[Elecraft] OT: Beam info, please

[George]

Hello all:  

I have a three element Telrex 20 meter monoband (Yagi) beam at 75 feet.  I successfully added two 15 meter elements in between the three 20 meter elements in the same plane and on the same boom as the 20 meter elements.  As best I can measure, the addition of the 15 meter elements had no effect on the 20 meter beam.  ( no detectable swr change)  Both beams work very well.  

So here's my question:  I would like to remove the 15 meter elements and replace them with two elements for 30 meters, using a gamma match system.  Will the 30 meter elements have a detrimental effect on the 20 meter beam??  If so, I can move them up the mast and use a boom just for this new beam.  If I do that, what is the minimum seperation I should use?

Thanks 

George, n4ym
K2 # 4758
K3 # 340