If you go out to a proper antenna test range you will see the antenna under test up off the ground at least 1/4 wavelenght to reduce ground distortions to the pattern. We are interested in airborne range, so unless you plan to fly at 100 meters distance at less than 6 feet up I don't see the placement on top or bottom as a problem. Yes, the ground range check will be affected, but the flying range will be more dependent on metallic and active elements within the plane such as wires and pushrods in proximity to the antenna. For the record, I fly scale and install the antennas internally. (Most full size aircraft have the antenna elements inside a housing too)

Be more concerned of the metal and active wires in proximity to the antenna than if the antenna is out the top or bottom in flight.

quint