Quote Originally Posted by W9JEF View Post
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Looks can be deceiving. The NVIS loop and DRRR are both
constructed of large diameter conductors because of the
lower radiation resistance they share. But other than that,
they have little in common. The DRRR requires a ground plane,
and is vertically polarized, the exact opposite requisite for NVIS.

Although a vehicle top is usually made of metal, a loop antenna,
by its nature, does not not require a ground plane.
Depends on how it's being fed. A tuned loop design can drive the loop against a ground via series feed (resulting in vertical polarization) or via a balanced feed, resulting in horizontal. In the case of a balanced feed being worked very close (in terms of wavelength) to a ground plane, there will be a significant high-angle radiation lobe.

While not a true DDRR, for purposes of discussion the latter method is very similar in concept.

An ideal implementation would use vacuum relays to change the feed method and a pair of motor-driven capacitors to resonate the loop in each mode.