C band SATCOM question

[senmeis]

Hello,

I’m reading the article “Satellite Basics” at **broken link removed**. The following can be read for C band:

Because of its weaker power it requires a larger antenna …

Question: why weaker power? Does it relate to C band?

Senmeis

 

[jesuschrist]

It is not weaker it is about the wavelength of the C band. For parabolic reflectors the directivity equation is G=10*log_10(k*((pi*D)/lambda)^2)) where k= efficiency factor generally between 0.5-0.8, D= diameter of the reflector, lambda=speed of light/frequency.
Now you can get the idea that when the frequency goes up wavelength goes down, because lambda is in denumerator of the equation when lambda goes down gain increases. So for the same reflector size, C band has the lowest gain.
So lets say you want to get a 30 dBi gain from your antenna at C band and Ka band.Assuming both of your antennas have %70 efficiency(k=0.7) you would need 1.25m antenna for Ka band transmission at 29GHz , for the same gain you would need a 6.3 meter antenna for C band transmission at 5750 MHz.

 

[senmeis]

On the attached slide you’ll find this description:

(C band ) Requires larger antennas than L-, Ku-, or Ka- band

According to your theory, L band should be weaker than C band, but on this KVH slide L band comes with Ku and Ka bands together.

Senmeis

 

[Azulykit]

This is a subject that causes a great deal of confusion. The presence of a frequency variable in the space loss calculation just adds further confusion. The power received by an antenna is related to the power density over the antenna and the antenna surface or aperture area. It is true that gain increases with frequency. That is an opportunity to add more confusion. Neither power density nor antenna aperture are frequency dependant.

ETS-Lindgren Inc. provides a discussion in their technical notes that goes a long way at explaining the issue you raise (lower frequency results in weaker signals). Run it down and study the couple of pages discussing the issue.

I like to remind myself that spherical spreading of a signal does not depend on frequency. More confusion results if one fixes gain. Now the physical size of an antenna decreases with increasing gain.

It would be helpful to get a firm grasp of effective radiated power (sometimes described as effective isotropic radiated power).

Look up the reference and watch the fun as I expect these comments to excite a discussion.

Regards,

Azulykit

 

[senmeis]

Could you please provide the link to this reference? There is a big archive. In fact I agree with all your description in the last posting. Right now L band in the second slide seems to generate the biggest confusion.

Senmeis

 

[ktr]

Waiting for the link about the topic. Interesting discussion.

ETS-Lindgren Inc. provides a discussion in their technical notes that goes a long way at explaining the issue you raise (lower frequency results in weaker signals). Run it down and study the couple of pages discussing the issue.