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Questions about microstrip array antenna

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promach

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1. Why is 100 ohm transmission line used in this microstrip array antenna ?

2. Why A quarter-wavelength transformer of 70 ohm is used to match the impedance line of 100 ohm to the impedance line of 50 ohm. ?

3. Would a butler matrix be better than these simple impedance transformers in terms of reducing MIMO diversity issue ?

4. Is this stacked array antenna difficult to realize ?

microstrip_array_antenna.png

microstrip_stacked_array_antenna.png
 

1. It is arbitrary chosen value, constrained by manufacturing process and lines placement. Probably via in the center is forming t-junction from 50 Ohm line on the other side of substrate.
2. They can't continue divide 100 Ohm line with t-junctions, because it will result in 200 Ohm lines, which may be too thin to manufacture. So before each new T-junction they lower impedance from 100 Ohm to 50 Ohm with Sqrt(50*100)≈70 Ohm. Point after 70 Ohm line is 50 Ohm virtual impedance point, which is immediately divided to two 100 Ohm lines
3. I do not know, but butler matrix is too big and requires more RF hardware
4. Not difficult, especially with help of computer simulation.
 
First of all, some of the arrows shown in the first picture are pointed on the wrong stub.
Here you can find all the equations and how this kind of stub splitter for patch array antenna works:
 
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