PROPAGATION IN S AND X BANDS IN LOW-LEVEL OCEAN DUCTS 



45 



legardiiig the effect on the two IjancTs was that on S 

 band antennas as liigh as the experiment would allow 

 gave the highest signal strengths. On X band, on 

 the other hand, the lowest antenna heights which were 

 available usually ga\e the strongest signals. 



Figure 18 is an S-band run made on March 15. It is 

 a coniposite run containing the results of both the 

 outward and the inward runs. Several of the curves 

 have been omitted for clarity. The highest curve is 

 for a combination of a 46-ft transmitting antenna 

 and 9-1-ft receiving antenna. The lowest curve is for 

 the two lowest heights, 16 and 14 ft. The slopes of the 

 curves are rather steef) for the first 80 miles or so, 

 the signal declining considerably less rapidly there- 

 after. Also, the variation of the signal with height is 

 shown here to be in the order in the extremes between 

 25 and 30 db. This interval from 80 to 50 db shows a 

 difference between the two extremes of 30 db. To trans- 

 late that into a radar situation, double that difEerence 

 to get a difEerence of 60 db, showing that on S band 

 the higher antenna combinations would provide con- 

 siderably better coverage for targets in the order of 

 100 ft high and with transmitters at the height of about 

 50 ft. Stated another way, the highest antenna com- 

 bination would provide coverage beyond that obtain- 

 able with the lowest in the order of 30 miles. 



There is as yet no reasonable explanation for the 

 extremely slow decrease in signal beyond 80 miles. 

 This feature is very distinctive in the S-band curves. 

 For the X band, it is generally not discernible except 

 on a few runs toward the extreme range portion. The 

 rate of decrease of signal with range in the region 

 inside 80 miles would be exponential if there were a 

 straight line on this figure. Considering it to be so, 

 averaging over a number of runs gives roughly 0.8 db 

 per nautical mile. That decrease is the total amount, 

 the 1/R variation not having been extracted from it. 

 Attempts to do so show that the resulting curve does 

 not, in a plot of this sort, fit a straight line as well 

 as the original values themselves, but if the 1/i? value 

 is taken out of the power relation the average attenua- 

 tion is then roughly between 0.5 and 0.6 db per nau- 

 tical mile. In this region (beyond 80 miles), on the 

 other hand, the decrease of signal with range is con- 

 siderably less, being between 0.15 and 0.2 db per nau- 

 tical mile. No satisfactory explanation for this be- 

 havior has yet been derived. 



Figure 19 shows the X-band results for the same 

 period. Antenna heights of 16-ft transmitting and 



6-ft receiving produced the highest curve, the lowest 

 curve being obtained on a 46-ft to 94-ft combination. 

 Note that succeedingly higher antenna combinations 

 produced successively lower signal strengths. There is 

 some variation, but when the curves are smoothed 

 to a straight line the attenuation is on the order of 

 0.33 to 0.5 db per nautical mile. Eemoving 1/B re- 

 duces the attenuation to roughly 0.2 db per nautical 

 mile. There is no sharp bend in the curve at about 80 

 miles, as was the case on the S band. The lo^vest (16- 

 ft to 6-ft) antenna combination showed more than 

 35 db greater signal strength than the highest (46-ft 

 to 94-ft) combination. Considering again the radar 

 ease, it is found that the higher antenna provides rela- 

 tively poor coverage compared to the lower. In terms 

 of range for a given signal threshold, the difEerence in 

 favor of the lower antenna is about 80 miles. 



Figure 20 shows an X-band curve obtained during 

 April 10 and 11, when a transmitting antenna height 

 of 8 ft was available. Eeceived signal powers for 6-, 

 14-, 24-, 54-, and 94-ft receiving antennas are shown. 

 The curves are somewhat scrambled, but the general 

 result is that the lowest antenna again produces the 

 greatest signal, with increasing antenna height pro- 

 ducing progressively smaller signals. This was not the 

 case without exception, as can be seen in Figure 4, 

 where the 6- and 14-ft antennas exhibit comparable 

 behavior. In that case the maximum range was ob- 

 tained on the 14-ft antenna. The a\erage sloj^e in 

 Figure 20 is somewhat less than that shown in Fig- 

 ure 19. Exact averages of all the runs have not yet 

 l)een worked up. 



Figure 21 shows a plot of received signal versus 

 range, made on a 3-cm radar, using a PC boat as a 

 target. The highest curve was obtained with a 6-ft 

 antenna height, using a 48-in. dish to obtain greater 

 gain and range. The other run with 6-ft antenna was 

 made using the regular 29-in. dish. There is a consider- 

 able spread in the values of received signal due to the 

 difficulty of measurement. However, the significant 

 thing is that the maximum ranges obtained are in 

 accord with the indications given by the one-way trans- 

 mission results. Striking an average slope shows the 

 decrease of signal with range to be aljout 1.0 db for 

 each 1.5 nautical miles. 



The important conclusions can be summarized as 

 follows : 



1. The surface duct is very persistent. 



2. The duct is very effective in extending the ranges 



