Ionosphere radio communication.

[Gazpar]

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.

[gwiz]

Short answer:

Ionospheric reflection depends on frequency, so they use frequencies that reflect less.

[Luke Pemberton]

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.

[Gazpar]

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

[ka9q]

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.

[ka9q]

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.

[ka9q]

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.)

[Gazpar]

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.

[Luke Pemberton]

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.

[Luke Pemberton]

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.

[Luke Pemberton]

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.

[Luke Pemberton]

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.

[Gazpar]

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

[Luke Pemberton]

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.

[Gazpar]

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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