Why do satellite use microwaves?

[GeekWizard]

Hello,

I know higher the frequency more likelihood of causing resonance in similar size molecules, thus increasing abosorption chances.

Then why do satellites communication use so high frequencies (L, K band etc.) ?

I want to know why these pass through the ionosphere and not abosrbed?

Many Thanks !

 

[BigBoss]

To my knowledge..
Because lower frequencies have scattering effect,higher frequencies have lower scattering issue ( like light beam ), therefore the necessary power is condensed on unit area of -for instance- a dish antenna..
If we have used lower frequencies, the signal will be scattered and it will never reach certain energy level on a unit area.
There are also some other constraints such as frequency planning, atmospheric absorbtion etc..

 

[tallface65]

There is also a key physics effect of the free electron density of the ionosphere. These free electrons are caused by the ionizing energies of the UV radiation and other high frequency particle radiation. Once these electrons are free, you can imagine that the ionosphere acts as a crappy conductor Since there will be a particular normalized density of free electrons, there will be a corresponding plasma frequency, just as in metals at optical and higher frequencies. If incident RF radiation is at a frequency less than the plasma frequency (~230MHz, see CHEMTRAILS - CONTRAILS) then the real part of the permittivity is less than zero and hence propagating waves are prohibited causing most of the radiation to be reflected. However, if the incident radiation is above the plasma frequency, the real part of the effective permittivity is greater than zero and thus propagating waves are permitted allowing for translucence at those frequencies.

A good example for comparison is the commonly encountered ITO (Indium-Tin-Oxide) which is used for low frequency modulation of optical phase modulators. At the modulating frequencies (~Mhz) ITO "appears" as a conductor and a bias can be induced since at these low frequencies the ITO acts as a conductor, however at the optical frequencies, the ITO acts as a dielectric and thus is transparent!!

Have Fun

 

[tony_lth]

Higher freq, higher data throughput. If you use 2G, the bandwidth available is only several tens MHz. For 18G, the bandwidth can be 2G. For 100G, the bandwidth can be 20G.

There are some windows for microwave signal (from 1G to 300G), even laser telecomm, to pass through atmosphere.

 

[flatulent]

The main problem is the frequency assignment issues. The lower frequencies are by law assigned to other uses. Another issue is beam width. You can get a narrow beam width with a reasonable size antenna at microwave. That way you do not waste your very expensive transmitted power by radiating it to the free space outside the earth.

 

[juppydu]

simple: higher frequency = small antennas... On Satellite the antenna size is of primary importance (ans mass as well).
Mission operating at lower frequency (< 1GHz) requires large antennas (for the same gain figure), this also implies high cost.

 

[RCinFLA]

In terrestial communications bandwidth is usually #1 limitation. In satellite comm it is power resources of the satellite that is #1.

Higher frequencies allows for better directivity control by reasonably sized antennas with the objective to spray power only where it is needed.

At geostationary satellite orbit, the earth looks a little smaller then a basket ball held at arms length. To hit a particular area requires a tight beamwidth.

Direct broadcast TV has started using Ka band (25 GHz). This allows DirectTV to paint tight patterns to provide expanded local HD TV station retransmission. Frequencies can be reused in other coverage areas.

 

[biff44]

Its cold in space, and the sattelite needs something to warm its coffee up with.