**Z**_{E }= Z^{2 }/ Z_{A}

**Z**_{E} = Impedance of
the output

**Z**_{A} = Impedance of
the antenna, in most cases 50 Ohm

**Z = Impedance of the matching
line with a length off odd multiples of λ/4**

__Example for stacking two Yagis:__

**The pieces L1 and L2 are the
quarterwave matching section, they must rise the 50 Ohm of each antenna to
100 Ohm at the connection point, because both resistances in parallel give
the wanted 50 Ohm for the coax to the station.**

**With the formula above, the matching
line must have an impedance of 70,7Ohm. There is no cable with that
impedance, therefore we use 75Ohm-coax, which is customary at the trade.
The additional SWR for the mismatch is < 1,15,**