4th Aug 2017, 8:27 pm | #61 |
Dekatron
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Hi Gents, don't forget that aircraft have been fitted with a VSI (vertical speed indicator) for many years. This is a gyro stabilised gyro instrument on which it is quite easy to read the common decent rate of 300ft/ mile after crossing the outer marker at a know altitude.
Ed |
7th Aug 2017, 11:57 am | #62 |
Dekatron
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Re: Standard Beam Approach Transmitter Radiation Pattern?
I am pretty sure there are no gyros in a VSI.
It is a simple pressure instrument. |
7th Aug 2017, 7:45 pm | #63 |
Hexode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
There are definitely no gyros involved in a Vertical Speed Indicator. Just to clarify things as GMB says it is a simple pressure instrument. It is essentially a corrugated capsule divided by a diaphragm into two chambers. One capsule is connected directly to the static vent on the side of the aircraft, the other capsule also "sees" the static pressure but through a much smaller calibrated leak. With the aircraft in level flight there is no difference in pressure between the two capsules so the diaphragm is central. The diaphragm is linked to the dial which then reads zero. IF the aircraft climbs or descends the capsule pressure connected to the static vent changes quickly, but the pressure in the other capsule changes much more slowly at a rate determined by the calibrated leak. The diaphragm moves one way or the other and via a linkage shows up on the dial as a rate of climb or descent as appropriate.
Ross |
7th Aug 2017, 10:58 pm | #64 |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Ed might have been thinking of the more complex IVSI, Instantaneous VSI...still no gyros though, not needed! To get the 300ft per mile descent multiply GS x 5. (roughly), not that we ever do...just hit c700fpm and lock onto the glide!
D |
7th Aug 2017, 11:10 pm | #65 |
Tetrode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
If it is any help, I have my maintenance training manual (in pdf) for the current ILS 90Hz and 150Hz aircraft system which I can send. (I was avionics maintenance while serving). It describes the ILS and glideslope theory in detail including antenna beam patterns. Send me a PM if interested.
Geoff |
8th Aug 2017, 8:48 pm | #66 |
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Hi Gents, my mistake, I was thinking of the turn & bank (its been some time since I flew part panel!)
Ed |
11th Aug 2017, 9:29 pm | #67 |
Hexode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Having read through this thread with a great deal of interest one thing rather scares me. If the original system required the pilot to start off with a steeper approach and then flatten it until meeting the threshold or breaking cloud and seeing the runway then it is an invitation to get into one of the standard ways of killing yourself on an approach by stretching the glide. Whilst it is avoided by feeding in power if the nose is raised without doing this then you are eroding your safety margin and asking to stall at a height that is not recoverable from.
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11th Aug 2017, 10:07 pm | #68 |
Nonode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Flying a heavy bomber in the 1940's was a bl**dy dangerous business for all sorts of reasons. If the early SBA enabled a returning aircraft to land safely in the dark, in foul weather, only fumes left in the tanks and a knackered crew then it was probably regarded as a godsend on balance!
Andy |
12th Aug 2017, 10:38 am | #69 |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Hi Robin
They did quickly stop using the 'Glide' path pretty soon - I'm still looking for the original reason stated formally in a contemporary document. The alternative - an altimeter initiated descent, often at a fixed rate - is also fraught. For a start you had to have been in radio contact with a local field in order to re-set your altimeter. If you consider the size of the figures on the setting dial, the accuracy that you could set the dial with gloves on, the accuracy of the altimeter and the distance of the dial from the pilot in a darkened cockpit on a big bomber then trusting a reading of 100ft must have been very stressful - if you thought about it at all! Then you would have to hope that the approach contained no trees in case you got too low, too soon. James |
17th Aug 2017, 1:19 pm | #70 |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Hi All
Here's another thought, again I'm not a radio expert but... The transmitting antenna is on the ground and is vertically polarised. The receiving antenna is also vertically polarised. Rotating the receiving antenna around the transmitter-to-receiver axis by 90 degrees would cause the signal level to fall to zero as I understand it, since now the receiving antenna is horizontally polarised. But how is the signal level affected by the antenna moving: a) about the axis of the wings - as it would if the aircraft dived or climbed b) around the shape of the radiation pattern - in theory the angle between the antenna axis and the tangent of the equistrength field lines changes considerably in the descent - the drawings show that this might be some 45 degrees. The signal level should therefore vary with position and pitch. Because the pilot is actively flying along what he is being told is a 'constant' signal level these changes should affect the path he flies. The text and drawings ignore this consideration - possibly for simplicity? So how would these physical effects affect the apparent steep dive at the start of his descent? Might they make it a little less steep (we know pilots did consider it steep)? What about his position at the start of the runway, might it keep him too high or perhaps just right? Cheers James Last edited by jamesinnewcastl; 17th Aug 2017 at 1:33 pm. |
20th Aug 2017, 12:17 pm | #71 | |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Quote:
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20th Aug 2017, 5:46 pm | #72 |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Hi Sparky
So on the Glide approach the pilots would need to descend quickly, not dive steeply. I can imagine that rapid height loss isn't what you want to do in fog. On the approach to the end of the runway the field line that he is following must become parallel-ish to the runway and almost perpendicular to the receiving antenna, this means that the indicated strength would fall and he would compensate by flying higher. In theory then the pilot would not be directed to fly at ground level. Just luck or a clever design feature? Cheers James |
20th Aug 2017, 6:41 pm | #73 |
Dekatron
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Would I be right in saying that the "Standard" in SBA stood for Standard Telephones & Cables?
There were other examples of anglicisation of German radio navigation technology, Sonne became Consol. Graham. G3ZVT |
21st Aug 2017, 10:32 am | #74 | |
Octode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Quote:
your initial paragraph refers to the "cross-polarisation" situation. The signal would theoretically fall to zero in such an instance if both transmitter and receiver were in free space. And there were absolutely no imperfections in either antenna. In real life, we are not in free space. In a landing situation we are approaching the ground, which is a very large object indeed. And then we have the plane itself, with a myriad of reflections off its metal surfaces. Even in situations where ground reflections (and other reflections) are kept to a minimum - such as in a point to point link with antennas both ends well above the ground, and with very directional antennas so little energy is actually hitting the ground - you only get a degree of "polarisation loss". I've heard of figures of 10 to 20dB being typical. I doubt in the SBA situation it would be anything like that. And why would any plane making a normal landing be banking, turning or doing any other drastic maneoevre? Surely straight flight, with gradual loss of height is what is required - and what is done? The other comment I would make is that as the plane approaches the SBA ground transmitter, the main effect on signal strength is going to be the rapid reduction in receiver-transmitter distance. Over a reflecting ground plane (i.e. the earth) the normal power law vs distance is to the 4th power of that distance. What that means is that the predominant effect as the plane approaches the runway (and thus the SBA transmitter) on a normal straight landing is that the signal power received is going to increase very dramatically indeed. Any ideas of holding the signal constant sound to me very fanciful indeed - simply because the d^4 law is going to swamp all other effects - reflections, polarisation etc. Richard |
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22nd Aug 2017, 8:12 am | #75 | |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Quote:
The radio equipment was bought from three different companies, not all providing all the kit, not looked into that deeply though. Certainly the equipment was originally licensed from the German manufacturer before the war, you have to wonder if the Germans tried to recind the license when war broke out. 'Standard Radio' supplied some of the kit - certainly some of the dials and the ground test equipment. The system was also called Standard Blind Approach but the name was changed. But you might be right - I'll keep an eye out when reading through the subject. Cheers James |
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22nd Aug 2017, 8:38 am | #76 |
Heptode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Hi Richard
This reflects back to my original point many posts ago hoping to relate signal strength/shape to distance. Remember that the full runway lies between the aircraft and the transmitter and that the pilot would lose interest in the indication once he had landed, possibly 1,400 to 2,000 yards from the transmitter - then you would also need to consider the power of the transmitter itself in order to gauge the strengths at each point. That would also need to be judged against the sensitivity/range of the SBA indicator. My point about the relative angle between the antenna and the field lines at low amplitude is still outstanding - no matter what the distortion, the lines have to be there. The question is I suppose at what distance do they curve upwards so as to be parallel-ish to the antenna at and beyond the start of the runway and at what height would they do so? And of course there is always the practical element in that they apparently did use and install the system, albiet for a short time so something was right-ish or it would have been discarded out of hand. Cheers James |
22nd Aug 2017, 11:59 am | #77 |
Octode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
James,
I think your best bet in terms of getting any sort of accurate calculation of field strength versus distance (and height) is to use some simulation software. This produces a plot of signal strength over a specific area around a transmitter, with parameters such as frequency, terrain features (e.g. hills) and receiver height that you specify. The only low cost (well free actually) simulation software that I know of is RadioMobile produced by Roger Coudé. The homepage is here. Its now years since I used the original version of this software to study some WWII era VHF links. Those were between two fixed points, but the software produces an area plot anyway, such as the one I have enclosed. That one may look very complex, but that is mainly because there are hills and other ground features producing reflections and shadows. The field strength is plotted by colour. For the relatively simple situation you have in mind, with no hills around, the plot should be fairly simple. You will probably have to run multiple plots to change the receiver (i.e. plane) height. You will need to watch out for program limits - its intended for mobile radio use, and a receiving antenna at say 2000' agl may be beyond the program's limits. That point obviously needs to be checked at the start! I should warn you that there was quite a steep learning curve using that software. It may well be that the newer version (it looks as though it might run in your web browser) is a lot easier, but I haven't tried it. Richard |
22nd Aug 2017, 12:01 pm | #78 |
Octode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
James
sorry I forgot to enclose my plot from my own use of RadioMobile years ago. Here it is. Richard |
22nd Aug 2017, 12:04 pm | #79 | |
Octode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
Quote:
I'm afraid I don't understand your words here. Phrases like "field lines at low amplitude......."?? And ".....at what distance do they curve upwards....."?? I just can't picture what you are driving at here. I suggest you do some sketches of what you think the field is like and post them, and maybe some of us can then comment. Richard |
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22nd Aug 2017, 12:21 pm | #80 |
Octode
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Re: Standard Beam Approach Transmitter Radiation Pattern?
James,
and what may also help you get some understanding of the radio propagation is to look at this slide presentation: https://www.slideshare.net/deepakecr...pagation-model Starting at slide 7 - plane earth loss - covers the SBA situation I think. (Note a lot of the stuff in this pack is irrelevant because its dealing with the more complex situation of a mobile radio in building clutter, etc). The equations he gives there can be used to produce some plots in Excel I guess. But you will need to use some care because the assumptions will not apply. For instance on slide 9 he says hm and hb << r (where r is the distance between transmitter and receiver, and hm and hb are the heights of each) won't apply, because clearly while hb (the transmitter) height is pretty small, hm (the mobile - in this case the plane) is going to be very much the same as r for a lot of the time. So you will need to run with the full equations. A further assumption on slide 10 is that the angle of incidence (that's the angle the propagating ray hits the ground) is small. When hb and hm are small that's true. But with a plane up in the sky, that assumption no longer holds. I don't know what the effect is going to be - which is another reason I have been rather cagey about trying to get into these theoretical models. Richard |