uwb, non dispersive radiation from the planarly fed leaky...

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UWB, Non Dispersive Radiation from the PlanarlyFed Leaky Lens Antenna. Part II: Demonstrators

and MeasurementsAndrea Neto, Member IEEE, Stefania Monni, Member IEEE, Frans Nennie

Abstract— We provide for the first time the experimentalcharacterization of a planarly fed ultra-wideband leaky lensantenna. The paper includes the description and the experimentalcharacterization of two antenna prototypes with radiating ele-ment realized in printed circuit board technology. The impedanceparameters and the radiation patterns of the antennas have beenmeasured showing excellent pattern quality and efficiency. Thelink between two antennas has also been characterized in thefrequency and time domains in terms of mutual coupling impulseresponse respectively. Unprecedented performance in terms ofpulse fidelity are demonstrated suggesting very high potentialsof these antennas for all applications that require preservationof narrow pulses.

I. INTRODUCTION

For most radar and communication applications that requirea wide frequency bandwidth the linearity of the phase ofthe antenna transfer function is of great importance. If theantenna is dispersive (i.e. the phase does not vary linearly withfrequency) compensations at system level must be included,such as sub-divisions of the bands, digital signal processing,inclusions of filters, etc. [1]. The most linear phase responsesare associated to antennas derived from TEM-like horns. Forinstance the Vivaldi antenna, which is a low cost implemen-tation of TEM horns, is a very successful example, see [2].However, it is well known that its phase center moves alongthe longitudinal axis. This implies that the phase linearity isonly limited to the broadside direction.

The stability of the phase center as a function of thefrequency is only a necessary condition for the linearity ofthe phase in all observation directions. The most successfulwideband antennas that present fixed phase centers are oftenderived from spirals or from the log periodic concept. As anexample, the Eleven antenna [3] has recently been proposedfor decade Bandwidth (BW) reflector’s excitation. However, asshown in [2] the log periodic concept is intrinsically associatedto a non linear phase variation of the transfer function. Thesame applies for spirals. Overall, it appears that no antennasare presently known capable of radiating non dispersively overthe entire main beam and over very large bandwidths.

When the attention is shifted from a single antenna toa radiated link, the linearity of the phase of the antennatransfer function is not sufficient for the reception of a nondistorted replica of the input impulse. Also the amplitude ofthe spectrum of the transmitted impulse must be preserved.

The planarly fed Leaky Lens antenna, described in paper[4], appears to be well performing with respect to both disper-

The authors are with with TNO Defense and Security, DenHaag 2597 AK, The Netherlands. E-mail: [email protected],[email protected],[email protected]

sivity and amplitude distortion. In [4], the concept and designof the antenna was presented. In this paper, correspondinghardware prototypes are described and experimentally charac-terized. To demonstrate the ultra-wide bandwidth performancewhile retaining manageable lens dimensions, 12 GHz has beenchosen as lowest operating frequency. The planar feed hasbeen realized in standard printed circuit board (PCB) technol-ogy. Both types of antennas discussed in [4], with and withoutmatching layers, have been manufactured. The matching layersreduce the reflection at the dielectric/air interface and thisis therefore the preferred implementation of the Leaky Lensantenna design. However, for applications at sub-mm wavefrequencies the realization of the matching layers poses serioustechnological challenges. Thus, the performance degradationof the planarly fed Leaky Lens antenna without the matchinglayers has also been quantified.

A link between two prototypes has been experimentallycharacterized. From the measured S-parameters, the pulsepreserving property of the antenna link demonstrates the phaselinearity and phase center stability of each antenna as well asthe spectral amplitude preservation of the link. While the phasepreserving properties arise from the Enhanced Leaky Lensradiation mechanism [4], the spectral amplitude preservationis strictly related to the specific geometry chosen for thedielectric lens.

The paper is structured as follows. After a short descriptionof the prototypes in Sec. II, Sec. III presents the S-parametermeasurement results for both structures, with and withoutmatching layers, covering the band from 5 to 70 GHz. Sec. IVdescribes the impulse response of the links between twoantennas. Time gating is applied to this response in Sec. Vto highlight the dominant wave phenomena characterizing thelinks. The measured antenna radiation patterns are plotted inSect. VI for the frequency range 33-75 GHz. Considerationson the results are provided in Sec. VII.

II. PROTOTYPE DESCRIPTION

A 3D radar imaging system operating with 30 GHz band-width is currently being designed at TNO. In the frameof this project, a planarly fed Leaky Lens antenna will beintegrated on the printed circuit board where the TX/RX front-end circuitry will be hosted. As preliminary step, two proto-types of the antenna have been designed and manufacturedto characterize the antenna performance independently fromthe rest of the system. According to the design guidelinespresented in [4] the prototypes have been manufactured tooperate efficiently as transmitter and receiver in the frequencyband 30 to 60 GHz. The dielectric lens has been made of a

2

(a)

(b)

Fig. 1. The planarly fed UWB Leaky Lens antenna prototype.

material for microwave applications (εr = 9.8, tanδ = 0.002)that is commercially available in cylindrical rods of up to100mm of diameter. The material could be machined at TNOworkshop with sufficient accuracy. The relevant dimensionsof the lens and of the feeding structure in the PCB can befound in [4] and will not be repeated here for the sake ofbrevity. Fig. 1 shows the lateral and top view of one ofthe prototypes. In Fig. 1(a) the holder consisting of a thickmetal plate (1mm) is also visible on top of a plastic blockthat was introduced to provide support to the antenna PCB.Because of the high front-to-back ratio of the planarly fedLeaky Lens antenna, this structure does not significantly affectthe radiation performance. A slab of foam material, ε ≈ 1, wasinserted to maintain a constant separation between the lensand the ground plane where the slot is etched and ensure theenhanced radiation mechanism. The micro-strip extends at theside of the lens, where it is fed through a Mini-SMP coaxialcable connector. To protect the mini-SMP, 1.85mm connectorsafers are used. As outlined in [4], to match the 80Ω antennaimpedance to the 50Ω impedance of the connector the micro-strip width has accordingly been tapered from 200µ to 300µ.The overall distance between the coaxial connector and thecenter of the slot is about 4cm.

After measuring the performance of the two antenna pro-totypes, two sets of curved matching layers conformal to theoriginal lens have been milled away from three dielectric roads

of nominal permittivity εr = 6.5, 3.5, 2.5 and tanδ = 0.002.The details of the stratification were described in Sec. IV-A.3of [4]. The matching layers have then been glued to each otherand to the lens of the original antennas and new measurementshave been carried out.

III. SCATTERING PARAMETERS

The antenna scattering parameters have been measuredusing an Agilent vector network analyzer, which is able toperform measurements up to 70 GHz. The reflection behaviorof each antenna has been investigated in terms of S11. Theperformance of the two manufactured antennas will be shownin Fig.3.

The transmit-receive antenna link has been characterized interms of S12 with the measurement set up depicted in Fig. 2.The antennas were placed at about 20cm distance, measuredbetween the two µstrip-slot transitions. The antenna alignmentwas optimized to provide maximum mutual coupling.

The antenna scattering parameters will be discussed sep-arately for the implementation without matching layers andwith matching layers.

Fig. 2. Measurement setup for the characterization of the transmit-receiveantenna link.

A. Without Matching Layers

The measured scattering parameters of the two antennas,S11 and S22, are shown in Fig. 3(a) and (b) respectively. Themeasurements were performed with the same setup used tocharacterize the antenna link in Fig. 2. However, since the dis-tance between the two antennas is relatively large and the S12is in the order of -23 dB, it is believed that the SII parametersprovide a very good approximation of the reflection coefficientfor each one of the two antennas in isolation. The amplitudesof all SII parameters are plotted in dB’s as a function of thefrequency in the range 5-70 GHz. The resolution obtainedwith this frequency range is so high, that the location of thestrongest reflection could easily be identified by performingthe Inverse Fourier transform (IFT) of the frequency domaindata and observing the time domain data. This reflectionwas, as expected, associated to the connector-µstrip transition.Accordingly, it has been possible to gate out this transition

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effect to characterize the matching of the two antennas withoutthe effect of the connector. Fig. 3(a) and (b) present also theresults pertinent to this gated situation (continuous line).

-30

-25

-20

-15

-10

-5

0

0 10 20 30 40 50 60 70

Freq (GHz)

S11

(dB

)

(a)

0-30

-25

-20

-15

-10

-5

0

10 20 30 40 50 60 70

Freq (GHz)

S22

(dB

)

(b)

Fig. 3. Measured Sii parameters for antennas (1) in a) and (2) in b) withoutmatching layers, dashed lines. For both antennas also the results pertinent tothe case in which the reflections at the connectors are gated out are included,continuous lines.

The curves obtained after time gating show very goodmatching performance for both antennas with SII lower than-10 dB from 15 GHz on. The rapid oscillation as a functionof the frequency can be associated to reflections inside thedielectric lenses. Simulations performed with the commercialFDTD-based software package CST Microwave Studio [5],not reported for brevity, are in excellent agreement with thesemeasured results. The simulations actually indicate that thissame structure should remain matched with SII better than -15dB up to at least 100 GHz (the increase of the computationalburden did not allow performing the simulations for higherfrequencies). When the connector effect is considered, thematching deteriorates significantly: the oscillations in the SIIparameters can be tracked to standing waves between the slotand the connector. Notably, the impact of the connectors differssignificantly in the two antennas. Being the designs exactly thesame it is apparent that the soldering of the coaxial connector

to the PCB is the most unreliable part of this antenna design.However, the use of such connector will not be necessary inthe final integrated design.

In Fig.4 the amplitude of the measured S12 is plotted as afunction of the frequency.

-30

-25

-20

-15

-10

-5

0

10 20 30 40 50 60 700Freq (GHz)

S12 (

dB

)

Fig. 4. Measured S12 parameter for antennas (1) and (2) without matchinglayers.

The S12 parameter presents significant oscillations due tothe reflections inside the lens and due to the mismatch asso-ciated to the transition from coaxial cable to µstrip. Moreoverthe S12 is impacted by the ohmic losses due to the µstrip lineslengths and by the ohmic losses in the dielectric composingthe lenses.

Estimated Efficiency Without Matching Layers

The causes of reduced efficiency for the antenna prototypeswithout matching layers can be identified and quantified bymeans of measurements and simulations:• Ohmic losses in the dielectric lens, varying from 0.5 dB

at 20 GHz to 1.5 dB at 70 GHz (calculated usingthe nominal value of the loss provided by the materialmanufacturer);

• Mismatch losses at the connector, measured with the Siiparameters;

• Ohmic losses in the µstrip line, varying from 0.5 dBat 20 GHz to 1.5 dB at 70 GHz (calculated using thenominal value of the loss in the dielectric as provided bythe manufacturer).

These losses, partly calculated and partly measured, aresummarized in Fig. 5, which shows the estimated efficiencyof antenna (1).

It should be noted that the antennas eventually will be usedwithout µstrips and connectors.

B. With Matching Layers

The two antenna prototypes equipped with matching layershave then been characterized. The S11 parameter of antenna(1) is shown in Fig. 6. In this case only the curve pertinent tothe situation in which the connectors are gated out is presented.

Also in this case excellent matching performance with S11lower than -10 dB from 12 GHz on is observed. The rapid

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0-5

0

-4

-3

-2

-1

10 20 30 40 50 60 70

Efficiency loss estimated on antenna (1)

dB

Freq (GHz)

Fig. 5. Overall expected loss of efficiency in dB for antenna (1).

-25

-20

-15

-10

-5

0

10 20 30 40 50 60 70Freq (GHz)

S11 (

dB

)

Fig. 6. Sii parameters for antenna (1) with matching layers.

oscillation as a function of the frequency is significantlyreduced in amplitude with respect to the corresponding casein absence of matching layers. Again simulations performedwith CST, not reported for brevity, are in very good agreementwith these measured results.

Fig. 7 shows the amplitude of the measured S12 as afunction of the frequency. In the whole frequency range theS12 is at least 5 dB higher than the value obtained withoutmatching layers. Moreover, the oscillations are now muchweaker, with the S12 amplitude varying in a 7 dB band (-15 dB to -22 dB) from 15 to 70 GHz.

Estimated Efficiency With Matching Layers

Based on the comparison between measurements with andwithout matching layers, one can clearly establish that thepresence of matching layers ensures an increase in gain ofthe order of 2-3 dB per antenna. This does not imply a muchhigher efficiency of the antenna in terms of losses. The ohmiclosses and the losses due to mismatch at the connector are forthese antennas essentially the same as those discussed for thecase of no matching layers. However, an increase of 2 dBin the transmission efficiency at the dielectric-air interface

-25

-20

-15

-10

-5

0

10 20 30 40 50 60 70Freq (GHz)

S12 (

dB

)

Fig. 7. Measured S12 parameter for antennas (1) and (2) with matchinglayers.

implies that the radiation pattern is better behaved in the sensethat more power is radiated into the main beam rather than inthe side lobes. This will be explicitly shown in Sec. VI.

IV. TIME DOMAIN FROM FREQUENCY DOMAIN

The link between a transmitting and a receiving antennacan be characterized in terms of its complex transfer functionH(f) = URX(ω)/UTX(ω), where URX(ω) and UTX(ω)are the spectra of the received and transmitted voltages. Thecoupling parameter in the frequency domain, S12(ω), is linkedto the complex transfer function of the antennas involved inthe link by [2]:

S21(ω) = HTX(ω)HRX(ω)jω

2πrce−jωr/c, (1)

where HTX(ω) and HRX(ω) are the transfer functions oftransmit and receive antenna and r is distance between theseantennas. In this section the impulse response of the linkS12(t) is derived over a 65 GHz BW, from 5 to 70 GHz,by means of the IFT of the measured S12(ω).

A parameter frequently used to quantify the pulse distortionis the fidelity factor. It is in general defined with respect to apredefined reference signal [6], [7]:

F = max[

∞∫−∞

sref (t)s12(t− τ)dτ

∞∫−∞

|sref (τ)|2dτ∞∫−∞

|s12(τ)|2dτ

].

In the following, the reference signal sref (t) will be asinc associated to the mentioned 65 GHz band, which canbe expressed as

sref (t) = FT−1[Sref (ω)] =12π

ω2∫

ω1

e−jωtdω

where ω1 = 2π5GHz and ω2 = 2π70GHz.

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A. With Matching LayersFig. 8 shows the time domain representation, normalized to

the maximum, of the amplitude of the S12 parameter measuredwith the setup in Fig. 2 for the case in which the lenses arecovered by matching layers.

The main peak of the signal arrives to the receiver afterapproximately 1.35 ns corresponding to 404 mm of equivalentfree space propagation. This propagation time can be explainedaccounting for 14cm of free-space propagation plus 6.4cm ofpropagation in the two lenses (equivalent to 18.5cm in freespace) and 8cm of micro-strip and coaxial cables propagation.A second peak of amplitude 22 dB smaller than the firstpeak appears 0.3ns after this, corresponding to approximately9.2cm of free space propagation. It can be associated to singlereflections inside the lens, as highlighted in the inset of thefigure. This effect could be considered as a measure of theringing. If as reference for defining the ringing an amplitudeof -13dB for the secondary peak is considered, also in thisrespect the leaky lens antenna outperforms the Vivaldi andthe log-periodic antennas reported in [2]. Since there are noother important images arriving at later times, the statementin Sec. IV.A.2 of [4] that the planarly fed leaky lens antennadoes not support double reflections inside the lens thanks toits directive pattern is fully validated.

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3

time(ns)

no

rma

lize

d S

12

main response(40.4 cm in fs)

Fig. 8. Normalized time domain representation of the S12 parameter for theantennas with matching layers.

A comparison between the response of the antenna link intime domain and the reference signal, delayed of 1.35ns topresent the maximum in correspondence of the main peak ofthe link impulse response is shown in Fig. 9. Note that thescale is expanded with respect to that in Fig. 8. The curvesshow impressive similarity and one can observe that the halfpower width of the measured pulse is only 0.002 ns longerthan that of the reference signal (0.015ns instead of 0.013ns). To more rigorously quantify the pulse distortion intro-duced by the combined antennas the fidelity factor has beencalculated. For the entire measured band (ω1 = 2π5GHz, andω2 = 2π70GHz) the fidelity factor of the link between twoleaky lenses with matching layers is 0.94. To the best of ourknowledge such a high fidelity factor on a frequency BW 1:14has never been reported for radiated links. Impulse radiatingantennas have been reported to present high fidelity factors,on smaller bands, but always when used in conjunction withresistive loadings (see [8] for a comparison). In the presentcase a non desired resistive loading is also present, due to

the ohmic losses in the dielectric, in the micro-strip and inthe connectors. However, this loading actually decreases thefidelity factor since it is the main cause for the diminishedantenna efficiency and for the reduced S12 amplitude in thehigh frequency portion of the band. A lossless leaky lensantenna link would provide a constant S12 link for even higherfrequencies. This will become clearer after the examination ofthe results of Sec. V.

0.6

0.8

1

1.2(5-70 GHz) pulse, fidelity=0.94

Received signalIdeal delayed signal

0.013ns

0.015ns

0

0.5

0.7

1

1.33 1.34 1.35 1.36 1.37

0

0.2

0.4

1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7

time(ns)norm

aliz

ed S

12

Fig. 9. Comparison between the measured impulse response of the combinedantennas and an ideal 65 GHz bandwidth (5-70 GHz) pulse delayed by1.35ns. The pulse is visible in the inset.

When one considers only the frequencies above the thresh-old of 15 GHz, after which the matching layers start tofunction, (see Sec. IV of [4]), the fidelity factors becomes 0.97,as visible in Fig. 10. The frequency band with fidelity higherthan 0.97 is in fact 1:5. Note that comparable performance interms of fidelity have been shown by Vivaldi antennas on BWin the order of 1:3 [7], but only in the broadside direction.

At the time of writing this paper, TNO measurement setup did not allow us to perform S-parameter measurements atfrequencies higher than 70 GHz. However it can be observedthat the S12 starts decreasing in the higher portion of the banddue to increase in the losses. Nevertheless simulations carriedout using CST Microwave Studio have shown that one canobtain, in relative BW higher than 1:7, the fidelity factor largerthan 0.97.

Phase Center Stability With Matching Layers

As explained in Sec. V of [4], the excellent dispersivityproperties that were presented in the previous Figures are dueto both the linear growth of the gain and the linear variation ofthe phase of the received signal as a function of the frequency.While the gain can be derived from the amplitude of theS12(ω), the phase variation over a certain frequency bandcan be read from the position of the peak of the link impulseresponse. Fig. 11 shows on the right scale the distance betweenthe two antennas composing the link as derived from theposition of the peak of the received signal in time domain overdifferent frequency bands. Thus, for instance, performing theIFT of S12(ω) by only retaining the frequency componentsfrom 5 to 21 GHz, one would observe that the peak of thereceived signal would arrive at a moment which corresponds to402.5 mm of propagation in free space. Similarly performing

6

0

0.2

0.4

0.6

0.8

1

1.2

1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65

(15-70 GHz) pulse, fidelity=0.97


time(ns)

1.7

no

rma

lize

d S

12

Fig. 10. Comparison between the measured impulse response of thecombined antennas and an ideal 55 GHz bandwidth (15-70 GHz) delayedpulse.

the IFT on the band 55 to 70 GHz, the peak of the receivedsignal would arrive after a time that corresponds to 404 mmof free space propagation. Dividing this phase delay by two,one can readily observe that the corresponding variation of thephase center in each of the antennas is less than ±0.32 mm.Considering that the slot width is 0.5 mm, one can state thatthe phase center does not move with frequency.

21.3 37.5 53.7 70

-0.25

0

0.25

0.5

freq (GHz)

Dfc

(mm

)

5

404.0

403.5

403.0

402.5

Dr12

(mm

)

Fig. 11. On the right scale: optimal delay that should be introduced torecover 16 GHz BW pulses centerd at different frequencies. On the left scale:corresponding phase center variation with respect to the average.

This experimental demonstration only holds for broadsidelinks. However, since the antenna behaves accordingly to thedominant ray picture provided in Fig. 12 of Sect. V of [4],the phase center stability can be implied for at least the entiremain beam.

Note that this antenna is the only one, to our knowledge,which is at the same time efficient, weakly dispersive and withstable phase center. Uniting this property with the intrinsicdirectivity, one can use the leaky lenses to excite a reflectorwith high F/D, maintaining the low dispersivity. Accordinglya focal plane arrangement, can be used to generate multi-ple secondary beams, hundreds, without significant off-focusdegradation. This was not possible until now, with any otherfeed.

The time domain link impulse response in the absence ofmatching layers is discussed in Appendix A.

V. FREQUENCY DOMAIN FROM TIME DOMAIN

The investigation of the time domain representation of theS12 parameter allows one to identify the critical time domain

zones in which the dominant wave mechanisms can be recog-nized. Fig. 12 shows a zoomed time window in which mostof the S12 signal is concentrated. The two curves correspondto the links including antennas with (solid line) and without(dashed line) matching layers. Note that here the amplitudesof the S12(t) are normalized to the maximum of the signal inthe presence of matching layers, to fully appreciate the relativedifference between the two signal intensities and qualities.It is apparent that for both links the bulk portion of thesignal arrives to the receiver between 1.3 and 1.4 ns. Alsoit is relatively simple to identify the zone between 1.6 and1.7 ns as the window where contributions arrive to the receiverafter having undergone a single reflection at the dielectric-airinterface and on the ground plane. Finally we can associatethe zone between 1.4 and 1.6 ns as the zone where furthercontributions associated to multiple reflections inside the lensor between the connector and the slot in antennas (1) and (2)are concentrated.

This subdivision suggests to adopt different gating of thetime domain signals to highlight the frequency signature ofeach contribution.

0.2

0.4

0.6

0.8

0

1

Lens reflectionzoneConnectors effects

zoneMain pulsezone

1.2 1.3 1.4 1.5 1.6 1.7 1.8nano seconds

S12 n

orm

aliz

ed to m

ax o

f m

atc

hed lens c

ase

window 1 (0-1.38 ns)



Fig. 12. Zoomed time window were most of the link impulse responseis concentrated. The responses for both antenna links, with (solid line) andwithout (dashed line) matching layers are plotted.

Fig. 13 shows the spectral signature of the main peaks of theimpulse response for the two antennas. A time gate between 0and 1.38 ns (window 1) is applied to neglect all contributionsfollowing the main peak. It is apparent that the amplitudedifference between the links with and without matching layeris essentially spread through the entire frequency range from 5to 70 GHz. A 6 dB band (-22 to -16 dB with matching layersand -28 to -22 dB without matching layers) extends from 10to 70 GHz for both links. This is the core graph that highlightsthe potentials of the leaky lens antenna for low dispersivity asclaimed in this series of articles. If ohmic losses were lower thedispersivity would also be lower. In fact one can observe thatwhile the tapering at the beginning of the spectrum is verysimilar for the links with and without matching layers, theantennas with matching layers are characterized by noticeablyhigher losses at high frequencies.

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0 10 20 30 40 50 60 70

-30

-28

-26

-24

-22

-20

-18

-16S

12

dB

Fig. 13. Frequency domain spectrum after windowing 1 for both links,including antennas with (solid line) and without (dashed line) matching layers.

With the window 2, the contributions associated to theconnectors are included. In Fig. 14, a ripple of intensity 1-2 dBand with approximately 4 GHz period appears for both antennalinks, independently from the presence of the matching layers.

0 10 20 30 40 50 60 70

-30

-28

-26

-24

-22

-20

-18

-16

S1

2 d

B


When finally the contribution associated to the lens-air inter-face is introduced (window 3) it appears that the reflections atthis interface are essentially insignificant for the antenna withmatching layers, Fig. 15. However they are very important forthe antenna without matching layers, as they induce 6-8 dBoscillations.

VI. RADIATION PATTERNS MEASUREMENTS

The measurements of the radiation patterns have beenperformed at the Near Field Antenna Test Range of TNO inthe frequency band 33 - 75 GHz. Both co- and cross-polarizedcomponents in the E and H planes have been measured fortwo antennas, one with matching layers and the other withoutmatching layers.

A. Co-polarised Patterns With Matching Layers

The co-polarised radiation patterns in the E-plane are shownin Fig. 16 (a) and (b) for the case with matching layers.

The normalized co-polarised radiation patterns in the H-plane are plotted in Fig. 17 (a) and (b), with matching layers.

0 10 20 30 40 50 60 70

-30

-28

-26

-24

-22

-20

-18

-16

S1

2 d

B


-30

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-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30q (degrees)

dB

Normalized Co-polar Patterns for E-plane

40

35 GHz40 GHz45 GHz

50 GHz

(a)

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-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30q (degrees)

dB

Normalized Co-polar Patterns for E-plane(ML)

40

60 GHz65 GHz70 GHz

75 GHz

55 GHz

(b)

Fig. 16. E-plane Co-polarized component of the field radiated by the lensincluding matching layers: a) frequencies 35 to 50 GHz, b) frequencies 55 to75 GHz.

8

-30

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-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30q (degrees)

dB

Normalized Co-polar Patterns for H-plane

40

35 GHz40 GHz45 GHz

50 GHz

(a)

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-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30q (degrees)

dB

Normalized Co-polar Patterns for H-plane(ML)

40

60 GHz65 GHz70 GHz

75 GHz

55 GHz

(b)

Fig. 17. H-plane Co-polar component of the radiated field by the lensincluding matching layers: a) frequencies 35 to 50 GHz, b) frequencies 55 to75 GHz.

The radiation patterns are circularly symmetric with sidelobes below 15 dB on the main planes, except for the H-plane in the higher frequency range. In this case the patternsalso appear less symmetric, probably due to an inaccuratealignment of the near field probe with the antenna.

The radiation patterns pertinent to the antennas withoutmatching layers are presented in Appendix B.

B. Gains

The absolute gains of both antennas have been measuredas a function of the frequency by comparison with referencestandard gain horns. These gains are reported in Fig. 18 forthe antennas with and without matching layers. The front-to-back ratio has also been separately measured and it has foundto be higher than 25 db at all frequencies, with and withoutmatching layers. This is also in agreement with predictionsfrom section IV.B.4 of [4].

12

14

16

18

20

10 20 30 40 50 60 70Freq (GHz)

dB

22

24

8

10

4

6

0

2Gain with ML

Gain without ML

Measured Gains

Fig. 18. Gains as a function of the frequency for the antenna withoutmatching layers (dashed line) and the antenna with matching layers (solidline). Note that the scale is the same as in Fig. 12 of [4] for easy comparisons.

C. Cross PolarizationThe antennas radiate a non negligible amount of power in

cross polarization. The exact shape of the patterns is of littlesignificance, but for all antennas with and without matchinglayers, and for all frequencies, the distribution of the crosspolarized fields (according to the third Ludwig definition) isqualitatively described by the schematic drawing shown inthe inset of Fig. 19. Four cross-polarized lobes emerge in thediagonal planes. For the case of lenses with matching layers,the maximum level of cross-polarized field with respect to themaximum of the co-polarized field at the same frequency isshown in Fig. 19 as a function of the frequency. The maximumof cross-polarized fields is lower than 9 dB for all frequencies.

-14

-12

-10

-8

-6

-4

-2

0

30 35 40 45 50 55 60 65 70 75

max cross pol. normalized to max co-pol at broadside

dB

Frequency (GHz)

x

y

z

E-plane

H-p

lane

Fig. 19. Maximum cross-polarized field as a function of the frequency forthe antenna with matching layer, normalized to the maximum co-polarizedfields at the same frequency.

VII. CONCLUSIONS

The measurements of the hardware presented in this papershow that the planarly fed UWB Leaky Lens antenna has, over

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a BW 1:5:• Directive radiation patterns that are circularly symmetric,• Low (with respect to other UWB non dispersive antennas)

cross-polarisation levels,• Constant phase center (less than ±0.32 mm variation),• Efficiency higher than 50% (85% in an integrated sys-

tem).The increase of efficiency from 50 % to 85 % in an

integrated system is due to the absence of the connectors andµ-strip lines.

A link realized using two leaky lens antennas has demon-strated a pulse fidelity F higher than 0.94 (with F=1 being themaximum) on the band 5-70 GHz. This pulse fidelity, overthe full 3 dB beamwidth and on such a large bandwidth, ismuch higher than ever reported for any radiated link (F=0.8on a 1-3 BW is often considered excellent). This pulse fidelityis indeed comparable to the one of TEM-guided links at lowfrequencies, opening a wide range of opportunities for systemdesigners. Since the link is composed of two nominally iden-tical antennas, each of the two antennas essentially radiatesnon dispersively.

The present Leaky Lens antenna, as it is also directive, canbe used as feed for reflector systems capable of operating overdecade bandwidths, even if imaging with hundreds of beamsis required. Moreover, minor design adaptations can lead to anomnidirectional antenna (only the shape of the lens needs tobe changed). This would render the concept suited for short-range communications and SAR systems. Finally, the antennais very inexpensive and simple to manufacture.

APPENDIX A:TIME DOMAIN LINK RESPONSE WITHOUTMATCHING LAYERS

Fig. 20 shows the time domain representation, normalizedto the maximum, of the S12 parameter when the two antennasof the link do not include matching layers. The main receivedsignal arrives to the receiver after 1.33ns corresponding toapproximately 399mm of equivalent free space propagation,(it was 404mm in the case of the matching layers). Thedifference corresponds to the matching layers thickness. Asecond peak can be observed 0.3ns after the main peak.0.3ns approximately correspond to 9cm in free space andthus to 3cm in the dielectric. Accordingly, this image can beassociated to single reflections in the lens. The amplitude ofthis second peak is 15 dB lower than the main one. Also in thiscase the antenna would be indicated as ringing free accordingto recent indications in the literature [2].

Fig. 21 shows an expanded view of the pulse received andthe reference signal, in order to highlight that the pulse is moredistorted than in absence of the matching layers (compare toFig. 9). In particular the pulse is slightly wider and the fidelityfactor on the band is F=0.91. When the attention is moved tothe band 15 to 70 GHz, Fig. 22 the pulse preservation improvessignificantly and can be quantified by F=0.96. Thus the lowerportion of the band, suffers most significantly from the pulsedispersion associated to the reflections at the dielectric-airinterface.

Phase Center Stability Without Matching Layers

Fig. 23 shows on the right scale the distance between thetwo antennas as measured using different frequency bands,

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3

time(ns)

no

rma

lize

d S

12

main response(39.9 cm in fs)

Lens reflections(39.9+9cm in fs)

Fig. 20. Normalized time domain representation of the S12 in the presenceof matching layers.

0.6

0.8

1

1.2(5-70 GHz) pulse, fidelity=0.91


0.013ns

0.016ns

0

0.5

0.7

1

1.31 1.32 1.33 1.34 1.35

0

0.2

0.4

1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7

time(ns)

no

rma

lize

d S

12

Fig. 21. Comparison between the measured impulse response and an ideal65 GHz BW (5-70 GHz) pulse delayed by a time corresponding to 399 mm.In the inset the pulse is visible.

0

0.2

0.4

0.6

0.8

1

1.2

1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65

(15-70 GHz) pulse, fidelity=0.96


time(ns)

1.7

no

rma

lize

d S

12

Fig. 22. Comparison between the measured impulse response and an ideal55 GHz BW (15-70 GHz) delayed pulse.

10

while the left scale shows the phase center movement. Thevariation of the phase center in each of the antennas is0.75 mm, in the lowest band, from 5 to 21 GHz, but thenstabilizes to levels comparable to the antenna with matchinglayers for the higher frequencies. Thus, again it appears thatthe dispersivity introduced by the absence of matching layersis mostly confined at lower frequencies.

21.3 37.5 53.7 70

-0.5

-0.25

0.

0.25

0.5

0.75

1

1.25

1.5

freq (GHz)

Dfc

(mm

)

5

400.0

399.5

399.0

398.5

398.0

397.5

397.0

396.5

396.0

Dr12

(mm

)

Fig. 23. On the right scale: optimal delay that should be introduced torecover 16 GHz BW pulses centered at different frequencies. On the leftscale: corresponding phase center variation with respect to the average.

APPENDIX B: RADIATION PATTERNS WITHOUT MATCHINGLAYERS

The normalized co-polarized radiation patterns in the E-plane are shown in Fig. 24 (a) and (b) for the case withoutmatching layers. The normalised corresponding co-polarisedradiation patterns in the H-plane are shown in Fig. 25 (a) and(b). Also in this case the patterns are mostly symmetric withrespect to broadside. While for low frequencies the patternsare relatively large, the beam width decreases with frequency.The measured side-lobes are lower than -10 dB.

REFERENCES

[1] J. D. Taylor, ”Ultra-Wideband Antenna Technology”, Introduction toUltra-Wideband Radar Systems, CRC Press, 1995.

[2] W. Sorgel and W. Wiesbeck, ”Influence of the Antennas on the Ultra-Wideband Transmission” EURASIP Journal on Applied Signal Processing2005, no.3, pp. 296-305.

[3] R. Olsson, P.S. Kildal, and S. Weinreb, ”The eleven antenna: a compactlow-profile decade bandwidth dual polarized feed for reflector antennas”,IEEE Transactions on Antennas and Propagation vol. 54, no.2, pp. 368-375, Feb. 2006.

[4] A. Neto, ”UWB, Non Dispersive Radiation from the Planarly Fed LeakyLens Antenna. Part 1: Theory and Design” IEEE Transactions onAntennas and Propagation, accepted for pubblication

[5] CST Microwave Studio, User Manual Version 5.0, CST GmbH,Darmstadt, Germany.

[6] A. Mehdipour, K. Mohammadpour-Aghdam and R. Faraji-Dana,“Complete Dispersion Analysis of Vivaldi Antenna for UltraWideband Applications”, Progress In Electromagnetics Research PIER77, 2007.

[7] E. Pancera and W. Wiesbeck, ”Correlation Properties of the PulseTransitted by UWB Antennas” Proc. of the 11th International Conferenceon EM in Advanced Applications 14 - 18 Sept. 2009, Torino, Italy.

[8] M. Manteghi, Y. Rahmat-Samii,“On the Caracterization of a ReflectorImpulse Radiating Antenna (IRA): Full Wave Analysis and MeasuredResults”, IEEE Transactions on Antennas and Propagation Vol. 54, no.3, pp. 812 - 822, March 2006.

-30

-25

-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30

35 GHz40 GHz45 GHz

q (degrees)

dB


40

(a)

-30

-25

-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30

50 GHz55 GHz60 GHz65 GHz70 GHz75 GHz

q (degrees)

dB


40

(b)

Fig. 24. E-plane Co-polarized component of the field radiated by the lenswithout matching layers: a) frequencies 35 to 45 GHz, b) frequencies 50 to75 GHz.

11

-30

-25

-20

-15

-10

-40 -30 -20 -10 0 10 20 30q (degrees)

dB

40

-5

0 35 GHz40 GHz45 GHz


(a)

-30

-25

-20

-15

-10

-5

0

-40 -30 -20 -10 0 10 20 30

50 GHz55 GHz60 GHz65 GHz70 GHz75 GHz

q (degrees)

dB


40

(b)

Fig. 25. H-plane Co-polarized component of the field radiated by the lenswithout matching layers: a) frequencies 35 to 45 GHz, b) frequencies 50 to75 GHz.

uwb, non dispersive radiation from the planarly fed leaky...

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