TRANSMISSION EXPERIMENTS OVER MASSACHUSETTS BAY 



supplied :ippi'oxiiuatcl\' 1 w to a paraljuluidal aiiteiiiia 

 30 ill. ill diameter. 



Tlie transmitter site for the otlier trausiiiissiou path 

 aiul the trucks liousiiig tlie tM'o radar sets were at 

 Race Point (Proviucetowii). The radar sets operate 

 on the 8 and X bands and are approximately 50 ft 

 above mean sea level. They both have antenna diam- 

 eters of 4 ft and a ratio of transmitted power to 

 minimum detectable power of approximately 167 db. 

 These sets were operated from August 1st through 

 October 20th. The 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 lip the coast line; in addition, jilnti iiosiliuii 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 

 rccei\ed ]iower. 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 SCE-371 

 tower. The house at the foot of the tower ser\ed as 

 operations headquarters, while the top house con- 

 tained the transmitters. The 117-mc antenna was a 

 five-element 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 

 liorn. Polarization was horizontal for practically 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 mc was one from an 

 SCR-624 VHP (very high frequency) communica- 

 tion set. Its frequency was quartz-crystal controlled. 



and it delixered aj)j)i'oximalcly 10 w of c-w power 

 into a balanced line connected to the Yagi antenna. 

 'I'lio outjiut power of tlie transmitter was monitored 

 continuously on an Esterline-Aiigus recording niil- 

 liammeter. 



The S- and X-band transmitters employed pulsed 

 magiietrons operating at a pulse recurrence frequency 

 of 700 c with a pulse length of 1.5 /i sec and a peak 

 pijwer 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 fivquency specti'um of the pulses by 

 means of the spectrum analyzer. 



Both S- and X-l)and transmitters were provided 

 with continuously recording monitors operating Ester- 

 line- Angus recording milliammeters. Several types 

 of monitor circuits were employed duiing the course 

 of the program, but tbe one wliich pi'oved most satis- 

 factory employed a thermistor bridge coupled by 

 means of wave selector, or directional coupler, to tlie 

 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, fn addition to recording 

 of average p(j\\'er output Ijy frequent checking of 

 spectrum and high-voltage pulse, the cathode current 

 of the magnetrons was also recorded. 



The K-band transmitting equipment differed from 

 the S- and X-liand 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 mc but no automatic frequency control [AFC], 

 with the consequence that it required constant attend- 

 ance to produce a satisfactory record. The receive)' 

 for the Deer Island circuit was a narrow-band c-w 

 receiver of the type used in last year's experiments 

 and described iu reference 1. 



