[TowerTalk] Interesting problem

[Mark Beckwith]

Hello sages of RF wisdom.  I have one for you.  Not being an RF engineer,
the solution escapes me:

I have two identical antennas on one tower at different heights.  They each
have feedlines coming down to the bottom of the tower.

If I wanted to feed these two antennas in phase, is there a way to figure
out how long a piece to add to the lower antenna's feedline so it is in
phase with the higher antenna?  The velocity factors of the feedlines are
unknown.  The feedlines are not the same material.

I have a dummy load, one of those Autek RF-1s and an MFJ antenna analyzer
with two meters (SWR and impedance).

Have at it.  Thanks in advance.

Mark, N5OT

PS If it matters, the antennas are for 10M and one is at 50 feet and one is
at 100 feet.