A Novel Beam Mapping Measurement
Technique for Cryogenic Detectors
E.J. Wollack, M. Limon, S.H. Moseley, D.T. Chuss
NASA/GSFC
September 30, 2003
1 — BACKGROUND AND OBJECTIVES
Recent advances in the development of antenna-coupled superconducting bolometers have been driven
by the interest in high sensitivity astronomical measurements at millimeter-to-submillimeter wavelengths.
Specifically, planar antennas are the most promising long wavelength technology candidates for the Inflation
Probe and SAFIRE, two high priority future NASA missions. In evaluating the performance of these devices for
such applications, characterization of the beam pattern and radiometric coupling efficiency in a controlled
environment is crucial. The superconducting properties of these sensors renders the traditional approach of
measuring antenna beam patterns at room temperature and then extrapolating to estimate cryogenic performance
ineffective. Attempting to measure the beam pattern through a window of a cryostat presents a series of
problems in relating the measured performance on the ground to that anticipated in the end flight application.
This is a result of the necessity to place the detector very close to the window in order to be able to measure as
much of the forward beam pattern as possible without vignetting the angular acceptance. The presence of the
window introduces systematic phase errors and residual reflections in the observed angular response. In addition,
the use of an anti-reflection coating on the window introduces a frequency and angle dependent transmission
with relatively narrow-band performance from a sensor validation perspective. Finally, the view though the
window can present saturation issues for high sensitivity devices.
In developing this measurement system, we propose a two-phase program. In the first phase of the
program, a simple proof-of-concept system will be built that will allow measurement of the beam pattern on a
laboratory bench. Once demonstrated, a version of the system that can be installed inside a cryostat capable of
sub-Kelvin sensor temperatures will be fabricated.
Our beam measurement concept is motivated by reflectometry methods used to study the spectral
baseline stability in radio telescopes. The general approach is depicted in Figure 1. Here, the source is a
reflective target element with absorber on its backside in order to minimize modulation of residual reflections.
The device under test in the figure, a feed horn, transmits light which is bounced off a target reflector and read
out with a HP 8510 network analyzer. The system is a quasioptical beam waveguide where the reflective target


presents an impedance mismatch that is a function of the angular reflectance,   G   exp  j 2k  r 8k r ,
where G() is the antenna gain, k is the wave number, and r is the distance from the feed horn to the reflective
target [1]. This approximation for the reflection amplitude is valid in the far field. By modulating the radial
position of the reflector target relative to the test antenna, it is possible to decouple the gain as a function of
angle. Residual reflections arising from imperfect absorption by the hemispheric beam dump are approximately
constant in time and drop upon application of a phase-sensitive lock-in that employs the target position as a
reference. By scanning the location of the reflecting target surface, the angular acceptance is mapped in the Eand H-plane directions. The system’s operation in the frequency range of 75-to-330 GHz is quite practical;
limited on the low frequency end by available space and on the high frequency end by the RF power available
from the network analyzer’s coherent source.
The fully implemented cryogenic version of this measurement system is shown in Figure 2. In addition
to the reflectometry operation mode, this configuration allows the use of an incoherent emitter such as a reverse
bolometer for the source. Modulation for a high sensitivity sensor is accomplished by rapidly varying the current
and hence the temperature of the source. In order to reduce the effect of changes in responsivity with source
power level, one can vary the modulation amplitude as a function of angle such as to maintain a constant power
on the bolometer could be employed. The variation of current as a function of angle then can be used to derive
the beam pattern. We note that mechanically modulating the distance of the reflecting target could be avoided
using an active antenna that can electrically insert a phase delay in the reflected signal. This approach would not
only simplify the mechanical design but also would be extremely useful to characterize polarization sensitive
elements.
2 — RESEARCH AND DEVELOPMENT PLAN:
Month from Project Start:
Design Warm Prototype
Warm Prototype Fabrication
Warm Prototype Testing
Cold System Design
Cold System Fabrication
Cold System Testing
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REFERENCES:
[1] Silver, Samuel, Microwave Antenna Theory and Design, 1986, Peter Peregrinus Ltd., London, UK, Section
5.10, pp. 155—158.
Figure 1: Beam Pattern Determination by Millimeter Wave
Reflectometry: The beam measurement configuration is
amendable to study at room temperature. The target reflector’s
position is modulated with small amplitude in the direction of
the source in order to decouple the effect of the reflector from
imperfections in the absorbers. The reflective element is
scanned along the detector’s E- and H-planes at an
approximately constant distance. This configuration can be
used to study waveguide-coupled versions of the antennas used
for millimeter wave detectors. By varying the target size the
return signal and mapping resolution can be varied.
Figure 2: Beam Pattern Determination with an Incoherent
Source and Cryogenic Sensor: For a high sensitivity sensor, a
thermal emitter is used as the source. The hemisphere is cooled to a
temperature representative of the background loading the detector
would see in an astronomical application. The filter is mechanically
coupled to the emitter to avoid the effects of oblique incidence. The
source is modulated with a modest change in current (i.e., source
temperature). As the source is moved in the E- and H-planes at a
constant distance from the detector, the source amplitude modulation
is varied in order to maintain constant power on the detector as a
function of scan angle. This approach minimizes the effect of
responsivity variations that are a function of detector loading.
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