MASSACHUSETTS BAY 
MICROWAVE TRANSMISSION IN 1944 
GENERAL DESCRIPTION* 
HIS PAPER describes the general features of the 
work on atmospheric refraction undertaken during 
the summer and fall of 1944; other papers by members 
of this group will describe specific phases. The results 
described must be considered strictly tentative. They 
are the outcome of a hasty survey of a large amount 
of experimental data which ceased to accumulate 
only a short time before this report was prepared. 
Consequently, it has not been possible to do more 
than abstract the most obvious information. 
The principal objectives of the present program 
were: 
1. To study the modification of continental air by 
the ocean surface and from this study to improve the 
technique of forecasting modified index curves at low 
altitudes over water. The reason for the detailed 
meteorological study is that when beginning this work 
we believed that the existing ideas of the physical 
phenomena involved in producing Jow-level modifi- 
cation were not on a sufficiently sound basis to allow 
a direct analytical approach. 
2. To study experimentally one-way and radar trans- 
mission through the range of refraction conditions 
varying from substandard to trapping. Particular em- 
phasis was to be placed on wavelength dependence, 
and, when possible, information: was to be obtained on 
vertical coverage patterns under these various refrac- 
tion conditions. 
Radio Program 
The radio part of the project employed a combina- 
tion of one-way and radar apparatus operating over 
Massachusetts Bay. Two paths were chosen for one- 
way transmission; one was the 22-mile path? from 
Deer Island (Boston Harbor) to Eastern Point 
(Gloucester) and the second a 41-mile path farther 
from the shore line (Eastern Point, Gloucester, to 
Race Point, Cape Cod; see Figure 1). Over the 22- 
mile path, transmission was on S band, while on the 
41-mile path one-way transmission was on 117 me and 
on 8, X, and K bands. Radar sets on S and X bands 
were placed at the transmitter site for the latter path. 
On the short path the terminals were placed so as 
to give approximately grazing incidence, but on the 
long path the terminals were well below the horizon. 
At the transmitting terminal of the one-way circuit 
were two radar sets on X and S bands; from this 
location they could scan the New England coast line 
to measute signal strength from fixed targets. 
Note that the short path is close to the coast line, 
while the longer path is considerably farther away 
and is so located that approximately westerly winds 
undergo appreciable modification by the time they 
have reached the transmission path. 
Transmitters 
The transmitter for the short path was located at 
Deer Island about 120 ft above mean sea level and 
supplied approximately 1 w to a paraboloidal antenna 
30 in. in diameter. 
The transmitter site for the ofher transmission path 
and the trucks housing the two radar sets: were at 
Race Point (Provincetown). The radar sets operate 
on the S and X bands and are approximately 50 it 
above mean sea level. They both have antenna diam- 
“Figures and tables on pp. 493 to 511. 
*By D. E Kerr, Radiation Laboratory, MIT. 
APPENDIX 
TRANSMISSION EXPERIMENTS * 
eters of 4 ft and a ratio of transmitted power to 
ininimum detectable power of approximately 167 db. 
These sets were operated from August Ist through 
October 20th. ‘Che measurements consisted of 
hourly determination of the strength of echo from 
four specially selected targets and of recording maxi- 
mum detection range on fixed targets over water look- 
ing up the coast line; in addition, plan position in- 
dicator [PPI] photographs were taken at hourly 
intervals. The performance of the radar sets was care- 
fully monitored by appropriate means for determina- 
tion of transmitted power and minimum detectable 
received power. All echo signal strengths were meas- 
ured in absolute values with a signal generator 
coupled to the system. The records of signal strength 
from the four selected targets and those of maximum 
detection range were plotted and returned to the 
laboratory on a weekly basis. 
The tower carrying the one-way transmitting equip- 
ment consisted of the bottom half of a 100-ft SCR-271 
tower. The house at the foot of the tower served as 
operations headquarters, while the top house con- 
tained the transmitters. The 117-me antenna was a 
five-elernent Yagi array projecting horizontally from 
the forward corner of the top of the house. The X- 
and S-band antennas were paraboloids 4 ft in diam- 
eter, made of close-spaced grid work designed to 
reduce wind resistance. The feed for each of these 
antennas was a dummy-dipole array excited from the 
open end of a wave guide projecting through the 
vertex of the paraboloid. The K-band antenna was a 
paraboloid 2 ft in diameter, illuminated by a small 
horn. Polarization was horizontal for practidally the 
entire period of the program. 
All the microwave antennas were provided with a 
scheme for rendering them independent of rain. A 
blast of air from inside the house was injected into 
the wave guide at the transmitter by means of a 
blower. This stream of air effectively prevented ac- 
cumulation of a film of water on the inside of the 
guide feed. 
The transmitter used on 117 me was one from an 
SCR-624 VHF (very high frequency) communica- 
tion set. Its frequency was quartz-crystal controlled, 
and it delivered approximately 10 w of c-w power 
into a balanced line connected to the Yagi antenna. 
‘The output power of the transmitter was monitored 
continuously on an Hsterline-Angus recording mil- 
liammeter. 
The S- and X-band transmitters employed pulsed 
magnetrons operating at a pulse recurrence frequency 
of 700 ¢ with a pulse length of 1.5 » sec and a peak 
power’ output of approximately 10 kw. The. output 
pulse from the modulator was continually checked by 
the synchroscopes, and a check was made of the trans- 
mitted radio frequency spectrum of the pulses by 
means of the spectrum analyzer. 
Both S- and X-band transmitters were provided 
with continuously recording monitors operating Hster- 
line-Angus recording milliammeters. Several types 
of monitor circuits were employed during the course 
of the program, but the one which proved most satis- 
factory employed a thermistor bridge coupled by 
means of wave selector, or directional coupler, to the 
wave guide between transmitter and antenna. Daily 
calibrations of the recording thermistor bridge cir- 
cuits were made, providing a constant check of power 
output in absolute values. In addition to recording 
of average power output by frequent checking of 
spectrum and high-voltage pulse, the cathode current 
of the magnetrons was also recorded. 
The K-band transmitting equipment differed from 
481 
the S- and X-band equipment only in matters of 
unessential detail. 
Receivers 
The receiving terminal of the one-way transmission 
circuit is located at Eastern Point, Gloucester; the 
receivers were mounted in a 100-ft tower similar to 
the one at Provincetown. There were two sets of re- 
ceivers, one approximately 136 ft above mean sea 
level in a house at the top of the tower and the other 
approximately 30 ft above in a house at the bottom 
of the tower. The receiving antennas are identical 
with those for the transmitters. 
The K-band receiver was a superheterodyne spe- 
cially constructed for this purpose and put into opera- 
tion late in the experiment. It had a bandwidth of 
14 me but no automatic frequency control [AFC], 
with the consequence that it required constant attend- 
ance to produce a satisfactory record. The receiver 
for the Deer Island circuit was a narrow-band c-w 
receiver of the type used in last year’s experiments 
and described in reference 1. i 
The X- and S-band receivers deserve mention be- 
cause of their special characteristics, which were de- 
veloped to meet the requirements of this work. They 
are provided with AFC circuits arranged to search 
for a lost signal automatically. Having found the 
signal, the circuit locks the receiver in tune and con- 
tinues recording. These receivers have specially de- 
signed automatic gain control circuits providing es- 
sentially logarithmic response of 70- to 80-db range 
well spread across the recorder scale. The minimum 
detectable power for these receivers is approximately 
110 db below 1 w for both S band and X band, and 
the minimum signal required for satisfactory opera- 
tion of the AFC is approximately 105 db below 1 w. 
The latter figures are important for this particular 
setup, since they determine the usefulness of the re- 
ceivers in studying signal strength near or below that 
encountered under standard refraction conditions. 
The receivers were calibrated daily by means of’ 
signal generators coupled permanently to the wave 
guide between the antenna and the receiver through 
wave selectors with known fixed coupling losses. Very 
close check on: performance was maintained so that 
the receivers at all times gave an accurate indication 
of the absolute value of received signal strength. 
The arrangement of S- and K-band receivers in 
the house on top of the tower was similar to that in 
the lower house, but the K-band receiver was of a less 
sensitive type requiring no tuning. There was a 117- 
me receiver in the top house but not in the bottom 
house, and there was no receiver for the Deer Island 
circuit in the top house. 
The outputs-of all eight receivers were wired direct- 
ly into an Esterline-Angus recording milliammeter. 
With this arrangement one operator was able to keep 
continuous watch on the performance of all receivers 
and was required to climb the tower only when major 
adjustments of the top receivers were necessary. 
The Gloucester station was the control station for 
the radio network formed by all the stations involved 
in the project. The transmitter station at Province- 
town, each of the radar trucks, the fixed meteoro- 
logical stations, the boat, and one of the airplanes 
were all equipped to operate radiotelephone on 3.5 me, 
thus allowing rapid and efficient exchange of informa- 
tion essential to the operation of all units involved in 
the program. 
Meterological Program 
The meteorological phase of our program con- 
sisted of two main parts: (1) meteorological meas- 
