For a parallel plate transmission line, let $v$ be the speed of propagation and $Z$ be the characteristic impedance. Neglecting fringe effects, a reduction of the spacing between the plates by a factor of two results in
- halving of $v$ and no change in $Z$
- no changes in $v$ and halving of $Z$
- no change in both $v$ and $Z$
- halving of both $v$ and $Z$