The value of string efficiency depends upon the value of K. Where K is defined as the ratio of shunt capacitance to mutual capacitance.
The lesser the value of K is, the greater is the string efficiency and more uniform is the voltage distribution.
K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased. This can be achieved by using longer cross-arms.