system worked at all! If you wanted to study this in more detail the way to do it would be to make a scale model of the system say at 1:100 scale. That would mean increasing frequency by x100 from 38MHz to 3.8GHz. A 1000' plane elevation would come down to 10' which would be doable with a tall step ladder. All the antennas would be 1/100 of their original size too. A 2 mile range would come down to about 105 feet, which is a typical large garden size. Quite an undertaking but its the sort of approach engineers have taken in the past, before very expensive software came along to attempt the job (no, I don't have any such software!). Of course, one benefit is that you don't have to find any planes to try things out....! And you can stop at a point in space with your step ladder and make detailed measurements.

Richard
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Old 30th Jul 2017, 12:01 pm   #55
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Richard

The audio file I have is from the IWM collection but there is no date. It was made by the MOD so you would imagine that this contemporary training item would have been correct. I may have interpreted it a little so here is a transcript direct from 1940 ish:

"Should the Pilot fly directly over the main beacon, he will hear two breaks in the steady tone of the signal known as the cone of silence:

<sounds> Steady tone - silence - steady tone - silence - steady tone

At 2000 feet and over this signal may not be so clearly heard"

(I love the use of official language from that era, so crisp and concise)

I don't know why there is that middle peep - I agree that it shouldn't be there, possibly it is due to the AGC but that would be a pretty 'clunky' and slow sort of operation!

Regarding your Uncle you should perhaps check first that the airfield had the SBA transmitter, very few did while loads of A/C seemed to be fitted with the receivers, certainly my Stirling of interest would have had. I too am researching a crash on landing - this time three miles out, but it transpires that Oakington did not have an SBA transmitter nor was the weather such that the pilot would have used it (the Met Office can give you a Day-by-Day account of the weather at the time). Very oddly the Stirling had a powerful landing light that would have illuminated the top of the trees he hit meaning that there was no need for reliance on altimeters or SBA (Altimeters being another source of error - they need recalibrating before landing). The landing light would have given you away to enemy fighters but what was the bigger risk?

If anyone is interested in REBECCA or EUREKA there is a film you can watch on the IWM website about the system in use

And I wish I had a garden.....

Cheers
James

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Old 3rd Aug 2017, 9:13 am   #56
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by jamesinnewcastl View Post
When the Pilot reached a certain point he switched the equipment in the cockpit to the 'Glide' Position. The receiver then displayed on a meter an 'absolute' but uncalibrated indication............... he either increased or decreased his hieght in order to keep the indicated field strength the same as the original setting as he flew to the runway.
What is the origin of this statement?

I looked again at the description of the system and I although I have not seen a description of how the GLIDE mode actually used, I would deduce that it was not like that. So is there an actual training book that describes it being used exactly like that?
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Old 3rd Aug 2017, 12:35 pm   #57
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi GMB

There are very many 'differently described' texts for this system in various documents and because it changed as time went by they are not all the same.

I'll dig out the document and quote directly from it when I get home, however the descriptions of the Glide are all in here:

http://www.vintage-radio.net/forum/s...d.php?t=129085

When the system was in 'Course' mode the AGC on the receiver was in operation - the texts all show that the pilot spends a great deal of time orienting himself and flying around in order to determine where he is on intial approach (he could be at any angle to the runway). He then needs to decide which end of the runway he is approaching from and correct if he is wrong (the system is symmetrical). I'd imagine that at this point he would not enjoy the signal level changing and he would want maximum range to his destination and so AGC is a good idea.

However, for flying down lines of equal field strength AGC is decidely unwanted, so it makes sense to switch to 'Glide' (switch off the AGC). Now you find an indicated field strength you like and stick to it, well keep returning to it anyway. Since the choice of field strength is totally arbuitary the dial is uncalibrated.

When Glide was abandoned the switch wording was changed to TEST and NORMAL. The Pilot left it in Normal.


Cheers
James
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Old 3rd Aug 2017, 5:58 pm   #58
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

In the documents I have seen, the term "equal field strength" refers not to the vertical signal strength meter, but to the concept of being in the centre of the beam, thus hearing equal signal strength dots and dashes (hopefully merging together).

Furthermore, the setup described in one manual explicitly states that the plane sets the vertical meter using the GLIDE (AGC off) mode to the bottom of the scale just as the plane passes over the outer marker. It makes the helpful suggestion to mark the gain control position so you can anticipate the setting. It warns that the reading mustn't go off the top of the scale on the approach thus telling us that you would see the reading go from bottom to top as you went down the glide path - not a constant reading at all.
Another manual mentions using the vertical scale to see if you are approaching the main beacon or going the wrong way, by the rise or fall of the reading. I note that a later feature was to reverse the beam from time to time to clarify which was the right way, maybe after they dropped the AGC-off mode so that the signal strength didn't vary much.

From all that it looks to me that the idea was so see the signal strength gradually rising. Maybe they hoped it would do it so nicely as to indicate your distance between the markers. We know that this was unlikely to have worked so smoothly, it would be bouncing up and down with multipath.
Perhaps the idea was that if you got too high or low then the reading would drop a little instead of continuing to rise, giving a clue that you were going wrong.

The idea that it was a constant field level approach makes no sense to me at all - that would take you way too high as you reached the field.
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Old 3rd Aug 2017, 9:14 pm   #59
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi George

You are forgetting the 'Elephant in the Room', the switch is marked Course/Glide, that is done for a reason I assume - why it is marked in that way? Ask yourself what the upper meter was for as it isn't used in any other description of the system where the 'glide' is just achieved by losing height and watching the altimeter. Why is it not calibrated? Why was the Course/Glide switch changed to test/normal and also not used?

What you are reading are the set-up instructions for the calibration of the equipment - they are not instructions for the pilot. I assume that the equipment is set up so that the Pilot can expect to be able to adust the Glide reading to a suitable point when he needs it and that the system does not prevent him from landing by having too low or too high a reading.

I urge you to look at the many explantions and diagrams of the subject that I have posted, these show where the lines of equal field strength are - they do seem quite clear. Also there are descriptions of pilots commenting on what exactly they are doing - I will look for more such descriptions and Pilot instructions, hopefully they will resolve the issue one way or the other.

Cheers
James

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Old 3rd Aug 2017, 9:50 pm   #60
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi All

I have now resolved the issue of the field appearing to have a 'stalk'. It doesn't of course.

Attached is a clip from the SBA Pilot Training Handbook AP 1751, the main receiver does indeed have a curious AGC quirk - I'll let the text do the explanation. Note that the AP does not have a date.

This odd quirk can't have helped the pilot use the system!

Cheers James
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