mains very nearly the same as the pulse is propagated to distances.
This is partially because the distribution of energy with frequency
near the source was different for Shot 3 and Shot 2, The curves also
show attenuation with distance at any frequency within the range analyzed. It is apparent that the higher frequencies are attenuated relatively more. It is interesting to compare Figure 13, which shows
spectral intensity values as received for Shot 2, and Figure 14, which
shows comparable values with the distance factor removed. Under the
conditions of the Castle tests, as distance increases, attenuation due
to distance becomes relatively less important.
Yie
d_ Predo
t Frequency.
There appears to be an approxi-
mate relationship between yield and frequency at which peak energy
occurs, and there is some theoretical justification for this relationship (Reference 13). Plotted as Figure 16 is a theoretical curve from
Reference 13, with points indicating frequencies with maximum energy,
as calculated from close-in waveforms, for some of the Upshot~Knothole
detonations, Castle Shots 2 and 3, and estimated frequencies for Castle
Shots 4, 5, and 6.
Narrow-Band Records.
Narrowband (approximately 200 cps) record-
ings were also made during the Castle series. This technique may be
necessary in making measurements under operational conditions, Values
of spectral intensity were also calculated for field strength narrowband values received for Shot 2 (Reference 11).
Figure 13 has narrow
band points indicated by crosses and it will be noted that in most
cases, the agreement with the corresponding broad-band spectral curve
is quite good. In some cases, the trigger level was somewhat high,
which resulted in missing the initial portion of the waveform. Other
reasons for discrepancies were variation in bandwidth with signal level
and aging of tubes.
' Peripheral Lightning Flashes.
Fast—frame moving picture photo-
graphy (3,000 or more frames per second) of Ivy Mike show what appears
to be lightning flashes between the natural cloud cover and the sea on
the periphery of the fireball.
This phenomena starts at about 5 msec
after the beginning of the nuclear reaction and continues for about
75 msec or more. These visible flashes were also evident on the highspeed photographic film recordings of the Castle series.
High-gain receiving equipment, with a bandwidth of about 1 ke to
1 Me, long-wire antenna, and a tape recorder were used at Enivetok in
an attempt to detect these flashes. No signals attributable to the
discharges were noted.
Loy-FrequencyTransmiters. An analysis of field strength mea-
surements of low-frequency transmitters has been carried out by NBS.
wee
ee
(References 4,12)
At widely-separated locations during Castle, NBS and
DRL monitored NSS, NLK, NPM and GBR to obtain field strength measurements. Narrow-band equipment was used; essentially the same as that
used for recording the narrow-band field strengths from mclear detona-
tions. Measurements were made at the same time at each location, once
each hour. Five selected plots of field strength versus time from
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