What is the relation between the frequency, speed and the distance in satellites?

Hi, all

One basic question...

I know that increase in frequency will decrease the range(R=k*Pt/(Pr*f)). But how can satellites communicate with base station, operating at higher frequency....

What is the relation between the frequency, speed and the distance...?

Thanks and regards
Kumar

Re: High freq range

disisku_22 said:

I know that increase in frequency will decrease the range(R=k*Pt/(Pr*f)). But how can satellites communicate with base station, operating at higher frequency....

Click to expand...

The equation given above is not correct. As there is a quadratic relationship between free space loss and distance it should read:

R²=(c/4Πf)²*Pt*Gt*Gr/Pr

with c=light speed, Gt=transmit antenna gain, Gr=receive antenna gain.

However, you have forgotten that Gt and Gr (which are part of your constant k) are factors which are proportional to frequency; thus the frequency dependence of the range R is cancelled as a rough approximation.

Re: High freq range

Dear LvW,

Ok, I know this Friss's equation, but I want to know, if we increase the frequency the distance will decreases correct.., so how can satellite(which uses high freq in order of GHz) is communicating with the base station which is very far range/distance. Is Tx power is play major role here.........?

Thanks in advance

Re: High freq range

this is a common belief, namely that there is some magic that causes space loss to increase with increasing frequency. I can argue both ways on this point.

It is tied up with the way Friis is cast. Usually transmit and receive gain numbers are set to the same value at different frequencies. In this case "space loss" increases with increasing frequency. This is really due to a decrease in transmitting and receiving aperture size with increasing frequency and not an increase in the insertion loss associated with propagation.

I would recommend deriviing the Friis equation for yourself. The issue (and confusion) will then be more clear.


Communication distance is a function of many factors. Look into the calculation of a "link budget" to get a feel for what is going on. B. Sklar's is a useful reference.

Be sure you know what the assumptions are when you look at this question.

Re: High freq range

disisku_22 said:

.........if we increase the frequency the distance will decreases correct.., so how can satellite(which uses high freq in order of GHz) is communicating with the base station which is very far range/distance. Is Tx power is play major role here.........?

Click to expand...

As I have mentioned in my first reply: With the frequency the gain of both receive and transmit antennas increase as well and - more or less (neglecting some parasitic influences) cancels the increased "free space loss". A calculation of the link budget considers all losses, gains, bandwidths and noise sources and arrives at a minimum power to be transmitted to the spacecraft.

Added after 16 minutes:

Azulykit said:

this is a common belief, namely that there is some magic that causes space loss to increase with increasing frequency........ In this case "space loss" increases with increasing frequency. This is really due to a decrease in transmitting and receiving aperture size with increasing frequency and not an increase in the insertion loss associated with propagation.

Click to expand...

When I am using the term "free space loos" I don´t mean something like an "insertion loss". Instead, this term is normally used to describe the power transfer between two ideal isotropic (spherical) radiating/receiving antennas. And then, "gain" of these antennas is nothing else than a facor which describes the improvement of this worst case due to beam forming. Physically, it is not a "gain" since the transmitted power never is higher than the power fed into the antenna.
Thus, antenna gain is only a factor of improvement if compared with the worst antenna system which radiates spherically.

Re: High freq range

fix gain and increase frequency and the antenna aperture will decrease. Smaller aperture means less power,

fix aperture and increase the frequency and the gain will increase. thus one will collect more power.

Space loss is often understood to represent insertion loss and thus all the confusion (lack of clarity? common sense?) about the physics of the situation.

Another concept (associated with this subject) is effective radiated power ERP (sometimes also effective isotropic radiated power EIRP). It is just power dinto the antenna times the gain.



"Physically, it is not a "gain" since the transmitted power never is higher than the power fed into the antenna."

Be careful here, the power density at a distance from a high gain antenna will be higher than one would see from an omni. It looks just like more power was being radiated than was input to the antenna port. If we are not using an antenna to create higher power densities then why bother with high gain antennas? They are too big anyway. Conservation of energy is not really part the space loss issue. Total energy radiated is always less than input.

Re: High freq range

This is a long time believe that "float" as credo. To believe
that somehow the LOS loss (Line of sight loss) equation means that
the losses increases with frequency. That is an oversimplistic way
to explain the physics of propagation.

Which is reinforced for the fact that short wavelength travel
longer distances. Although related does not means one implies
the other. Friis would be pulling his hear in Engineering Heaven.

Re: High freq range

hi

.................Which is reinforced for the fact that short wavelength travel
Longer distances.................

What is the relation between the wavelength and the distances.......... can you more brief on this

Thanks
kumar

Re: High freq range

disisku_22 said:

hi

.................Which is reinforced for the fact that short wavelength travel
Longer distances.................

What is the relation between the wavelength and the distances.......... can you more brief on this

Thanks
kumar

Click to expand...

Well that is a missword....No frequency travel better than any other....All travel
the same, granted some have more or less attenuation due to rain, vapor
absorption, etc, but there is no equation that can tell this is better than any other.
It is just that low frequency is prefered (for example: ham radio) because can
rebounce the ionosphere, or can have surface wave to propagate even beyond
the horizon, can rebounce from the moon, etc. Or rain attenuation is critical
at certain frequencies. But that is not a consequence of the LOS
equation. Thats is propagation mechanism.

Any propagation book can tell you that.

There is not relationship as in an equation(Ex. LOS) that summarizes everything.

Then there is people who take LOS losses and interpret it as if the losses increases
with frequency and the justification is given that way. In other words, the LOS
equation is not consequence of low frequencies bouncing off the ionoesphere, etc.

I have not seen any book which say "losses increases as the frequency increases
and the LOS equation proved". If someone find one, please let me know. So I can
scrap a bunch of others I have.

Re: High freq range

Let’s return to the initial questions.

First of all author was wrongly cited well known Friis formula. There is strong dependent on frequency: Pr=(Pt*Gr*Gt*(lambda)^2)/((4*PI)^2*d^2*L). Here Pr and Pt are receiving and transmitting antenna signals powers; Gr and Gt is receiving and transmitting antennas gain; lambda is wavelength; d is radio link distance; L is system losses. All values in numerical format, not in dB.

Secondly, when you design satellite link you need to carefully count many factors. All the losses along the links (there are at least two of the links and they often are not the same); the orbit parameters; time of the day or night; communication parameters and so on. Only losses worksheet can easily occupy a lot of pages for one link. Real numbers usually well behind 200+ dB for Earth satellites. For space ships that go far from Earth numbers become creasy, but fortunately in this case kTB is working for us. Often losses reduce with frequency. For example it is well known that it is possible to have stable long radio link at 30 MHz with 50 W, whereas to have the same link quality at 3 MHz you need much higher power. But definitely, in rain forest area for satellite link situation will be opposite.

When designer is modeling all the losses and receivers’ parameters are known for the satellite and Earth stations it is possible to balance between available RF transmitting power and realizable antenna parameters. All of mentioned above related to the initial question about power versus frequency and distance relation.

Antenna dependents on frequency is often not the main factor for satellite links because they usually are not very broadband and designed especially for link requirements.

Speed is different case. There may be a variety of cases and it is not easy to describe all of them here. It is something like adding the 3rd dimension to the problem and there is not only Doppler related issues. It is better to solve speed problem separately for each case.

It looks like you friends were lured by the simple LOS case when you discussed the satellite radio link design problems. The real life is totally different. My last satellite radio link project (only links, nothing else!) was about 250 pages of MathCAD worksheets. There were just the calculations and optimizations, almost without the texts, only necessary notes to the equations. And project itself was relatively easy: low data rate satellite system for collection the data to 7 meteorological satellites from several thousands Earth stations and then translating results to the main Earth station.

I can recommend to disisku_22 using good satellite link worksheet or create one and play with it. A lot of questions will disappear quickly. There are a lot of books available, surprisingly may be more than for RF system design. Antenna design for satellite link may be not easy, usually big parabolic structures with narrow beam are used, but this is the area where Azulykit can help. He is out for short vacation and should be back in about a week or so.

Re: High freq range

Azulykit said:

Another concept (associated with this subject) is effective radiated power ERP (sometimes also effective isotropic radiated power EIRP). It is just power dinto the antenna times the gain......
...................................

"Physically, it is not a "gain" since the transmitted power never is higher than the power fed into the antenna."
Be careful here, the power density at a distance from a high gain antenna will be higher than one would see from an omni. It looks just like more power was being radiated than was input to the antenna port. If we are not using an antenna to create higher power densities then why bother with high gain antennas?

Click to expand...

In order to avoid misundertsandings just to corrections for clarification:
1.) EIRP means NOT effective radiated power (in this case we would have a kind of power gain) but equivalent isotropic radiated power. That means, if we would have an isotropic radiating antenna it must radiate a value of EIRP in order to produce the same power density as the beam forming antenna with gain G.
2.) the power density at a distance from a high gain antenna will be higher than one would see from an omni.
I didn´t speak of power density. My only point was to make clear that "antenna gain" does not mean "power gain".

Question to disisku_22: Is your question answered ? Are you satisfied ?

Re: High freq range

I realy appreciate the reply of you all ! it realy helped me to clear on this topic..

thanks
kumar

Re: High freq range

Effective or Equivalent is the same thing,

https://en.wikipedia.org/wiki/Equivalent_isotropically_radiated_power

**broken link removed**

https://en.wikipedia.org/wiki/Effective_radiated_power

Even IEEE called effective:

**broken link removed**

I am also for a good use of terms but in this case I think is irelevant.
I like to think is relevant when someone change dBm for dB, or
dBM or DBM or DBm, etc......