If   the   wires   of  an   aerial   are   widely

spaced  apart,  the  capacity  is  greatly  increased,  and  the  total  inductance  somewhat  increased,
due  to  the  diminishing  of  the  mutual  inductance  between  adjacent  wires.  But  the  increase
of  capacity  exceeds  the  decrease  of  inductance,  and  the  natural  wave  length  will  therefore
be  increased  considerably.  In  practice,  it  is  the  custom  to  space  the  wires  so  they  will  be
mutually  inductive,  the  distance  between  adjacent  wires  not  exceeding  three  feet.
114.  Fundamental  Considerations. — In  the  design  of  a  wireless  aerial  the
important  points  to  be  considered  are:

(1)  The  aerial  wires  must  possess   great  tensile  strength  and  be   of  good
conductivity;

(2)  There  should  be  several  parallel  wires;

(3)  Adjacent  wires  should  be  spaced  2  to  3  feet;

(4)  The  wires  must  be  thoroughly  insulated  at  all  points  of  support;

(5)  If  possible,  the  aerial  should  be  erected  in  a  clear  space  and  at  least  a
half-wave  length  from  all  metallic  structures.
The  matter  of  antenna  insulation  is  extremely  important;  the  antenna  insula-
tors should  not  only  possess  high  specific  resistance,  but  they  should  also  be  of
considerable  length  to  prevent  the  high  voltage  current  discharging  over  them  to
some  nearby  metallic  conductor.  Particular  care  must  be  taken  to  insulate  the  free
end  of  an  aerial  because,  owing  to  the  fact  that  the  voltage  and  current  are  not
uniformly  distributed  in  vertical  aerials,  a  very  much  greater  potential  exists  at
the  top  than  at  the  base.  This  non-uniform  distribution  exists  to  some  extent  in
all  types  of  aerials,  but  is  less  manifest  in  the  flat  top  aerials  than  in  vertical  types.
