908 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



spaced corner reflectors whose relative reflecting properties are known. 

 The free-space attenuation is readily calculated and any measured at- 

 tenuation in excess of this represents absorption by the atmospheric 

 gases. 



A description of the method and the apparatus is followed by a dis- 

 cussion of data taken in the wavelength range 5.1 to 6.1 mm (which in- 

 cludes the long wavelength skirt of the oxygen absorption band centered 

 at 5 mm). These data, when compared with the theory,^ indicate that 

 the line-broadening constant of oxygen at atmospheric pressure is of 

 the order of 600 mc. Some rain and fog attenuation measurements at a 

 wavelength of 6.0 mm are included. 



METHOD 



The experimental setup is shown in Fig. 1. It consists of a high-gain 

 antenna for both transmitting and receiving and a pair of spaced corner 

 reflectors. Corner reflectors can be built to have good mechanical and 

 electrical stability, and their reflecting properties are relatively insensi- 

 tive to slight misalignments. The reflectors are mounted well above the 

 ground to ensure free-space propagation conditions. 



At the outset, the relative reflecting properties of the corner reflectors 

 are measured by placing them side by side at a convenient distance (cfi 

 for example) from the antenna. By alternately covering one and the other 

 with absorbent non-reflecting material and measuring the reflected sig- 

 nals, the relative effective areas are determined. The reflectors are then 

 separated as shown and consecutive measurements are made of the sig- 

 nals returned from each reflector. From these measurements, knowing the 

 distances di and d^ and the calibration of the reflectors, one determines 

 the attenuation over the path d2-di in excess of the free-space attenua- 

 tion.* This excess, in the absence of condensed water in the air, repre- 

 sents absorption by the atmosphere. 



The power received from the reflector at distance di is, 



A' A,'' 



Pi = Pt -tt^- Q(K dd 

 X di 



where A and Ai are the effective areas of the antenna and corner-reflector respec- 

 tively, and Pr is the transmitted power; Q(\, d\) is a loss factor which accounts 

 for atmospheric absorption. A similar relation holds for the power received from 

 the reflector at distance ^2 . The ratio of the received powers is then, 



'2 [aJ \dj 



f^= (t^) It) QlKid^"- d.)\ 



