Whilst thinking of how to word it correctly, i thought sod it, copy and paste!
"Radar altimeter antennas have a fairly large main lobe of about 80° so that at bank angles up to about 40°, the radar detects the range from the aircraft to the ground (specifically to the nearest large reflecting object). This is because
range is calculated based on the first signal return from each sampling period. It does not detect slant range until beyond about 40° of bank or pitch. This is not an issue for landing as pitch and roll do not normally exceed 20°." (From Wikipedia).
https://en.wikipedia.org/wiki/Radar_altimeter