Why do the satellite use high frequency ?

[pfijk]

The high frequency spectrum get wide bandwidth .


On the view of wave propagation

The path loss of EM wave is large in high frequency .

So .. Why do the satellite still use high frequency ? (even in long distance)

welcome to any comments .

 

[drabos]

DBS:
Uplink : 14 GHz
Downlink: 10-12.4 GHz

Maybe, because of the antenna. (parabolic has a big gain)

And the wide bandwidth.

 

[IanP]

The main issue is the throughput - the more bits can be sent the more economical it is ..
On top of that, (g)satellites are always (except of the N and S poles .. ) in the line of sight ..

Regards,
IanP

 

[masadi]

how bout size of the RF circuit and antenna, higher freqency gets smaller realizable size?

 

[flatulent]

part of it is legal. The spectrum was already allocated for earth links. The satellites had to be assigned the higher not allocated frequencies. Some of the allocations are used for point to point earth links. The paths to the satellites are perpendicular to the earth links and so the two do not interfere with each other.

 

[electronics_kumar]

Always, satellite will be placed at a high altitude..high frequency can travel upto very long distance.. so that only we allocated high frequency for that communication

 

[sidra]

hi...sidra here...actully i think there are many issues to be disscussed on satellite's high frequency.....e.g
on high frequency datarate is high.
the high frequencies are highly directional
as the satellites are above ionosphere...the high frequencies are not disturbed by it....
n there are many more reasons

 

[mshareef]

Hi,

Sidra is right. There are various issues involved in using the high frequencies for the satellites. The most important is the distance involved and the coverage provided by a Satellite. The reason for not using the lower frequencies is due to the bending of the rays and the influence of the atmospheric effects.

Hope the following note will make it more clear:

Radiowaves, suitable as carriers of information with a large bandwidth, are
found in frequency ranges where the electromagnetic waves are propagated
through space almost in conformity with the law of optics, so that only line-ofsight
radio communication is possible. As a result, topographical conditions
and the curvature of the earth limit the length of the radio path. Relay
stations, or repeaters, must be inserted to allow the bridging of greater
distances. Skyway radar uses the ionosphere, at height of 70
to 300 km, to transmit information beyond the horizon and may not require
repeaters. However, transmission suffers from ionospheric distortions and
fading. To ensure that appropriate frequencies are optimally selected, additional
monitoring equipment is required to sample the ionospheric conditions
instantaneously.

A communication satellite in orbit around the earth exceeds the latter
requirement. Depending on the orbit’s diameter, satellites can span large
distances almost half the earth’s circumference.

 

[rautio]

Hi Pfijk -- I spent 4 years working on communications satellite design in the 1980's. Some of the comments above are accurate, others are a little bit off.

When using parabolic dish antennas, the "link budget" (i.e., how much signal is recieved for a given transmitter power) is not dependent on the frequency. Yes, the path loss increases, but the the dish gain (for a given dish area) increases just exactly enough to compensate for that.

What does change at higher frequencies is the antenna beam width, it becomes narrower. Thus, in crowded synchronous orbit, you can space the satellites closer together. You also can get wider bandwidths at higher frequencies. The tradeoff is the dish surface needs to be more accurate (even under extreme thermal cycling) and it is harder to get high power and lower LNA noise figures at higher frequencies. In addition, when you get above about 10 GHz, rain attenuation can cause outages. Around 60 GHz (I don't recall the exact frequencies), you can get big time attenuation due to things like oxygen. Lot's of tradeoffs, very interesting field to work in.

 

[ssankurathri]

hi rautio
does this high frequency has anything to do with length of the antenna?
i think if frequency is high,it will have high energy to travel long distances.is it true?
cheers
skr

 

[hermin]

rautio,
I just wanna ask a question about your post, Ive studied link budget in college and frequency of the signal is considered in the budget,

 

[springf2000]

reduce the antenna size and improve the antenna gain!

 

[electronics_kumar]

hi rautio
does this high frequency has anything to do with length of the antenna?
i think if frequency is high,it will have high energy to travel long distances.is it true?
cheers
skr

\


for maxmimum radiation, the height of antenna should be 'lamda/2'.. so high the frequency lower the height

 

[rautio]

Hi Folks -- Very tired now, spent most of last 10 hours talking to many different people, about to hit the sack, but will reply to the above posts. Sounds like quite a bit of interest!

ssankurathri -- The energy of the photon has nothing to do with how far it will travel through empty space. If the energy (i.e., frequency) matches a resonant mode of a molocule, it will get absorbed while other photos with different energies (both above and below) pass with no trouble. One such molucule is O2, with a resonant frequency of around 60 GHz (as I recall). If you isotropically (for example) transmit one watt at 1 MHz, or at 1000 GHz, it will travel the same distance, at the same speed, with the same flux (watts per meter squared) at all distances, provided it is traveling through empty space.

herman -- Yes indeed, the frequency appears in the link equation. But that is assuming you have a dipole (or isotropic) antenna for both transmit and receive. The higher frequency dipole (and equivalent isotropic) has a smaller size, and thus smaller capture area. This where the lambda squared term in the link equation comes from. It makes it seem like free space strongly favors lower frequencies.

However, if you use a parabolic dish, it has the same area at all frequencies, and because the flux density (see above) is independent of frequency, the dish's received power will be the same at all frequencies. If you use a dipole for a receive antenna, the lower frequencies are better because the dipole capture area is larger, it can pick up more of the watts per square meter because it has more square meters.

It will be a couple days before I can check postings again. See y'all!

 

[arunkumar]

ssankurathri -- The energy of the photon has nothing to do with how far it will travel through empty space. If the energy (i.e., frequency) matches a resonant mode of a molocule, it will get absorbed while other photos with different energies (both above and below) pass with no trouble. One such molucule is O2, with a resonant frequency of around 60 GHz (as I recall). If you isotropically (for example) transmit one watt at 1 MHz, or at 1000 GHz, it will travel the same distance, at the same speed, with the same flux (watts per meter squared) at all distances, provided it is traveling through empty space.

Rautio -- Let me understand your above statement. Generally higher the frequency, lesser the wavelength. That is why AM being lower in frequency can travel longer distance whereas FM (high frequency) decays at a faster pace and can cover only a shorter distance. If we transmit AM & FM with the same power, will they reach the same distance?? What is your stand in this statement?

 

[masadi]

frequency of AM is below the cut off frequency of ionosphere, so it's reflected back to earth, frequency of AM is higher than cut off freq of ionosphere, so it passes through ionosphere away from earth.

 

[rautio]

Hi Arunkumar -- Masadi's comments are correct for long distnace HF and MF propagation. In fact, last night on my ham radio I talked to a station on Aves Island, off Venezuela on 1.8 MHz. The long distance was possible because the signal reflects off the ionosphere. This is called a skywave. I would also like to talk to him on 50 MHz. But the ionosphere does not have sufficient ionization to reflect 50 MHz. So that is not presently a possibliity.

Another way to look at it, if one frequency could travel farther than another frequency in empty space, then one frequency or the other, or both, would no longer fall off with 1/r². Please consider what that means for conservation of energy...

 

[plasma]

In the past technology work with lower freq. so to get sufficient gain it involved with
Big antenna and big load => big satellite => big money
Now technology improved so you can get more gain form smaller antenna by increasing the freq. => get small antenna and small load => little satellite => less money thats all friend MONEY

pl

 

[Dr_mas]

first, i agree with the one who say that at high frequency we can transmit the signal with small size antenna and also with narrow beamwidth so the gain will be high

second, as the frequency increase the attenuation due to the free space increase
but as we see the satellite communications used first 4-6 GHz (c-band) to communicate then they use 12(downlink)-18(uplink) GHz ???!!

so why they increase the freq although they know the attenuation will be high!!
the most important reason is the increase of the number of the application at the c-band freq which make a great interference with the satellite's signals.

third, they looked to the attenuation due to the atmospheric gases as water vapour , oxygen, carbon dioxide..and found that there is a little attenuation for those at 12 and 18 GHz, so they choose it

 

[hermin]

ssankurathri -- The energy of the photon has nothing to do with how far it will travel through empty space. If the energy (i.e., frequency) matches a resonant mode of a molocule, it will get absorbed while other photos with different energies (both above and below) pass with no trouble. One such molucule is O2, with a resonant frequency of around 60 GHz (as I recall). If you isotropically (for example) transmit one watt at 1 MHz, or at 1000 GHz, it will travel the same distance, at the same speed, with the same flux (watts per meter squared) at all distances, provided it is traveling through empty space.

Rautio -- Let me understand your above statement. Generally higher the frequency, lesser the wavelength. That is why AM being lower in frequency can travel longer distance whereas FM (high frequency) decays at a faster pace and can cover only a shorter distance. If we transmit AM & FM with the same power, will they reach the same distance?? What is your stand in this statement?

and also AM radio are on vertical polarization, meaning the EM waves, that also helps it go further in distance, if anyone objects on this statement please say so because its been a while since I had this subject in college,