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# array antenna amplitude tapering

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#### dvdb

##### Newbie
Hello
I am designing microstrip array antnna.
Number of the array is 4, and amplidute distribution of each array is 0.3, 1, 1, 0.3(W).
I want to design divider for feeding of each array antenna like below.
For designing divider, I want to know how to calculate the difference between S21 and S31 in dB scale.

Best regards,
Thank you

As I see you want to reduce the amplitude of the signals going to antennas on ports 2 and 5 by increasing the length of the feeding lines.
This is wrong, because you will change the phase also, and you don't want this.
All 4 lines must have the same length, and place some resistive attenuators on 2 and 5 to reduce the amplitude.

I increased the feeding line of port 2 and port 5 to keep the same phase for each array.
What I want to know is how to calculate difference between S21 and S31 to meet amplitude ratio of 0.3 : 1.
For example if the difference between S21 and S31 is -7dB, then if S21 is -5dB, S31 must be -12dB.

You can make a lossless distribution network by utilizing transmission line transformers. The intended phase relation (e.g. equal phase as suggested by vfone) must be defined and implemented in the design.
--- Updated ---

What I want to know is how to calculate difference between S21 and S31 to meet amplitude ratio of 0.3 : 1.
For example if the difference between S21 and S31 is -7dB, then if S21 is -5dB, S31 must be -12dB.
Are you asking how to calculate dB (trivial) or how to design the transmission line transformer for 0.3:1 ratio? By the way, post #1 says power ratio, now you are asking about amplitude ratio.

Thanks for helping me.
I am asking how to calculate dB (trivial) for 0.3:1 power ratio. (Sorry, not amplitude)
If I know the difference in dB, I can design transmission line transformer.
I mean the power ratio of 0.3:1

power ratio log10(0.3)*10 = -5.23 dB

Just an observation: Both amplitude/power distributions and phase distributions are very important in array performance. Amplitude/power tapering is usually done to fool with sideline level and phase is manipulated usually for scanning. You do not say describe your objective. Take care as arrays can be tricky, particularly when element patterns and spacing is in the game.

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