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
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
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 Conceptualizing Noise ... 24 6.3.3 M easurement Algofithm ... 25 6.4 FINDING A V ... 27 6.4.1 Splicing ... 27 6.4.2 Analyzing data2 ... 29
6.4.3 Processing datal and data3 ... 30
6.4.4 Re-joining data ... 33
7. LIM ITATIONS ... 52
7.2 ENVIRONM ENT ... .. ... . ... 52
7.3 SUBJECTIVITY OF JUDGEMENT ... ... 53
8. FUTURE WORK. ... 54
8.1 V ARYING 0 , 0 FOR A GIVEN . . -... ... ... 54
8.2 OTHER DIRECTIONAL ISSUES... 55
8.3 N OISE FACTORS ... ... .... ---... 55
9. CONCLUSION ... 56
APPENDIX A: GRAPHS OF POWER (V) VS DIVISION NO... 57
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 Polar-orbiting Observational Environmental Satellite System (NPOESS) Aircraft Satellite Testbed (NAST). The platforms for deployment are currently the NASA ER-2 high-altitude 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 solar-geophysical 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
The NAST-M has two total-power, multi-channel radiometers. The first has a single-sideband 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
2. RADIOMETER OVERVIEW
2.1 The Radiometer
A superheterodyne total-power single-channel radiometer following main elements:
" Antenna
* Local Oscillator " Mixer
" IF Amplifier * Filter
typically consists of the
* Detector * DC Amplifier
A Schematic is shown below in Fig 2-1.5
Mixer IF Amplifier Filter Detector DC Amplifier
V~ Tr + Ta T aL R F
OIFr
LO
Local Oscillator
Fig. 2-1. Total-power superheterodyne radiometer
Antenna
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 physical layout of the assembly.6
Frcnt V4 Qq Z~-~ 1 ty 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
\b- \26 Cl\ 2_ h fy)
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 gain pattern, 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
7Jansenp
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.
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,$) at each of the 22 scan angles will suffice. This will result in an antenna pattern. For instance, measurements for a particular frequency
/
could be normalized relative to G(f,0,0) at scan angle 0, = nadir. In this case, G(f,0,) is taken to be 1 and all other measurements of G(f,6,$) 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, and 0,
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) Center frequency, 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
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 someinteger multiple of
f,,.
f
0 , could then be tweaked until it was a desired centerfrequency,
f,. f,
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 -~
H
7
/ 30dB SNK o6.servec4 S Pe0-L~O
C1 -z
Fig. 4-2. Test-signal measurement
N
To overcome the frequency limitations of the spectrum analyzer, the mixer on the NAST-M was used (effectively separate from the instrument itself) to downconvert the incoming
signal ( f,, ) to some final frequency, ffinal flu, was fed directly into the RF terminal of
the mixer and 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 = f0U, -46 GHz
and since the range of
fu,
is as follows:50.3 GHz <
f,
< 56.02 GHzit 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
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
All 8 values of
f
successfully corresponded to the 8 values off
.
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 10.02 115. 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 NAST-M'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
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-Centers of Corrugated Feed
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 center that was chosen in the case of the NAST-M was the beam-mode
phase-center". 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
11Wylde et al, p 1697
(0.6435)21ka2 2H
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):
k
2( 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:
Table 5.1: Relevant Parameters and their Values
d, was thus found to be 19.2cm.
Parameter Value K 1131 /m A 3.175 cm H 30.0 cm A 0.79 m go = 41r x10-7 H/m Eo = 8.84 x10~ F/rm T 0.64
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.
tO tbe(q -e
\
-burotO'f
\
.O
CV l,4
777/
7
NTA
/ Q00 /~ ,// ecV~c~ ~xt\Vzo
N 7 \, &O'~e
f
Fig. 6-1. Transmitter and NAST-M
- 31b2e \egJ
CVnA \ K
-Oaf .--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
(Test-Signal Generation). As done previously, The HP3640A frequency synthesizer was
programmed to generate some input frequency,
f
, so that the desired output signal from the WR-15,f
0 was an appropriate test signal (one of the channel center frequencies,f).
4 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.
i
t&\C
ti a f 11--tCb\j 1-i Q~zy~ -0 lot-~rt QVOLu-
Yae
\l
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
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
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
9- 8- 7- 6- 5-4 3 2 1 0 0 channel 4 data 1 500 1000 1500 sample no 2000 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
4
I1
jj1jIfj~r~~
-
k~ _lL~
k IThus 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:
0.3 0.29- 0.28-0.27 0.26- 0.25- 0.24- 0.23-0.22 0.21 0 datal I I' HI i~) ~II
V
' tvAi
, (X\. N: 50 100 150 200 250 300 350 400 450 500 ~vi ~~ Lf~Pi
Fig. 6-4. datal I I I I I I I I I Idata2
K
I
K
200 400 600 800 1000 1200 1400 1600 1800 2000 Fig. 6-5. data2 9 8 7 6 5 4 3 2 1 0 0-
]
-data3 0.26 r---.---.---.---....---,---r---.---~-___., I
~'l
rv I tI
0.24I
I
Ii iI~
i
I ! i[
I
i j 0.22~~
l~
o 50 100 150 200 250 Fig. 6-6. data3.
~~~
I1
I
300 350 400 i iWr
450datal 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
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
I
Ii.MnEEEEEEEEEEEEEIIIUIU 7 RIUInnn IIUIRIIEEEEI EEIEIEE IEIM
-- M1ii
20 30 40 50 60
10
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.
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
noise-signal and noise-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:
Channel 4 signoisel
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Fig 6-8 signoisel (from datal) 0.35 0.3 0.25- 0.2-0.15 -0.1 0.05 -0
Channel 4 signoise3
4 5 6 7
IIII"II
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 code finalmerge.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
0.35 0.3 0.25 -0.2 F 0.15 I 0.1 0.05 F 0 1 2 3 16 17 Appendix B Appendix B
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 0 III I I II 7- 6- 54 -0 3- 21 -01 0 5 10 15 20 25 dhvsion no 30 35 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.m 1 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:
Channel 4 Sample 0- -25- -15- -20- -25- -30--35 0 5 10 15 20 25 30 35 40 45 division no
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*log 10 (as might be expected, the vertical axis being in Volts). This was because the voltages read off the NAST-M were proportional to 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.
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 0 7 6 5 04 3 2 1 01 - I _ .mUUUUUUUU 0 5 10 15 20 25 division no 30 35 40 45
Fig. 6-12. Channel 4 sample 1 (V)
Channel 1 Sample 1 M *inMEMEM 0 5 10 15 20 25 division no 0 -5 -10 ca '~-15 -20 -25 -30
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:
Channel 1 Sample 2 0 -5- -10- -15--30 -35 0 5 10 15 20 25 30 35 40 45 division no
0 -5 10 0 5 10 15 20 25 division no 30 35 40 45
Fig. 6-15. Channel 2 sample 1
Channel 2 Sample 1 C ii CD -15- -20- -25- -30--35
Channel 2 Sample 2 M- I'-nEEMEN 0- -5- -10- -15- -20- -25- -30--35 0 5 10 15 20 25 division no 30 35 40 45
Fig 6-16. Channel 2 sample 2 0
I I I I I
Channel 3 Sample 1 --- -I I I I I 5 10 15 20 25 division no 30 35 40 45
Fig. 6-17. Channel 3 Sample 1 0 -5 -10 -15 ---20 -25 -30 -35 -40 0
Channel 3 Sample 2 . r- -m . -15 -20 -U. 5 10 15 20 25 division no 30 35 40
Fig 6-18. Channel 3 sample 2 -35
Channel 4 Sample 2
II I I I I 1j
5 10 15 20 25 30 35 40 45
division no
Fig. 6-19. Channel 4 sample 2 0 -5 -10- -15-(9 -20 1--25 -30 -35 -40 L 0
0 -5 -10 -15 m -20 0 -25 -30 5 10 15 20 25 division no 30 35 40 45
Fig. 6-20. Channel 5 sample 1
Channel 5 Sample 1
-35
--40
Channel 5 Sample 2
-, --
-5 10 15 20 25
division no
30 35 40 45
Fig. 6-21. Channel 5 sample 2 0 -5 -10- -15- -20- -25-Go ca -30 -35 L 0 ' '
Channel 6 Sample 1 Mmm =inmm -10- -15- -20- -25- -30- -35-5 10 15 20 25 division no 30 35 40 45
Fig. 6-22. Channel 6 sample 1 0
-5
0
0 -15- -20--25 --30 --35 0 5 10 15 20 25 30 35 40 45
Channel 7 Sample 1 M--, -MEMEN 0- -5- -10- -15- -20-0 -25- -30- -35--40() 5 10 15 20 25 division no 30 35 40 45
Fig. 6-24. Channel 7 sample 1
I I I ~ I I
0 -5 -10 -15 _--20 -25 -30 5 10 15 20 25 division no 30 35 40 45
Fig. 6-25. Channel 7 sample 2
Channel 7 Sample 2
I=_T =I
-35
f--40'
Channel 8 Sample 1
T--,=mnr
5 10 15 20 25
division no
30 35 40 45
Fig. 6-26. Channel 8 sample 1 0- -5--10 - -15--20 - -25- -30--35 -0 I I I I I
0--5 -10f 0 5 10 15 20 25 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.
Channel 8 Sample 2 -15 C-0-20 -25 -30 --35
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.
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.
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.
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.
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.
APPENDIX A: Graphs of Power (V) vs division no.
Channel 1 sample 2 9 8 7 6 I I ~ I I >5 -0 0. 4 - 3- 2f-1K
0 --- _mE 0 5 10 15 20 25 division no IEMMMEMEMM . 1 30 35 40 45 I I tChannel 2 sample 1 -r ... 0 5 10 15 20 25 division no 30 35 40 45 8 7 6 5 04 0 3 2 1
Channel 2 sample 2 7 6 -> 5- S43 - 211 -0 5 10 15 20 25 30 35 40 45 division no
Channel 3 sample 1 > -0- 4 3 2 1 -01 -0 5 10 15 20 25 30 35 40 45 division no I 1 1
Channel 3 sample 2 8- 7-6 ->5 - 3.- 2- 1-0 5 10 15 20 25 30 division no 35 40 45 9
Channel 4 sample 2 I I
j
21 -01 - ' - --- J III 0 5 10 15 20 25 division no 30 359
8 7- 6->5 -01 3 -40 45Channel 5 sample 1 8 - 7 7- 6- 5-4 3- 2-1 5 10 0 5 10 U 15 20 25 division no 30 35 40 45
-Channel 5 sample 2 I I 10 15 20 25 division no 9 8 7 6 >5 0 CL4 3 0 5 Bit 30 35 40 45
Channel 6 sample 1 10 lb 20 2b division no 30 35 40 45 5 4 0 3 2 1 0' -0 5
Channel 6 sample 2 8 5 04 >1 01 0 5 10 15 20 25 division no 30 35 40 45
Channel 7 sample 1 I I I I I I 8 7 6 5 04 0 3 2 1 0 - 1 30 35 40 45 10 15 20 25 division no I - m 0 5
Channel 7 sample 2 01 ---0 5 10 15 20 25 30 35 40 45 division no 8 7 6 5 04 3 2 1
Channel 8 sample 1 7 - 6- 53 - 01- 0-0 5 10 15 20 25 division no 30 35 40 45
Channel 8 sample 2 9 8 71- 6-> 5 1 4 n 3-00 5 10 15 d20so n25 30 35 40 45
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 §o no
%% iEnd: The index where we end looking, must correspond to noise
%% threshold: Noise is below This :hreshold, sIgnal Is above 7h1s
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 for the current region
%% Iterate over all -f the given data samples
for iData=iStart:iEnd
if( -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 t aor data valu me
%% The last region is noise so we output itos average output(iOutput)= nTotal / count;
function x = manualmerge(data, indexarray) x = [1; i = 1; while i < (max(size(indexarray)) + 1) x = [x mean(data(indexarray(i) :indexarray(i+1)))]; i = i + 2; end
function x = finalmerge(signoisel, signoise2, signoise3)
datal = siglessnoise(signoisel); data2 = siglessnoise(signoise2); data3 = siglessnoise(signoise3); x = [datal data2 data3];
function x = SigLessNoise(signoise)
x = []; i = 2;
while i < max(size(signoise))
x = [x (signoise(i) - mean([signoise(i-1), signoise(i+1)]))]; i = i+2;
funcrion x = dbscale(nologans)
x = 10*log10(nologans/max(nologans)); end
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". Massachusetts Institute 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
