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It all depends. The volume of the antenna is related to the gain. If you want less diameter you will have to increase thickness by using an array of yagis. This is very difficult at higher gains and frequencies. The famous antenna book by Kraus describes this constant volume effect.
I suspect that the receiving system in question alread has a 1 or 2 dB noise figure as these are relatively cheap these days. This means that even using liquid nitrogen cooling only 0.6-1.3 dB of SNR could be gained.
Your only other choice if you want to reduce the antenna gain is to switch to a lower data bit rate if this is possible.
I do not agree with you. For low noise systems the important thing does not the noise figure itself, gut the G/T for the antenna system (where G is the antenna gain and T is the noise temperature of the system). So using 0.5 dB noise fig. amp. instead of 2 dB means much more than the 1.5 dB difference !
The definition of noise figure is the amount that the signal to noise ratio is reduced by the given amplifier compared to a ideal noisless amplifier. The T you used is for the entire system and what the antenna is pointing at including side lobes pointing to earth.
as a receiving antenna, it is possible to reduce antenna size using lower antenna temperture techniques. But when we want to transmit, the antenna pattern must meet ITU requirements. As you know, the sidelobe and cross polarization can be suppreased by changing aperture field distribution, but how to narrow antenna beamwidth is a big problem. does anybody have ideas on it?
You are constrained by the laws of physics. Gain, antenna size, and beamwidth are all related. You can control the beamwidth in the two (vertical and horizontal) planes independently. This can either be done with reflector dimensions or by number of radiating elements in a phased array being different in each direction. Another factor you have control of is the taper of dish illumination or the weighting of the phased array elements. These control the sidelobe level and the main beam width. Again, the laws of physics pretty much hold you in.
You also have the problem of keeping the antenna pointed as the mobile vehicle moves about. One very expensive solution is to have a phased array that has nulls in the direction of the adjacent satellites. This requires extra receivers to monitor the signals from these satellites and adjust the null in their direction. What makes this worse is if the TX and RX frequencies are different. Could you give me more details. Either in public or by PM? Your customer may be trying to use the wrong satellite service. One designed for mobile users will have antenna requirements more usable on vehicles.
It looks like your only choice is a phased array with nulls in the directions of the adjacent satellites. This will require a two dimensional array of possibly printed antennas with different lengths of transmission line to each one. If the TX frequency is different from the RX frequency, the nulls should be positioned at the TX frequency and you will take your chances on RX with your desired signal being high enough commpared to the other ones that you can demodulate it properly. If this will not work, you will have to have two sets of antenna elements that may be able to be on the same plane (different spacing than the TX array) with their own set of phasing transmission lines to null out the other satellites on RX as well.
The phased array may be able to pass the required specifications by putting nulls at the other satellite angles instead of the minimum radiation at all angles usually specified in the regulations. I am not sure if the rules you have to meet will allow this. This meets the spirit of the law, but not the letter. If you can make the array part of some surface that already exists, you have extra degrees of freedom to play with.
For what concerns patch antenna array, I can say that you can use slotted patch antennas inspite of standard configurations such as rectangular, squared circular patches.
I've obtained wider bandwidth(!!), shift of working frequency (size reduction), similar Ellipticity Bandwidth but lower Gain.