ApolloHoax.net
Off Topic => General Discussion => Topic started by: Gazpar on June 28, 2015, 09:33:31 AM
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I have a question about waves propagation.
We know that radio stations use the ionosphere to bounce their radio signals and cover a large area with broadcasts.
But how do we make those contact with satellites, ISS, etc If they are above ionosphere, which makes signals bounce or refract?
Low energy-long wave signal be the answer? Since they dont have enough energy to interact with the ionized atoms in that layer.
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Short answer:
Ionospheric reflection depends on frequency, so they use frequencies that reflect less.
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I have a question about waves propagation.
We know that radio stations use the ionosphere to bounce their radio signals and cover a large area with broadcasts.
But how do we make those contact with satellites, ISS, etc If they are above ionosphere, which makes signals bounce or refract?
Low energy-long wave signal be the answer? Since they dont have enough energy to interact with the ionized atoms in that layer.
ka9q will know a lot about this. At a very basic level, waves below 30 MHz begin to reflect more strongly. Those between 30 MHz and 30 GHz are transmitted. Waves above 30 GHz are scattered easily by water droplets or are absorbed by dust, so are only suitable for short range communication.
I don't wish to patronise, but BBC Bitesize offers a good summary.
http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/space_for_reflection/satellite_communication/revision/1/
The military use very low frequency waves to communciate with submarines around the world. There's an old nuclear bunker about 30 miles from me, and the radio trasmitter sited in the bunker was used to communicate with HMS Conqueror during the Falkland War. In the event of a nuclear war, the transmitter would be used to communicate with British Nuclear submarines around the world by radiowave reflection.
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I have a question about waves propagation.
We know that radio stations use the ionosphere to bounce their radio signals and cover a large area with broadcasts.
But how do we make those contact with satellites, ISS, etc If they are above ionosphere, which makes signals bounce or refract?
Low energy-long wave signal be the answer? Since they dont have enough energy to interact with the ionized atoms in that layer.
ka9q will know a lot about this. At a very basic level, waves below 30 MHz begin to reflect more strongly. Those between 30 MHz and 30 GHz are transmitted. Waves above 30 GHz are scattered easily by water droplets or are absorbed by dust, so are only suitable for short range communication.
I don't wish to patronise, but BBC Bitesize offers a good summary.
http://www.bbc.co.uk/schools/gcsebitesize/science/triple_ocr_gateway/space_for_reflection/satellite_communication/revision/1/
The military use very low frequency waves to communciate with submarines around the world. There's an old nuclear bunker about 30 miles from me, and the radio trasmitter sited in the bunker was used to communicate with HMS Conqueror during the Falkland War. In the event of a nuclear war, the transmitter would be used to communicate with British Nuclear submarines around the world by radiowave reflection.
That makes sense, but 30 mhz isnt a lot of energy for a wave? Its almost like gamma radiation
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Longer answer (this is what you get for asking a radio ham):
The ionosphere is a plasma. Plasmas have a "critical frequency", fc, above which EM radiation passes through with a delay. At frequencies well above critical, that delay is negligible. As the frequency approaches fc from above, the delay increases until it hits infinity; at that frequency and below, the radiation is totally reflected.
The critical frequency depends on the total electron content along the path, and that of the earth's ionosphere varies with time of day and solar activity. It is typically within the HF band (3-30 MHz). Satellites use frequencies well above fc not just to punch reliably through the ionosphere, but also to gain the benefit of much lower noise levels (which drop rapidly with frequency) and wider bandwidths.
Delay is usually not an issue except for navigation satellites like GPS; even at 1575.24 MHz, which is way above fc, the ionospheric delay is usually the largest single error source for single-frequency civilian receivers.
Oh, btw, the reason metals are shiny is because they're electron "seas" -- at least one electron from each atom is so loosely bound that it floats freely from atom to atom, somewhat like a plasma only far more dense. This puts the critical frequency above that of visible light -- therefore the material reflects light. It's also why they conduct electricity.
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That makes sense, but 30 mhz isnt a lot of energy for a wave? Its almost like gamma radiation
No, 30 MHz is actually pretty low. It has a wavelength of 10 meters. Gamma radiation has a wavelength of less than 10 picometers, 12 orders of magnitude shorter and more energetic. 10 pm is on the order of the size of an atom.
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Waves above 30 GHz are scattered easily by water droplets or are absorbed by dust, so are only suitable for short range communication.
Things are a little more complex than that. It's true that water vapor generally absorbs more strongly as you go up in frequency, but there are other interactions with other atmospheric gases. Oxygen has very strong resonances at 60 GHz and 120 GHz, so the atmosphere is almost opaque near those two frequencies. (But not totally -- new 802.11 wireless LAN standards have been specified for 60 GHz, which is a very good use of that band since interference is automatically kept from going very far.)
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That makes sense, but 30 mhz isnt a lot of energy for a wave? Its almost like gamma radiation
No, 30 MHz is actually pretty low. It has a wavelength of 10 meters. Gamma radiation has a wavelength of less than 10 picometers, 12 orders of magnitude shorter and more energetic. 10 pm is on the order of the size of an atom.
Lets see:
λ=c/v
λ: wave lenght
c:speed of light
ν:frequency
λ=(300.000.000m/s)/(30.000.000hz)=10 meters.
You are right.
But how the signal does bounce through the ionosphere? Not enough energy to ionize atoms of the atmosphere? I though they were ionized already, given the name of the layer.
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That makes sense, but 30 mhz isnt a lot of energy for a wave? Its almost like gamma radiation
Speed of electromagnetic wave = 300 000 000 m/s (as an approximation)
Gamma has a wavelength in the 10 pm range, or 10-11m
frequency = wave speed / wavelength
frequency = 300 000 000 / 10-11m
This is 3 x 1019Hz, so 12 orders of magnitude more energy than radiowaves.
Despite ka9q's discussion of critical frequency, waves with even higher frequency, such as x-rays and gamma ray do not follow the critical frequency rule. They have much higher frequencies than the frequencies used in satellite communication. They tend to be attenuated by the atmosphere thanks to things like Compton scattering, excitation events and pair production. The interaction of electromagnetic waves with the atmosphere is complex.
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Things are a little more complex than that.
Absolutely, like in the infra-red, there are certain frequencies of infra-red that interact with the gases in the atmosphere, so if we analyse the infra red spectrum we see absorption bands that correspond to different gases.
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Oxygen has very strong resonances at 60 GHz and 120 GHz, so the atmosphere is almost opaque near those two frequencies.
I recall those frequencies being discussed when Phil Webb produced his videos on Jarrah's Exhibit D and shot him down on his understanding of satellite communication. It was the usual thing where Jarrah took a broadbrush statement to prove his case but lacked the detail to know about absorption in the microwave band, as usual he had the wrong conclusion as he did not have all the facts available. I need to look that up as it was an absolute howler.
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But how the signal does bounce through the ionosphere?
It doesn't bounce through the atmosphere, its transmitted through the atmosphere.
Not enough energy to ionize atoms of the atmosphere? I though they were ionized already, given the name of the layer.
Exactly, the region is already ionised and behaves as a weak plasma. The frequency at which waves are transmitted will depend on the charge density of the plasma. As ka9q as explained, there is a critical frequency, above which waves are transmitted. The critical frequency changes with the charge density, so will change according to space weather and time of day.
If the charge density increases then so does the critical frequency. Metals have much higher critical frequencies because their charge density is much higher than the plasmas in the ionosphere, owing to the free electron charge density. In fact, when light is incident on a metal it sets up plasmon oscillations in the free electrons, making the surface of the metal opaque to light and reflecting the light from its surface.
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But how the signal does bounce through the ionosphere?
It doesn't bounce through the atmosphere, its transmitted through the atmosphere.
Not enough energy to ionize atoms of the atmosphere? I though they were ionized already, given the name of the layer.
Exactly, the region is already ionised and behaves as a weak plasma. The frequency at which waves are transmitted will depend on the charge density of the plasma. As ka9q as explained, there is a critical frequency, above which waves are transmitted. The critical frequency changes with the charge density, so will change according to space weather and time of day.
If the charge density increases then so does the critical frequency. Metals have much higher critical frequencies because their charge density is much higher than the plasmas in the ionosphere, owing to the free electron charge density. In fact, when light is incident on a metal it sets up plasmon oscillations in the free electrons, making the surface of the metal opaque to light and reflecting the light from its surface.
Charge density is electrons per cubic centimeter? Other than that I understood all you stated above
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Charge density is electrons per cubic centimeter? Other than that I understood all you stated above
cubic metre if you are using SI. I hope the replies have helped you understand.
As ka9q and I have pointed out, EM waves and the atmosphere is a complex field.
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Charge density is electrons per cubic centimeter? Other than that I understood all you stated above
cubic metre if you are using SI. I hope the replies have helped you understand.
As ka9q and I have pointed out, EM waves and the atmosphere is a complex field.
Also Interesting
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I am also a radio ham, currently inactive. I used to work the 10m band some years ago, regularly working contacts in Hawaii and the western States of the USA, particularly Arizona, California, Nevada and Colorado. Many of these contacts were members of 10-10 International, although I never was.
The lower bands, 15m and 20m were a lot easier to work, but 10m was a challenge because the skip was a lot less frequent being so close to fc (which varied according to atmospheric conditions and location). 10m propagation was also quite dependent on solar activity. You could try for weeks and get nothing and then all of a sudden the band would open up and "go mad" for a few days.
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Charge density is electrons per cubic centimeter? Other than that I understood all you stated above
What actually matters is Total Electron Content - TEC. That's the total number of free electrons you encounter as you penetrate the plasma. In this case, the plasma is the earth's ionosphere.
There are actually multiple ionospheric layers created by multiple mechanisms, and some are more relevant than others at a given frequency. For example, the D layer is formed by solar UV at relatively low altitudes, 60-90 km. The recombination rate is high (electrons and ions easily find each other) so it disappears at night. It is significant mainly below 5 MHz or so and it absorbs more than it reflects, and this is why distant AM broadcast stations (0.54 - 1.7 MHz) disappear during the daytime. The 80 meter (3.5-4.0 MHz) and 160 meter (1.8-2.0 MHz) ham bands are also usually closed during the daytime, with local propagation only.
The F layer(s) at 150-500 km are the most important layers for long-distance "skywave" propagation. The air at those altitudes is so thin (the ISS flies in this region) that when ion/electron pairs form, they last a long time before recombining. They persist through the night, reflecting low frequency signals but letting high frequency signals escape to space.
Whether a signal reflects or goes through depends on both the critical frequency and the incidence angle. A signal might be reflected if it hits the ionosphere at a shallow angle but penetrate at a high angle. You can observe this phenomenon optically by going to the bottom of a swimming pool and looking up at the surface. At high angles you can see right through the surface, but at shallow angles (below the Brewster angle) the surface looks like a mirror.
This sets a Maximum Usable Frequency (MUF) for communication with some particular station. This explains the "skip" effect: you can only hear stations beyond a given distance because nearer signals hit the ionosphere at a high angle and go right through into space instead of being reflected. This is a very common (routine) phenomenon on the ham bands from 20 through 10 meters; very often you can only hear one side of a conversation. 20 meters is usually open to someplace on earth 24 hours/day, making it the most popular band for "DX" (working distant stations). The higher bands are often closed at night, with local propagation only, because the sun doesn't maintain sufficient ionization density in the F layers. Hams get very adept at picking frequency bands, usually a high frequency (short wavelength) during the daytime and a low one (long wavelength) at night.
Luke is quite right that at extremely high frequencies (above visible light) the atmosphere attenuates signals before they even get to the ionosphere. Most UV is absorbed by ozone and/or oxygen, and X- and gamma rays are absorbed entirely. It seems counter-intuitive that radiation famous for penetrating solid objects would be stopped by mere air, but it's true. Radio interacts only with free electrons, but ionizing radiation (anything shorter than mid-UV) will knock any electrons it finds loose even if they're bound to atoms. That takes energy from the radiation.
This is why lead is good shielding; it's not the mass per se but the high electron density that goes with it. Air has a sea level density of about a kilogram per cubic meter, and that can really add up. This is how nuclear weapons produce fireballs near the surface; most of their energy comes out as soft X-rays that are absorbed by the surrounding atmosphere, heating it to incandescence.
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Nice read.
This is why lead is good shielding; it's not the mass per se but the high electron density that goes with it
Is there a reason why aluminium was used as shielding in apollo? Lead has 82 electrons compared to 13 electrons from aluminium. I have read from clavius that high energy protons where easily shielded against but what about cosmic rays or x-rays in outer space and I dont think a material with few electrons could protect the astronaut from such a ionizing radiation.
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Nice read.
This is why lead is good shielding; it's not the mass per se but the high electron density that goes with it
Is there a reason why aluminium was used as shielding in apollo? Lead has 82 electrons compared to 13 electrons from aluminium. I have read from clavius that high energy protons where easily shielded against but what about cosmic rays or x-rays in outer space and I dont think a material with few electrons could protect the astronaut from such a ionizing radiation.
Both for purposes of weight reduction and avoiding inducing too much Bremsstrahlung.
From http://www.clavius.org/envrad.html
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I have read from clavius that high energy protons where easily shielded against but what about cosmic rays or x-rays in outer space and I dont think a material with few electrons could protect the astronaut from such a ionizing radiation.
Taking some of these points:
Apollo did not have sufficient shielding to protect against galactic cosmic rays (GCR), but then the dose from such radiation would not be problematic on a short trip. GCR fluctuates with the solar cycle, and at solar maximum GCR tends to be lower. Apollo flew through a solar maximum. GCR becomes a problem on a trip to Mars (or beyond).
X-rays. Indeed there is an X-ray flux from the sun to consider. However, the X-rays produced in solar events are predominately soft X-rays which are readily attenuated by a several cms of air, so not a problem for the Apollo hull. There are X-rays with higher energies to contend with, but their flux levels are extremely low.
The main problem for the astronauts is a Solar Proton Event, or SPE. These events have a distinct definition, and non occurred during an Apollo flight, not that I am aware of, and I have trawled the literature to look for them. In fact I have cross referenced that much literature in a search to prove the CTs are talking BS, it has given me great joy in learning new ideas and concepts.
Solar Proton Events are generated by an event called a shock driven Coronal Mass Ejections (CME). A large proportion of CMEs are harmless and are ejected at speeds comparable to the solar wind. However, once in a while the speed of the CME exceeds a critical value (if I recall 500 km/s), and this results in charge separation through the plasma which creates an enormous electric field. This electric field accelerates protons in the solar plasma and causes massive solar storms. NOAA provides a frequency of such events, and I can assure you, that the astronaut killers do not occur often in each solar cycle. In fact, our CTs are quite adept at citing NOAA data, but miss out some rather telling facts from NOAA that shatter their arguments into splinters. :)
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I did understand all, thank you for your responses!
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Is there a reason why aluminium was used as shielding in apollo? Lead has 82 electrons compared to 13 electrons from aluminium.
High-Z (high atomic weight) materials like lead are good shielding for ionizing electromagnetic radiation (photons), which was the subject of my last message. But as Luke explained, ionizing photons aren't a serious problem in space as they are easily stopped by ordinary structural materials. You only have to be careful when you deliberately open a window to these photons for observation. Skylab astronaut Owen Garriott told me that they had a quartz window for taking pictures of the sun in ultraviolet, and all sorts of alarms would sound if that window was left uncovered when it wasn't in use.
At ~6000K the "surface" of the sun is simply too cold to generate much in the way of far UV or X-rays. Only solar flares generate a lot of hard photons because of the extreme heating (to millions of kelvins) of the ejected material by energy stored in a local magnetic field. Sometimes the far UV/X-rays are strong enough to rapidly and heavily ionize the D layer of the earth's ionosphere, causing an ionospheric radio blackout on the day side of the earth. The same thing happened during nuclear tests in space; fortunately these were banned long ago.
As Luke also explained, the main hazards to astronauts comes from energetic charged particles, not photons. Here it turns out that low-Z materials like aluminum and even hydrogen are better shields, per unit weight, than high-Z materials like lead. The Apollo command module structure was stainless steel and aluminum, and the whole thing was covered with a phenolic resin that contained a lot of carbon and hydrogen. Many of the plans for interplanetary spacecraft use their fuel and water tanks as shielding against solar mass ejection events. These are particularly effective shields because of their high hydrogen content.
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Is there a reason why aluminium was used as shielding in apollo? Lead has 82 electrons compared to 13 electrons from aluminium. I have read from clavius that high energy protons where easily shielded against but what about cosmic rays or x-rays in outer space and I dont think a material with few electrons could protect the astronaut from such a ionizing radiation.
Note that the spacecraft designers didn't add a layer to the capsule and say, "this is the radiation shielding." They built a capsule structure that would withstand the thermal and physical stresses of launch, spaceflight, and landing; and it had adequate radiation shielding without a dedicated, separate radiation protection system. Aluminum is a far better aerospace construction material than lead, which is soft and melts easily.