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

only±0.07;_ up to p[a = 1.77. Beyond this the error
increases to about X/4 at p[2 = 2, but there is very little
signal plane area at this value of the abscissa. Thus, the
image quality should be quite good. The situation
rapidly improves as we go to higher frequencies.
• _ I I I T I' I I I I
(o)
pyramid at the highest frequency used. Alternatively,
the radiative imager could use short helical antennas.
These also permit broadband operation and the two
polarizations used could then be left and right handed
circular. An advantage of helical antennas is that the
phase of the radiation can be changed simply by rotating
the helix about its axis. This phase changing capability
could well simplify the construction of the imager and
reduce the phase tolerances needed since it would be
easy to trim each element.
f.,c,_ \ \ \ \
-_ I _ I I I I I I I
(b)
.I
O. IOta
-.I 1. 57m
f • 1.42CHZ
-.2 i i i i t
.I I( ' _ ' ' ' ' _ ' '
--. I I I I I I I I _ I _I
(d)a IOta, /.120m
f-" 3GHZ
--.I I I I I I I I I I I_"I
o I 2
Figure 11-20. Spherical aberrations with parabolic
p/o
delay.
Further studies might reveal that some other system,
such as using a fixed
curved signal array plus variable RF
delay, would give improved results, but the system
described seems entirely adequate. We are, after all, not
attempting high-resolution imaging by optical standards;
there are only 32 independent picture elements per line.
Another advantage of microwave imaging is that both
polarizations can be imaged simultaneously. It is a
simple matter to mix the IF signals (if necessary) to
obtain signals representing vertical and horizontal polarization.
They can then (after heterodyning to the imaging
frequency) be radiated as such by crossed dipoles or
loops. Good broadband performances can be realized
with the crossed Vee antennas shown in Figure 11-21. If
the height of the vanes is one-third their separation the
characteristic impedance is 300 _. The length of the
sides should be _/2 greater than the altitude of the
With both polarizations imaged we can either add the
intensities to decrease integration times, or subtract
them to detect polarized sources. For the latter purposes
we should be able to rotate the polarizations displayed,
by using goniometers in the IF or RF lines.
The microwave imager just described requires a
building with a clear volume about 80 ft high by 80 ft
wide by 550 ft long. This is a volume of about 3 million
cubic feet and a floor area of 41,250 square feet. The
total surface area is 176,000 square feet. The building
must be completely shielded and lined with microwave
absorbing material to provide an anechoic chamber.
Very thin copper sheet will provide adequate shielding,
provided there are no cracks. The skin depth of
copper at l GHz is about 0.08 mil, so 5-mil thick sheet
will provide about 62 nepers or 500 dB of attenuation.
All doors must be sealed with contacting fingers and
probably should be double with absorbing material
between them. All signal and power leads entering and
leaving must be shielded and equipped with low pass
filters, as is done in screen rooms. Air vents must have a
section, containing an egg-crate structure of waveguides
below cutoff, that is soldered all around to the building
shield.
3
Figure I 1-21. Dual polarization broadband radiator.
150