LABORATORY MEASUREMENTS OF DIELECTRIC PROPERTIES 



185 



IVALUES OBTAINED AT 3.01 CM ON 

 (TWO DIFFERENT SAMPLES 

 _VALUES OBTAINED AT 9.18 CM 



1.0 

 0.9 



0.5 

 0.4 

 0.3 

 0.2 



-40 -35 -30 -25 -20 -15 

 TEMPERATURE IN DEGREES C 



Figure 26. Absorption index (/cxlO+') versus tem- 

 perature for ice. 



Yonnker,-- using the standing wave ratio method 

 at 1.35 cm, found at about — 15 C 



£, = 3.3 and a = 0.011, or a = 0.013 mhos/m. 



These data can be compared with tliose obtained at 

 3.01 cm,*^ where 



£,. = 3.0G and £,; = 0.00080, o- = 0.00044 mhos/m. 



The difEerence at tliese two wavelengths between tlie 

 conductivities appears mucli too large and further 

 studies should clear up this discrepancy. The dielectric 

 losses in ice in the centimeter region are, however, 

 \cry snuill. 



At much lower frequencies the dielectric behavior 

 of ice is given in Figure 37. These data refer to a 

 temperature of — 13 C.*^ 



Attenuation Due to Watek Vapor 



In order to determine the attenuation due to water 

 vapor, Saxton endeavored to measure the refractive 

 and absorption indices of water vapor.*^ Using the 

 resonator Q method, he found that by passing from 

 9 to 3.3 cm the real part of the dielectric constant 

 changes from 1.0056 to 1.0051. According to a general 

 relation.ship connecting the real and imaginary parts 

 of the coniiDlex dielectric constant,'"' the indicated 

 variation of (e,- — 1) given by Saxton should be accom- 

 panied by a tremendous absorption by water vapor in 

 the microwave region as pointed out by Van Vleck.*" 

 This is contrary to the data available and rules out 

 tlie frequency variation of (t,. — 1) given by Saxton. 



1.0 



150 



50 



10' 



10° 



1 



10* 10' 10' 10' iC 



FREQUENCY IN C 



Figure 27. Loss factor and dielectric constant of ice. 



According to Van A^leck, in the microwave region out- 

 side of any resonance region, the refraction [(^r — 1), 

 n being the refractive index] or (£,. — 1), must be 

 appreciably constant over the microwave region to 

 account for the absence of any large absorption co- 

 efficients. 



The conclusion is similar in case of resonance 

 which occurs for both Oo (0.35 cm and the 0.5-cni. 

 band) and H^O (—1.30 cm). The refractive index of 

 the atmosphere free of condensation should be con- 

 stant throughout the microwave region. The refrac- 

 tive index for intinitely long waves or the static dielec- 

 ti'ic constant can lie used here. In the presence of con- 

 densation, clouds, fog, and rain, the attenuation is 

 increased considerably, and the refraction or (£,. — 1) 

 might then differ from the static value. But under 

 standard conditions, the refraction of the atmosphere 

 should not change by more than a few parts in a 

 thousand in the microwave reaiou. 



1"^ LABORATORY MEASUREMENTS OF 

 DIELECTRIC PROPERTIES* 



The working committee of Ultra Short Wave Panel 

 has presented a report dealing with the measurement 

 in the lalioratory of 



1. The dielectric constant and loss angle in super- 

 heated steam for wavelengths in the S, X, and K 

 baiuls. 



'By F. Hoyle, Ultra Sliort Wave Panel, Ministry of Supply, 

 England. 



