This paper provides insight into the difficult analytical issue for launch vehicles and spacecraft that has applicability outside of the launch industry. Radiation from spacecraft or launch vehicle antennas located within enclosures in the launch vehicle generates an electromagnetic environment that is difficult to accurately predict. This paper discusses the test results of power levels produced by a transmitter within a representative scaled vehicle fairing model and provides preliminary modeling results at the low end of the frequency test range using a commercial tool. Initially, the walls of the fairing are aluminum and later, layered with materials to simulate acoustic blanketing structures that are typical in payload fairings. The effects of these blanketing materials on the power levels within the fairing are examined

Stanley, James

Trout, Dawn

Wahid, Parveen

This paper provides insight into the complicated issue of launch vehicle resonate cavity effects that has applicability outside of the space industry. Radiation from spacecraft or launch vehicle antennas located within the enclosures of a launch vehicle generates an electromagnetic environment that is difficult to predict accurately. This paper discusses the test results of power levels produced by a transmitter within a representative scaled vehicle fairing model and provides preliminary modeling results at critical frequencies using a commercial tool. Initially, fairing walls are aluminum and are then layered with materials to simulate acoustic blanketing structures that are typical in payload fairings. The effects of these blanketing materials on the power levels within the fairing are examined. Industry data is also reviewed for comparison purposes. © 2009 IEEE

Trout, Dawn H.

Wahid, Parveen F.

Stanley, James E.

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

Electromagnetic Cavity Effects From Transmitters Inside A Launch Vehicle Fairing

NASA Technical Reports Server

Electromagnetic Cavity Effects from Transmitters inside a 
Launch Vehicle Fairing 
Dawn H. Trout, Parveen F. Wahid, James E. Stanley 
Abstract: 
This paper provides insight into the difficult analytical issue 
for launch vehicles and spacecraft that has applicability 
outside of the launch industry. Radiation from spacecraft or 
launch vehicle antennas located within enclosures in the 
launch vehicle generates an electromagnetic environment that 
is difficult to accurately predict. This paper discusses the test 
results of power levels produced by a transmitter within a 
representative scaled vehicle fairing model and provides 
preliminary modeling results at the low end of the frequency 
test range using a commercial tool. Initially, the walls of the 
fairing are aluminum and later, layered with materials to 
simulate acoustic blanketing structures that are typical in 
payload fairings. The effects of these blanketing materials on 
the power levels within the fairing are examined. 
Background: 
Intentional and inadvertent activation of spacecraft or vehicle 
emitters while inside a launch vehicle fairing is a long-
standing issue in the space industry. The primary intentional 
RF emitters for spacecraft and launch vehicles operate at 
frequencies in the UHF, S, C and X Bands. It is often a 
requirement to evaluate the effects of spacecraft radio 
frequency (RF) transmission while enclosed within the 
payload fairing. Requirements for this evaluation can stem 
from a desire on the part of the spacecraft to transmit status 
signals to respective ground stations during launch, because of 
a desire not to change states while in the fairing, or due to 
insufficient inhibits to show that transmission will not occur. 
In addition, since at fairing separation launch vehicles 
typically change from external fairing antennas to internal 
antennas, there is often a transient RF cavity effect produced 
within the fairing until the fairing is completely deployed at 
separation. Because of short wavelengths and typically small 
antenna dimensions relative to spacing between emitters and 
sensitive equipment, analysis of the RF environment for the 
typical external antenna case can be performed in the far field 
under free space conditions thus making the calculations 
straight forward. In the enclosed fairing, however, multiple 
reflections, scattering and resonances occur that complicate 
this analysis. Since accurate modeling of these fields requires 
a mesh with no more than 1/10 ofa wavelength and fairing 
dimensions can be several meters, the computational 
capabilities required are enormous. With recent expansion in 
computational processing capability at KSC and the use of a 
Plexiglas fairing model used in a computational fluid 
dynamics modeling efforts in [J], a new study is underway to 
use actual test data to improve the ability to model this 
complicated cavity problem. 
Existing Data 
Separate testing and or analysis have been performed 
independently by launch vehicle providers. Moreover, this 
data is proprietary. Consequently, no consistent approach has 
been used across the launch vehicle community to deal with 
this cavity problem. This paper is using a generic test vehicle 
with generic materials that are typical of launch vehicles to 
evaluate this issue in a publishable forum. 
Hallet and Redell [2] sought to quantify these fields using 
Poynting's theorem to solve for an equivalent wave that 
would dissipate the transmitted power in the surface areas of 
the surrounding fairing, blanketing materials and associated 
apertures ("the power supplied by the transmitter is equal to 
the power leaving through a closed surface." 
The equation below is used to calculate the field [2] 
where 
~------------------
P, 
I----------~------~= 
n=k [77n J 2IAn • real 2 
n=1 1770 + 77 n I 
1 ~ l[ 4 An . 77 n J 
- L,.rea 
2 n=1 1770 +77J 
Equation 1. 
770 is the intrinsic impedance of the media (air) in the fairing volume. 
77 n is the impedance of the surface area exposed to the media 0 (air). 
An is the surface area in square meters. 
n is a designator for each surface type. 
A scaled fairing model as well as an empty cavity was used to 
validate this method with some reasonable correlation. The 
actual test data reported in this reference was limited due to 
the proprietary nature of the study. The author's caution "this 
technique yields an assumed uniform field strength, not an 
exact solution of the field distribution." Significant modeling 
of equivalent complex impedances of typical fairing materials 
was performed for the study. This modeling has shown 
promise, but since spacecraft are expensive with often delicate 
instruments, it is necessary to have a more precise method of 
predicting the RF environment as testing to higher levels 
without adequate proof that these levels exist near these 
sensitive instruments is extremely unpopular. 
One significant contribution of this report was the technique 
of accounting for layered materials. Commercial tools that 
our group has evaluated that are capable of performing 
computational electromagnetic analysis of this scale do not 
allow specification oflayers at a boundary. This report used 
equivalent impedances to allow the entire layered boundary to 
be represented as one equivalent boundary material. This 
technique will be utilized with the commercial tool and 
presented in future publications to evaluate the blanketed test 
case presented here. The prediction method and to a lesser 
extent the test data shows a standing wave effect when layered 
https://ntrs.nasa.gov/search.jsp?R=20110011478 2019-08-30T15:41:25+00:00Z
---------------------
blanketing is used at specific frequencies where the distance 
between the layers with some conductivity approach multiples 
of one quarter of a wavelength. 
Test Configuration: 
The test article shown in Figure I used for this testing was a 
Plexiglas enclosure in the shape of a representative fairing 
bolted at the edges and supported by a metal frame. The 
internal cavity was lined with industry aluminum foil and 
smoothed as much as possible to simulate aluminum fairing 
walls of a typical launch vehicle and the perfect electric 
conductor of the computer model. 
Figure 1. Scaled Fairing Plexiglas Model with Aluminum 
Foil 
Baseline tests were performed in this configuration before 
other lining materials were added. A low input level of zero 
(0) dBm or one milliWatt (1 mW) from an Agilent E8257D 
Signal Generator was used for most of the test, although an 
amplifier was used to generate higher levels to verify that the 
power level chosen would not affect the results. This data 
showed linear correlation with power input increase therefore 
only the 0 dBm data will be presented here. A Spectrum 
Analyzer was used to measure the received power from the 
pick-up antenna. This set-up is shown in Figure 2. 
Figure 2. Pro_E model of half-cut on fairing to show Test 
Set-up 
The EM CO 3115 double ridge guide hom antenna used for 
both transmit and receive antennas at the top and bottom of 
the test fairing is shown in Figure 3. 
Figure 3. EMCO 3115 Double Ridge Guide Horn 
Test Results 
Data showing the frequency response of multiple power levels 
in the aluminum lined mock fairing configuration is given in 
Figure 4. Although there is variation across the 750 MHz to 
18 GHz Frequency range, the data shows that these 
measurements are scalable with power level. 
300E·Ol ,.... _________ ..... _--------, 
2 00e · O, J.I+lh------------------i 
oooe·oo 
-700E.O' .J.----------..... ..-.----...... """"'----.... 
Figure 4: Power Measurements with Bare Aluminum at 
varied power levels. 
Kapton 377® manufactured by DuPont was used as a liner for 
the cavity since this material is specifically designed for RF 
absorption and Kapton is a material frequently used for launch 
vehicle and spacecraft blanketing materials for its thermal 
insulating properties. Standard I" foam was used to separate 
the blanketing material. The test cases were studied for walls 
(i)Aluminum (ii) Aluminum-Kapton (iii) Aluminum-Kapton-
Foam, and (iv)Aluminum-Kapton-Foam-Kapton. Data was 
recorded for each test case from 750 MHz to 18 GHz and 
shown in Figure 5. 
Buildup Received Power CO"1larison 
~ 0 0 0 0 0 ~ 0 
ILl ILl W W ~ W W ILl 
'" '" 
N 
'" '" 
..: ..: ~ ~ ..:: ..: ..: ,.; ,.; 
-4.00E+01 ~--L-~----------""""\r-+----';1 
-4.50E+01 -!----.::---------------"---1 
.5.00E+01 J..... ____ ~~_~~~~=~~~~~~-..t 
Aluminum lining 
- Alluminum/Kapton/Foam 
Frequency [Hz] 
--+E- Altuminum/Kapton 
- Aliuminum/KaptonlFoam/Kapton 
Figure 5: Power Measurements with Bare Aluminum and 
Various Blanketing Materials 
The test data obtained for the aluminum lined fairing shows 
wide variation in power levels. This case has predominately 
the highest envelope except at frequencies in the upper range 
of the test data (above 14 GHz) where the addition offoam 
and Kapton resulted in slightly higher levels. In the 4 - 8 
GHz range the aluminum lined case is 10 to 20 dB above the 
power data from all other lined cases. Each additional layer 
caused less fluctuations in the power levels especially at 
higher frequencies . Minimal effects were observed from 
adding the foam only which is expected since the impedance 
of this material approaches free space. A profound smoothing 
effect was observed with the multiple layers of Kapton and an 
approximately 20 dB decrease with this layering above 14 
GHz. 
Ref2 also showed attenuation in field strengths with similar 
blanket materials from the bare fairing wall case by 8-26 dB. 
Similar results were found here, however the standing wave 
pattern predicted from the separation between the layers of the 
blanketing materials was not nearly as pronounced as in Ref2. 
Modeling 
The test data collected is then compared to the simulation data 
generated by the electromagnetic simulation software, EM 
Software & System's FEKO. 
A Pro-E model of the plexiglass structure was developed and 
imported into FEKO. The fairing walls were modeled as 
perfect electric conductors in the FEKO model. The Antenna 
was built in FEKO to match the antenna structure of EM CO 
3) 15 shown in Figure 3. The pattern from this model was 
then obtained as shown in Figure 7 and used in the modeled 
fairing structure. 
z 
Figure 6. FEKO Model of EMCO 3115 Double Ridge 3-D 
The results compared nicely with the published EMCO gain 
pattern data. One plane pattern is shown in Figure 7. 
r 
I 
Electric lar Ileia 
I- , ........ "E.J~, I 
'80 
200&-ro-21I : Bo/ICOJ I1~tmlgH-v4 
Figure 7. EM CO 3115 Double Ridge Plane Antenna 
Pattern 
The bare fairing wall case was simulated at 1 to 1.9 GHz in 
100 MHz increments using a hybrid Method of 
Moments/Physical Optics model. 
Power Simulated Rx (Watts) 
0.000284 
0.000282 
U) 0.00028 
::: 0.000278 CIS 
~ 0.000276 
~ 0.00027 4 +---~""",,~ ........... Jt+---~ 
~ 0.000272 #~~.......,....,.,........,.,.....~...,.,........,.,.....~ 
c.. 0.00027 ~.....r...,.,..........,..... ......... "......,........,.....:-..,..,-~ 
0.000268 -I-~""""'---"":-"""-"":-"""~ 
0.000266 +-----.....-~--____1 
1.00E+09 1.50E+09 
Frequency (Hz) 
Figure 8: Simulated Received Power 
2.00E+09 
Figure 8 shows received power from the simulated case. 
Although there is a general increase in the power received 
with frequency , the variation is only 0.22 dB over the range 
from 1.0 to 1.9 GHz. The power losses shown at particular 
frequencies in the test cases were duplicated in these results. 
Model Vs. Test Data 
Frequency (Hz) 
Figure 9: Differences between test data and model 
The model predicts the peaks of test data within 1 dB when 
test data is adjusted for the typical high VSWR in the 1-2 GHz 
range. The power losses in the cavity are not predicted in the 
model as accurately and the most significant loss at 1.7 GHz 
test case was approximately 8 dB lower than the model case. 
Comparisons to the Free Space Case at 1 GHz 
To evaluate the validity of the test results from a general 
sense, the free space predicted value was calculated. At I 
GHz, using 2D/A "2 criteria, this antenna would be in free 
space. Where D is the largest dimension of the antenna and A 
is the wavelength. 
Pr = Pt*(Al41tf),,2*(G),,2 = (I mW)(0.3/(41t( 1.57»)"2*( 4.09)"2 
= 3.8 /-lWatts 
Where: 
Pr = Power Received 
Pt = Power Transmitted 
R = distance between transmit and receive antennas in meters 
G = gain of the antenna at I GHz 
The measured power at 1 GHz was 203 /-lWatts. The 
difference between these values can be attributed to 
amplification from the cavity. 
1 O*log (20313 .8) = 17.27 dB 
Comparing this value to an approximate Volume Q for PEC 
per reference 3, we obtain: (The approximate volume is 
0.3632 m"3) 
Q= (161t"2V)/A " 3 = 2124 = 33 dB 
Accordingly, it is reasonable that the measured power levels 
within the cavity are significantly higher than the free space 
calculated values. 
Conclusions: 
This study shows how the fields increase in the fairing model 
lined with a good conductor, the effects of the various 
blanketing materials that typically line the fairing walls in 
launch vehicles, and the effects of adding a conductive 
spacecraft. Losses contributed to blanketing materials were 
shown to be comparable to the available industry reports. 
Preliminary modeling results from 1.0 to 1.9 GHz range 
which showed peak levels were well matched. 
The variations in the power data in the aluminum only case 
was expected due to multiple cavity reflections. The layered 
RF absorbing liners test cases had the biggest reduction on the 
observed power levels. Although there was variation with 
frequency, no pronounced standing wave pattern appeared as 
with Ref 2 data. 
Future Work: 
Testing should be modified to examine more points in the 
cavity to identify changes in eigenmodes especially with the 
addition of a spacecraft model. Reference 4 shows mUltiple 
examples of various eigenrnodes for a conical structure. An 
omni-directional antenna will also be used to remove 
uncertainties regarding increases in field from the gain of the 
antenna. Further investigation into the differences in power 
absorption modeled versus test case is warranted as well as an 
extension of the modeled frequency range and cases 
considered. 
References: 
I. "CFD Validation with LDV Test Data for Payload/Fairing Internal 
Flow", AIAA, Kandula, Hammad, Schall horn 
2. Technique for Predicting the RF Field Strength inside and 
Enclosure, M. Hallett, J. Redell, NASAlTP - 1998-206864 
3. D. A. Hill, M. T. Ma, A. R. Ondrejka, B. F. Riddle, M. L. 
Crawford, and R. T. Jonk, "Aperture excitation of electrically 
large, lossy cavities," IEEE Trans. Electromagn. Compo!., vol. 36, 
no. 3, pp. 169- 177, Aug. 1994. 
4. " Resonant Modes of Conical Cavity", Greg Egan, Website 
www.gregegan.net/Science/Cavity/Cavity.html. 


Electromagnetic Cavity Effects from Transmitters Inside a Launch Vehicle Fairing

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Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=20130012824 2019-08-31T00:45:50+00:00Z
Electromagnetic Cavity Effects from Transmitters inside a 
Launch Vehicle Fairing 
Dawn H. Trout, Parveen F. Wahid, James E. Stanley 
Abstract: 
This paper provides insight into the difficult analytical issue 
for launch vehicles and spacecraft that has applicability 
outside of the launch industry. Radiation from spacecraft or 
launch vehicle antennas located within enclosures in the 
launch vehicle generates an electromagnetic environment that 
is difficult to accurately predict. This paper discusses the test 
results of power levels produced by a transmitter within a 
representative scaled vehicle fairing model and provides 
preliminary modeling results at the low end of the frequency 
test range using a commercial tool. Initially, the walls of the 
fairing are aluminum and later, layered with materials to 
simulate acoustic blanketing structures that are typical in 
payload fairings. The effects of these blanketing materials on 
the power levels within the fairing are examined. 
Background: 
Intentional and inadvertent activation of spacecraft or vehicle 
emitters while inside a launch vehicle fairing is a long-
standing issue in the space industry. The primary intentional 
RF emitters for spacecraft and launch vehicles operate at 
frequencies in the UHF, 5, C and X Bands. It is often a 
requirement to evaluate the effects of spacecraft radio 
frequency (RF) transmission while enclosed within the 
payload fairing. Requirements for this evaluation can stem 
from a desire on the part of the spacecraft to transmit status 
signals to respective ground stations during launch, because of 
a desire not to change states while in the fairing, or due to 
insufficient inhibits to show that transmission will not occur. 
In addition, since at fairing separation launch vehicles 
typically change from external fairing antennas to internal 
antennas, there is often a transient RF cavity effect produced 
within the fairing until the fairing is completely deployed at 
separation. Because of short wavelengths and typically small 
antenna dimensions relative to spacing between emitters and 
sensitive equipment, analysis of the RF environment for the 
typical external antenna case can be performed in the far field 
under free space conditions thus making the calculations 
straight forward. In the enclosed fairing, however, multiple 
reflections, scattering and resonances occur that complicate 
this analysis. Since accurate modeling of these fields requires 
a mesh with no more than 1/10 of a wavelength and fairing 
dimensions can be several meters, the computational 
capabilities required are enormous. With recent expansion in 
computational processing capability at KSC and the use of a 
Plexiglas fairing model used in a computational fluid 
dynamics modeling efforts in [I], a new study is underway to 
use actual test data to improve the ability to model this 
complicated cavity problem.
Existing Data 
Separate testing and or analysis have been performed 
independently by launch vehicle providers. Moreover, this 
data is proprietary. Consequently, no consistent approach has 
been used across the launch vehicle community to deal with 
this cavity problem. This paper is using a generic test vehicle 
with generic materials that are typical of launch vehicles to 
evaluate this issue in a publishable forum. 
Hallet and RedelI [2] sought to quantify these fields using 
Poynting's theorem to solve for an equivalent wave that 
would dissipate the transmitted power in the surface areas of 
the surrounding fairing, blanketing materials and associated 
apertures ("the power supplied by the transmitter is equal to 
the power leaving through a closed surface." 
The equation below is used to calculate the field [2] 
El_I	
r2n=l
- 2 A,, real_ 	 2 	
reai4A,,• 2
frlo+'i,,l 	 11h,+) 7,, 
Equation 1. 
where
h o is the intrinsic impedance of the media (air) in the fairing volume. 
17,, is the impedance of the surface area exposed to the media 0 (air). 
A,, is the surface area in square meters. 
n is a designator for each surface type. 
A scaled fairing model as well as an empty cavity was used to 
validate this method with some reasonable correlation. The 
actual test data reported in this reference was limited due to 
the proprietary nature of the study. The author's caution "this 
technique yields an assumed uniform field strength, not an 
exact solution of the field distribution." Significant modeling 
of equivalent complex impedances of typical fairing materials 
was performed for the study. This modeling has shown 
promise, but since spacecraft are expensive with often delicate 
instruments, it is necessary to have a more precise method of 
predicting the RF environment as testing to higher levels 
without adequate proof that these levels exist near these 
sensitive instruments is extremely unpopular. 
One significant contribution of this report was the technique 
of accounting for layered materials. Commercial tools that 
our group has evaluated that are capable of performing 
computational electromagnetic analysis of this scale do not 
allow specification of layers at a boundary. This report used 
equivalent impedances to allow the entire layered boundary to 
be represented as one equivalent boundary material. This 
technique will be utilized with the commercial tool and 
presented in future publications to evaluate the blanketed test 
case presented here. The prediction method and to a lesser 
extent the test data shows a standing wave effect when layered
'I 
Figure 3. EMCO 3115 Double Ridge Guide Horn 
Pro_E model of half-cut on fairing to show Test 
blanketing is used at specific frequencies where the distance	 T 
between the layers with some conductivity approach multiples 
of one quarter of a wavelength. 
Test Configuration: 
The test article shown in Figure 1 used for this testing was a 
Plexiglas enclosure in the shape of a representative fairing 
bolted at the edges and supported by a metal frame. The 
internal cavity was lined with industry aluminum foil and 
smoothed as much as possible to simulate aluminum fairing 
walls of a typical launch vehicle and the perfect electric 
conductor of the computer model. 	
L 
Figure 2 
Set-up 
The EMCO 3115 double ridge guide horn antenna used for 
both transmit and receive antennas at the top and bottom of 
the test fairing is shown in Figure 3. 
Figure 1. Scaled Fairing Plexiglas Model with Aluminum 
Foil
Baseline tests were performed in this configuration before 
other lining materials were added. A low input level of zero 
(0) dBm or one milliWatt (I mW) from an Agilent E8257D 
Signal Generator was used for most of the test, although an 
amplifier was used to generate higher levels to verify that the 
power level chosen would not affect the results. This data 
showed linear correlation with power input increase therefore 
only the 0 dBm data will be presented here. A Spectrum 
Analyzer was used to measure the received power from the 
pick-up antenna. This set-up is shown in Figure 2.
Test Results 
Data showing the frequency response of multiple power levels 
in the aluminum lined mock fairing configuration is given in 
Figure 4. Although there is variation across the 750 MHz to 
18 GHz Frequency range, the data shows that these 
measurements are scalable with power level.
-1 
-4	 O
Modeling 
The test data collected is then compared to the simulation data 
generated by the electromagnetic simulation software, EM 
Software & System's FEKO. 
A Pro-E model of the plexiglass structure was developed and 
imported into FEKO. The fairing walls were modeled as 
perfect electric conductors in the FEKO model. The Antenna 
was built in FEKO to match the antenna structure of EMCO 
3115 shown in Figure 3. The pattern from this model was 
then obtained as shown in Figure 7 and used in the modeled 
fairing structure.
Z 
Figure 6. FEKO Model of EMCO 3115 Double Ridge 3-D 
The results compared nicely with the published EMCO gain 
pattern data. One plane pattern is shown in Figure 7.
2O 
power data from all other lined cases. Each additional layer 
caused less fluctuations in the power levels especially at 
higher frequencies. Minimal effects were observed from 
adding the foam only which is expected since the impedance 
of this material approaches free space. A profound smoothing 
effect was observed with the multiple layers of Kapton and an 
—	 approximately 20 dB decrease with this layering above 14 
GHz. 
Ref 2 also showed attenuation in field strengths with similar 
blanket materials from the bare fairing wall case by 8-26 dB. 
Similar results were found here, however the standing wave 
pattern predicted from the separation between the layers of the 
blanketing materials was not nearly as pronounced as in Ref 2. 
Figure 4: Power Measurements with Bare Aluminum at 
varied power levels. 
Kapton 377k manufactured by DuPont was used as a liner for 
the cavity since this material is specifically designed for RF 
absorption and Kapton is a material frequently used for launch 
vehicle and spacecraft blanketing materials for its thermal 
insulating properties. Standard I" foam was used to separate 
the blanketing material. The test cases were studied for walls 
(i)Aluminum (ii) Aluminum-Kapton (iii) Aluminum-Kapton-
Foam, and (iv)Aluminum-Kapton-Foam-Kapton. Data was 
recorded for each test case from 750 MHz to 18 GHz and 
shown in Figure 5. 
Buildup Received Power Comparison 
OXE*O(. 	 ,-_..._ ..-. ........ 	 -.-_. .................... 
5.00E*00
	 '	 -Si-
tee w lii Lii U Lu U Lu U U U Lu LU Lb Lu U U U Lu 
-1.00E+01 a--
-1.50E+0  
2 50E+01 
-
3 OOE+01 I __ 
-450E4-01 
-5.00E.-01 -	 --	 --------------

Frequency [Hz) 
Ahsmn,sn LnirLg	 —LU-- AHLuThnt%n/KaplOn 
Alhum,num/Kapton FoaLu	 AIIumnumIKapton/Foam/Kapton 
Figure 5: Power Measurements with Bare Aluminum and 
Various Blanketing Materials 
The test data obtained for the aluminum lined fairing shows 
wide variation in power levels. This case has predominately 
the highest envelope except at frequencies in the upper range 
of the test data (above 14 GHz) where the addition of foam 
and Kapton resulted in slightly higher levels. In the 4 - 8 
GHz range the aluminum lined case is 10 to 20 dB above the
Electric Tar field 
I
(
lie 
Figure 7. EMCO 3115 Double Ridge Plane Antenna 
Pattern
Model Vs. Test Data 
E
	
i/.oa	 -kog
 --
Fr.qosncy (liz) 
Figure 9: Differences between test data and model 
The model predicts the peaks of test data within I dB when 
test data is adjusted for the typical high VSWR in the 1-2 GHz 
range. The power losses in the cavity are not predicted in the 
model as accurately and the most significant loss at 1.7 GHz 
test case was approximately 8 dB lower than the model case. 
Comparisons to the Free Space Case at I GHz 
To evaluate the validity of the test results from a general 
sense, the free space predicted value was calculated. At I 
GHz, using 2D/X^2 criteria, this antenna would be in free 
space. Where D is the largest dimension of the antenna and ? 
is the wavelength. 
Pr = Pt*(AJ4 1tr)f 2*(G)I 2 = (1 mW)(0.3/(4it( I 57))A2*(4O9)'2 
= 3.8 l.tWatts 
Where: 
Pr = Power Received 
ower Transmitted The bare fairing wall case was simulated at I to 1.9 GHz in 	 Pt = P 
100 MHz increments using a hybrid Method of	 R = distance between transmit and receive antenna 
in of the antenna at I GHz
s in meters 
Moments/Physical Optics model.
	 G = ga 
Power Simulated Rx (Wafts) 
0.000284	 ........................................................ 
0.000282 
— 0.00028 
0.000278 .......... 
. 0.000276 
0.000274 
	
0.000272	 - 
	
- 0.00027	 J 
0.000268 r— 
0.000266
The measured power at I GHz was 203 j.tWatts. The 
difference between these values can be attributed to 
amplification from the cavity. 
(203/3.8) = 17.27 dB l0*log 
Comparing this value to an approximate Volume Q for PEC 
per reference 3, we obtain: (The approximate volume is 
0.3632 m"3) 
Q=(l6itt 2V)/)3 = 2124 = 33 dB 
L	 1.00E+09	 1.50E+09	 2.00E+09 Frequency (Hz)	 -	 Accordingly, it is reasonable that the measured power levels -	
-	 within the cavity are significantly higher than the 
ted values. 
usions:
free space 
calcula Figure 8: Simulated Received Power
Concl Figure 8 shows received power from the simulated case. 
Although there is a general increase in the power received 	 This study shows how the fields increase in the fairing model 
with frequency, the variation is only 0.22 dB over the range	 lined with a good conductor, the effects of the various from I .0 to 1 .9 GHz. The power losses shown at particular 	 blanketing materials that typically line the fairing walls in frequencies in the test cases were duplicated in these results, 	 launch vehicles, and the effects of adding a conductive 
spacecraft. Losses contributed to blanketing materials were
shown to be comparable to the available industry reports. 
Preliminary modeling results from 1.0 to 1.9 GHz range 
which showed peak levels were well matched. 
The variations in the power data in the aluminum only case 
was expected due to multiple cavity reflections. The layered 
RF absorbing liners test cases had the biggest reduction on the 
observed power levels. Although there was variation with 
frequency, no pronounced standing wave pattern appeared as 
with Ref 2 data. 
Future Work: 
Testing should be modified to examine more points in the 
cavity to identify changes in eigenmodes especially with the 
addition of a spacecraft model. Reference 4 shows multiple 
examples of various eigenmodes for a conical structure. An 
omni-directional antenna will also be used to remove 
uncertainties regarding increases in field from the gain of the 
antenna. Further investigation into the differences in power 
absorption modeled versus test case is warranted as well as an 
extension of the modeled frequency range and cases 
considered. 
References: 
I.	 -CF13 Validation with LDV Test Data for Payload/Fairing Internal 
FloW, AIAA, Kandula, Hammad, Schallhorn 
2. Technique for Predicting the RF Field Strength Inside and 
Enclosure, M. Hallett, J. Redell, NASA/TP - 1998-206864 
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Electromagnetic Cavity Effects from Transmitters Inside a Launch Vehicle Fairing

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