I have seen a lot of information regarding striplines and microstrips,   
but what about two traces routed as a pair without ground planes.  It is   
quite common in a Printed Circuit Board that there is a need to route a   
pair, like a tip and ring of a T1.  These should be a controlled   
impedance to ensure minimize reflections,  these also need to be void of   
ground or power planes to minimize noise and impedance to ground.  Does   
there exist an equation to determine the impedance between two traces   
without ground planes?  

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