It is common for a monopole antenna on a coax feed line without a ground plane to exhibit weird radiation patterns & behavior like this. Walkie-talkies use monopoles but their feedlines are vanishingly short so they escape these effects. In a metal airplane, the coax shield is connected to the fuselage skin to form a ground plane or counterpoise and the antenna behavior is thereby tamed and made predictable.

The usual way to furnish a fuselage-mounted monopole with a counterpoise in a nonmetallic plane is to glue a square of copper sheeting to the inside of the skin and mount the antenna in the center of the square, with the coax shield connected to the copper.

You can also glue long strips of copper tape in a radial pattern extending outward from the antenna mount to accomplish the same effect.

In either case, the dimensions of the copper sheet or the lengths of the copper radials need to be adjusted to get the best electrical match between the feed line and the antenna, which will suppress radiation from the feedline shield. This is called pruning and shortwave enthusiasts who make their own monopole antennas have to do this.

Suppressing radiation from the feedline is important because it is accompanied by high voltages on the shield and significant amounts of radiation inside the fuselage. You can get bad shocks and RF burns if you touch the shield, and the radiated energy will seriously disrupt other electronic devices in the cockpit.

It is common for an unbalanced monopole antenna on a coax feed line without a ground plane to exhibit weird radiation patterns & behavior like this. Walkie-talkies use monopoles but their feedlines are vanishingly short so they escape these effects. In a metal airplane, the coax shield is connected to the fuselage skin to form a ground plane or counterpoise and the antenna behavior is thereby tamed and made predictable.

The usual way to furnish a fuselage-mounted monopole with a counterpoise in a nonmetallic plane is to glue a square of copper sheeting to the inside of the skin and mount the antenna in the center of the square, with the coax shield connected to the copper.

You can also glue long strips of copper tape in a radial pattern extending outward from the antenna mount to accomplish the same effect.

In either case, the dimensions of the copper sheet or the lengths of the copper radials need to be adjusted to get the best electrical match between the feed line and the antenna, which will suppress radiation from the feedline shield. This is called pruning and shortwave enthusiasts who make their own monopole antennas have to do this.

Suppressing radiation from the feedline is important because it is accompanied by high voltages on the shield and significant amounts of radiation inside the fuselage. you can get bad shocks and RF burns if you touch the shield, and the radiated energy will seriously disrupt other electronic devices in the cockpit. Baluns are commonly used by ham radio guys to block "RF in the shack" but their design and use is something of an art.