394 AUSTIN: RADIOTElvEGRAPHY 



their wires not too widely spaced, can be very approximately 

 represented by the formula 



c = (4Va + 0.88 a/h) X io~^ (i) 



where c is the capacity, a the area, and h the mean height in micro- 

 farads and meters. For antennas having a length / more than 

 eight times the breadth b, the above formula must be multiplied 

 by an elongation factor, and we have 



c = (4Va + 0.88 a/h) (i + 0.015 l/b) X io~'' (2) 



Equation (i), while derived empirically, is in reality the sum 

 of the usual expressions for the capacity of a disk in space and 

 that for a two plate condenser, disregarding edge effect. These 

 equations can be depended upon to give results correct in general 

 to 10 per cent for the antenna top, to which must be added the 

 capacity of the downleads and that due to metal towers, etc. 

 The poorest agreement is found in the case of umbrella antennas, 

 where the amount of wire is often not sufficient to give full capacity. 



The closeness of wire spacing required to give approximately 

 full capacity, differs very much with antennas of different shapes 

 and sizes, the required spacing being closer the smaller the an- 

 tenna. For long parallel wire antennas, this may be calculated 

 from the Bureau of Standards formula already mentioned. In 

 the case of some of the larger antennas, remarkably few wires 

 are required. For example, on a certain triangular antenna 

 about 300 meters on a side, flat tops composed of five wires about 

 one meter apart, strung around the sides of the triangle gave 

 nearly 90 per cent of the capacity obtained when the whole tri- 

 angular area was filled in. With parallel wire antennas of medium 

 dimensions, a spacing of one meter will generally insure over 90 

 per cent of the possible capacity. 



Table i gives some observed values of capacity for elongated 

 parallel wire antennas, and a comparison of the capacities calcu- 

 lated according to the Bureau of Standards formula and equa- 

 tion (2). The data for calculation are given below table i. 



Table 2 gives observed values of capacity for antennas of various 

 shapes compared with the values calculated according to equa- 

 tion (i). 



