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Old 24th Aug 2016, 1:10 pm   #1
jamesinnewcastl
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Default Standard Beam Approach Transmitter Radiation Pattern?

Hi All

I'm trying to work out/show by maths or construction the field strength/radiation pattern of the WW2 SBA transmitter in an attempt to see how the inventors justified its 'Glide Path' function. I've made some calculations but they show completely the wrong thing!

Just for those unfamiliar with this equipment it is a 1930/40s device for providing pilots with a guide to get to the start of the runway in conditions of very poor visibility. It has many functions but I would like to explore the following...

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 of the RF field strength (it switched off any AGC in fact). Next the pilot adjusted a manual gain control to set the meter reading to some arbitary position (say midway on the meter). Next 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.

The principle now was that the transmitter was radiating such a field that lines of equal field strength all converged at the start of the runway and were shaped pretty much like the path he would have taken to land in daylight. So following any old equi-strength path would get him to the runway (but not to help him perform the subsequent landing......) I'll add a drawing of that tonight but for now I'm wondering about the shape of the radiation pattern.

Attached is a picture of the aerials used and a circuit diagram of the operation. Basically there is a centre fed dipole with unpowered dipoles on each side, these dipoles are a slightly different length to the centre aerial and are shorted in turn to reflect the field to the right or the left and shape it such that in a direction perpendicular to the aerials the patterns overlap to produce a 'beam' that had a constant amplitude (thats the bit the pilot will fly along)

My questions are:

1. What is the field pattern in the vertical plane (elevation?)
2. What is the effect if any of the unpowered aerial on the combination of powered and shorted aerials? I'm thinking of the effect of a 'director' which I believe is normally a little shorter/longer than the main aerial and helps produce more a directional pattern.
3. I believe that directors have to be shorted but wouldn't the presence of the aerial wires have some effect, even if not shorted?
4. What effect would the ground have? I assume at a distance the ground wasn't an issue but was there an effect closer to the aerial?

I've looked at the shape being a hemisphere with one edge touching the main antenna but that give equi-strength lines that would cause the aircraft to 'hop' rather than to slide down.

I've Googled this as much as I can but I either get something 'different' or so complex that I have to reach for the Ibuprofen.

I wouldn't expect anyone to do loads of work on this but it's possible that someone may already know of a good link or document, etc. Hopefully it may be of general interest too.


Cheers
James
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Old 24th Aug 2016, 2:39 pm   #2
Jon_G4MDC
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I think the antennas were only concerned with azimuth not elevation.
The only thing that matters is they have a rapid rate of gain change down the centre line of the runway and one pattern is the exact mirror image of the other.

Glide slope was predetermined and checked when flying over the outer and inner markers transmitting on a different frequency.
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Old 24th Aug 2016, 4:54 pm   #3
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Jon

Thanks for the reply. The SBA system spanned a good few years and changed over that time this included the operational useage too.

It's true that at one point in time the pilots would be at 2000ft and start a fixed decent rate after passing over the outer marker but that is dependant on the accuracy of one of the Altimeters, the pilots reading of the descent rate meter, the accuracy of that meter and on the pilot getting a pressure reading from the tower and then calibrating the Altimeter correctly. You can imagine how many errors could slip in doing that and it seems unikely that the inventor would have put that method forward in the original invention.

As promised here is the article from FLIGHT 1936 that explains the glide path principle. The question, among others, is how are the radio glide paths the shape they are?


Cheers
James
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Old 24th Aug 2016, 5:46 pm   #4
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I suspect a second frequency and a completely different set of antennas would drive the glide path meter. Good luck in your search.

On further thought I think I see what you are getting at.
Why is the glide path a curve when a straight line would be expected in free space. It could be reflections from the ground playing a part in that if it really was a curve.

A little bit in here http://www.radarworld.org/flightnav.pdf

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Old 24th Aug 2016, 7:55 pm   #5
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Jon

Thanks for the link, it mentions the SBA system in passing really, it moves rapidly onto the much more complex, and better systems.

I have the AP for the ground based system and there is only one main transmitter, see attached. There is a radiation diagram in the AP but as always it's the azimuth one, not the elevation.


Cheers
James
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Old 24th Aug 2016, 8:17 pm   #6
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I agree.

This is what you need to follow up in my opinion.
"Even our Glide Slope Patent of 1937 is still used in ILS of today. Two intersecting vertical beam patterns are created
by reflectioned beams from the ground. The beams are transmitted by two antennas located one above the
other. And the intersection of the patterns form the straight glide path. "
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Old 25th Aug 2016, 12:02 am   #7
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

1. What is the field pattern in the vertical plane (elevation?)
Just the normal pattern that a low-band VHF dipole would generate with the aerial being fairly near the ground.

2. The passive aerial with the switch closed at the centre point receives and re-radiates the signal so think of the main aerial and the shorted one as a pair of dipoles (phased array) and you get an interference pattern producing beams.
The pattern shifts when the other aerial is doing it. The open aerial has little effect as it is out of tune.
.
3. I believe that directors have to be shorted.
Forget Yagis or whatever. Just look at it as a pair of dipoles.

4. What effect would the ground have?
The usual one, i.e. the ground reflection pushes the radiation pattern upwards. By how much depends on how high the dipoles are off the ground.

It is a pretty crude and simple system. I think that is why it worked!
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Old 25th Aug 2016, 12:53 am   #8
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hello,
If you are talking about the Lorenz system in the Flight article I think a similar system is variously described in the chapter starting on page 55 and also p114 of this 1941 book.
https://babel.hathitrust.org/cgi/pt?...iew=1up;seq=67
Yours, Richard

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Old 25th Aug 2016, 7:43 am   #9
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

With 2-beam systems there has to be differences in the signals in the two beams, otherwise there is no way for the receiver in the plane to the pilot to tell which beam he is in, or to gauge the balance between the two beams.

The current ILS uses 90Hz and 150Hz modulations to distinguish the two beams. The original Lorenz navigational aids switched between beams with only one on at a time and an unequal mark-space ratio so that one beam sounded like Morse dots, the other like dashes. The equi-signal zone produced a continuous tone. Pilots learned to fly slightly to one side so they could hear changes.

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Old 25th Aug 2016, 9:20 am   #10
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Thanks all for the replies!

Richard - That's a nice article and the exact system I'm referring to, it also concentrates on the left/right aspect but there is a passing nod to the hieght control (which I would have thought just as important) and a note of lines of equal strength.

GMB - Apparently the unpowered antennas could be adjusted in length a little. This, and the notes in Richards reference means that the two fields might not always be equal and so the central 'beam' could be rotated. Quite by how much and how they measured/calibrated the system is interesting, you would imagine they would need some sort of ground test system.

I am preparing some diagrams with numbers on, hopefully people will take a look and show me where I'm going wrong with regard to the vertical field.

Cheers
James
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Old 25th Aug 2016, 9:50 am   #11
G4YVM David
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

I am absolutely confident that you are barking up the wrong tree completely. The glide path was NEVER allocated to field strength, which is why you cant fathom it. GP was always controlled by the height at the markers and vertical speed (RoD) as shown on the VSI (vertical speed indicator).

My guess is that the field strength was either a backup (though as a commercial pilot myself I would say "stuff that for a game of soldiers") or as a (short lived) trial.

I say this because the diagram shown shows a curved descent profile, which is jolly hard to fly and impossible to assess - unlike a RoD based on the very simply calculated and monitored maths associated with groundspeed and rate of descent (5 x Gnd Spd). This gives a constant RoD, a constant airspeed and a stable approach.. This is / was corrected by check heights at the three marker beacons. Trying to fly an approach with a constant airspeed with constantly changing attitude is really hard and unreliable; trying to correct height with pitch whilst also flying a constantly varying attitude...well, rather them than me Gungadin. It's nigh on impossible for a human to do.

I have no recollections,*knowledge or hearsay whatsoever of a glideslope being calculated via fieldstrength. Ive read that article on Lorenz and it is flawed: pilots do NOT fly constantly changing curved glide paths. Note also that the article references the horizontal beams (correct) and then marker beacons. THESE are the primary means of height control.

All the diagrams shown in this thread also seem to suggest that fieldstrength was never the primary tool, or even a workable one:bearing in mind that some journos are more capable of making up copy for the editor than they are at understanding things, it might not even have ever been an existing one!

Im a life long aviator and career commercial pilot. If Im wrong Im wrong, but in this case, I am sure you are misled by diagrams.

David
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Old 25th Aug 2016, 10:44 am   #12
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Absolutely.

It seems there are two purposes for the vertical meter which indicates the signal strength:

It indicates that the system is actually receiving a signal at all.

It helps to make sure you are going the right way. You are supposed to be seeing the signal strength gradually increasing, so anything else is bad.

I think because the display has a rather similar look to later ILS displays some authors may have assumed it was displaying the same kind of thing.
The old SBA meter display is really quite different.

It would be interesting to get hold of the training notes on SBA. I expect RAF London museum archive will have a copy.
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Old 25th Aug 2016, 11:10 am   #13
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi David

Thanks for your reply, I know little of radio or flying so its good to contact people who know about both on a single forum and it demonstrates the value of the forum in sharing information.

I hold no fixed views about any of the SBA system, indeed I am challenging the idea of the field strength concept myself since I can't get the maths to work. The only steady course I am holding is to stay with the exact make/design of the equipment that I am referring to and presenting contemporary documentation relating to the same.

In trying to keep on topic I have deliberately not presented all of the documents available as they are wide ranging, but I think you might find the attached interesting as it is written by an amateur pilot flying the system for a review in FLIGHT magazine. I have had to chop it up a little to make it readable but it is available on the Flight Global website under historic June 18 1936.

Cheers
James
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Old 25th Aug 2016, 11:22 am   #14
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi All

On another note, and again straying off-topic, the switch I mentioned which switches the AGC of the main receiver on and off has its wording changed at one point in the use of the system, this corresponds to changes in the receiver and the way it works. I can't remember exactly but initially it was Glide/Course and later became Test/Normal.

I know too that the National Film Library have a few reels of film showing training on the SBA, but as usual they haven't quite got around to converting it and no I can't go and view it!

Cheers
James
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Old 25th Aug 2016, 11:50 am   #15
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

The paper I linked definitely describes a glide path controlled using Lorenz beams at a UHF frequency and it is the patent of 1937 to which my post referred.

Apart from distortion of radiation patterns by ground reflection it would produce an equi signal zone which is a straight line so it would lend itself to a descent as David YVM describes.

It would be interesting to find a picture of the antennas used for it - no success with that yet. Just a statement that they were separated vertically which only makes sense.

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Old 25th Aug 2016, 12:27 pm   #16
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Jon

You are absolutely correct, it's just not the same system that I am referring to. I suspect the confusion is in the word Lorenz which may abstractly refer to the use of overlapping 'beams' to produce a discernable path.

Cheers
James

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Old 25th Aug 2016, 8:21 pm   #17
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James, that few pages is intriguing; clearly there WAS a system using fieldstrength but you'll note that "expert pilots allow the needle to fall away in the first kilometer or so" because the path is too steep, flattening in the latter stages. This will be why they didnt develop this method...too darned hard!

I wish you luck with your developments, its an intriguing thing to attempt.

D
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Old 25th Aug 2016, 9:20 pm   #18
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hello,
This short biography of Ernst Kramar gives a very good summary of early developments in ILS from 1907! onwards and explains most of the points already mentioned.

http://ieeexplore.ieee.org/stamp/sta...number=4502167

Yours Richard

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Old 25th Aug 2016, 9:27 pm   #19
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

James,

in the book posted by Mr Moose, on page 114, you get a fair summary of the system, which is quite explicit that the main beacon was not used very much for vertical guidance. See the photo of the relevant text.

Richard
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Old 25th Aug 2016, 10:03 pm   #20
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hello,
This from 1930 predating Lorenz is perhaps the key to the puzzle.

http://www.pnas.org/content/16/11/678

Yours, Richard
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