All this discussion about RDF overlooks the issue of
polarization. If you make an array of verticals
with a certain RDF (assuming noise comes from all
directions uniformly), the array will be better than an
individual vertical by the RDF factor. However, what
I have found is that a horizontally polarized antenna,
such as a low dipole frequently receives
considerably better than a vertical. In that
case, you would be better off using an array of
low dipoles. The reason why horizontal polarization
can be better is that the horizontal component of
terrestrial based noise is highly attenuated over
distance as a ground wave.
Rick N6RK
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