Chapter 4 

 TRANSMISSION EXPERIMENTS AT ANTIGUA, WEST INDIES 



41 PROPAGATION IN S AND X BANDS 

 IN LOW-LEVEL OCEAN DUCTS 



*'^'^ General Description'' 



THE EXISTENCE of low-lyiiig ducts over the seas of 

 the world, particularly in the trade wind belt, has 

 been known for the past 2 years. Measurements made 

 by the British and by Washington State College and 

 the Naval Eesearch Laboratory have consistently in- 

 dicated the presence of duets ranging in thickness 

 from 20 to 50 ft in regions where the trade wind 

 followed a long over-water trajectory. These ducts 

 are known to vary in intensity and thickness with 

 wind velocity during the trade wind season. It was 

 considered advisable to investigate the possibility 

 that such ducts would permit greatly extended ranges 

 on surface craft and very low-flying aircraft by prop- 

 erly sited radar installations. 



Discussion by representatives of the Chief of Naval 

 Operations, NDKC, and the Naval Eesearch Labora- 

 tory resulted in organization of a project to make an 

 experimental investigation of meteorological and prop- 

 agational conditions in an area of the Caribbean 

 theater where such ducts are persistent, with a view 

 to determining their operational usefulness. It was 

 decided that a one-way ship-to-shore transmission 

 path over water would provide the most direct data 

 for analysis, and sucli a system was set up, using 

 transmitting and receiving equipment provided by 

 the Eadiation Laboratory. The transmitters were in- 

 stalled in a patrol craft assigned for the project, there 

 being no larger vessel available, with transmitting an- 

 tenna heights of 16 and 46 ft. 



The site chosen for the receivers at the land-based 

 end of the link was at Judge Bay on the island of 

 Antigua in the Leeward Island group of the British 

 AVest Indies. Antennas were installed on a tower 50 

 ft from the water's edge, at heights of 14, 24, 54, and 

 94 ft, for both S- and X-band receivers. 



Antennas for both S- and X-band transmitters were 

 installed on the patrol craft at heights of 16 and 46 

 ft. These consisted of parabolic reflectors arranged 

 to permit transmission forward or astern, so that 



'By^Lt. R. W. Bauchman, U. S. Naval Research Labora- 

 tory. 



transmissions could be made on both the outward 

 and inward legs of the runs. The S-band transmitter 

 peak power output was 42 kw, and its antenna pro- 

 vided a measured gain of 27 db. Output on X band 

 was 31 kw, the antenna providing a measured gain 

 of 29 db. Later in the experiment an S-band antenna 

 was installed at a height above the water of 8 ft. 

 Tests were made with this antenna on two runs. Ad- 

 equate switching arrangements to permit tests with 

 the difl:erent antennas were provided, and power out- 

 puts were measured by means of directional couplers 

 and thermistor bridges. 



Meteorological measurements from the ship con- 

 sisted of detailed temperature and relative humidity 

 readings taken on a rigging running from a boom 

 extending out over the water amidsliip to the yard- 

 arm about 46 ft above the water. Low-level sounding 

 equipment of "Washington State College design was 

 used for all meteorological measurements. Balloon 

 ascents to heights of 600 ft from the stern of the shif) 

 were also made when conditions permitted. Hourly 

 observations of sea temperature, wind, and sling psy- 

 chrometer readings from tlic bridge were made. It was 

 impossible to obtain satisfactory soundings on the 

 rigging or by use of balloons and kites when running 

 away from the tower into the wind because of the 

 large amount of water taken over the bow and the 

 resulting salt spray. Shipboard observations during 

 outward runs were therefore confined to the hourly 

 wind velocity, sea temperature, and sling psychrom- 

 eter readings. On return runs with the wind, bal- 

 loon and rigging soundings were made. It was neces- 

 sary to estimate the height above the surface for 

 readings taken below 10 ft because of the severe 

 pitching and rolling motion of this type of ship, and 

 therefore very few such readings were made. 



At the receiving end of the radio path, the antennas 

 for S band were 48-in. parabolic dishes with a gain 

 of about 30 db. The X-band antennas were 48-in. 

 dishes cut to 2 ft in the horizontal dimension to 

 broaden the horizontal acceptance angle. This was 

 done to eliminate the effects of minor deviations of 

 the ship from a radial course. These antennas had a 

 measured gain of 35 db. Midway in the experiment 

 an X-band antenna was mounted at the base of the 

 tower at a height of 6 ft, since results up to that 



33 



