the station was moved to Eniwetok Atoll, about 320 km from the Bikini
detonation locations. The Shot 6 detonation point was within line-ofsight, but the records obtained were of questionable value, probably
because of spurious pick-up on lead-ins,
Since most of the electromagnetic energy recorded from a nuclear
detonation is in the VLF band, the ground waves recorded at 320 lam have
substantially the same shape as a closer recording with, of course, a
decrease in magnitude. For example, from References 8 and 9, decreases
in ground wave amplitude to be expected over a sea water path between
two locations 23 km and 320 km from the origin vary with frequency as
follows: 1 kc, 27 db; 10 kc, 24 db; 500 kc, 29 db; 2 Mc, 33 db; and
5 Mc, 39 db.
At distances, where no ground wave was recorded, the wave shape
was an additive combination of more than one sky wave, and the result
was usually similar to a damped sinusoid with higher frequencies atten~
uated relatively more. From Reference 10, for a typical daylight path,
sky-wave attenuation in decibels per 1,000 km is about 20 at 1 kc,
about 2 at 10 ke and then rises gradually to about 50 at 1 Mc.
Field strength, especially at distant points, is only a very ap—
proximate measure of yield. However, a rough estimate of yield within
about an order of magnitude may be obtained from broad-band fieldstrength measurements. Figure 5 is a plot of field strengths from the
Castle shots for stations on an east-west path recording broad-band
waveforms. Probable curves are drawn in for three of the stations.
Figure 6 is a plot of field strengths of three stations on a north-south
path. Field strength data from Guam, Shemya and Pt. Barrow are generally low. The reasons are not definitely known and these anomalies are
being investigated. Contributing causes may be interference between
sky-wave modes, ionospheric absorption, ground constants, and, in the
case of Pt. Barrow, attenuation due to auroral absorption. It is believed that the Shemya field strength data, additionally, may be low
because of local conditions at the receiving site.
Re
»
Figures 7 through 12 are selected Castle electromagnetic
records (References 4,5).
Records from all shots recorded are shown
from Iniwetok; Guam; Maui, T. H.; Boulder, Colorado; and Ft. Belvoir,
Virginia to illustrate changes with distance. Additionally, some typical records from other locations are reproduced.
At Enivetok, for Shots 2, 3, 4 and 5, the sweep time was long
enough to record not only the ground wave, but also one or more sky
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waves (Figures 8, 9, 10, 11). At Guam, for Shot 3 (Figure 9), several
sky waves are shown.
Close-in waveforms are broad band. The oscilloscopes had the
capability of faithfully recording frequencies in the range from essen-
tially de to 13 Mc.
This was more than adequate, since the maximm
equivalent frequency recorded was about 3 or 4 Me. Two broad-band vertical electric component waveforms are included for each shot, except
Shot 1. when all film on Enyu was exposed to radiation.
At least at a distance of 320 km some detail is preserved in the
first hop sky wave (Figure 9, Eniwetok).
22
The arrival times of the first
que>