Hi everyone , I'm recently interested in RTWO(rotary traveling wave oscillator). According to a thesis, the coupled strips are used in the circuits structure, and oscillation frequency can be calculated as follows,
fosc=Vphase/(2N*l),
l is the distance between two transistors, N represents  the cross coupled pairs or the inverters. However, the author provided a formula for the cut off frequency,
fc= Vphase/(pi*l)



So the my questions are,
1.why there will be a cut off frequency in coupled strips?
2.how to calculate the cut off frequency in coupled strips and why is the above equation?
3.why people use coupled strips in the RTWO rather than just transmission lines? Since the even propagation mode is the one we try to avoid , can we insert a ground strip between the two strips to decrease the coupling? I don't think the coupling between contribute any positive effects to the oscillation.  

if you have any suggestions or thoughts about these problems, please leave a message, thank you very much!