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#1
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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? (See AIM 1-1-8)and then order some room service! ;-) Antonio |
#2
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"Antoņio" wrote in message news 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? Yes, it does logically follow that at the higher altitude the VOR would be able to be received further out. Do you now understand why the distance is less at higher altitude? |
#3
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Steven P. McNicoll wrote:
Yes, it does logically follow that at the higher altitude the VOR would be able to be received further out. Do you now understand why the distance is less at higher altitude? I am not sure. Is it because the radiation pattern is spherical and not line-of-sight? Antonio |
#4
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"Antoņio" wrote in message ... I am not sure. Is it because the radiation pattern is spherical and not line-of-sight? You're looking at just one VOR at a time. There are about 1000 VORs in the US and just 100 VOR frequencies. The service volume has to ensure not only usable reception of the desired VOR, but non-reception of undesired VORs on the same frequency. As an example, let's say you're flying from EAU VOR to LAN VOR at 15,500'. They're about 320 miles apart but at that altitude line-of-sight distance is about 180 miles so you should receive LAN before losing EAU, even though you're well outside of the standard service volume of forty miles. When you're about halfway you switch from EAU to LAN, but you're unable to get a reliable signal. The problem may be that you're closer to ESC and RFD VORs than you are to LAN, and they all operate on 110.8. |
#5
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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
Depends on the antenna radiation pattern, and interference from other emitters. Jose -- Money: what you need when you run out of brains. for Email, make the obvious change in the address. |
#6
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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 |
#7
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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. |
#8
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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. |
#9
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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 |
#10
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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 |
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