Turns out I got prematurely excited on this issue without having all the
data. Unlike the Ryan 9900A, the B is highly directional with 4
receivers vs 2 in the A model and the antennae are quite different also.
Oh well, thought I could save a few bucks and I should have known
better.

In any event, it turns out that the 9900A is incompatible with the MX20
display so my plane is being shoved into yet another higher cost
bracket! And yes, their antennae need to be mounted top and bottom with
ground planes. According to Ryan, the ground plane issue is neatly
solved by spraying conductive paint on the skin surface with the antenna
in physical contact with the paint.

Angier Ames