Standard Beam Approach - Find the Aerial!

[jamesinnewcastl]

Hi All

This question is a mix of SBA history and radio common sense, it also takes up the R1082, T1083 and the TR9 as fitted to the Stirling Bomber.

The Standard Beam Approach system for guiding aircraft to just about the start of the runway in fog had a main transmitter with a 3-split beam that produced either a constant tone if the pilot was on the beam or different morse tones if he went either side. So all he had to do was fly with a constant tone to be in line with the runway.

To control his height there were two 'marker' beacons, one placed a mile or so away from the start of the runway and one placed close to the start. The markers transmitted directionally upwards and the pilot would only hear them for a short while as he passed over. The first meant 'start your decent' the second meant 'the runway should be about.....NOW', hopefully the fog wouldn't be too thick....

The antenna on the aircraft that picked up the marker tones was a long horizontal di-pole slung under the fuselage - this is well documented on the Stirling. The aerial for the main beam was moved to be a whip aerial on the top of the fuselage, that is also well documented.

I have the modification notes for the Stirling that details the addition of the new whip aerial and notes that the old aerial connection was to the main W/T mast, this is the big thick mast near the cockpit that carries the long antenna wire going back to the tail.

Finally my question, or series of questions and suppositions.

1. Is it likely that the long wire aerial was used for the SBA main aerial? I'm pretty certain that that cannot be the case as there is a feed to the long wire aerial coming from just behind the wireless operator (about 1/3 along the wire length). There are two 'eggs' which I take to be insulators in the long wire aerial - one is just in front of the tail and I assume that this is to insulate the aerial from the airframe, the other is in front of the W/O feed. So I assume that the bulk of the long wire aerial is used for the R1082 and T1083 which the W/O used (and he can also use the trailing aerial but that is clear enough to ignore). Also I assume that the signal was vertically polarised and the long wire aerial is horizontal (caution - radio idiot talking).

2. If not the long wire aerial then where is the main SBA aerial? Come to that where is the TR9 aerial? this does connect to the bottom of the aerial mast and so might it use the part of the long wire aerial between the mast and the first insulator?

3. If the TR9 uses that wire then what is left for the SBA aerial?? Could there be an aerial inside the mast? I assume not as I assume that it would be aluminium and a 'Faraday Cage' thus preventing any signal pick-up.


I realise that I have left much information out but initially I am hoping that someone will know the answer, failing that I can tease out frequencies that should help to match aerial wire lengths, etc. (I'll have to do this later)

Finally for this post I attach a photograph that may make you sigh and wonder why I was asking the question at all - the photograph may show the obvious situation that of a whip aerial mounted on top of the mast - or it may be a hair on the negative... Equally it doesn't seem as long as the later whip aerial though a slight wind when taking the photograph may have made the end go missing!

The reason I am in two minds is that the modification to provide a fuselage mounted whip aerial is clearly visible on many photographs but the attached is the only image that I have ever found of what might be the first whip aerial and since it took over a year to start the modification there should be more - although the resolution of the photos at that time is far too poor to show a really thin wire, especially one set against the sky.

Also why move the whip aerial? Could it be that there wasn't enough of a 'ground plane' to give good reception? (Here I am assuming 'ground planes', I am no radio enthusiast but have some hardware electronics).

Hopefully this might provide some interesting Sherlock type logic to come up with a reasoned conclusion. I'll try to add more details later tonight.

Thanks for reading this far!!


James

[Radio Wrangler]

They wouldn't use the trailing wire antenna close to landing. That had to be stowed safely or else they got torn off on fences etc leaving wire entanglement on the runway for the next unfortunate plane.

What frequency did they use for SBA?
If it was the original Lorenz system, then it was 33MHz. (I'm not sure because I design stuff for the current system). Also what polarisation was it?

The transmitter in line with the runway used 3 antennae, but only powered the centre one and one of the side ones at any one time. The use of a pair of antennae transmitted a signal with a sharp null, which was adjusted to one side. The other antenna pairing put the null to the other side. This is because nulls can be made much sharper than beams. The antenna pattern was switched on a 3:1 ratio. It wasn't a case of morse dots being sent to one side, the null business meant that morse dots were NOT sent to the OTHER side. A plane on the centre line got equal signal from both patterns and the dots filled in the gaps of the dashes (and vice-versa) giving a continuous tone. Fly to one side and the dots would get weaker, leaving the pilot to hear dashes rising out of the continuous tone, fly to the other side and the dashes would get weaker leaving the pilot the impression of dots rising out of the continuous tone.

So that's why there were three transmitter aerials, although the system is usually thought of as having two beams.

David

[GMB]

I think you need to refer to the wire above the fuselage as the "fixed aerial" to avoid confusion - I am guessing that was what you were referring to.

The blind approach main beam is vertically polarised and in the 30-40MHz range.

The AP on this equipment says that the aerial is a short vertical dipole which may have a loading coil as part of it, supplied by "special" coaxial feeder - by which I think it means it was double screened.
It goes on to say that it is retractable - so maybe you don't see it normally. It looks likely that it was quite near the receiver as there was an issue if the feeder exceeded 20 feet.

[GMB]

As for the TR9, I think it always uses a wire like the main fixed aerial.

I think I have heard that on larger planes which had a fixed aerial for the long range radio then there would have been another wire strung from the same mast, in the gap, for use by the TR9.

Another arrangement was a wire running from wing-tip to the tail.

[jamesinnewcastl]

Thanks for the replies!

Dave - you are right but I'm really only concerned with the main aerial on the Stirling Bomber itself. The ground transmitter had only one powered aerial, there were two similar passive aerials on either side, one or the other was shorted out in order to deflect the beam, I suppose that it is a bit like a shorted turn absorbing energy from one side of the transmitted beam. That was pretty smart as it did not involve more that one transmitter, no switching of RF power and a simple cam driving a relay meant that the Morse could readily be 'programmed' with a notched wheel.

GMB - The fuselage mounted whip aerial was retractable, the text you have may reflect that. I can't see anything that would evidence the Mast aerial retracting.

Since I have got home and started preparing more data I've realised that the long wire aerial is more complex and yet simpler than I had thought. It looks like the wire was divided by insulators into two sections with the Wireless Operators aerial isolated as one section and the Pilots TR9 as the other. Do the aerial lengths look right? Does the long feed wire from the radio unit play a part?

The picture of the bottom end of the Mast shows two pipes for the Pitot tube, a connection to the TR9, and earth bolt and what could be the connection to the SBA main receiver. No sign of any way to retract the aerial

The three circuits show the wiring - on the SBA circuit the aerial connection is happily called the 'Main Aerial', on later circuits it does refer to a whip aerial. So is it possible that the SBA main aerial is a fixed vertical wire inside the Mast? I don't know what the mast is made of, there is a de-icing 'bag thing on the leading edge - might it be possible that the mast had an insulated panel at the front to allow the SBA signal in while the back part was metal?


James

[trh01uk]

Originally Posted by jamesinnewcastl

3. If the TR9 uses that wire then what is left for the SBA aerial?? Could there be an aerial inside the mast? I assume not as I assume that it would be aluminium and a 'Faraday Cage' thus preventing any signal pick-up.

James,

I suspect that the mast itself is probably the aerial. In other words the mast does not contain the aerial, but the mast is possibly isolated electrically from the airframe and the HF wire aerial, and thus can be fed with coax to the SBA system, if its made of aluminium as you suggest.

The above is only conjecture, but I will try and ask someone who should know the answer - when I can get hold of him.

Richard

[Andrewausfa]

Originally Posted by jamesinnewcastl

the photograph may show the obvious situation that of a whip aerial mounted on top of the mast - or it may be a hair on the negative...

Or the tip of the Rotol whirly thing on the port inner.

Andrew

[jamesinnewcastl]

Hi Richard

I really don't know what the mast is made of - I am now suspecting a hybrid construction. Possibly the hybrid construction was still enough of a 'cage' to attenuate the signal resulting in its departure to a better place?

I don't know much about radio but if the mast were the aerial then wouldn't the small gap between the mast and the fuselage present a path easily bridged by moisture and ice?

I don't have any real opinion at the moment but a non-faraday cage mast would be likely, and I seem to be so desperate to see what isn't there.......

Andrew -

and relief too as that one pic was obviously distorting the issue - or rather I was! I vote for the prop too, thanks for pointing it out!

James

[jamesinnewcastl]

Hi Richard

I've had it suggested that possibly the mast was made of wood and a skin of metal used as the antenna which marries up with your suggestion and would allow a fair insulation gap to negate my concerns.

I assume that it also possible to just cut a rebate into a wooden mast and run a wire along the length, add-in some putty or resin and the aerial disappears - literally into the woodwork! There could be a metal collar on the mast to secure it to the fuselage.

The SBA could work at 30 to 40 MHz - would anyone care to suggest a suitable aerial length? The mast would need to be at least that long plus some additional length to allow for a collar and the Pitot metalwork.

Cheers
James

[Radio Wrangler]

A resonant whip would be about 14ft tall. Shorter ones will work with a loading coil in their base, though the shorter the whip the less signal, but landing aids only need to work close to the runway = close to the transmitter, so sensitivity isn't an issue.

Incidentally, someone mentioned a Faraday cage earlier. A Faraday cage (-shield -screen) is a special RF shield designed to stop electric fields and to pass magnetic fields. They do NOT shield things from radio signals (which have both electric and magnetic field components) they let half of it through! Ordinary screens (-shields) are ones intended to stop all of a radio wave. Always adding the Faraday prefix is a bit like always prefixing the word 'tyres' with the word 'punctured'

David

[GMB]

I just found a description of the early vertical beam approach aerial:

It was made of 3/4" stainless steel tube and is retractable. When deployed it projects 32 inches above the skin of the plane.
The retractor has a handle which is turned a quarter turn to unlock it and is then pulled down. (Note all this implies the aerial sticks upwards). It has a loading coil as part if the mechanism.

Further notes clarify this more...
It was early installations that used the retractable aerial. "Current" ones just have a simple whip aerial says a document dated March 1945. Curiously the loading coil is often referenced as an optional device that was fitted to improve the matching when the feeder exceeded 20 feet. It is a receive-only aerial so it may not have been a resonant length but needed better treatment when the feeder was long. I'm guessing that where possible the aerial would have been very close to the receiver.

Given the frequency used a resonant quarter wave aerial would be about 7 foot long but I take the strange comments about the loading coil to suggest that maybe it wasn't and hence wasn't a great match to the feeder. The retractable 32 inch aerial shows what it can cope with albeit with a loading coil which there seems more part of the mount, so maybe the whip aerial was similar in length and they only added the loading coil if they got an unfortunate reflection on the feeder.

[jamesinnewcastl]

Hi

David, I don't know much about radio so perhaps my comment about Faraday cages was erroneous - but if you take the electric field away from a radio signal what is left that could be called a radio signal or used as one? I thought the two swapped energy as the radio wave propagated.

GMB - The mast on the Stirling was about 8 foot away from the receiver which sat on the floor essentially between the W/O and the navigator. If I am getting the right messages the length of the SBA antenna was not a 'natural' one but was modified by the use of this 'loading coil', in other words it could have been any length. I take it that 3.5 foot (1/8th wave?) would not have been sensible? It's not too important as all I was wondering was if it was linked to the height of the mast.

Incidentally the whip aerial on the mid fuselage was angled backwards for some reason - I do have a drawing of it in situ if you like - I liked to angle the telescopic antenna on my MKI Cortina, but for a very different reason!

How about the lengths shown for the TR9 and the R1082 and T1083 wires? Do these seem about right?

I think that many of my questions have been answered, so thanks to all!


James

[Radio Wrangler]

My goof was quoting a half wave height from a standard formula, of course the resonant whip should be a quarter wave, about 7 foot!

Sorry!

If you get fairly deeply into radio wave theory, you'll come across Maxwell's equations. These are student-scarers par excellance. What they say is thar the magnetic component of a wave induces an electric component to match it, movng along in the same direction, and that the electric wave similarly induces an electric one. So they usually travel in pairs, of balanced amplitudes. You can stop one component or the other with a specialised screen, and let the other component continue merrily on its way, but once that component has got past your screen, you''ll see the missing component slowly grow, and the component that got through diminish somewhat, until you have a balanced pair again. It's generally reckoned to take about 20 wavelengths of travel for the pair to get fully balanced to their usual ratio.

So special screens like Faraday screens are useable close to sources or to receiving antennae, but once you get a distance beyond them, their work gets undone.

An ordinary screen blocks everything, and the everything stays blocked.

David

[trh01uk]

Just to confirm what others have already said, here's a bit more info on SBA aerials from my mate, Tony:

The beam approach marker antenna antenna was the same for all the heavies, a wire around 3 foot 3 inches long under the body of the aircraft towards the rear.

The localizer or main SBA antenna was a whip type 137 protruding under the airframe often used with a loading unit and the only stipulation seems that it must be around 12 feet from the propellers. The loading unit is prescribed when the coax feeder to the receiver exceeds 20 feet in length.

Early SBA installations used a fixed rod aerial Yype 3 that could be retracted in flight presumably to reduce drag.


The references to specific aerials may help you locate more information. Tony says he also has the relevant APs - though can't immediately lay hands on them.


Richard

[M0FYA Andy]

The AP for SBA is AP2534B, which I have in PDF form.

It has some generic information on the aerials, but not Stirling-specific.

James, I seem to recall sending this to you a good while back, or am I mistaken?

Andy

[GMB]

If I am getting the right messages the length of the SBA antenna was not a 'natural' one but was modified by the use of this 'loading coil', in other words it could have been any length.

Aerials can be any old length you want. Many people think there is something magic/natural about "resonant length" but actually there isn't much at all.

The thing with aerials is to match them to the feeder and ultimately the equipment at the end. The only magic thing about a resonant aerial is that they often sort-of match standard feeders without any help (but not always) and they tend to have simple radiation patterns (but not always).

Short whips are capacitive and have a low resistive component (the part that does the work). This capacitive reactance dominates their performance unless tuned out by a series coil - the loading coil.

But the coil mentioned in the APs is doing something else connected with long feeders and at present I haven't quite worked out what it is for. The retractable aerial had a series loading coil as part of the assembly - no surprise given the length. I don't know how long the later whip aerial was - they may have gone for the 7 foot quarter-wave but that sounds a lot to drag through the air!

When aerials don't match feeders there is scope for making use of this to create a transformer action. I wonder if that was going on here. It would be nice to see a real installation and take some measurements.

[jamesinnewcastl]

Hi Andy

Yes you did send me AP2534B, but it's 1945 and details a much later version of the SBA than the one on my Stirling of interest. Recently I took delivery of AP1186 Vol 1 Part IV which perversely has a big section on the SBA receivers and (deep joy) the first ever photograph of the long Marker Aerial (that I have seen anyway) - this is attached along with the base of the retractable aerial (not the one I am after....) I wonder if the SBA stuff was pulled from 1186 and given its own AP?

The radio APs seem to be different every issue and items pop up all over the place - I have some pages of AP 1186A which I got from the RAFM and it was big so I only had time to get pics of the SBA ground station equipment - I did get the index page however and there is apparently a whole section on Aircraft Aerials!!

You are welcome to a copy of AP1186 that I have, and I can let you have all the ground station pages that I have if you like.


Hi Richard

The earliest AP (Dec 1938 - but Issued June 1940) I have lists the Type 3 aerial as the one used, it may well be inside the mast but I suspect I won't know unless I can dig deeper into some Stirling Information.

Thanks All
James

[M0FYA Andy]

James, I think AP1186 set out to cover all things radio-related, but as the world of of electronic systems ever expanded, each system started to get its own AP. So I guess I shouldn't be surprised to hear that SBA was also in AP1186!
I would certainly be very grateful for a copy of the version of AP1186 that you have. As you rightly say, every issue is different!
Many thanks,
Andy

[M0FYA Andy]

Further to my last reply, I've just had a look at the version of AP1186 which is available on a CD from Malcolm Goosey. This is dated 1938, reprinted with amendments in 1941 and 1943. Indeed, there in Section 3 Chapter 7 is 'Blind Approach Receivers R1124A and R1125A'. There is a list of numerous aircraft types, but, Sod's Law, not the Stirling!
Andy

[Radio Wrangler]

In the WWII era, receivers commonly weren't matched to antennae, they presented a fairly high impedance, and there was an antenna trimmer on the panel to allow the reactance of the antenna to be trimmed out because it would pull the first tuned circuit off-tune. This worked reasonably well. There was usually a reduced signal level over what an accurate match would give, but in most cases there was plenty of signal and nearby interferers were similarly treated.

Controlled-impedance coaxial cable was then a new thing and 34MHz was significantly shorter wavelength than was routinely used for HF AM and CW communications, so the worry was whether the length of coax could happen to be resonant on any of the frequencies used. If the antenna is a poor match to the coax, and the receiver is also a poor match to the coax, then there are two reflectors, one at each end of the coax, and there will be some of the signal bouncing backwards and forwards between the reflecting mismatches. Sometimes this will peak a signal, sometimes it will reduce it, possibly with a deep null.

So to make a system which can be depended on across a range of frequencies and a range of cable lengths, the way out is to improve the matching. Any improvement at either end will help.

So the use of an antenna without loading coil provided the coax feeder is short fits in with the way things had long been done, and then the addition of an antenna loading coil to improve its feed impedance for longer cables is sensible.

At that frequency, about 7 foot is a quarter wave in air, but waves travel slower in coax, depending on the dielectric. 60% of the speed of light in the then new polythene stuff, so 7 foot becomes 4.2 foot. A quarter wave antenna cable would have a profound change in the impedance of a signal, it takes it half way round a Smith chart, it changes capacitive impedances into inductive ones and changes very low resistive components into very high ones (and vice versa) A cable on an odd multiple of quarter waves does just the same, but the effect is narrower-band.

The maths of transmission lines and the effects of mismatches and characteristic impedances had worked out long before, for telegraph and telephone cables. The tale of William Thomson (later became Lord Kelvin) and the first transatlantic telegraph cable makes very interesting reading. It's one of those cases where elderly respected scientists have closed minds and they act like complete twazzocks despite someone there who has worked out what has gone wrong. The young guy who understood what was happening went on to become an extremely distinguished scientist, but seems to have kept his brain functioning.

David