Development and Use
of an Antenna Gain Pattern test-system
for the NAST-M Microwave Radiometer at -54GHz
by
Hua Fung Teh
Submitted to the Department of Electrical Engineering and Computer Science
in Partial Fulfillment of the Requirements for the Degrees of
Bachelor of Science in Electrical Science and Engineering
and Master of Engineering in Electrical Engineering and Computer Science
at the Massachusetts Institute of Technology
May 23, 2001
Copyright 2001 Hua Fung Teh
The author hereby grants to M.I.T. permission to reproduce and
distribute publicly paper and electronic copies of this thesis
and to grant others the right to do so.
Author
_47
Department of Electrical Engineering and omputer Science
May 23, 2001
Certified by
David4i. Staelin
Professor of Electrical Engineering
Thesis Supervisor
Accepted by
Arthur C. Smith
Chairman, Department Committee on Graduate Studies
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
JUL i 12001
LIBRARIES
Development and Use
of an Antenna Gain Pattern test-system
for the NAST-M Microwave Radiometer at ~54GHz
by
Hua Fung Teh
Submitted to the
Department of Electrical Engineering and Computer Science
May 23, 2001
In Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science in Electrical Science and Engineering
and Master of Engineering in Electrical Engineering and Computer Science
ABSTRACT
The NAST-M is a passive microwave spectrometer employed on the National Polar
Orbiting Observational Environmental Satellite System (NPOESS) Aircraft Satellite
Testbed (NAST). It has channels in the oxygen absorption bands near 54 GHz and 118.75
GHz. The 54 GHz band consists of 8 channels.
This thesis deals with the design, development and usage of a system capable of
measuring the NAST-M's antenna gain pattern for the 54 GHz channels. Through the use
of a frequency synthesizer and a harmonic mixer, appropriate test-frequencies were
generated and directed at the NAST-M at angles of incidence varying from -20 degrees
to 20 degrees.
A method of manual timed sampling was implemented to give alternating
readings of signal and noise. The signal readings were averaged and corrected via
adjacent noise values and the results were normalized and plotted on a logarithmic scale.
Although MATLAB code was extensively used, much of the process was manual due to
the irregularity (and hence complexity) of the pattern, especially at larger angles of
incidence. The result was a gain pattern in units of decibels.
There were 16 data sets in all - two samples for each channel. Readings were
fairly consistent with what was to be expected from an effective, workable system. The
system provided good resolution, evidenced by sidelobes that dropped as low as -35 dB.
There was also relatively good consistency in the shape of the patterns for each channel,
and abnormally large sidelobes on the outer edges of the patterns were significantly
absent.
Thesis Supervisor: David H. Staelin
Title: Professor of Electrical Engineering
ACKNOWLEDGEMENTS
I would like to thank a number of people (and entities) who helped make this thesis
possible. Firstly, I would like to thank Professor David H. Staelin for being a great
advisor and for teaching me many lessons about life along the way. The patience and
grace he demonstrated was much appreciated. I would also like to thank R. "Vince"
Leslie for showing me the ropes and for taking me under his wing without hesitation.
Many late nights would have been unbearable without him to share the delirium with.
Without his help, completion of this thesis would have been impossible.
I would also like to express my gratitude to Dr. Philip Rosenkranz, William "Bill"
Blackwell and Frederick "Fred" Chen for rendering me technical and theoretical
assistance when I needed it the most. Thanks for being smart guys!
Nikhil Sadarangani and Jeremy Todd were my primary tutors in MATLAB and a big
hug (no kiss though) goes out to them too.
I also want to thank Lillian Wu for her help with the graphics and looking pretty.
Finally, I would like to thank Starbucks coffee for the sleep deprivation it enabled me to
endure throughout the course of research.
Development and Use of an Antenna Gain Pattern test-system for the
NAST-M Microwave Radiometer at ~54 GHz
by
Hua Fung Teh
I
1. INTRODUCTION .............................................................................................................................. 4
1.1
BACKGROUND ........................................................................................................................ 4
1.2
OBJECTIVE ............................................................................................................................. 5
2. RADIOM ETER O VERVIEW ............................................................................................................ 6
2.1 THE RADIOMETER ........................................................................................................................... 6
2.2 THE ANTENNA ................................................................................................................................ 7
2.3 ANTENNA TEMPERATURE AND GAIN PATTERN ................................................................................. 8
2.4 SIGNIFICANCE ................................................................................................................................. 8
3. ANTENNA PATTERN M EASUREM ENT ...................................................................................... 10
3.1 OBJECTIVE ................................................................................................................................... 10
3.2 TASK BREAKDOWN ....................................................................................................................... 10
4. TEST SIGNAL GENERATION ....................................................................................................... 11
4.1 TARGET TEST SIGNALS .................................................................................................................. 11
4.2 THE SOLUTION ............................................................................................................................. 12
5. SETTING UP .................................................................................................................................... 16
5.1 FAR-FIELD REGION ....................................................................................................................... 16
5.2 FAR-FIELD PHASE CENTER ............................................................................................................ 17
6. M EASUREM ENT AND ANALYSIS ............................................................................................... 20
6.1 THE TRANSMITTER ....................................................................................................................... 22
6.2 SECURING NAST-M ..................................................................................................................... 22
6.3 M EASUREMENT M ETHOD .............................................................................................................. 23
6.3.1 Overview .............................................................................................................................. 23
6.3.2 ConceptualizingNoise .......................................................................................................... 24
6.3.3 MeasurementAlgofithm ........................................................................................................ 25
6.4 FINDING A V ............................................................................................................................... 27
6.4.1 Splicing................................................................................................................................ 27
6.4.2 Analyzing data2 .................................................................................................................... 29
6.4.3 Processingdatal and data3.................................................................................................. 30
6.4.4 Re-joining data..................................................................................................................... 33
7. LIM ITATIONS ................................................................................................................................ 52
7.1 HUMAN ERROR ............................................................................................................................. 52
7.2 ENVIRONM ENT ..................................................................
....... . .............................................
7.3 SUBJECTIVITY OF JUDGEMENT .............................................................
8. FUTURE WORK.
8.1 V ARYING
0, 0
.............
............................................................................................................
FOR A GIVEN. . -.............
....................
...........................................................
8.2 OTHER DIRECTIONAL ISSUES..............................................................................
8.3 N OISE FACTORS .....................................................................
9. CONCLUSION
.............
....
---............................
.................................................................................................................
52
53
54
54
55
55
56
APPENDIX A: GRAPHS OF POWER (V) VS DIVISION NO..........................................................
57
APPENDIX B: MATLAB CODE................................................................................................
71
3
1. INTRODUCTION
Remote Sensing using Microwave radiometry is used to measure atmospheric parameters
such as humidity and temperature. The NAST-M is an example of an airborne instrument
employed for this purpose. High-Altitude observations are carried out by the NAST-M at
absorption frequencies of atmospheric molecules such as oxygen and water. Atmospheric
sensing is a well-developed science with applications dating as far back as 1960. For
example the TIROS (Television and Infra-red observation satellite) series of weather
satellites launched in the early 1960s were polar-orbiting and carried infra-red sensors.
The goal of this thesis is to design, develop and use a test system capable of mapping the
antenna gain patterns of the NAST-M's scanning sub-assembly at ~54 GHz. The
scanning sub-assembly is the primary component responsible for coupling incoming
electromagnetic radiation to a measurable and discernable electrical signal that will be
analyzed in later stages.
1.1 Background
The NAST-M is a passive microwave spectrometer employed on the National Polarorbiting Observational Environmental Satellite System (NPOESS) Aircraft Satellite
Testbed (NAST). The platforms for deployment are currently the NASA ER-2 highaltitude research plane' and Scaled Composites' Proteus aircraft.
The NPOESS is required to provide a:
"Remote sensing capability to acquire and receive in real-time at field terminals, and to
acquire, store and disseminate to processing centers, global and regional environmental
imagery and specialized meteorological, climatic, terrestrial, oceanographic and solargeophysical and other data in support of civilian and national security missions"2
This is basically a modified U-2 spyplane. The U-2 was originally designed by Lockheed.
2 Cummingham, slide
4
4
The NAST-M has two total-power, multi-channel radiometers. The first has a singlesideband receiver and has 8 channels spaced evenly between 50.2 GHz and 56.2 GHz.
The second radiometer has a double-sideband receiver and has six functioning channels
with center frequencies from 118.75±0.8 GHz to 118.75 ±3.5 GHz. 3
NAST-M has a scanning assembly that makes 19 measurements within a range of ± 64.8
degrees (hereafter abbreviated as deg) from nadir. Each measurement has a spacing of 7.5
deg. The rays from an antenna reflector are effectively fed into one of two feedhorns
(depending on the incident frequency). Each scan also views 3 calibration spots: an
internal heated blackbody, an ambient internal blackbody and a reading from the zenith
direction (this is effectively a reading of the cosmic background radiation). The nominal
integration time is 100 ms. 4
1.2 Objective
The objective of the thesis research was to design and implement a test system capable of
measuring the gain pattern of the NAST-M's scanning sub-assembly for the channels
centered around 54 GHz.
3 Leslie, p 3
4 Blackwell, p 1
5
2. RADIOMETER OVERVIEW
2.1 The Radiometer
A superheterodyne total-power single-channel radiometer typically consists of the
following main elements:
" Antenna
*
Local Oscillator
" Mixer
" IF Amplifier
*
Filter
*
Detector
*
DC Amplifier
A Schematic is shown below in Fig 2-1.5
Antenna
Mixer
IF Amplifier
Filter
Detector DC Amplifier
OIFr
T aL
RF
V~ Tr + Ta
LO
Local Oscillator
Fig. 2-1. Total-power superheterodyne radiometer
5 Leslie, p 3
6
2.2 The Antenna
The scanning assembly (hereafter referred to as simply the "antenna") for the NAST-M
consists of a reflector and a set of 2 feedhoms as mentioned before. Fig 2-2 highlights the
6
physical layout of the assembly.
Frcnt
V4 Qq
Z~-~ 1
ty
I\b- \26 Cl\ 2_ h fy)
Oaks Ot
_j
r~e~CO(
4
VAOT TO Z\LE.
Fig. 2-2. NAST-M scanning sub-assembly
The antenna serves as an interface between free space and the receiver (i.e. it is an
impedance matching device) and it basically provides "selectivity in the angular
distribution of the radiation".
6 Blackwell,
fig 4
7
Each feedhorn has its own reception characteristic and this generally involves a main
lobe (with maximum reception in the straight ahead direction) and lobes on the side.
These lobes represent the directional gain of the antenna. When coupled with the
reflector, the overall assembly will have some effective characteristic (or gainpattern, as
will be discussed later), which also consists of a main lobe and a number of subsidiary
sidelobes.
2.3 Antenna Temperature and Gain Pattern
The antenna and its sensitivity to direction determine the net power received. Generally,
the antenna can be characterized by its gain pattern, G(f,O,#), which embodies the
coupling effect the antenna has on the incident radiation. This is usually only dependent
on frequency and direction, (f,6,4). Since we are interested in the microwave region, it
is instructive (and common) to express power in terms of temperature because of the
Rayleigh-Jeans approximation. The antenna temperature is given by:
Ta(f) =
4--fT (f,O,O)G(fO,O)dQ
This is measured in units of degrees Kelvin (K) and accounts for the brightness
temperature T, integrated over the entire gain pattern of the antenna. It is the primary
parameter we are interested in as far as the antenna is concerned.
2.4 Significance
The accurate measurement of T at all scan angles is essential to the success of the
radiometer. Differences in G(f,6,#) over the scan range give rise to inconsistencies in
the readings. For example, the presence of spurious sidelobes when looking at a
7
Jansenp 17
8
particular spot would give a higher-than expected reading of T,. These differences arise
for a number of reasons, including:
" Imperfect coupling between reflector and feedhorns
*
Effect of background noise on material characterictics
" Changes in the physical temperature of the system
It is thus important to know the form G(f, 0, 0) takes at all 19 + 3 scan angles as this will
allow us to accurately predict the errors involved in each reading and effectively correct
for them using math. These variations in antenna gain have yet to be characterized at this
point.
It is hard to provide an analytic solution to the problem as the ray optics involved cannot
account for all the extraneous factors involved. This is why we have to do it manually.
9
3. ANTENNA PATTERN MEASUREMENT
3.1 Objective
Since we are not interested in the absolute antenna gain but rather the relative changes in
gain between scan angles, a relative measurement of G(f,6,$) at each of the 22 scan
angles will suffice. This will result in an antenna pattern. For instance, measurements for
a particular frequency
/
nadir. In this case, G(f,0,)
could be normalized relative to G(f,0,0) at scan angle 0, =
is taken to be 1 and all other measurements of G(f,6,$) for
any 0, can be expressed as a fraction (note that this is not necessarily less than one).
The goal is thus to design and implement a system to enable the accurate and precise
measurement of normalized G(f,0,$) for all 22 values of,.
3.2 Task Breakdown
Given the variables in question, the task can be broken down into 3 separate stages, all of
which involve design and implementation:
*
Generate test signals of appropriate frequency and adequate strength, and at low cost
*
The accurate and precise measurement of the incoming test signal by the NAST-M
" The ability to vary 6,
8 Note
and 0,
that we are only interested in frequencies in the 2 regions: near 54 Ghz; near 118.75 Ghz
10
4. TEST SIGNAL GENERATION
4.1 Target test signal
In the 54-GHz region (the region of interest), there are 8 channels. Each channel is
sensitive to a range of frequencies. The target test signals that need to be generated are
basically the 8 center frequencies of those channels. These channels, their frequency
ranges, and their center frequencies are shown in Table 4.1 below.
Table 4.1: Channels and frequency ranges for the NAST-M 54 GHz system
Channel
Frequency range (GHz)
Centerfrequency, f, (GHz)
1
50.21 - 50.39
50.3
2
51.56 - 51.96
51.76
3
52.6
-
53
52.8
4
53.63 - 53.87
53.75
5
54.2 - 54.6
54.4
6
54.74 - 55.14
54.94
7
55.335 - 55.665
55.5
8
55.885 - 56.155
56.02
There were a number of significant constraints faced in generating these 8 values offc.
Firstly the frequencies were all in the neighborhood of 54 GHz, which is relatively high.
The HP83640A frequency synthesizer available had an upper limit of 40 GHz, which was
about 14 GHz short of the desired frequency region. A frequency doubler or tripler was
an option but turned out to be too expensive.
There was also an obstacle faced in measuring the signal. The only available spectrum
analyzer was the HP8563E and it had an upper limit of 26.5 GHz. This meant that even if
11
generating a signal as high as 54GHz was possible, There was no direct way of verifying
this through measurement of the signal.
4.2 The Solution
The following schematic illustrates the apparatus that was set up to generate a signal in
the 54 GHz region:
Fig. 4-1. Signal Generation Setup
The idea behind the setup was to use the harmonic-generating qualities of the HP 11970V
mixer to generate higher harmonics in the 54 GHz region. The HP3640A frequency
synthesizer was programmed to generate some input frequency, f,. This signal was fed
into the IF terminal of the HP3640A. A WR-15 standard gain horn was attached to the
RF terminal of the HP3640A such that the RF frequency radiated,
f.,
would be some
12
integer multiple of
frequency, f,.
f,
f,,. f0 ,
could then be tweaked until it was a desired center
could be 52.8 GHz (channel 3), for example.
Given this value off,,, f,, would then depend on which harmonic we are interested in.
The harmonic was chosen such that the RF signal, f , was at its strongest. The value of
this harmonic, N, varied with the channel no. In this way we were able to overcome the
upper frequency bound of 40 GHz on the HP3640A.
The RF signal,
f,,,,,
, was measured as illustrated in Fig. 4-2:
NP~T
-~
30dB SNK o6.servec4
N
7
H
/
S Pe0-
L~O
C1 -z
E:::
Fig. 4-2. Test-signal measurement
13
To overcome the frequency limitations of the spectrum analyzer, the mixer on the NASTM was used (effectively separate from the instrument itself) to downconvert the incoming
signal ( f,, ) to some final frequency,
the mixer and
ffinal
flu, was fed directly into the RF terminal of
ffinal
was read directly off the IF terminal (it was noted, however, that
there was some internal gain of 20 dB before we were able to tap the signal externally).
The mixer had an LO frequency of 46 GHz. Thus
ffinal
is given by the following
equation:
finai =
f0 U, - 46 GHz
and since the range of fu, is as follows:
50.3 GHz < f,
< 56.02 GHz
it follows that:
4.3 GHz < ftnai < 10.02 GHz
Hence
ffina
conveniently fell within the operating range of the HP8563E spectrum
analyzer and was easily measurable.
f,.a,
basically appeared as a spike in the frequency
spectrum being scanned. Since there was a one-to-one relation between final and f, ,
the corresponding RF frequency f.,
could be computed easily from the results. This
verified that the harmonic mixer was generating the required electromagnetic test signals.
All 8 values of
f,
were achieved.
Manual sampling revealed the signal strength of each harmonic (using a dB scale on the
HP8563E) and the strongest harmonic for each channel was picked as the one to be used
14
for the final test-system. Table 4.1 shows the chosen values for fi,
f.t ,
final
and N, the
harmonic no.
Table 4.1
Channel No
f,
(GHz)
f, (GHz)
frial
(GHz)
N
1
5.589
50.3
4.3
9
2
5.751
51.76
5.76
9
3
5.867
52.8
6.8
9
4
5.972
53.75
7.75
9
5
6.104
54.4
8.4
9
6
4.9945
54.94
8.94
11
7
5.045
55.5
9.5
11
8
6.224
56.02
10.02
11
All 8 values of
f
successfully corresponded to the 8 values of
f .
15
5. SETTING UP
This section deals strictly with the actual method of measurement (taking readings off the
NAST-M) itself Variation of Os, 0 and 0 are discussed in Section 7.
Measuring G can be broken down into 3 parts:
" Finding the appropriate far-field distance so the transmitter and NAST-M can be
placed at an appropriate radius from each other.
*
Taking appropriate readings for G and corresponding noise levels
*
Correcting the readings for G for noise
5.1 Far-field Region
The far field region of a radiation source is the region where wavefronts of the
transmitted signal are effectively parallel and not spherical. If the source has a maximum
overall dimension D that is large compared to the wavelength of interest, the far-field
region is taken to exist at distances greater than 2D
2/A
. We
are interested in the far-field
distance of the NAST-M antenna as the actual targets it will be dealing with in practice
will be about 60, 000 feet away and are probably going to be in the antenna's far-field
region. Thus it is vital that we ensure that our test signal source is placed in the NASTM's far-field.
In the case of the NAST-M, D is taken to be the diameter of the 54 GHz feedhorn. This
was measured and found to be 6.35 cm. Since the feedhorn will be accomodating a range
of frequencies, there will exist a range of far-field distances as well. For experimental
purposes, the effective far field distance will be taken as the upper limit of the range,
which corresponds to the highest receivable frequency, 56.155 GHz. Thus the
experimental far-field distance, d.far, is given by the following calculation.
16
2(0.0635)2
1.51 m
c /(50.21E9)
where c is the speed of light in free space.
5.2 Far-field Phase Center
Although we now know the far-field distance of the antenna, this does not mean that we
can simply place the test-signal source at that particular distance from the instrument. We
need to know where the origin of radiation is. The emanating wavefronts from a feedhorn
are spherical. The common center of curvature that all these phase-fronts share is the
phase center. The far-field phase center is thus the apparent phase center for waves
received in the far-field. Even though the far-field was defined to be the region in which
the waves are effectively parallel, it does not change the fact that the waves are curved at
the source and thus the phase-center still must exist. It is from the phase-center that the
far-field distance will hence be measured.
There are various ways to calculate where the phase center lies. These are well
documented in "Gaussian Beam-Mode Analysis and Phase-Centersof CorrugatedFeed
Horns"9. There are a number of potentially admissible apparent phase-centers and the
one that is chosen will be the most suitable for a selected performance criterion". The
phase-center that was chosen in the case of the NAST-M was the beam-mode phasecenter". This is simply the best-fit Gaussian phase-center .
This phase center was calculated by first computing the dimensional parameter of the
horn, A 12:
9 Wylde et al, IEEE Transactions on Microwave Theory and Techniques, Vol 41 No. 10, October 1993
10 Wylde et al, p 1698
11
Wylde et al, p 1697
12Wylde et al, p 1698
17
(0.6435)2 1ka
2H
2
where a is the diameter of the aperture, k is the spatial frequency and H is the horn length.
Incidentally, k can be calculated using the dispersion relation (note that the temporal
frequency,f was taken to be 54 GHz):
k2
( 2Y
Let d,, be the distance of the phase-center from the apex of the horn. Using the
calculated A , T was then looked up on the graph of T vs A (fig 6-1)3, where:
T =
H
d,, was found from the preceding equation. Table 5.1 shows the important parameters
and their values:
13
Wylde et al, p1698, Fig 10.
18
Table 5.1: Relevant Parameters and their Values
Parameter
Value
K
1131 /m
A
3.175 cm
H
30.0 cm
A
0.79 m
go
=
Eo =
T
41r x10-7 H/m
8.84 x10~ F/rm
0.64
d, was thus found to be 19.2cm.
19
6. MEASUREMENT AND ANALYSIS
Now that the far field distance and the phase-center had been determined, we needed to
position both transmitter and NAST-M
in a way that would enable accurate
measurements. The process consisted of the following basic elements:
" Setting up a transmitter that outputs the test signals
*
Setting up the NAST-M such that the scanning sub-assembly is straight ahead of the
transmitting aperture
*
Placing both the transmitter and the NAST-M relative to each other such that their
mutual angular orientation can be changed.
The NAST-M and transmitter were set up in Room 26-356 at MIT. Echo-absorber was
placed on the floor midway between the NAST-M and the transmitter. 9 square-shaped
pieces of the absorber were arranged to form a large square. This was to reduce the effect
of unwanted noise as a result of reflection. The Echo-absorber, however, was not moved
as 0 was varied. The limitations due to this are discussed in Section 7.
The positioning of the transmitting unit and NAST-M are illustrated in Fig 6-1:
20
\
\ -burotO'f
tO
tbe(q
.O
-e
--
--
- 31b2e \egJ
CV l
,4 777/
7
NTA
t\Vzo
,//
CVnA \ K /
Oaf .
ecV~c~ ~x
N
7
Q0 0
\,
/~
&O'~
ef
Fig. 6-1. Transmitter and NAST-M
21
The purpose of the setup was to allow the transmitter to move in a radial path relative to
the NAST-M, transmitting test signals at various angles of incidence. The NAST-M
would be stationary throughout the process. Fig 5-2 shows a side view of the apparatus:
6.1 The Transmitter
The primary engine of the transmitter was the apparatus described in Section 4 (TestSignal Generation). As done previously, The HP3640A frequency synthesizer was
programmed to generate some input frequency,
the WR-15, f0
f).
4
f
, so that the desired output signal from
was an appropriate test signal (one of the channel center frequencies,
The harmonics used were the same as illustrated in Table 4.1.
This time however, the HP 11970 Mixer/WR- 15 assembly was connected to the HP via a
flexible HP5061-5458 cable. This was to provide some freedom in the orientation of the
components. The instruments were set up on a trolley (refer to Fig 5.1) so that the
transmitter would be laterally mobile. This was necessary, as we needed to vary the angle
of transmission in order to measure G(f,0,0).
6.2 Securing NAST-M
The NAST-M was placed on its brace vertically (refer to Fig 5-2 below) with the
receiving aperture facing the transmitter. The upper section of the brace (which was now
facing upwards) was then chained to the legs of a laboratory table. This was necessary to
prevent the NAST-M from toppling over. Effort was made to ensure that the NAST-M
was vertical and not tilted in orientation relative to the ground.
14
Refer to Fig 4.2
22
i t&\C
QVOLu
- Yae \l
ti
a
f
11--tCb\j
1-i
Q~zy~ -0 lot-~rt
Fig 6-2: Securing NAST-M
6.3 Measurement Method
6.3.1 Overview
The NAST-M
flight
computer, MTS, was programmed
to make continuous
measurements of incident power, P. This incident power is the total power received
integrated over the entire surface of the antenna feedhorn. The output reading was a
voltage, V, which was proportional to P. MTS was configured to make 1000
measurements (integrations) per second, and stored these readings as a matrix with 8
columns, each column representing one of the eight channels. Since we were only
transmitting one frequency at a time, we were only interested in one of the eight at any
23
one instant. The matrix was stored as a file and the channel of interest noted. These
readings would be analyzed later.
With the transmitter outputting some
fe,
the trolley was moved radially along the arc as
illustrated in fig 6-1. The arc was drawn using chalk on the room floor and for our
purposes, was divided into 41 points spaced 1 deg apart. A protractor and thread were
used to determine where each division lay. The center of the arc was the far-field phase
center determined in section 5.2. The radius of the arc was 2.8 m, which was sufficiently
greater than the far-field distance of 1.51 m. The orientation of the trolley relative to the
curvature of the arc was kept constant. This was achieved through the use of 2 rigid
cardboard pointers that always pointed down to any 2 points on the arc. The cardboard
pointers were attached to the bottom of the trolley. This was to ensure that the direction
of the transmitted signal was always along a radial line of sight. 6 was the angle of
incidence.
6.3.2 Conceptualizing Noise
The voltage, V, given by MTS is proportional to the brightness temperature at the
antenna. This is effectively given by TA + TR, where TA is the temperature due to the RF
test signal and TR is the internal noise. This noise comes from the NAST-M itself Thus
the following equations hold:
V oc TA+TR
TA = TS+TB
where:
TB= Background noise at the antenna
Ts = Temperature solely due to the source (transmitter)
24
Note that both these values depend heavily on 6 and 0 as they are the result of
integration and coupling with the directional gain of the antenna.
Therefore we conclude that:
V oc TS +TB+ TR
Hence if
TB
and
TR
remain constant, V will give a direct indication of Ts . However,
TR
constantly fluctuates due to irregularities in the gain of the NAST-M and this makes it
hard for V to accurately describe Ts - a simple direct measurement will obviously not
suffice. What was required was a way of determining some average noise for each signal
measured and finding some way to subtract that noise from the signal.
6.3.3 Measurement Algorithm
Readings were generated in the following manner with MTS running:
*
Turn transmitter on for a period of approximately 4 seconds
" Turn transmitter off for a period of approximately 4 seconds and move trolley along
arc by Ideg within those 4 seconds
" Repeat process
In this way we get 2 kinds of readings for V - when the transmitter is on and when the
transmitter is off
Fig 6-3 gives an example of a data set from channel 4:
25
channel 4 data 1
9-
8-
4
7k
I1
65-
4
3
-
2
k~_
lL~
1
0
jj1jIfj~r~~
0I
2000
1500
sample no
1000
500
2500
3000
Fig. 6-3. channel 4 data set
The horizontal axis is sample no, where each sample took a time of 1 millisecond. The
vertical axis is the received power of a corresponding sample. The units are given in
volts, as the MTS outputs a voltage proportional to the power received.
From section 6.3.2, we know that:
VON
>c TS + TB +
TR
VOFF oc TB + TR
AV =
VON
-
VOFF
>- Ts
26
Thus in order to eliminate noise we had to find a sensible way to measure AV, where
AV is simply the difference between VON and
VOFF.
6.4 Finding AV
6.4.1 Splicing
The first step to this process was to take the data set (as illustrated above) and laterally
splice it into 3 parts: data1, data2 and data3. These are shown below:
datal
0.3
I
I
I
I
I
II
I
I
I'
I
I
0.29-
0.28-
HI
0.27
0.26-
i~)
0.25-
~II
0.240.23-
V
0.22
~vi ~
N: (X\.
0.21
0
50
~ Lf~Pi
',tvAi
100
150
200
250
300
350
400
450
500
Fig. 6-4. datal
27
data2
9
8
7
6
5
4
-
3
]
2
I
1
0
0
200
400
600
800
1000
K
1200
K
1400
1600
1800
2000
Fig. 6-5. data2
28
data3
0.26 r------.-------.-----.-----....--------,------r------.---~-___.,
~'l
I
rv
I
t
I
I
0.24
.~~~1
i
I
I
iW
I ii
I~i
!
I
i
j
I
r
i
[ I
~~ l~
0.22
o
50
100
150
200
250
300
350
400
450
Fig. 6-6. data3
datal and data3 were basically the data points on the left and right-hand sides of Fig 6-3 .
They were a consequence of choosing data2.
6.4.2 Analyzing data2
data2 was chosen such that all signal pulses within lay above some minimum threshold
noise level, thresh. The MATLAB code in janalyze2.m enabled us to define data2 by
29
entering the entire data set and the beginning and ending indices. It then used the
appropriate thresh value (that had been observed manually) to decide what was signal
and what was noise. Basically points that were above thresh were considered signal and
anything below was taken to be noise. Since signal and noise were alternating in data2, it
was easy to split up the signals and noises up discretely. It then averaged all these signal
and noise values and returned the following discrete pattern:
averaged noises and signals for data2
0
0
0
-- M1ii
nEEEEEEEEEEEEEIIIUIU
7 RIUInnn
10
20
I
Ii.M
IIUIRIIEEEEIEEIEIEE
30
40
IEIM
50
60
Fig. 6-7. Average Noise and Signal Values (signoise2)
Now we had constant noise and signal values, which would make the job of noise
correction easier. This new data set was saved as signoise2.
6.4.3 Processing datal and data3
30
datal and data3 were essentially sets of readings that were too close to the noise floor for
there to be any discernable threshold noise value that was less than all the signal pulses.
The signal pulses in these 2 regions were also less distinct in visibility as a result. Hence
the observation and averaging of signal and noise had to be done manually.
The average signal and noise values were found by first manually noting where the noisesignal and signal-noise transitions occurred and storing these indices in an array on
MATLAB. This was done so that we always started with noise and ended with noise (of
course, this meant that the noise on some of the borders would intersect with part of
data2, but it had no effect on the results). There was a pair of indices for every signal and
for every noise. We then ran this array and the respective data splice through
manualmerge.m15 , and it returned a similar graph of averaged signal and noise values.
The data sets returned by data] and data3 were accordingly stored as signoisel and
signoise3. These are shown below:
15
Appendix B
31
Channel 4 signoisel
0.35
0.3
0.25-
0.2-
0.15
-
0.1 -
0.05 -
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Fig 6-8 signoisel (from datal)
32
Channel 4 signoise3
0.35
0.3 -
0.25 -
0.2 F
0.15 I
0.1
0.05 F
0
IIII"II
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Fig. 6-9. signoise3 (from data3)
Now we had 3 data sets of the same form and all we were left with was correcting for
noise and appending the data sets back together.
6.4.4 Re-joining data
The final two steps were correcting for noise and rejoining the three graphs. This was
achieved through the written MATLAB codefinalmerge.m . Taking signoisel, signoise2
and
signoise3 as
arguments,
finalmerge.m internally
calls
another
method,
siglessnoise.m'7 , which takes each signal peak and subtracts the average of its 2 adjacent
noise values. This gives the noise-corrected value for each signal. Now we have 3 signal
data sets minus the adjacent noise regions. finalmerge.m the appends all 3 sets in the
16
Appendix B
17
Appendix B
33
appropriate order to give a graph of Power (of the signal) vs division no. This is shown
below in Figure 6-10:
Channel 4 sample 1
I
III
0
I
II
30
35
7-
6-
5-
4 0
3-
2-
1 -
01
0
5
10
15
20
25
dhvsion no
40
45
Fig. 6-10. Power vs division no
This gave a good picture of what the antenna gain pattern looked like, but we wanted a
pattern that could be read in decibels. This result was thus run through dbscale.m1 8 ,
which divided all the signal values by the largest value in the set and scaled the resulting
values by 10*1og 10 . This yielded the antenna gain pattern in decibels, as desired:
18
Appendix B
34
Channel 4 Sample
0-
-15-25-
-20-
-25-
-30-
-35
0
5
10
15
25
20
division no
30
35
40
45
Fig 6-11: Antenna gain pattern in dB
It is important to note that we scaled the data in Fig 6-10 by 10*log
and not 20*log10
(as might be expected, the vertical axis being in Volts). This was because the voltages
read off the NAST-M were proportionalto power received, so the extra factor of 2 had
already been taken into consideration when the signal was being processed.
The horizontal axis, division no, is directly related to the angle from which the radiation
was being directed - each division represents a particular angle. There are 41 divisions in
all, and the transmitter was swept at regular intervals from -20 deg to 20 deg. This means
that division 1 is equivalent to -20 deg, division 21 is equivalent to 0 deg and division 41
is equivalent to 20 deg.
35
Two sample data sets were taken per channel, each sample producing 2 gain patterns: one
in Volts and one in decibels. Fig. 6-12 and Fig 6-13 show another set of graphs for
channel 1 after data analysis and noise correction:
Channel 4 sample 1
I
I
0
I
I
I
30
35
40
7
6
5
04
3
2
1
01
0
-
I
5
_
.mUUUUUUUU
10
15
20
25
division no
45
Fig. 6-12. Channel 4 sample 1 (V)
36
Channel 1 Sample 1
0
M
*inMEMEM
-5
-10
ca
'~-15
-20
-25
-30
0
5
10
15
20
25
division no
30
35
40
45
Fig. 6-13. Channel 1 sample 1 (dB)
Both samples (channel 4 and channel 1) produce gain patterns that have sidelobes that go
about 30 dB - 35 dB down, as evidenced in the figures.
The other 14 antenna patterns (in decibels) are illustrated in Figures 6-14 to 6-27:
37
Channel 1 Sample 2
0
-5-
-10-
-15-30
-35
0
5
10
15
25
20
division no
30
35
40
45
Fig. 6-14. Channel 1 sample 2
38
Channel 2 Sample 1
0
-5
10
-15C
ii
CD
-20-
-25-
-30-
-35
0
5
10
15
25
20
division no
30
35
40
45
Fig. 6-15. Channel 2 sample 1
39
Channel 2 Sample 2
0-
M-
I'-nEEMEN
-5-
-10-
-15-
0
-20-
-25-
-30-
-35
0
I
I
5
10
I
I
II
15
II
20
I
25
II
30
I
35
I
40
45
division no
Fig 6-16. Channel 2 sample 2
40
Channel 3 Sample 1
0
-
------
-5
-10
-15
--- 20
-25
-30
-35
-40
0
I
I
I
5
10
15
I
I
20
25
division no
30
35
40
45
Fig. 6-17. Channel 3 Sample 1
41
Channel 3 Sample 2
.
r- -m
.
-15
-20
-
U.
-35
0
5
10
15
25
20
division no
30
35
40
45
Fig 6-18. Channel 3 sample 2
42
Channel 4 Sample 2
0
-5
-10-
-15-
-20
1-
(9
-25
-30
-35
-40 L
0
I
I
II
5
10
15
20
I
25
30
I
35
1j
40
45
division no
Fig. 6-19. Channel 4 sample 2
43
Channel 5 Sample 1
0
-5
-10
-15
m -20
0 -25
-30
-35
-
-40
0
5
10
15
20
25
30
35
40
45
division no
Fig. 6-20. Channel 5 sample 1
44
Channel 5 Sample 2
0
-,
--
-
-5
-10-
-15Go
ca
-20-
-25-
-30
-35 L
0
'
'
5
10
15
20
25
division no
30
35
40
45
Fig. 6-21. Channel 5 sample 2
45
Channel 6 Sample 1
0
=inmm
Mmm
-5
-10-
-15-
0
-20-
-25-
-30-
-35-
I
I
I
5
10
15
I
I
25
20
division no
I
I
I
30
35
40
45
Fig. 6-22. Channel 6 sample 1
46
0
-15-
-20-
-25
-
-30
-
-35
0
5
10
15
20
25
30
35
40
45
Fig. 6-23. Channel 6 sample 2
47
Channel 7 Sample 1
M--,
-MEMEN
0-
-5-
-10-
-15-
-200
-25-
-30-
-35-
-40()
I
I
5
II
10
II
15
~
I
I
20
25
division no
II
30
II
35
I
40
45
Fig. 6-24. Channel 7 sample 1
48
Channel 7 Sample 2
0
-5
-10
-15
_--20
I=_T
=I
-25
-30
-35
f-
-40'
0
5
10
15
25
20
division no
30
35
40
45
Fig. 6-25. Channel 7 sample 2
49
Channel 8 Sample 1
0-
T--,=mnr
-5-
-10 -
-15-
-20 -
-25-
-30-
-35 -
0
5
10
15
I
I
25
20
division no
I
30
I
35
I
40
45
Fig. 6-26. Channel 8 sample 1
50
Channel 8 Sample 2
0-
-5
-10f
-15
C-
0-20
-25
-30 -
-35
0
5
10
15
25
20
division no
30
35
40
45
Fig. 6-27. Channel 8 sample 2
Most of these graphs looked reasonable. It was not often that both samples form a
particular channel looked very alike. This was probably a result of random noise and
human error. However, there were no significantly abnormal sidelobes present, which
was fortunate.
The system was thus effective in achieving its aim of mapping the antenna pattern of the
NAST-M.
The 16 absolute gain patterns (measured in V) can be found in Appendix A.
51
7. LIMITATIONS
There were some obvious limitations faced in the design, testing and development of the
system. Most of these were a result of a large part of the system being manual, and the
environment under which the experiments were carried out.
7.1 Human Error
The manual nature of the measurements rendered the system prone to some degree of
human error. Firstly, sampling of signal and noise values were done manually, which
means that the sample time wasn't the same each time the transmitter was turned on and
off. This is evidenced by the varying widths of each signal and noise portion.
The transmitter-on-wheels was also somewhat cumbersome and moving it round the arc
created some inaccuracy as well. It is possible that each time the transmitter traversed the
arc, it was pointing in a slightly different direction for the same division. Care was taken
to ensure that the transmitter was aligned consistently with the curvature of the arc, but
since the system was not automated or mechanized, there was no way of ensuring that the
angles of incidence were exactly the same each time.
7.2 Environment
Ideally, the environment under which such a system should be tested and developed is a
room free of external sources and reflective surfaces, so that all the radiation reaching the
NAST-M is entirely and directly from the transmitter.
52
The testing and development in our case took place in room 26-356 of MIT. There were
many objects such as shelves, tables, chairs, boxes and electrical equipment surrounding
our area of operation, meaning that there could potentially have been a great deal of
reflection. There was also a possibility of cosmic radiation entering from the windows.
There was no way in which these sources of interference could have been cancelled or
accounted for in the process.
7.3 Subjectivity of Judgement
Another factor contributing to inaccuracy was the fact that parts of the data analysis
relied on human judgment. The processing of data] and data3 involved manually
deciphering where signal regions and noise regions lay. There were times where it wasn't
obvious at all where the transitions were, and personal judgment had to be used in
assigning index values. This was inherently subjective.
There were also certain data sets which lacked a noise region at the end. When
constructing the various graphs for signoise, the missing average noise value was filled in
with the last averaged noise value. Values for noise levels close to each other were
deemed to be close enough such that this would not affect the final result significantly.
53
8. FUTURE WORK
8.1 Varying 6, 0 for a given o,
An issue that was not addressed in the thesis research was the complete variation of the
incident angle of radiation. In our experiments, we kept 0 constant at zero and only
varied 0 (for a given scan angle,
,). Being able to vary both 0 and 0 efficiently would
be a very pertinent addition to the system. We also want to increase the range of 0 variation from ± 20 deg.
The receiving sub-assembly can be looked upon as a square aperture (closed on the other
side) and we are only interested in its response in the half-sphere that the reflector faces.
Thus we will limit both6 and 0 to a range of-90 deg to +90 deg.
We want the transmitter facing the antenna from a particular direction and to be able to
change 0 or 0 slightly without any difficulty. This would ensure that we can measure G
for any direction we please. The intuitive way to do it would be to keep the antenna
stationary while the transmitter radiates energy in the direction we desire. The transmitter
of course, would always have to point towards the phase-center in order to get the
correct 0 and $.
However, this would be difficult as we would have to find some way to "float" the
transmitter. A better idea would be to keep the transmitter mobile along the 6 -axis while
allowing the NAST-M to be mobile in the 0 -direction. This might require some
machining or the use of a strong gun-mount.
54
8.2 Other Directional Issues
Another significant issue is the polarization of the transmitted radiation. In our system we
kept the transmitting horn oriented the same way throughout. Thus the incident rays were
consistently polarized in one plane only.
It would be instructive to experiment with different variations of horn-orientation, so that
the effects of the various polarizations can be studied in detail.
Also, due to the complex nature of the scanning sub-assembly, G will very likely vary
with the scan angle, O.. All readings in the current research were taken at 0, = 0. It
would hence be vital in future work to take readings at different values of 0. This would
culminate in a more complete picture of the antenna gain pattern of the NAST-M.
8.3 Noise factors
As mentioned in Section 7, the environment in which the experiments took place was
less-then-ideal. Future experiments might be carried out in a less noisy environment. This
can be achieved by better strategic use of Echo-absorber or setting up the apparatus in an
anechoic chamber.
55
9. CONCLUSION
A test-system was designed from scratch to map the antenna gain pattern of the NAST-M
passive microwave radiometer at 54 GHz. The work began with generating the desired
test-frequencies at appropriate strengths, and calculating the parameters involved in the
positioning of the measurement apparatus.
The next step was sampling the signal strengths at varying angles of incidence and
performing noise-correction on these measurements. The sampling was done using timed
transmission interspersed with non-transmission, giving rise to alternating signal and
noise values. The noise-correction was carried out with the help of the attached
MATLAB code.
Two samples were taken from each channel, giving a total of 16 data sets. The graphs of
interest were plotted on a dB scale. Most of the data sets did not exhibit unexpected
behavior. The patterns were evenly distributed and the sidelobes were observed to reach
values of-25 dB to -35 dB.
Although inherent time constraints limited the sophistication of the test-system, the
results were generally good and there was a high degree of agreement between the
generated patterns and what was to be expected from successful research. It is hoped that
this thesis will serve as a strong foundation or starting point for parties conducting
antenna-pattern measurement.
56
APPENDIX A: Graphs of Power (V) vs division no.
9
I
I
~
Channel 1 sample 2
I
I
I
I
t
8
7
6
>5 0
0.
4 -
3-
2f-
1
0
K
0
---
5
_mE
10
IEMMMEMEMM
15
20
25
30
35
.
40
1
45
division no
57
Channel 2 sample 1
8
-r
...
7
6
5
04
0
3
2
1
0
5
10
15
20
25
30
35
40
45
division no
58
Channel 2 sample 2
7 -
6-
> 5S4-
3-
21-
1-
0
5
10
15
20
25
division no
30
35
40
45
59
Channel 3 sample 1
1
1
30
35
40
-
>
0-
I
4
-
3 -
2-
1 -
01
0
-
5
10
15
25
20
division no
45
60
Channel 3 sample 2
9
87-
6 -
>5 -
3.-
21-
0
5
10
15
20
25
30
35
40
45
division no
61
j
Channel 4 sample 2
9
I
I
8
7-
6-
>5 01
3 -
2-
1 -
01 0
'
5
-
---
10
J
III
15
25
20
division no
30
35
40
45
62
8
-
-
7
Channel 5 sample 1
U
7-
6-
5-
4
3-
2-
1
0
5
5
10
10
15
20
25
30
35
40
45
division no
63
Channel 5 sample 2
9
I
I
8
7
6
>5
Bit
0
CL4
3
0
5
10
15
20
25
30
35
40
45
division no
64
Channel 6 sample 1
5
0
4
3
2
1
0'
0
-
5
10
lb
20
2b
30
35
40
45
division no
65
Channel 6 sample 2
8
5
04
>1
01
0
5
10
15
20
25
30
35
40
45
division no
66
8
I
I
10
15
Channel 7 sample 1
I
I
I
I
35
40
7
6
5
04
0
3
2
1
0I
0
-
m
-
5
20
25
division no
30
1
45
67
Channel 7 sample 2
8
7
6
5
04
3
2
1
0
1
0
---
5
10
15
20
25
30
35
40
45
division no
68
Channel 8 sample 1
7 -
6-
5-
3 01-
0-
0
5
10
15
20
25
30
35
40
45
division no
69
Channel 8 sample 2
9
8
71-
61
>5
n
4
3-
00
5
10
15
d20so n25
30
35
40
45
70
APPENDIX B: MATLAB CODE
71
function [output]= janalyze2( data, iStart, iEnd, threshold
%% function [outpu-]= janalyze2( data, star*, end, threshold
%% janalvze2: OutpLts an array of averages
aer
data: The channel data to analyze
% iStart
: The incex
hIere we star- looking , must
orrecpod
no
§o
%% iEnd: The index where we end looking, must correspond to noise
%% threshold:
Noise is
below This :hreshold, sIgnal
Is above
threshold
%% Example: output= janalyze2( data, 700, 2300, 0.66, 0.68
bSignal= 0;
iOutput= 1;
count= 0;
nTotal= 0;
% Set when we're in a signal
region
% The current output sample
% The inumber of data samples in the current region
% The running 6otal
%% Iterate over all
-f
for iData=iStart:iEnd
if(
7h1s
for the current
region
the given data samples
-bSignal & data(iData)>=threshold
%% Transition to data region
fprintf( 1, :ransItion from
se
data:
fo
%d
, iData );
output(iOutput)= nTotal / count;
iOutput= iOutput + 1;
nTotal= 0; count= 0; bSignal= 1;
elseif( bSignal & data(iData)<threshold
%% Transition to noise region
fprintf ( 1,
Triansition from data to
:se:
d
,
iData
output(iOutput)= nTotal / count;
iOutput= iOutput + 1;
nTotal= 0; count= 0; bSignal= 0;
end % if
transition to noise region
nTotal= nTotal + data(iData);
count= count + 1;
end
taor data valume
%%The last region is noise so we output itos average
output(iOutput)= nTotal / count;
72
function x = manualmerge(data, indexarray)
x
i
=
=
[1;
1;
while i < (max(size(indexarray)) + 1)
x = [x mean(data(indexarray(i) :indexarray(i+1)))];
i = i + 2;
end
73
function x = finalmerge(signoisel, signoise2, signoise3)
datal = siglessnoise(signoisel);
data2 = siglessnoise(signoise2);
data3 = siglessnoise(signoise3);
x = [datal data2 data3];
end
74
function x = SigLessNoise(signoise)
x = [];
i = 2;
while i < max(size(signoise))
x
i
=
=
[x (signoise(i) - mean([signoise(i-1),
i+2;
signoise(i+1)]))];
end
75
funcrion x = dbscale(nologans)
x = 10*log10(nologans/max(nologans));
end
76
REFERENCES
1.
J. Cummingham. "Future Role of NPOESS in Earth Environmental Remote
Sensing". October 2000 (presentation)
2. V. Leslie, "Three-Point Calibration of the NAST-M
Passive Microwave
Spectrometer". MassachusettsInstitute of Technology
3. M. Jansen. "Atmospheric Remote Sensing by Microwave Radiometry". John
Wiley and Sons Inc. 1993
4. W. Blackwell, J. Barrett, P. Rosenkranz, M. Shwartz, D. Staelin. "NPOESS
Aircraft Sounder Testbed-Microwave (NAST-M): Instrument Description and
Initial Flight Results". IGARSS 2000. 2000
5. D. Fink, D. Christsansen. "Electronic Engineer's Handbook". McGraw Hill. 1982
6. R. Wylde, D. Martin. "Gaussian Beam-Mode Analysis and Phase-Centers of
Corrugeted Feed Horns ". IEEE Transactions on Microwave Theory and
Techniques. Vol 1, No 10. Oct 1993
77