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Old 26th Aug 2016, 8:41 am   #21
Jon_G4MDC
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

It's clear there were 2 systems.

One a single beam and flying followed a path of constant field strength - curved path of descent.
The other overlapping beams where flying followed the equisignal with an essentially straight glide path.

I have to admit I don't like the idea of the single beam very much. You are trying to fly under the antenna radiation pattern to make up for the reduction in 20Logfd as your approach reduces d. (That is if 20Logfd applies in this scenario)
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Old 26th Aug 2016, 9:00 am   #22
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi All

Thanks for all the replies!

I've been investigating the equipotential strength line method expounded by the texts of the day and seeking to discover what the switch marked Course/Glide on the SBA control panel was for. I've never vaunted anything for the usability, popularity or longevity of the system.

I've redone my graph and it looks very much like the lines of equi-strength RF are exactly the same as those lines you might draw if you were showing how the signal leaves the aerial and radiates outward! Now I'm thinking that I should have come to that conclusion intuitively....

In order to meet the lines at the right point, the aircraft has to be very low relative to the whole radiation field. I assume that the aircraft is placed in the right place by dint of setting the transmitter field strength, the distance of the outer marker and the approach hieght.

I've a good few documents on the system from various sources, it seems that calibration and maintenance were always a problem for the RAF. The two neon indicators that lit to indicate the markers were too bright and the neons were often simply removed, in any event the pilots seemed to prefer using the system aurally.

The SBA system itself has many different aspects and some interesting circuitry, including a non-linear meter for the left-right indicator (or 'kicker) and a 'simple' cumulative-grid detector with anode voltage controlled Reinartz reaction (possibly the subject of another question!). In general aircraft were diverted rather than landed using the system and FIDO was always implemented on the runway with SBA (if fitted). SBA also took much of the time of a Wing Commander Blucke, a very interesting individual, I recall being told that he flew the aircraft involved in the first british radar trials and he was certainly a key player in the Battle of the Beams.

Thanks again for all your input.


Cheers
James
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Old 26th Aug 2016, 9:25 am   #23
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I kicked around a few numbers for the single beam - just out of interest.
If you take a runway of about 2km long and put the single transmitter at 1/2 way along it is 1km from the threshold.
Say an approach started at 5km out from threshold thats 6km from the transmitter down to 1km at touchdown. I make that a signal range of about 15dB if 20logfd applies. Not as much as I expected.

3dB off on the TX Power or RX sensitivity would be bad news all the same. In the equi-signal case most errors of that sort just cancel out - a big advantage to that technique.
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Old 26th Aug 2016, 9:33 am   #24
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

It was back in the day when numerous ideas were being tried out and they were finding out what worked and what didn't. This one obviously didn't and was soon supplanted by something a lot better. It was easily forgotten about.

Workload is very high on approach and machinery has to make the pilot's task easier not harder. There is no room for ambiguities.

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Old 26th Aug 2016, 10:02 am   #25
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Quote:
Originally Posted by Jon_G4MDC View Post
I kicked around a few numbers for the single beam - just out of interest.
If you take a runway of about 2km long and put the single transmitter at 1/2 way along it is 1km from the threshold.
Say an approach started at 5km out from threshold thats 6km from the transmitter down to 1km at touchdown. I make that a signal range of about 15dB if 20logfd applies. Not as much as I expected.
What happens if you put the system into the real world; say runway of 3km and a/c starts approach at 15km/ 2500ft.?

What happens now of course is that the glide antenna is displaced to one side of the runway at the threshold, the localiser at the far end on the centreline.

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Old 26th Aug 2016, 10:17 am   #26
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Glide TX located at threshold in a single beam constant field strength system would be bad. The distance at touchdown would be negligible and the signals would be very strong.

If glide TX was again located 1/2 way down the 3km runway, it is 1.5km away at threshold and 16.5km at start of approach a distance ratio just over 10:1.
In free space that would be about 21dB.

Put the glide TX at far end of runway and it becomes 18/3=6 Back at about 15dB again.

Last edited by Jon_G4MDC; 26th Aug 2016 at 10:22 am.
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Old 26th Aug 2016, 3:22 pm   #27
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

If you want to see the real detail of the system in use then this appears to answer a lot of questions: Isle of Man ATC in the 1950s

So the main beam was at the end of the runway - but not the end the aircraft is approaching!

The inner marker is right close to the threshold where you are supposed to be at 300'.

I don't think there was any intention that the system was for blind landing, just blind approach (the clue is in the name) so it only had to get you vaguely near the runway so you could complete your visual landing.
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Old 26th Aug 2016, 5:32 pm   #28
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

I have had a go at modelling the aerial system.

The documentation talks of the "beam" being 3-4 degrees wide.
By "the beam" I think they mean the area where you get a continuous tone as there is no way the total RF is beamed that sharply.

From this I get about 1.5dB difference at 5 degrees off course going to almost 3dB at 10 degrees off so that seems to work I think.
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Old 26th Aug 2016, 5:34 pm   #29
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Having been a passenger in a helicopter landing in thick fog I was very glad said aircraft had the option of UP without needing airspeed. I saw the ground at some 100 feet and the pilot said 'are there it is' (calm as you like). An experience I do not want to repeat but glad to have done. I can't imagine what it would have bee like in a fixed wing aircraft.

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Old 26th Aug 2016, 5:51 pm   #30
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

It depends what category of ILS you and the airport have. CAT3A allows automatic landings.
The lesser ones have various decision thresholds where if something inviting hasn't appeared it's a case of power back on and nose up to go looking for somewhere with better vis.

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Old 31st Dec 2016, 9:51 pm   #31
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Default SBA Aerial Field Pattern - Reprise!

Hi All

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?

Cheers
James
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Old 31st Dec 2016, 11:40 pm   #32
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Default Re: SBA Aerial Field Pattern - Reprise!

There are no "distances to the transmitter".
It is all about angles.

What you have is a plot of the relative magnitudes in all directions, not some some kind of model of the actual transmitter. So be careful not to ascribe it physical quantities that are not there!

dB are power ratios expressed in logarithms for convenience (adding them is multiplying the power)
The actual power ratios are 10^(dB x 0.1)
So 10dB is 10 times the power, 3dB is about double the power etc.
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Old 1st Jan 2017, 11:45 am   #33
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Default Re: SBA Aerial Field Pattern - Reprise!

Sounds like what you need James is something called a 'link budget' calculator, which takes into account Tx power, Tx feeder/coupler losses, Tx antenna gain, free-space loss, Rx antenna gain and Rx feeder/coupler loss, to determine the signal strength (in dBm) expected at the receiver input. Calculators are available on the web.

When these parameters are quoted in dBm for the Tx and Rx, dBi for the antenna gain and dB for feeder, coupler and free-space propagation losses it's a simple job to add them together to find the required info. Gains add, losses subtract. Note in antenna gain figures there is a 2.15dB difference between dBi and dBd...0dBi is -2.15dBd.

The radio frequency in use will have an effect. There may also be something called a 'fade margin' to apply, which takes into account varying propagation losses.

If not quoted in dBm, the aircraft receiver sensitivity figures and required minimum signal-to-noise (SINAD) ratio for the SBA device to work reliably in the aircraft will then determine what the allowable free-space loss in dB (ie distance and height variables from the Tx) is. Again dBm / voltage conversions are available on the web.

The radiation pattern of BOTH the Tx and Rx antennas and their dBi gain/loss figures (ie relative to an isotropic radiator) in azimuth and elevation will have an effect on this, of course.

I think initial assumptions will need to be made that the aircraft in centered on the beam, and in-line with it, and at a certain elevation.

As an aside I recently did a little research via Google on the early WW2 Lorenz SBA system when looking into the E.Bl1 and E.Bl2 marker and beam SBA (and possibly beam bombing) receivers fitted in the Dornier Do.17 recovered from the Goodwin Sands...

Last edited by Sparky67; 1st Jan 2017 at 12:03 pm.
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Old 2nd Jan 2017, 12:15 am   #34
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Both

Thanks for the replies! I hadn't thought to much about the receiver or the other details but I agree they need to be accounted for.

For the main signal, the aerial on the Stirling was a vertical one contained in the radio mast. I don't know if it was a dipole or end fed. In the early Stirlings this was vertical - now I'm thinking about the polarisation of that antenna - as the aircraft approached then surely the polarisation would have been correct but as it got closer to the ground, and effectively 'underneath' the donut then the angle the field was making to the antenna would have approached 90 degrees and so reducing the received amplitude? I note that later the antenna was angled backwards and in the middle of the fuselage - possibly to compensate for change in polarisation?? I assume that the metal body of the aircraft will have played a part in the reception too?

The marker antennas, both transmit and receive, were the same polarisation and type and always effectively parallel.

I am building a 3D model that I hope will allow all to see what is in my head since words can be difficult, but also to demonstate more clearly to non-radio types (which I appreciate actually includes me!)

With regard to the way other objects would have distorted the field I've attached a page from the AP which gives some idea of the practical position taken for the system. Would I be right in assuming that the antenna supports would necessarily be made of wood? Images of them attached too.

Notice that the antenna length can be varied - I'm assuming that once a fequency was chosen for a particular SBA location then the antenna was set and left for the duration. Transmitter power was 500W and the frequency between 30.5 and 40.5 MHz.

Cheers
James
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Old 9th Jan 2017, 12:22 am   #35
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

As a comparison, the German Do.17 also used a vertical blind landing (azimuth) receiver antenna fitted inside the radio mast (on top of the cockpit), which was used with an E.Bl1 receiver (around 31MHz) to intercept and then centre on the L/R beams. A dipole, with both elements in line, was mounted under, in line with and quite close to the rear fuselage, and was used by an E.Bl2 receiver (38MHz) for reception of the marker beacon frequency. Each beacon had a different audio tone freq and repetition rate to identify it. The installation was known as the Fu.Bl1.

From what I have read the aircraft would likely find the approach beam at an angle, then, using a L/R instrument in the cockpit, make a series of turns until it was centred on the beam. It would then approach the outer marker (positioned on the extended centre-line 3km from the runway) at a predetermined height and as it crossed the marker start to descend at a predetermined rate to fly down the required 'glide slope' angle. The inner marker was positioned 300m from the runway so a final height and position check could be made. Both distances were later increased to provide a shallower glide slope. The Do.17 blind landing system didn't provide any elevation information, only when the aircraft was passing a marker beacon and its azimuth in relation to the runway's extended centre-line. One thing which puzzled me was a vertical scale on the associated L/R instrument. This turned out to be a signal strength indicator which provided a rough indication of the distance from the transmitter. The usable range of the blind-landing beam is quoted as between 35 and 50km, dependent on the source.

The E.Bl1 receiver could also used for beam-bombing over short distances. The E.Bl3 (a much more sensitive multi-channel version of the original 2-channel E.Bl1 Rx) was later introduced, which could track a Knickebein beam over the UK. A timing marker was provided by an intersecting beam, also detected by the E.Bl3 receiver on a second frequency. The crew would have to switch between the two freqs when approaching the cross-beam. When the first Luftwaffe 100KGr pathfinder aircraft were shot down and inspected the boffins expected to find a separate beam-following receiver. It was the unusual sensitivity of the blind-landing E.Bl3 Rx which finally gave the game away - much more sensitive than it needed to be just for an airfield beam-approach receiver.
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Old 9th Jan 2017, 9:52 am   #36
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Sparky

I've not looked into the German system in detail but I have seen some photos and block diagrams and I'm pretty certain that the early system is the same as the SBA version I am looking at, it's just that all the equipment is physically a different shape, the actual circuitry being equivalent.

You didn't explain why the vertical scale on the Pilots indicator was confusing, it was for me too as it had no calibration numbers. On the early version it only became a field strength meter for the descent, prior to that it had agc and was used for 'bulk' detetction of the main antenna. Once the outer beacon sounded a switch was operated by the Pilot to return to field strength, then all you needed to do was to keep the needle at the same level as you were at when you crossed the outer beacon.

Later the original operation was ditched in favour of the principle of "Come in at a predefined level, hit the outer beacon and descend at a fixed rate till you see the runway". That seems like total madness - for a start the sensuitive altimeter that they would have used to judge the hieght had to be recalibrated on approach by calling the tower and getting the local atmospheric pressure - setting that on the altimeter in the dark using the tiny numbers on the dial then hoping that the altimeter was accurate, which apparently they weren't. If the altimeter read 1000 feet FSD, then 10% error is 100 feet - very substantial I would have thought! Also, you would have to hope that no wind or stick jiggling would change your hieght as no accurate correction would be possible. With the original system you could deviate, or be deviated by winds, as long as you eventually got the vertical indicator back to the original reading noted at the outer beacon (and providing that the radio transmitter kept the field stable). Incidentally, I have one of the sensitive altimeters and it's interesting to just drive around and see just how sensitive it is.

I'm not sure how clear the picture is but if you look at the picture in my post #13 (and note the text) I believe that the pilot is actually flying the German version of SBA as the Pilots instruments are the same - possibly it is the one you have researched?

I think too that the SBA 'beam' wasn't actually any sort of beam, but just used the overlap of two donut shaped fields, but the beams used for bombing were as they used aerial 'arrays' to actually produce a shaped 'beam' - but I'll freely admit I am not at all a radio engineer so pplease don't quote me!

I'm still working on my 3D model and hopefully I will be able to demonstrate that distances do play a big part in the operation of the system. This year I'm travelling to the RAF Museum to get the original publications regarding the system to get information from the horses mouth so to speak.

Cheers
James
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Old 1st Feb 2017, 12:32 am   #37
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Again

Its taken some time to draw up the radiation pattern but has given rise to some interesting theories.

Attached is the distorted radiation pattern of (say) the Dot field as generated by the software program and shown as a 3D 'shell', I am concluding that any 'normal' to the surface of this shell is the same signal strength as any other point on the shell. That strength for the moment lets assume as arbuitary.

The next image shows the Dot and Dash fields together, bearing in mind that they are mutually exclusive in reality.

It is only where the two shells meet that the Dot and Dash field strengths are equal and so only at those points would the pilot hear a continuous tone, this suggests:

1. The actual path that the pilot would follow if he kept to a continuous tone is a dead straight line right down the runway centre line!! So it's not a flared 'beam' and is actually extremely accurate.

2. The actual shape of the radiation pattern doesn't matter - just as long as one pattern overlaps the other and they are both 'balloon like!!(This only applies to the horizontal guidance part of SBA of course - it does matter for the vertical)

I think that these points are a missed sublety that the German engineers might have been quite pleased to realise in their invention.

OK it's not going to be that simple, my thoughts are:

1. The width of the 'beam' will broaden if the ear/radio equipment cannot exactly discern the difference between continuous tone and one with a tiny part of dot/dash imposed on it.

2. The 'beam' would sharpen if the Dot/Dash fields were only just overlapping as the fall-off when moving off-beam would be greater.

3. It should have been quite difficult to fly exactly down the beam without hearing dots and dashes frequently (especially with a strong wind)

More thoughts later but I would be pleased to hear your views/ideas.

Cheers
James
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Old 2nd Feb 2017, 8:05 am   #38
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi James, have you tried contacting Newcastle Flying School ? A least one of the instructors may have had experience of this system. It was certainly mentioned as background when I was learning to fly with the University air squadron in the late 60's.
I believe the Heaton bookshop the specialises in Air, Rail, Naval books may have some volumes in, worth a look.

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Old 2nd Feb 2017, 1:37 pm   #39
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

Hi Ed,

Thanks for the tips, I'll follow both up!

I know that the BFI Film Archives in London have 3 reels of an SBA training film but it seems that they aren't bothered about copying it to electronic media or letting me have a private screening (I'd load the film myself!). I'd imagine that that film is possibly the best record of an SBA actually in use complete with sound and commentry, one day perhaps.


Cheers
James
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Old 2nd Feb 2017, 4:10 pm   #40
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Default Re: Standard Beam Approach Transmitter Radiation Pattern?

RV Jones' "The battle of the Beams" has a segment with William Woolard demonstrating it in a light aircraft. https://www.youtube.com/playlist?lis...blM-xOPk97KM85
 
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