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#21
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On Fri, 13 May 2005 19:08:15 -0400, "Morgans"
wrote in :: but a signal is never too weak, with the right receiver. True, provided the random background noise doesn't significantly exceed the signal strength of the source you want to receive. |
#22
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Antoņio wrote:
Hilton wrote: Yes, if we all had extremely efficient receivers, but we don't. The FAA and some radio guys got together and decided on applicable distances. Once they figured that out, they had a bunch of semi-spheres. While it would have been 'correct' to define the service volumes are a semi-sphere, it wouldn't have been all that useful to us (pilots). So the FAA made them (mostly) cylinders (and ensured that the cylinder lay within the semi-sphere) to make it easy for pilots to figure out whether or not they were in the service volume. i.e. it is a combination of radio effectiveness and pilot usefulness that describes the service volume. I just made that up, but it sure sounds convincing, logical, and almost as good as if I had stayed at a Holiday Inn last night... instead of working on software. Hilton If the FAA simply depicted a cylinder of theoretical signal strength within the actual "semi-sphere" of service, I would completely follow the analogy. However, the FAA has depicted cylinders of various diameters stacked upon each other. Given that the VOR is line-of-sight, I did not understand why, for example, a VOR would be received 130nm out at FL180 yet only be received 100nm at FL500. Doesn't it logically follow that at the higher altitude the VOR would be able to be received further out? No, then it wouldn't fit in the semi-sphere. Signal strength drops off (non-linearly I believe) as you move away from its source, so the further you go, the weaker it becomes, hence the semi-sphere. Since the sphere tapers off at the top, so too do the cylinders. Hilton |
#23
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But Hilton, that doesn't explain the effect observed. What DOES explain it
is the antenna pattern that the FAA chose for the VORs. In their infinite wisdom they never considered that aircraft would fly much above FL250. Remember, this was in the late 40s and early 50s that the system was designed. Therefore, they "squished" the antenna pattern to squirt more signal at a lower radiation angle than an isotropic ("all angles") radiator. Think of it as a ball of dough (isotropic) that has been squished to become a pancake (low angle radiation). If you are ABOVE the pancake, you receive less signal strength than if you are in the dough, so to speak. There are two effects here. One is "radio horizon" which limits low altitude reception to what the antenna can "see". The equation for this is that radio horizon (in miles) equals the square root of the aircraft altitude above the VOR (in feet). Thus, an aircraft near San Diego receiving SAN VORTAC (which is on an island near Pacific Beach, damn near as close to sea level as you can get) at an altitude of FL180 will have a radio horizon of 134 miles, almost exactly what the fellow said, and will be almost in the dead center of the antenna "beam" pattern. However, take that same aircraft in the same geographic spot and honk it up to FL500, the radio horizon moves to 224 miles, but you have climbed yourself way above the beam and the signal strength has dropped below usable.. Howzat? (Signal strength, BTW, falls off as the SQUARE of the distance.) Jim "Hilton" wrote in message .net... However, the FAA has depicted cylinders of various diameters stacked upon each other. Given that the VOR is line-of-sight, I did not understand why, for example, a VOR would be received 130nm out at FL180 yet only be received 100nm at FL500. Doesn't it logically follow that at the higher altitude the VOR would be able to be received further out? No, then it wouldn't fit in the semi-sphere. Signal strength drops off (non-linearly I believe) as you move away from its source, so the further you go, the weaker it becomes, hence the semi-sphere. Since the sphere tapers off at the top, so too do the cylinders. |
#24
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"RST Engineering" wrote in message ... But Hilton, that doesn't explain the effect observed. What DOES explain it is the antenna pattern that the FAA chose for the VORs. In their infinite wisdom they never considered that aircraft would fly much above FL250. Remember, this was in the late 40s and early 50s that the system was designed. The engineers may have been more farsighted than you give them credit. Since the radiation pattern is reduced at higher altitudes, there is less chance of frequency congestion and receiving a signal you don't want. With limited frequencies available, you have to depend on other limits to prevent unwanted reception of other signals. |
#25
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On Thu, 12 May 2005 at 14:29:01 in message
NKJge.73931$NU4.3092@attbi_s22, Grumman-581 wrote: "Antoņio" wrote in message ... I'm not sure of what that remark means. The earth is round... Radio travels line of sight, which means a straight line...Draw a large circle with a protractor... Choose a point on the circumference at the top of the circle... Draw a line tangent to the circle through this point... The line is horizontal... If an object is above this line, it will be able to 'see' the original point, if it is below the line, but above the circumference, it will not be able to 'see' the original point since the body of the circle (i.e. the earth) is getting in the way of the signal... The greater the distance the object is above the circumference of the circle, the more of the circle it is able to 'see'... Now, extend this concept into three dimensions... There is a very simple formula for the distance to the horizon from a given height above the surface for a smooth sphere of 4000 miles radius. Height (Feet) Distance (miles) 0 0.0 6 3.0 20 5.5 50 8.7 100 12.3 150 15.1 500 27.5 1000 38.9 2000 55.0 4000 77.8 8000 110.1 16000 155.7 32000 220.2 64000 311.4 128000 440.4 256000 622.8 The real earth is not of course that flat except over the oceans! Also the further away you go the closer the horizon distance gets to being the same as the height. It is obvious that from the moon you can almost see the entire hemisphere. -- David CL Francis |
#26
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"David CL Francis" wrote in message
... The real earth is not of course that flat except over the oceans! Actually, as long as we're being pedantic, it's not even flat over the oceans. It's much flatter, but the Earth simply is not an ideal "smooth sphere" anywhere on its surface. Also the further away you go the closer the horizon distance gets to being the same as the height. It is obvious that from the moon you can almost see the entire hemisphere. Almost. But the difference is significant enough to matter when you really care whether you can see the entire hemisphere (astronomy, for example). |
#27
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On Wed, 11 May 2005 21:10:13 -0700, Antoņio
wrote: I was at a CFI safety meeting today and the subject of VOR service volumes came up. The AIM describes the Standard High Service Volume as providing positive course guidance at varying distances depending on your altitude--40nm at 1000ft., 100nm at 14,500ft, 130nm at 45,000ft, etc. Hi... To try to make a realllllly long explanation short....(er) Those are the 'design' or 'advertised' service volumes. This is the required minimum coverage they are flight checked for. Will the FAA guarantee coverage outside of that? Nope. Will you pick up a useable signal outside of that? Possibly....maybe...depending on altitude/distance/terrain But they don't guarantee xx nm at xxxx feet. --Don Byrer Don Byrer Instrument Pilot Commercial/CFI Student Electronics Technician, RADAR/Data/Comm @ CLE Amateur Radio KJ5KB "I have slipped the surly bonds of earth; now if I can just land without bending the gear..." |
#28
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Hilton wrote:
Signal strength drops off (non-linearly I believe) as you move away from its source, so the further you go, the weaker it becomes, hence the semi-sphere. Since the sphere tapers off at the top, so too do the cylinders. Hey! I think I finally got it ! That makes sense to me now...thanks! Antonio |
#29
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RST Engineering wrote:
But Hilton, that doesn't explain the effect observed. What DOES explain it is the antenna pattern that the FAA chose for the VORs. In their infinite wisdom they never considered that aircraft would fly much above FL250. Remember, this was in the late 40s and early 50s that the system was designed. Therefore, they "squished" the antenna pattern to squirt more signal at a lower radiation angle than an isotropic ("all angles") radiator. Think of it as a ball of dough (isotropic) that has been squished to become a pancake (low angle radiation). If you are ABOVE the pancake, you receive less signal strength than if you are in the dough, so to speak. There are two effects here. One is "radio horizon" which limits low altitude reception to what the antenna can "see". The equation for this is that radio horizon (in miles) equals the square root of the aircraft altitude above the VOR (in feet). Thus, an aircraft near San Diego receiving SAN VORTAC (which is on an island near Pacific Beach, damn near as close to sea level as you can get) at an altitude of FL180 will have a radio horizon of 134 miles, almost exactly what the fellow said, and will be almost in the dead center of the antenna "beam" pattern. However, take that same aircraft in the same geographic spot and honk it up to FL500, the radio horizon moves to 224 miles, but you have climbed yourself way above the beam and the signal strength has dropped below usable.. Howzat? (Signal strength, BTW, falls off as the SQUARE of the distance.) Jim Very comprehensive and "engineer like" elaboration to Hilton's answer. I think I finally get the picture. Thank you! Antonio |
#30
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Don Byrer wrote:
Those are the 'design' or 'advertised' service volumes. This is the required minimum coverage they are flight checked for. Don Byrer Instrument Pilot Commercial/CFI Student Electronics Technician, RADAR/Data/Comm @ CLE Amateur Radio KJ5KB "I have slipped the surly bonds of earth; now if I can just land without bending the gear..." Ahhh...that angle had not entered my mind. .....and I just loved your Sig line ! Antonio |
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