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Old 24th Aug 2017, 2:34 pm   #101
GMB
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by jamesinnewcastl View Post
There was no beam at all in SBA
Confusing the nomenclature again. There is a beam, but it is not a beam of RF energy. The term beam refers to the narrow region where you get equal signals from the two lobes.

A number of the terms used in the old documentation have slightly different meanings to how we might use them today.
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Old 24th Aug 2017, 3:05 pm   #102
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
There is a beam, but it is not a beam of RF energy. The term beam refers to the narrow region where you get equal signals from the two lobes.
It's not truely a beam. The equal signal line is just a temporal construct, for 99.999999% of the time the radiation pattern is either a left facing or right facing donut, no beam as such. But used as intended it is the sharpest 'beam' you can have as it is purely mathmatical. Right down the runway centreline.

It's only imperfections in the system and our ears/brains that make the beam appear to have any width. So for practical purposes we can, and they did, treat it as a beam and indeed many texts will also refer to three 'beams' (dots, constant, dashes)

Again hats off to the invention of a smart system, simple & clever I think.

Cheers
James
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Old 24th Aug 2017, 7:46 pm   #103
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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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.
James,

I t 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.

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.

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.

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.

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?


Richard
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Old 24th Aug 2017, 7:51 pm   #104
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by rambo1152 View Post
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,

first you can dispense with the inverse square law. That only applies in free space - which is not the situation here. We have the plane earth case, which has an inverse fourth power law. In other words signal strength increases in proportion to the fourth power of the distance between transmitter and receiver. See my many posts above this one setting out the detail.

James in his post that you quote was quite correct. Its the reduction in height as the plane approaches the runway that gives the reduction in signal power at the plane to offset the increase due to the decreasing distance. Its a complex situation - but I have finally nailed it I think in my post #96. The maths does predict what was claimed all along - you can fly with constant signal strength and it gives you a curved glide path, as shown in some of the historic documents.

Richard
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Old 24th Aug 2017, 10:58 pm   #105
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

My father's WW2 RAF 'Descent Through Cloud' pilot training notes show a lot of WT contact between the aircraft and the ground, initially to obtain bearings so that the aircraft would fly over a DF station, from which procedural turns and descents were made. But no mention of markers or beams unfortunately. Although the pattern flown would have allowed them to intercept an azimuth beam outside the outer marker. 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)?
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Old 25th Aug 2017, 7:39 am   #106
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Martin,

If we knew the power of the transmitter we could work out the field strength at certain ranges and heights of the plane. We have the circuits of the receivers, and we can probably work out (maybe even look up) the sensitivity.

Armed with that information we can make a reasonable stab at working out how far away the SBA signals could be detected.

Richard
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Old 25th Aug 2017, 7:52 am   #107
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Folks,

one way of finding out what the pilot was supposed to do would be to note this sentence in AP1186 Vol.2 Sec.3 Ch.7:

"A full description of the methods of using blind approach apparatus is given in AP1751, Blind Approach, Pilot's Handbook."

Anyone got AP1751?

Richard
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Old 25th Aug 2017, 8:07 am   #108
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Folks,

another interesting snippet of information in AP1186 is in clause 104. It talks about testing the main beam way beyond the outer marker. It says:

"The aeroplane should be flown at a constant air speed directly across the beam at a distance of 20 miles from the transmitter and at a height of about 3000 feet."

It then gives details of measuring the beam width at that distance, but it tells us that an SBA transmitter was readily detectable way outside the normal usage zone. That was why they needed multiple channels in the R1124A to avoid confusing one SBA system with another.

Richard
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Old 25th Aug 2017, 8:14 am   #109
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by trh01uk View Post
if we knew the power of the transmitter we could work out the field strength at certain ranges and heights of the plane. We have the circuits of the receivers, and we can probably work out (maybe even look up) the sensitivity.
Here you go - 500W, markers 5W. I did look up the receiver sensitivity but I don't think I found any - I'll have another dig around.


I do have AP 1571 - but it is of a date that does not include the use of the Glide Path - so if anyone has a very early version?

The problem with the AP is that while they are great documents they move on rapidly. The RAFM may well have earlier and more info on SBA and I have a bit but at the time I visited I was more interested in Stirling stuff.


James
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Old 25th Aug 2017, 8:39 am   #110
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi All

It's taken some time but I think I may have a good analogy - If you imagine the radiation pattern shown in post #31 as some sort of fruit and then imagine a small fly flying towards it and landing. The fly can crawl anywhere on the surface of that fruit - his radio would indicate the same volume at any point.

Now consider where he could go - he can crawl under to reach the core, crawl over to get to the same point or just go round and round. (Our pilot would land low on the fruit and take the underneath path to get close to the core of course).

The analogy extends quite well to my last point - as long as the flys body sits on the surface of the fuit and is north south then he gets the full V/m amplitude - however if he rotates to be horizontal to the ground he loses the signal as he is now horizontally polarised - Now he still has another degree of freedom in that he could burrow his head into the fruit and face the transmitter directly (while still being in the N-S plane) - he must also lose amplitude in that orientation.

Thus it does matter what the alpha and beta angles are - it's important that this is not forgotten as it part in the landing at a critical point - getting close to the ground.

Cheers
James
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Old 25th Aug 2017, 8:41 am   #111
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Richard

Fully deserving of a seperate 'well done' is your work on the calculations of the field strength curves!


Cheers
James
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Old 25th Aug 2017, 9:25 am   #112
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I have a copy of AP1751, "Blind Approach, Pilot's Handbook", but unusually I can't find any date or issue number anywhere!
Andy
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Old 25th Aug 2017, 9:30 am   #113
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Default Re: SBA Aerial Field Pattern - Reprise!

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I'm still looking at the field pattern for the SBA aerials. Inspired by GMBs modelling I downloaded the free version of EZNEC and by entering the physical parameters of the antenna I got similar results to GMB and also a 3D model (which I will need) - see attached.

All very fine, but I now need to put some actual distances to this field drawing so can anyone interpret the plots for me please? I'm confused by the 0dB at the extremes and the fact that the closer you get to the transmitter the smaller the dB ratio? I confess I never really understood dB.

For what I need, I'd like to be able to know the field strength at any distance from the transmitter and I can then match that to the minimum sensitivity of the receiver and so determine how far out the aircrafts SBA equipment would have first 'heard' the signal.

Also the aircraft would need to have been quite low relative to the plot in order not to have been confused by the strange 'waisting' in what would have otherwise been a simple squashed donut! So in order to validate the plot the field might have to go up to 12000 feet or so. So again dimensions are important.

That's the plan but now I need a good dose of 'expert opinion'! Can anyone put dimensions to the plot please?
James,

I can see your post #31 needs further answers. I can now see why you are having problems relating this antenna radiation pattern to what has been said so far, and the calculations I have done of field strength at the plane.

If we assume the ground around the runway and the approach to the runway - within the SBA area out as far as the outer marker - is horizontal, then we get the following angles at the transmitter antenna:

With the plane at the outer marker, height = 1000', the angle is 3.8 degrees

With the plane at the inner marker, height = 200', the angle is 2.7 degrees

With the plane at the start of the runway, height = 100', the angle is 1.9 degrees.

According to your simulated plot (see enclosed) of the vertical radiation pattern of a dipole (right hand half of plot) the power output of the transmitter is at -3dB (half power) at a vertical angle of 5 degrees. And the power falls off very rapidly at smaller angles.

If your simulation was correct, then certainly there is a serious (catastrophic?) problem here. The transmitter antenna has no power output in the region of the sky we are interested in - so the system cannot work (at all).

But is your simulation correct? No - clearly there is something wrong with it. I suggest you publish the geometry of the transmitting antenna that you assumed for your EZNEC file, and we can take a look at it.

As a more general comment, if a vertical dipole above ground (which is an extremely common situation for numerous radio systems, such as mobile radio) were to have the vertical radiation plot as you have shown, none of them would work. For instance, if our receiver was a standard mobile radio in a car, the transmitter radiation would simply head off into the sky and never reach the car at all. Clearly it does not do that - there is a perfectly usable signal at ground level (essentially at 0 degrees to the horizontal), and therefore it follows that your radiation plot is nonsense.

The question, of course, is why?

Send us the full geometry of the SBA transmitting antenna - and we can consider this further.


Richard
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Old 25th Aug 2017, 9:34 am   #114
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by jamesinnewcastl View Post
It's taken some time but I think I may have a good analogy - If you imagine the radiation pattern shown in post #31 as some sort of fruit and then imagine a small fly flying towards it and landing. The fly can crawl anywhere on the surface of that fruit - his radio would indicate the same volume at any point.

Now consider where he could go - he can crawl under to reach the core, crawl over to get to the same point or just go round and round. (Our pilot would land low on the fruit and take the underneath path to get close to the core of course).

The analogy extends quite well to my last point - as long as the flys body sits on the surface of the fuit and is north south then he gets the full V/m amplitude - however if he rotates to be horizontal to the ground he loses the signal as he is now horizontally polarised - Now he still has another degree of freedom in that he could burrow his head into the fruit and face the transmitter directly (while still being in the N-S plane) - he must also lose amplitude in that orientation.

Thus it does matter what the alpha and beta angles are - it's important that this is not forgotten as it part in the landing at a critical point - getting close to the ground.
James,

I am having some difficulty following your analogy here. At no point does the plane (assuming it stays the right way up at all points during the landing and comes to rest the right way up even have the SBA receiver antenna ever having anything other than vertical polarisation. In other words the vertical antenna stays vertical at all times. So I am not sure what your reference to horizontal polarisation is all about?

I still don't understand what your alpha and beta angles are. You need to do a clear drawing showing them - not just a sketch of a plane with an arbitrary line next to it!

Richard
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Old 25th Aug 2017, 9:35 am   #115
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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I have a copy of AP1751, "Blind Approach, Pilot's Handbook", but unusually I can't find any date or issue number anywhere!
Andy,

OK - but I am not sure the date or issue number are of interest at the moment?

What we want to know is what was the procedure for using the signal strength meter?

Richard
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Old 25th Aug 2017, 9:55 am   #116
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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.

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Old 25th Aug 2017, 9:56 am   #117
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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OK - but I am not sure the date or issue number are of interest at the moment?
Because James said "I do have AP 1571 (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
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Old 25th Aug 2017, 10:03 am   #118
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

The Contents page of my AP1751 includes "Chapter VIII The Glide Path (to be issued later)".
Needless to say there is no Chapter VIII present in the document.

So it looks like a later version is required, not an early one!

Andy
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Old 25th Aug 2017, 10:04 am   #119
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by jamesinnewcastl View Post
Here you go - 500W, markers 5W. I did look up the receiver sensitivity but I don't think I found any - I'll have another dig around.

I do have AP 1571 - but it is of a date that does not include the use of the Glide Path - so if anyone has a very early version?

The problem with the AP is that while they are great documents they move on rapidly. The RAFM may well have earlier and more info on SBA and I have a bit but at the time I visited I was more interested in Stirling stuff.
James,

right thanks for that. I can see a reference to "aerial power of 500W". That's a little bit vague - but I will take it that if we disconnected the aerial and stuffed a power meter in its place it would read 500W.

A dipole has a little bit of gain - theoretically 2.2dB. Which means the radiated power could be 830W.

At the moment I have an absolute path loss of about 80dB with constant signal on the glide path (but I need to double check that absolute figure). This would give a received power of 8.3 microwatts if the receiver antenna had a gain of 1.

In practice the antenna gain is not 1 because the plane's SBA antenna appears to be a very much shortened dipole (AP1186 clause 65). No length is given but it appears to be around one foot long based on comparing it to other parts of the mechanism, which is about 0.03 wavelengths (a dipole should be 1/2 wavelength long). I don't know the gain of such a short antenna, but its probably -10dB to -20dB or worse. If we said its -20dB, then our signal power at the receiver is around 0.083 microwatts.

That's equivalent to about 1millivolt. And that is a pretty big signal by any modern standard. At a guess (for the moment) the SBA receiver worked OK with any signal from around 30 microvolts upwards - that would be fairly typical of early VHF receivers.

What that means is that in our scenario we have about 30dB signal power "in hand" (that's the ratio of 1mV to 30uV). Or we could say that we could lose up to 30dB of signal at the receiver before we ran into problems with not having enough signal.

My judgement is that is a respectable margin. The system should work!

All the above is subject to the plane remaining within the main beam of the transmitter antenna. In other words, the receiver is getting the full output of the transmitter. We are considering the shape of that "main beam" elsewhere in this thread.


Richard
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Old 25th Aug 2017, 10:07 am   #120
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

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Originally Posted by M0FYA Andy View Post
The Contents page of my AP1751 includes "Chapter VIII The Glide Path (to be issued later)".
Needless to say there is no Chapter VIII present in the document.

So it looks like a later version is required, not an early one!
Andy,

so does the AP bits you do have not provide any explanation as to the purpose of the signal strength meter?

Or what the pilot should do with the gain control?

Richard
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