As a general rule for a yagi antenna (assuming a fixed number of
elements) the longer the boom length the greater the gain. Forced by
economic matters yagis are more often squeezed into a smaller boom size
than is optimum for best performance. Hence I don't think it is a any
problem for the designers of the Step IR to optimize the performance of
a ten meter antenna using the element spacing and boom length of a 20
meter antenna.