When will it read -20ft, that is negative radio altitude of 20ft?
It obviously means you are flying 20ft underground !
No, its to do with instrument calibration delays. On a given installation it may read say +/- 4 ft (with a/c on the ground) the actual value displayed depends on the antenna slant distance (height) above ground. In a specific installation instrument setup software may allow you to calibrate out that delay to near zero to represent wheel height - which,then, if repeated on a shorter legged a/c it would read negative.