-
Exchangeable Bases for Rapid Prototyping of
Carbon Fiber Reinforced Polymer Antenna Cavities
Gerald Artner∗ Christoph F. Mecklenbräuker∗
Abstract — Chassis antenna cavities were recentlyintroduced as
large, hidden, antenna modules. Test-ing different antenna
configurations inside the mod-ule would require cutting several
holes into the cav-ity floor. Manufacturing of several prototypes
isnot feasible, especially for carbon-fiber reinforcedpolymer
chassis; flexible solutions are desired. Ex-changeable cavity bases
for chassis antenna cavitiesare proposed, designed and
manufactured. Antennameasurements show the feasibility of the
concept.
1 INTRODUCTION
Chassis antenna cavities have recently been pro-posed for
concealing antenna modules inside ve-hicles’ hulls [1]. A prototype
of a chassis cav-ity was manufactured from carbon fiber
reinforcedpolymer (CFRP) and it was shown that near-omnidirectional
radiation from the cavity is possi-ble with monopole and inverted-F
antennas. In [2]it was proposed to use chassis cavities in
automo-tive applications. The cavity replaces roof mountedshark-fin
modules, and keeps the car roof as pre-ferred antenna location.
Such a cavity is preferableover hidden antenna placement in side or
rear viewmirrors, spoilers, roof pillars, etc. as it is dedi-cated
antenna space, which allows antenna designindependent from other
functional units. A patternreconfigurable antenna inside a chassis
antenna cav-ity, which can be electronically switched to
radiatetowards the front, back, left or right side of a car,is
measured in [3].
The chassis antenna module follows the designparadigm to conceal
automotive antennas. By hid-ing the antennas in the vehicle hull,
the antennas nolonger influence the drag coefficient or the
aestheticdesign of the vehicle. An aperture in the roof wasproposed
in [4]. [5] presents a stacked combinationof a Satellite Digital
Audio Radio (SDARS) andGlobal Positioning System (GPS) antenna.
Anten-nas in truck side mirrors are described in [6]. Con-formal
antennas on a car side mirror are designedin [7]. A telephony and
WLAN antenna hidden onthe rear roof end is developed and measured
in [8].
A very practical problem appears when measur-ing antenna
prototypes inside chassis antenna cavi-
∗Institute of Telecommunications, Technische Uni-versität Wien,
Gußhausstraße 25, 1040, Vienna,Austria, email:
[email protected],[email protected].
Figure 1: Assembled setup with CFRP chassiscavity, exchangeable
aluminum base and conicalmonopole antenna.
ties. The cables of the antennas need to be con-nected through
the cavity bottom or the cavitywalls. This requires, that a hole is
cut into thecavity in order to connect the antennas with
themeasurement equipment in the anechoic chamber.Measurements of
multiple antennas in the cavityor measurements at different
locations are thereforenot possible, as too many holes will alter
the results.Manufacturing of large CFRP parts is quite expen-sive,
as the laminate needs to be laid by hand andcured in an autoclave.
Building a separate cavityprototype for each measurement would be
expen-sive and unreasonable. In mass production this isnot an issue
as the positions of the antennas arethen already fixed.
Exchangeable cavity bases are proposed for therapid prototyping
of antennas inside chassis cavities(Fig. 1). Exchangeable metal
bases are cheap andfast to manufacture, as they can be laser cut.
Usinga metal of sufficient thickness allows attachment ofcoaxial
cable flanges to the cavity base, which isnot possible with thin
CFRP sheets. Previously,the flanges were attached to an additional
metalground plane that was then placed inside the cavity(see
[1-3]). The bases can be used in drive tests asthe bases and
antennas are securely attached withscrews. With the proposed method
it is also possi-ble to attach advanced prototypes of whole
antennamodules inside chassis cavities, which also containradio
frequency hardware, structures for antennadecoupling, protective
cover, etc.
-
A
4,0020 28,75 28,75 28,75 28,75 28,75 28,75 28,75 28,75 28,75
28,75 28,75 28,75 28,75 28,75 28,75 28,75 20
2027,5
27,527,5
27,520
(500,00)
(150,00)
4,50
A
6,11
(8,64)
1,604,20
most and medium number of screwsall prototypes
propotype with most screws
Figure 2: Technical drawing of the exchangeable cavity bases.
All dimensions are in millimeter.
2 EXCHANGEABLE CAVITY BASES
The floor of the CFRP cavity from [2] is replacedby rectangular,
exchangeable, metal plates. Theplates are fastened to the cavity
with counter-sunkscrews along the plates’ borders. Such plates
arecheap and fast to manufacture with laser cutting.To show
feasibility, a conical monopole is measuredin the center of the
cavity. This was done in [2] toobtain a broadband characterization
of the influ-ence of the antenna cavity on monopole antennas.But
note that in [2] the conical monopole antennaneeded to be placed on
a separate metal groundplane, which was lowered into the cavity and
placedon the cavity floor, as the CFRP sheet was toothin to thread
and attach flanges. In this paper theconical monopole antenna is
measured again, butthis time the broadband measurements are used
tocompare exchangeable cavity bases to the resultsfrom [2]. Coaxial
cables are again connected withSubminiature Version A (SMA) coaxial
connectors.This time the flanges are directly screwed to thecavity
bases. A technical drawing of the exchange-able cavity bases is
depicted in Fig. 2. Three ex-changeable base prototypes were built,
which usedifferent amount of screws. Only the holes shown ingreen
in Fig. 2 are used for the base with the leastamount of screws,
green and blue holes are used forthe base with the medium amount of
screws andthe base with the most screws uses all holes. Theconical
monopole antenna is a standard antenna de-
sign. The antenna, its dimensions and influences ofCFRP onto
such antennas are discussed in [9, 10].The three aluminum bases
with different holes areshown in Fig. 3 together with the brass
cone.
Figure 3: Exchangeable cavity bases and conicalmonopole
antenna.
Electric contacting between cavity and exchange-able bases is
straightforward for metal cavities.The electrically conductive
bases are then directlyscrewed to the conductive cavity. For carbon
fiberreinforced polymer (CFRP) cavities this is not
sostraightforward, as the conductive carbon fibers areburied under
an insulating epoxy layer. For mea-surements in anechoic chambers
metal cavities canbe used instead of CFRP, as metals result in
com-parable antenna performance [10]. For drive testmeasurements
this is not possible, as the car chas-sis can not simply be
replaced. As CFRP are al-
-
ready used as chassis material in mass producedcars, the focus
in this paper is therefore on CFRPcavities. A rectangular hole was
cut into the cavityfloor by water jet, as laser cutting is not
suitablefor CFRP. The holes placed along the cutout fit theones in
the bases, as is depicted in Fig. 4. Waterjetcutting caused
delamination on some holes, but forthe antenna performance this is
of little interest, asthese regions are covered by the aluminum
basesanyway.
Two things are problematic in this exchangeablebase design and
require that prototypes are eval-uated by measurement. Firstly, the
exchangeablebases are placed on the CFRP’s surface epoxy
layer.Electrically, the bases are only (poorly) connectedto the
cavity’s carbon fiber layers via the screws.Advanced contacting
methods via the screws as in[11] are barely feasible for
prototyping. Secondly,note that the distance between the screws is
quitelarge compared to wavelength at higher frequencies.Connections
can not easily be spaced as close, as itis possible for example on
printed circuit boards,where vias are typically placed λ/10
apart.
Figure 4: Cut bottom of a CFRP chassis antennacavity module for
exchangeable bases. Waterjetcutting caused delamination, which is
circled redto increase visibility.
3 MEASUREMENT RESULTS
The directivity is a good indicator, whether the pat-terns on
the different ground planes diverge overfrequency. Fig. 5 shows the
measured directiv-ity up to 10 GHz. The directivity on the
differentbases is quite similar up to the frequencies wherethe
monopole antenna is no longer useful inside thecavity. The
normalized gain patterns at 5.9 GHzfor dedicated short range
communication are com-pared in Fig. 6 and Fig. 7. For the cut along
thelong side of the cavity the patterns on the differ-ent bases are
quite similar (Fig. 6). On the cutalong the short cavity side the
patterns on the basewith the fewest screws divert a few decibel
from
the other patterns. Results on the small aluminumground plane
imply, that the cone was slightly tiltedduring measurement. This is
now improved withthe exchangeable bases, as the ground planes
arenow properly mounted in the cavity. At lower fre-quencies, the
radiation patterns are similar (theseplots are not shown). The use
of the exchangeablebase with at least the medium number of screws
isrecommended.
2 3 4 5 6 7 8 9 105
10
15
20
Frequency / GHz
Dire
ctiv
ity /
dBi
Small aluminum groundExchangeable base, most screwsExchangeable
base, medium screwsExchangeable base, fewest screws
Figure 5: Directivity of a conical monopole antennameasured on
exchangeable cavity bases with differ-ent number of screws.
θ = 0 °15°30°
45°
60°
75°
90°
105 °
120 °
135 °
150 °165 ° 180 ° 165 °
150 °
135 °
120 °
105 °
90°
75°
60°
45°
30°15°
−30
−20
−10
0 dB
Small aluminum groundExchangeable base, most screwsExchangeable
base, medium screwsExchangeable base, fewest screws
φ = 90° φ = 180°
Figure 6: Normalized, measured gain patterns.Vertical cuts at
5.9 GHz, polar angle θ for azimuthϕ = 0◦.
4 CONCLUSION
Measuring multi-antenna systems or antennas atdifferent
positions inside a chassis antenna cav-ity requires holes for
coaxial cables. The proper
-
θ = 0 °15°30°
45°
60°
75°
90°
105 °
120 °
135 °
150 °165 ° 180 ° 165 °
150 °
135 °
120 °
105 °
90°
75°
60°
45°
30°15°
−30
−20
−10
0 dB
Small aluminum groundExchangeable base, most screwsExchangeable
base, medium screwsExchangeable base, fewest screws
φ = 0° φ = 180°
Figure 7: Normalized, measured gain patterns.Vertical cuts at
5.9 GHz, polar angle θ for azimuthϕ = 90◦.
contacting of carbon fiber reinforced polymer lam-inates is
problematic, as the conductive carbonfibers are buried in epoxy.
While it is possibleto perform measurements inside anechoic
chamberswith metal cavities instead of CFRP cavities, theCFRP
chassis of a car can not be replaced for drivetests.
Exchangeable cavity bases are proposed for pro-totyping of
chassis antenna cavities. The bases arescrewed to the cavity floor,
which increases stabil-ity – as it is required for drive tests.
Measurementsin an anechoic chamber suggest, that the base withjust
six screws influences antenna performance andis not suitable for
prototyping.
Acknowledgments
The financial support by the Austrian Federal Min-istry of
Science, Research and Economy and theNational Foundation for
Research, Technology andDevelopment is gratefully acknowledged.
References
[1] G. Artner, R. Langwieser, R. Zemann andC. F.
Mecklenbräuker, “Carbon Fiber Rein-forced Polymer Integrated
Antenna Module,”in IEEE-APS Topical Conference on Antennasand
Propagation in Wireless Communications(APWC), Cairns, Australia,
2016.
[2] G. Artner, R. Langwieser and C. F. Meck-lenbräuker,
“Concealed CFRP Vehicle Chas-
sis Antenna Cavity,” IEEE Antennas andWireless Propagation
Letters, 2016. DOI:10.1109/LAWP.2016.2637560
[3] G. Artner, J. Kowalewski, C. F. Meck-lenbräuker and T.
Zwick, “Pattern Reconfig-urable Antenna With Four Directions
Hiddenin the Vehicle Roof,” in International Work-shop on Antenna
Technology (iWAT), Athens,Greece, 2017.
[4] L. Low, R. Langley, R. Breden andP. Callaghan, “Hidden
Automotive An-tenna Performance and Simulation,” IEEETransactions
on Antennas and Propagation,vol. 54, no. 12, pp. 3707-3712,
2006.
[5] J. Kammerer and S. Lindenmeier, “InvisibleAntenna
Combination Embedded in the Roofof a Car with High Efficiency for
Recep-tion of SDARS - and GPS - Signals,” 2013IEEE Antennas and
Propagation Society In-ternational Symposium (APSURSI),
Orlando,Florida, 2013.
[6] L. Marantis, K. Maliatsos and A. Kanatas, “ES-PAR Antenna
Positioning for Truck-to-TruckCommunication Links,” in European
Confer-ence on Antennas and Propagation (EuCAP),Davos, Switzerland,
2016.
[7] J. de Mingo, C. Roncal and P. L. Carro,“3-D Conformal Spiral
Antenna on Ellipti-cal Cylinder Surfaces for Automotive
Applica-tions,” IEEE Antennas and Wireless Propaga-tion Letters,
vol. 11, pp. 148-151, 2012.
[8] S. Hastürkoǧlu and S. Lindenmeier, “A Wide-band Automotive
Antenna for Actual and Fu-ture Mobile Communication 5G/LTE/WLANwith
Low Profile,” in European Conferenceon Antennas and Propagation
(EuCAP), Paris,France, 2017.
[9] G. Artner, R. Langwieser and C. F. Meck-lenbräuker, “Carbon
Fiber Reinforced Polymeras Antenna Ground Plane Material Up to
10GHz,” in European Conference on Antennasand Propagation (EuCAP),
Paris, France, 2017.
[10] G. Artner, P. K. Gentner, J. Nicolics andC. F.
Mecklenbräuker, “Carbon Fiber Rein-forced Polymer With Shredded
Fibers: Quasi-Isotropic Material Properties and Antenna
Per-formance,” International Journal of Antennasand Propagation,
2017.
[11] J. Brettle, K. J. Lodge and R. Poole, “Theelectrical
effects of joints and bonds in car-bon fibre composites,” in
Proceedings NATOAGARD, no. 283, Portugal, Lisbon, 1980.
