302 RADIO WAVE PROPAGATION EXPERIMENTS 
According to Van Vleck, in the microwave region out- 
side of any resonance region, the refraction[(n’—1), 
n being the refractive index] or (e,—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 O, (0.25 em and the 0.5-cm. 
band) and H,O (—1.30 cm). The refractive index of 
the atmosphere free of condensation should be con- 
stant throughout the microwave region. Tlie refrac- 
tive index for infinitely long waves or the static dielec- 
tric constant can be 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 region. 
LABORATORY MEASUREMENTS OF 
DIELECTRIC PROPERTIES* 
The working committee of Ultra Short Wave Panel 
has presented a report dealing with the measurement 
in the laboratory of 
1. The dielectric constant and loss angle in super- 
heated steam for wavelengths in the S, X, and. K 
bands. 
2. The dielectric constant and conductivity of bulk 
water for the K band. 
These experiments have been carried out by Saxton. 
The method adopted on S and X bands in (1) was to 
allow superheated steam to isue freely through a 
resonator into the air. The pressure throughout the 
apparatus was accordingly the atmospheric pressure. 
The temperature of the steam was measured before 
tBy F. Hoyle, Ultra Short Wave Panel, Ministry of Supply, 
England. 
entering and after leaving the resonator. The temper- 
ature difference between these measurements was no 
more than 2 C, so it seems clear that no condensation 
of water droplets could occur within the resonator. 
A somewhat different technique was employed for 
X band in that the resonator was replaced by a wave 
guide. If ¢« is the dielectric constant, then the best 
representation of the results is obtained by plotting 
(e—1) against 1/7, for each wavelength where T 
is in degrees absolute. It was found that the results 
for different values of T fitted very well to a straight 
line as they should theoretically, but the value of 
(e—1) for all values of T was found to be systemati- 
cally about 10 per cent less than would have been 
expected on the basis of previous measurements of the 
dielectric constant of steam at much longer wave- 
lengths. This discrepancy was found on X and K 
bands. The reason for the discrepancy is not yet under- 
stood. It is known, however, that the reduced value of 
(e—1) does not arise from strong dispersion occur- 
ring in the microwave band since there is no evidence 
of any abnormal absorption. 
The method adopted in (2) was to measure first the 
attenuation factor of radiation passing through water. 
This was done by placing a transmitting horn above 
a large shallow trough and a receiving horn below 
the trough. The attenuation factor was measured im- 
mediately by varying the depth of the water in the 
trough. Second, the reflection coefficient of electromag- 
netic waves incident normally on a plane surface of 
water was measured. These two observations were 
sufficient to determine the dielectric constant and 
conductivity of water. The results obtained for a 
wavelength of 1.58 cm were: 
Dielectric constant about 40; 
Conductivity about 4.10 esu.14 
The same values were obtained for both tap water and 
distilled water, showing that the presence of salts in 
the water had little effect on the value of the con- 
ductivity. 
