One of the things that will cause nulls in the supposed
omni-directional pattern of an antenna is antenna currents on the
shield. This comes about whenever the coax lead, on non-metal aircraft,
does not come away perpendicular from the antenna, or a balanced
antenna is fed unbalanced from a coax. It is always wise in these
installations to place one or two clamp-on ferrites, such as those
Radio Shack sells, to the coax as close as possible to the antenna.
Another way is to form a balun by running the coax up one leg of the
antenna through a tube or a piece of sheet aluminum covering the coax
and attached to the leg. Leaky coax and connectors will also add to the
over-all pattern, giving additions and cancellations. As Jim pointed
out, these nulls, which are frequency-sensitive, will occur as various
aspect angles around the plane. It's interesting that you can have a
signal running on the outside of a coax that is different from the one
on the inside! Skin-effect and internal coupling, you know! Will
wonders never cease! Paul