Tiny NVIS and DX antenna

[W9JEF]

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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.

[N8YX]

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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.

[W9JEF]

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.

To change from DRRR to a true loop would require
more than relays and motor-driven capacitors.
The DRRR "loop" axis is vertical, but a real loop's axis
is horizontal, with no ground plane.

[N8YX]

To change from DRRR to a true loop would require
more than relays and motor-driven capacitors.
The DRRR "loop" axis is vertical, but a real loop's axis
is horizontal, with no ground plane.

You might want to argue that point with Art Bell and a number of others who successfully employ horizontal loops at their stations.

[W9JEF]

You might want to argue that point with Art Bell and a number of others who successfully employ horizontal loops at their stations.

How large are these loops, and how high up?

The DRRR is definitely not what is considered a loop.