This paper presents a measurement system used for w-band complex permittivity measurements performed in NASA Langley Research Center's Electromagnetics Research Branch. The system was used to characterize candidate radome materials for the passive millimeter wave (PMMW) camera experiment. The PMMW camera is a new technology sensor, with goals of all-weather landings of civilian and military aircraft. The sensor is being developed under a NASA Technology Reinvestment program with TRW, McDonnell- Douglas, Honeywell, and Composite Optics, Inc. as participants. The experiment is scheduled to be flight tested on the Air Force's 'Speckled Trout' aircraft in late 1997. The camera operates at W-band, in a radiometric capacity and generates an image of the viewable field. Because the camera is a radiometer, the system is very sensitive to losses. Minimal transmission loss through the radome at the operating frequency, 89 GHz, was critical to the success of the experiment. This paper details the design, set-up, calibration and operation of a free space measurement system developed and used to characterize the candidate radome materials for this program

Fralick, Dion T.

English

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NASA Contractor Report 201720
/iV- M_//
W-band Free Space Permittivity
Measurement Setup for Candidate
Radome Materials
Dion T. Fralick
Lockheed Martin Engineering & Sciences, Hampton, Virginia
Contract NAS1-96014
August 1997
National Aeronautics and
Space Administration
Langley Research Center
Hampton, Virginia 23681-0001
https://ntrs.nasa.gov/search.jsp?R=19970026944 2020-06-16T01:25:50+00:00Z
The use of trademarks or names of manufacturers in this report is for
accurate reporting and does not constitute an official endorsement,
either expressed or implied, of such products or manufacturers by the
National Aeronautics and Space Administration.
Introduction
A system to characterize the permittivity of candidate
radome materials for the passive millimeter wave (PMMW)
camera experiment was developed in support of the NASA LaRC
Electromagnetics Research Branch (ERB). The PMMW camera is
a new technology sensor, with goals of all-weather landings
of civilian and military aircraft. The sensor is being
developed under a NASA Technology Reinvestment Program (TRP)
with TRW, McDonnell-Douglas, Honeywell, and Composite
Optics, Inc as participants. The camera operates at W-band,
in a radiometric capacity and generates an image of the
viewable field. Because the camera is a radiometer, the
system is very sensitive to losses. The transmission loss
through the radome at the operating frequencies, 84 GHz - 94
GHz, was of particular interest to the camera developers. As
a goal, the PMMW camera designers assigned a transmission
loss figure of 1.0 dB through the radome to be used during
the flight test. A test of the NASA LaRC B-757 radome, at
the PMMW camera operating frequencies, generated
transmission losses between 7.0 and 8.0 dB, well above the
tolerable loss for this new sensor. Development and
characterization of low-loss composite materials, at W-band
frequencies, became crucial to the successful development
and flight testing of the PMMW Camera.
System Design
Because of the severe time constraint associated with this
task, a robust, precision system which had been under design
was temporarily tabled in favor of a system that could be
put together quickly, for little cost, with reasonable
accuracy. The accuracy requested by the radome designers
was 10% in e', with no particular preference provided for
A free-space technique was used for its obvious advantages
over waveguide techniques at W-band frequencies. In
addition, all of the pieces necessary to build this "short-
term" solution were already branch resources except for the
sample holder. The measurement system was centered around
the HP 85106 mmW Measurement System, W-band external mixers
for the same, and Millitech Gaussian optics lens antennas.
The antennas used were standard, off-the-shelf, WR-10 fed,
conical horns with 3 inch diameter lenses with infinite
focal lengths. A table which had been fabricated previously
for the W-band transmission loss measurements on the NASA
-I-
LaRC B-757 radome was used to support the antennas,
waveguide and mixers. A simple sample holder was then
designed and fabricated on-site at NASA LaRC to complete all
the components necessary for this system.
A picture of the complete system set-up is shown in figure
i. The antennas were separated by a distance of
approximately 40 inches. This would allow the sample to be
illuminated by a column of rays with a diameter
approximately the size of the 3 inch aperture of the
antennas. I A sample size of 12 inches square was used based
upon the Gaussian optics lens antenna parameters and the
separation distance. A sample this size would be large
enough so the edges would not be greatly illuminated and
essentially all of the power radiated would propagate
through the sample. A simple isolation test with a 12 inch
square piece of 4 inch thick absorber provided an isolation
of greater than 70 dB and verified the above assumption.
Still, not having adequate hardware to verify the cross
section of the beam at the sample plane, the sample holder
was designed such that the edges of the sample would be
overlapped by about 1/4 inch of absorber. This would reduce
diffraction from the sample edges in the event they were
illuminated at greater levels than assumed. The absorber
plugs which were cut out and removed from each side of the
sample holder were saved for use as calibration standards.
After the sample holder was in place, a more rigorous
isolation test was performed using the cut out plugs. This
produced an isolation level of better than 78 dB, which was
more than sufficient for measuring low loss materials.
Sample frames were made from, General Plastics Manufacturing
Co., Last-A-Foam® to support the samples during the
measurement process.
System Alignment
Although there's really no substitute for precision, the
lack of precision was made up for, in part, by very careful
procedure. The alignment of the sample holder and
boresighting of the antennas was very tedious and
accomplished to our satisfaction only after several trials
and several hours of work. The alignment of the sample
holder to the beam was accomplished with a reflecting plate
and the boresighting was accomplished by peaking the
receiving signal level using a thru measurement.
-2-
System Calibration
A technique to calibrate the system was necessary for
operation of the free-space measurement set-up. Other free-
space systems that have been developed 2.3have successfully
used a thru-reflect-line (TRL) _ method as a calibration
technique. The drawback of the TRL method for our
application was that the "line" calibration standard
required changing the path length between the horn antennas
by k/4 at mid-band. Since several hours had been spent
tweaking the boresight, and precision adjustments were not
available, the desire was to not move the antennas after the
system was peaked. These facts and the reality that k/4 was
on the order of 0.025 inches at the frequencies of
operation led us to use a similar technique, the thru-
reflect-match (TRM) 3,_ technique. The TRM technique allowed
us to not move the antennas after boresighting and utilize a
"match" calibration standard in place of the "line"
standard. The match standard was accomplished by inserting
the absorber plugs, which had been cut out to form the
sample opening, as mentioned earlier. The reflect standard
was a nominally 3/8 inch thick aluminum plate with its
opposite faces milled flat and parallel. This was the same
plate used to align the sample holder as discussed earlier.
The thru standard was simply the empty sample holder. The
standards were defined to the HP 8510 based system using
the internal TRL/TRM measurement model.
Operation
Once calibrated, the measurement system was operated under
PC based control using the HP 85071 Material Measurements
Software. Several measurement models exist within this
software package. The precision and fast models were
exercised and results compared. The precision model was
chosen for the measurements of the candidate radome
materials and coatings since it appeared to perform better
for the low loss materials and provided more consistent
results for the imaginary portion of the permittivity. Time
domain gating was used to reduce the effect of multiple
reflections between the sample faces and the transmit horns.
To verify adequate calibration and operation, samples with
(relatively) known permittivities were measured. Air, a
0.062 inch thick sample of DuPont Teflon® and a sample of
0.I inch thick acrylic sheet were measured. Results of some
of these measurements are shown in figures 2(a), 2(b), 3(a),
3(b), 4(a), 4(b). Each of the figures shows data from two or
-3-
three independent measurements. The accepted value for air
is 1.0006 for the real part and zero for the imaginary part,
for all frequencies. The measured values shown in figures
2(a) and 2(b)are very close to these values. The values,
both real and imaginary, for Teflon® are very close to those
accepted for this material at much lower frequencies and are
shown in figures 3(a) and 3(b). A small sample, cut from the
same sheet, of acrylic was sent to Claude Weil at the
National Institute of Standards and Technology. He performed
permittivity measurements using a cavity resonance technique
in the 60 GHz range and provided the data 5 for comparison.
His measurements indicated a real value of permittivity
around 2.62 - 2.64 and an imaginary value ranging from 0.018
to 0.020. The measured values for the acrylic sample are
shown in figures 4(a) and 4(b) and agree closely but are
slightly higher than those provided by Weil. This could be
due to the different frequency range since permittivity does
change slightly with frequency or simply measurement error
due to our system set up.
The candidate radome materials and coating materials were
then measured. The results of these measurements are
documented and discussed in reference 6. Calibrations were
re-accomplished and verified on a regular basis throughout
the performance of the measurements due to the observed
sensitivity to temperature and surroundings.
Conclusions
A W-band free space permittivity measurement set-up was
designed and constructed for little cost and was operational
in just two weeks. This measurement system met the
requirements put forth by the radome designers for the PMMW
camera experiment. This will allow for the successful design
and construction of a radome to be used for the PMMW camera
flight tests later this year.
References
[i] J.Musil and F.Zacek, Microwave Measurements of Complex
Permittivity bv Free Space Methods and Their Applications,
Elsevier Science Publishing Company, 1986
[2] D.K. Ghodgaonkar, V.V. Varadan, V.F. Varadan, A Free-
Space Method for Measurement of Dielectric Constants and
Loss Tanqents at Microwave Frequencies, IEEE Transactions on
Instrumentation and Measurement, Vol 37, No. 3, June 1989
4-
[3] D. Blackham, Free Space Characterization of Materials,
Digest of the 15 th Annual Symposium of the Antenna
Measurements Techniques Association, Oct 4-8, 1993, Dallas,
Texas
[4] Hewlett Packard Product Note 8510-8A, Network Analysis,
Applyinq the HP 8510 TRL Calibration For Non-coaxial
Measurements, 1992
[5] C. Weil, National Institute of Standards and Technology,
Antennas and Materials Metrology Group, Personal
Communication, May 7, 1997
[6] R.L. Cravey, Complex Permittivities of Candidate Radome
Materials at W-band, NASA TM 110344, May 1997
-5-
iE
cJ_
-6-
1.001 ....
Real
Air
Permittivity
............................... !.................................... i................................... _ .............................. --; .............................................. ::::::::::i:, :£ ......... : ........
..........................i .............................................................................................................................................................T.....................................................................
0.999
79 84 89 94 99
Frequency (GHz)
Figure 2a
0.001
0.0005
Air
Imaginary Permittivity
0
79
,...................................i...................................................................._.................................._ .................,...................................
84 89 94 99
Frequency (GHz)
Figure 2b
-?-
2.1
Teflon ®
Real Permittivity
2.05
2
' i
......... i......................... _ ......................... " .............................. ! ......
.......................... i ......................
79 84 89 94 99
Frequency (GHz)
Figure 3a
0.05
Teflon®
Imaginary Permittivity
0.025
0
................................. i .................................. i ........................................................................................................................................................................... "_................................. i
..................................................................i .........................../::iiiii i
79 84 89 94 99
Frequency (GHz)
Figure 3b
-8-
_?.,!
3
2.8
2.6
Acrylic Sheet
Real Permittivity
........................................................................................................................................i i i
79 84 89 94 99
Frequency (GHz)
Figure 4a
0.06
Acrylic Sheet
Imaginary Permittivity
0
/ \
0.04 ....................._ ,.............................._- ...............................................................i / ----_
0.02 .....__ ..................................S....::_7::::::---: .......................................i.._A,, i........................1
79 84 89 94 99
Frequency (GHz)
Figure 4b
-9-
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4. TITLE AND SUbTiTLE
W-band Free Space Permittivity
Radome Materials
6. AUTHOR(S)
Dion T. Fralick
August 1997
Measurement Setup for Candidate
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Lockheed Martin Engineering & Sciences
Hampton, Virginia
9. SPONSORING/ MONITORINGAGENCYNAME(S)ANDADDRESS(ES)
National Aeronautics and Space Administration
Langley Research Center
Hampton, VA 23681-0001
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Langley Technical Monitor: Melvin C. Gilreath
Contractor Report
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WU 522-33-11-04
NAS1-96014
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AGENCY REPORT NUMBER
NASA CR-201720
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13. ABSTRACT (Maximum 200 words)
This paper presents a measurement system used for w-band complex permittivify measurements performed in
NASA Langley Research Center's Electromagnetics Research Branch. The system was used to characterize
candidate radome materials for the passive millimeter wave (PMMW) camera experiment. The PMMW camera
is a new technology sensor, with goals of all-weather landings of civilian and military aircraft. The sensor is
being developed under a NASA Technology Reinvestment program with TRW, McDonnell- Douglas, Honeywell,
and Composite Optics, Inc as participants. The experiment is scheduled to be flight tested on the Air
Force's "Speckled Trout" aircraft in late 1997. The camera operates at W-band, in a radiometric capacity and
generates an image of the viewable field. Because the camera is a radiometer, the system is very sensitive to
losses. Minimal transmission loss through the radome at the operating frequency, 89 GHz, was critical to the
success of the experiment. This paper details the design, set-up, calibration and operation of a free space
measurement system developed and used to characterize the candidate radome materials for this program.
;14.SUBJECT ERMS
Free Space Measurements; Permittivity; W-Band
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W-Band Free Space Permittivity Measurement Setup for Candidate Radome Materials

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