TRANSMISSION EXPERIMENTS 27 
very wide fluctuations of intensity, the rate and 
amount again increasing with the frequency. 
In the course of these experiments a certain amount 
of meteorological study was carried out and fore- 
casting of propagation conditions was done on a 
tentative basis. The general results again fore-shad- 
owed the more complete data obtained by later 
studies, and a description of the details will be 
omitted here. 
Similar experiments were carried out simultane- 
ously by the Bell Telephone Laboratories [BTL] on 
optical paths near New York City. The wavelengths 
employed were 10, 6, and 3 cm.1®°!74 Here we find 
clearly established the different signal or fading 
types that are described in detail below. 
A very extensive program of transmission measure- 
ments was carried out by the Radiation Laboratory 
of Massachusetts Institute of Technology [MIT]. 
The meteorological records were made in cooperation 
with the U. 8. Army Air Forces. The first measure- 
ments were made in 1942, and experiments on a very 
large scale were carried out in 1944. 3:1 12) 188,183 Two. 
optical transmission paths were operated in 1943, 
a 22-mile path over the sea and a 45-mile path over 
land. A 10-cm continuous signal was used, and the 
strength was monitored by means of thermistors. The 
antennas were dipoles with 30-in. parabolic reflec- 
tors. The received signal was automatically recorded 
on meters having a range of 60 db. The signals 
received were correlated with meteorological observa- 
tions, the results of which will be given below. 
In the spring of 1944 a new over-water transmission 
path was installed which was operated simultaneously 
with the 22-mile one. This path was nonoptical, 41 
miles long, and crossed Massachusetts Bay from the 
southern tip of Cape Ann to the northern tip of Cape 
Cod near Provincetown. Transmission over this path 
was carried on with 256-cm waves, 10-cm S band, 
3-em X band, and 1.25-em K band. The 256-cm 
equipment used Yagi antennas and operated with 
continuous waves. The microwave transmitters used 
pulses with a repetition frequency of 700 c and used 
parabolic reflectors as antennas. 
The transmitter for the short path was about 120 
ft above mean sea level, and the transmitters for the 
long path were at a similar height. The two receivers 
were about 136 and 30 ft above mean sea level. The 
transmitter power was monitored and continuously 
recorded during the experiments while the receivers 
had automatic frequency control with apparatus 
which searches for the signal if it is lost. The auto- 
matic gain control of the receivers was arranged to 
give a spread of the signal over 70 to 80 db. The 
receivers were directly calibrated by means of signal 
generators and a very close check was kept on their 
performance throughout. The rectified output of all 
receivers was fed directly into recording milliammeters. 
Coincident with the operation of these transmission 
paths there was a very extensive meteorological 
program determining sea and air temperatures and 
atmospheric humidities by means of fixed installa- 
tions, captive balloons, ships, and airplanes. The 
distribution of the refractive index along the trans- 
mission path was thus known in considerable detail 
during practically the whole course of the experi- 
ments. Concurrently with ‘these measurements, a 
program of forecasting the transmission conditions 
was carried out. 
RESULTS 
The results obtained on the various transmission 
paths on the east coast of the United States are 
rather closely similar to each other, and the graphs 
presented here may be taken as being characteristic 
of all of them. 
Figure 3 shows the signal types observed at the 
microwave frequencies, S band and X band. The 
first type is well above the standard level with high 
signal on the average. It has roller fades with periods 
of from 2 min to an hour or so which may go down 
to the minimum detectable level. These periods are 
generally shorter at any time on the X than on the 
S band. When this type of signal was present on the 
S band, it was almost invariably present on the X 
band and on both the short and long paths. It always 
occurred simultaneously on the high and low receivers 
at any frequency. 
The second type is high and steady at anywhere 
from 5 to 30 db above the standard, generally higher 
on the X than on the S band. Most of the time this 
type occurred simultaneously on both bands, but 
there were some occasions when the S-band signal 
was of the high and steady type while the X one 
was of the first type, high with roller fades. 
The third type of signal is about standard and 
fairly steady which may be a limiting case of the 
high and steady variety. It does not necessarily occur 
on both frequencies and on both high and low © 
. receivers at the same time. 
The fourth type is standard on the average, with 
scintillations of more than 10 db. The reason for the 
difference between this and the preceding type has 
not yet been established. The scintillations may 
occur on either the S or X band while at the same 
time the other signal is steady. 
The fifth type, known as “blackout,” is far below 
standard and shows strong scintillations. In general 
- it occurs simultaneously on both frequencies, both 
paths, and on both high and low receivers. 
Figuze 4 shows a similar set of signal types as 
observed with 256-cm waves. These are distinct 
from those observed at the microwave frequencies 
not only in appearance but also in times of occur- 
rence. In general no relation has been found to exist 
between the signal type at this frequency and that 
observed simultaneously on S or X band, although 
on rare occasions such a relation is indicated; the 
