Project Cyclops, A Design... - Department of Earth and Planetary ...

addedto theother IF channel. The two IF bands, one
extending from 75 to 175 MHz and the other from 325
to 425 MHz, are then combined with the 250-MHz pilot
frequency for transmission over the coaxial cable.
arrange to have each IF band experience the same cable
loss and dispersion. In fact, we would like this to be true
for each frequency within each IF band.
With straight through transmission the loss experienced
by any frequency is
E fO
S = SO = SOVq-z-& (5)
f) 1/2
Figure 10-4. IF transmission spectrum.
The spectrum occupancy for the IF cable is indicated
schematically in Figure 10-4. The shaded blocks extending
from fl to f2 and from f3 to f4 represent the two IF
bands. Centered between these at fo is the 250-MHz
pilot. It would certainly be possible to send these signals
in the relationship shown over the entire distance; this
was one of the alternatives seriously investigated. This
requires the design of equalizers to compensate for the
cable loss versus frequency characteristic and probably
the provision of other pilot frequencies (at the outer
edges of the band) to servo those equalizers as the cable
temperature changes. The attenuation of 7/8-in. coaxial
cable is about 24 dB/km at the 250 MHz pilot
frequency, or 120 dB for a 5-km length. Since the loss
varies as/1/2 the total loss in 5 km would be 66 dB at fl
(75 MHz), 100 dB at f2 (175 MHz), 137 dB at f3 (325
MHz) and 156 dB at f4 (425 MHz). Thus about 90 dB of
equalization is needed from f_ to f4 with about 34 dB of
this occurring between fl and f2 and 19 dB betweenf3
and/"4. These loss figures are at room temperature and
are roughly proportional to the square root of absolute
temperature.
While the equalization needed is well within the state
of the art, the symmetry of the Cyclops transmission
problem causes us to favor another approach. The two
IF channels representing orthogonal polarizations are to
be combined in various ways to obtain other polarizations
(Chap. 9). If a signal is received in a mixed
polarization it might be transmitted as a frequency near
f_ in one IF channel and near f4 in the other. We would
like these signals to have suffered identical loss, phase
shift, and delay in each channel, so that when they are
demodulated to the same frequency at the central
station, the polarization represented by their relative
amplitudes and phases is the same as would have been
determined at the antenna. This suggests that we should
f4
where 8 = (fifo) - 1, and So is the cable loss at fo-
Rather than using the total loss s, we define a normalized
relative loss P = (a/s0) - 1. Then in terms of p,
equation
(5) becomes
p = X/1+5 -1 (6)
8 82 53 564
_---
2 8
+
16 128
+"" (6a)
If now we arrange to invert the entire spectrum at the
midpoint of the line, as indicated in Figure 10-5, so that
a frequency transmitted as [ for the first half is sent as
2fo -]'for the second half, we will have handled both 1F
channels in a similar fashion. The relative normalized
loss is then
1
P =2 (_/1+5 + 41-5)-I (7)
52 554
....
8 128
+"" (7a)
which shows far less variation, and is symmetrical about
fo.
_
c_c_d
"-to o
...... 21
DISTANCE
Figure 10-5. Midpoint frequency inversion.
Finally, a further improvement is realized if we
perform this inversion twice at the one-quarter and three
quarter points along the line and, at the midpoint, swap
o
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