P5 - Radio-waves & Microwaves Flashcards
Why are electro-magnetic waves with different frequencies used?
• Different frequencies mean they behave differently in the atmosphere.
What are the different examples of electro-magnetic frequencies & how they behave in the atmosphere?
- Below 30 MHz, Radio-waves are reflected off a layer of the atmosphere called the ionosphere. This allows the wave to travel longer distances and deal with the curvature of Earth.
- Between 30 MHz and 30 GHz, Radio-waves are and microwaves pass straight through the atmosphere, so transmission must be by line of sight, because they cannot reflect off the atmosphere.
- Above 30 Ghz, Rain and dust in the atmosphere absorb and scatter microwaves. This reduced the strength of the signal, so the highest frequency that can be used for satellite signals is about 30 GHz.
What is the highest frequency of satellite signals that can be used?
30 GHz, anything above will be scattered and absorbed by the rain and dust in the atmosphere, that will reduce the strength of the signal.
What is ideal frequency for communicating with satellites?
• Between 30 MHz and 30 GHz, these waves will pass straight through the atmosphere in a straight line.
What is the ideal frequency for communicating long distances across the Earth?
• Below 30 MHz, These radio-waves are reflected by the ionosphere, this allows them to travel longer distances and deal with the curvature of the Earth.
What is a narrow gap?
A narrow gap is one which is about the same size as the wavelength.
What happens as waves pass through a narrow gap or past an object?
• All waves tend to spread out ( diffract ) when they pass through a narrow gap or past an object.
What does a “narrow” gap depend on?
The narrow gap depends on the wave, a narrow gap for a long wavelength radio-wave may be a huge gap for a microwave.
What is the correlation between wavelength and diffraction?
• The larger the wavelength of a wave, the greater the diffraction amount.
When do you get maximum diffraction?
You get maximum diffraction when the size of the gap is equal to the wavelength of the wave.
Why are long wavelength radio-waves sometimes used for communication?
- Long wavelength radio-waves diffraction easily over hills and into buildings, these wavelengths will be received by a hill at the bottom of a house.
- Shorter wavelength TV and FM radio waves do not diffract much.
How do regular ( terrestrial ) TV receive radio signals?
• Regular TV and radio-signals using radio-waves are received using an aerial.
Why are microwaves not received by an aerial?
• Wavelength of microwaves is too short for aerials to be effective at receiving them.
How are satellite TV signals and radio-waves received?
- Wavelength of microwaves is too short for aerials to be effective at receiving them.
- So dishes are used.
Why are dishes used & why do transmitting and receiving dishes need to be carefully aligned?
- Dishes are many times larger than the wavelength of the microwaves, so the microwaves don’t diffract much, this produces a narrow beam that doesn’t spread out.
- This means the transmitting and receiving dishes need to be carefully aligned so the signal is picked up.
