REFLECTION COEFFICIENTS 267 
sults are given in Figures 16, 17, 18, and 19. These 
show that theory and experiment check satisfactorily 
for vertical polarization, but for horizontal polariza- 
tion the experimental values of reflection coefficient 
generally fall well below the theoretical values based 
on the assumption of a smooth sea. Whereas over sea 
a regular interference pattern existed, over land 
(Orlando, Florida) no specular reflection was ob- 
served. The lobe structure was absent in the observa- 
tions over land. 
Another experiment® carried out over land was 
performed using X-band waves between Beer’s Hill 
and Deal, New Jersey. The reflection coefficient of 
the ground is expected to change with the seasons 
on account of seasonal vegetation changes on the path. 
One series of measurements lead to reflection coefti- 
cients of 0.17 and 0.20 for horizontally and vertically 
polarized radiation respectively. 
Specular Reflection and Scattering 
Ordinarily neither the sea nor the land are ideally 
smooth, and one would expect always nonspecular re- 
flections which tend to perturb the interference pat- 
tern of the direct and reflected rays from a smooth 
surface. 
It has been pointed out® that the condition which 
has to be fulfilled for specular reflection to occur is 
that thé grazing angle y be such that sin y = A/g, 
where g is the wavelength of the sea waves. Clearly 
this is a kind of limiting condition and assumes the 
perfect regularity of the sea waves. It is seen that 
the above condition expresses the fact that the smaller 
the grazing angle, the smaller are the apparent irregu- 
larities of the sea and, if these apparent irregularities 
are much closer than the wavelength of the incident 
radiation, it is to be expected that specular reflec- 
tion should predominate. 
A direct consequence of this condition is that the 
echoes from a target, that is, a ship, will not be 
drowned by the clutter from the sea waves for large 
distances between the target and observer. Whereas at 
closer distances (large grazing angles) the echo from 
the target might be drowned by the irregular reflec- 
tion, i.e., scattering from the sea. To this effect, a report 
is quoted in which it is stated that ships could only 
be detected beyond a certain distance from the shore. 
It is also thought!’ that the discrepancies observed 
between the theoretically predicted and measured sea 
reflection coefficients (horizontal polarization) could 
be attributed to scattering. The irregular reflections 
have the effect of decreasing considerably the ratio of 
the successive maxima and minima of the interference 
pattern developed. The discrepancies referred to are 
those discussed by the Radiation Laboratory workers. 
{n this connection, Hckersley mentions some experi- 
ments by Hoyle on sea reflections in which no corre- 
lation could be observed on the voltage registered by 
two aerials a few inches apart. This tends also to sug- 
gest the existence of scattering from the sea. 
In another series of transmission experiments’? it 
was observed that the contrast between maxima and 
minima was poor. Here the experiments were carried 
out at 200 me over sea at a distance of 100 miles be- 
tween an airplane and a ground station. The diver- 
gence of the observed from the calculated values of 
reflection increases as the grazing angle increases. 
This seems to be in agreement with the results accord- 
ing to which the sea surface may be considered as 
formed by a number of corrugations which, for small 
grazing angles, appear to be so close together that the 
reflection is mostly specular. 
As to the frequency variation of scattering one 
would expect more and more scattering with increas- 
ing frequency. 
The effect of uneven ground on the reflection co- 
efficient was investigated by the British workers al- 
ready mentioned.1? The reflecting ground consisted 
of an artificially prepared series of uniform ridges 
placed along, across, and at 45° to, the direction of 
TABLE 8. Reflection coefficient of ground ridged at 45° with the direction of transmission. \ = 9 cm.!,? 
Grazing Vertical polarization Horizontal polarization 
angle, Level D=60cem D=120cem D =120cm Level D=60cem D=120cm D = 120cm 
degrees |estimate h=14em h= 10cm h= 5cm estimate h =14em h= 10cm hkR= Scm 
22 0.08 0.07 0.09 0.13 0.65 0.14 0.18 0.30 
36.5 0.13 0.04 0.05 0.07 0.51 0.04 0.06 0.16 
46.5 0.22 0.04 0.04 0.04 0.45 0.07 0.06 0.10 
TABLE 9. Reflection coefficient of ground ridged along or across the direction of transmission. > = 9 cm. 
; Vertical polarization Horizontal polarization 
Grazing Along Across Along Across 
angle, Level D=60cem D=60cm D=120cm Level D=60cm D=60cm D=120cm 
degrees | estimate h=14cem h=16cm h= 12cm estimate h=14cem h=16cm h= 12cm 
12 0.23 0.20 0.10 0.30 0.86 0.4 0.2 0.4 
22 0.06 0.03 0.05 0.12 0.76 0.07 0.10 0.18 
36.5 0.28 0.03 0.02 0.06 0.64 0.10 0.12 0.16 
46.5 0.36 0.04 0.08 0.08 0.58 0.04 0.08 0.14 
