Mechanisms of energy coupling between transmission lines?

Hello
I am currently working with transmission line filter design (stripline filter, to be specific)

I always see books/papers/other articles saying something like below when analyzing filters with parallel transmission lines:

(1) SL1 and SL2 couple the major portion of signal energy between the I/O ports at close to the center frequency (see Figure 1 in attachment)
(2) see Figure 2 in attachment.

and many other papers/books/articles

To me, this magical "coupling" seems to be a very helpful tool in designing transmission line filters. But I don't understand the mechanism behind it. And I couldn't find good references explaining it.

So could anyone kindly explain to me briefly why/how the EM energy can be coupled between parallel adjacent transmission line?
A QUALITATIVE explaination will be enough! I have prepared Figure 3 and 4 for your explaining convenience. Plz explain e.g. whether and why the arbitrary structures in Fig 3, Fig 4 can have a band-pass behaviour from energy coupling point of view.

Or could anyone suggest any good reference, which provides a QUALILTATIVE explanation? I feel quantitative analytical methods based on Maxwell's equations are too much for me now. An overkill.

This 'coupling' concept has been haunting me like forever. Plz help!!
Thank you so much!!
Adun

Re: Mechanisms of energy coupling between transmission lines

Hi can anyone recommend me a forum where I can ask the above question plz?

You can use distribute lumped elements to analysis the transmission line. because transmission line can be equivalent as RLCG cascade ciruit, and between adjacent two lines exists mutual capacitances and mutual inductances, and then the coupling effects appears. another way is using the microwave theory, because the line transmit signals, there would be electromagnetic fields around the lines, when the frequency is low, the distance between two adjacent lines is quite little compared to the wavelength, then the coupling effects can be neglected, but as frequency goes higher, then the distance between adjacent lines is quite big compared to the wavelength, then the energy of the signal carrier line constrains around the line, so the coupling effects can't be neglected. i think it is the mechanism of the energy coupling between transmission lines. hope it can help you