CkAP. 10] 



ELECTRICAL METHODS 



655 



which is first oriented in the plane of polarization and' then rotated until 

 minimum signal is obtained. The axis of the double antenna is then in 

 the direction of the minor axis of the eUipse. Ninety degrees from this 

 position a maximum signal will be observed. The ratio of the maximum 

 and minimum signals gives the ratio r. The inclination of the antenna in 

 the position of minimum signal gives the tilt angle of the ellipse and, 

 therefore, the ratios of the horizontal and vertical components and the 

 phase shift between them. From these, the effective ground conductivi- 

 ties and dielectric constants may be determined by trial and error with 



k.'tf 



MtitfSurfac* Lai/tr 

 OJm tkkk; Pj' id* if em 



Sround Waftr (pt •Z1o*elmcm. /e-80) at: 

 5 m I 50m \ 100 m 



No 6round water 

 Pf • to* Sicm:k,-Z 



Fig. 10-20. Ellipses of polarization for surface and ground waves, for various fre- 

 quencies, and for various conductivities and dielectric constants (after F. Hack). 



the help of curves referred to in the last footnote. Changes in dielectric 

 constants are of less effect than changes in conductivity. For both long 

 and short waves it is observed that with an increase in conductivity the 

 horizontal component vanishes and that the yhase shift approaches 4^°. 

 The ground wave referred to before may be considered as a surface wave 

 having penetrated into the ground, with concomitant modification of 

 wave front and polarization due to the electrical properties of the surface 

 strata. The ellipse of polarization takes different forms, depending on 

 whether the surface beds are dry to the depth of penetration, a moist 



