Topband: Shunt feed question

[Marty Ray]

I am shunt feeding a 70 ft Trylon tower with a Tennadyne T12.10-30HD LPDA at 70 ft and a full size 40m rotatable dipole at 79 ft, (the top of the mast is ~85 ft). Both antennas have relays that electrically bond them to the tower when the shunt feed is in use. 

I have tried two shunt tap points, one at 65 feet and another at 45 feet. Using a Rig Expert AA-55 Zoom, the Rs measured a little over 100 ohms on the 65 foot version and 49 ohms on the 45 foot version. In both cases, adding the shunt capacitor caused Rs to drop by approximately 50 percent, (to around 60 ohms and 23 ohms respectively).

I expected Rs to not change much, if any. I tried a vacuum variable, an air variable and a silver mica. Same result. 

Has anyone seen this happen before?

Regards,
Marty N9SE