  The effect of quenched random ferromagnetic bonds on the antiferromagnetic
correlation length of a two--dimensional Heisneberg model is studied, applying
the renormalization group method to the classical non--linear sigma model with
quenched random dipole moments. It is found that the antiferromagnetic long
range order is destroyed for any non--zero concentration, of the dipolar
defects, even at zero temperature. Below a line T ~ concentration, the
correlation length is independent of T, and decreases exponentially with
concentration. At higher temperatures, itdecays exponentially with an effective
stiffness constant which decreases with concentration/T. The results are used
to estimate the three--dimensional N\'{e}el temperature, which decays linearly
with $x$ at small concentrations, and drops precipitously at a critical
concentration. These predictions are compared with experiments on doped copper
oxides, and are shown to reproduce successfully some of the prominent features
of the data.
