I see nobody has responded to your question.  I am struggle to know really what you are asking.  I "think" you are asking about an evanescent coupling mechanism to feed a resonator or an antenna.  Assuming this is the case, they are definately frequency sensitive.  You can read a bit about directional couplers as I think this is the method you are asking about.  The short story is that when to modes (think of two waveguides) are in close proximity and their evanescent fields overlap, they can begin to exchange power.  This can be used to form a splitter, feed an antenna, etc.

I have designed and done measurement for proximity-coupled microstrip antennas of various shapes taking feed line widths from 1 mm to 8 mm at different frequencies from 1 GHz to 6 GHz. There is no such upper or lower limit of the width, impedance matching is the main thing. If in computation/simulation return loss is less than 20 or 30 dB, then the feed width is acceptable. But be careful that the width of the feed line should not be less than the diameter of the inner conductor (0.2 to 0.3 mm)  of co-axial SMA connector.

Simple reason for more bandwidth in proximity couple feed is : this feeding technique involve more that one dielectric layer and therefore overall high/thickness of the substrate increase and therefore the bandwidth also increases. Another factor is since, multiple dielectric layers are involved the effective dielectric constant will reduce  therefore the over all Q factor also reduces (one can check simple relation between Q, Energy stored, and parallel plate capacitance).  I think these are the reasons that proximity couple feed give more bandwidth.