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Nuclear Instruments and Methods in Physics Research A 520 (2004) 390–392

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doi:10.1016

Antennas for bolometric focal plane

Alexey Goldina,*, James J. Bocka, Andrew E. Langeb, Henry LeDuca,Anastasios Vayonakisb, Jonas Zmuidzinasb

aJPL, 4800 Oak Grove Drive, Pasadena, CA 91109, USAb61 W. Bridge, M/C 59-33, Caltech, Pasadena, CA 91125, USA

Abstract

The future Cosmic Microwave Background (CMB) experiments will require large focal planes with hundreds of

bolometric receivers. For CMB detectors, the field of view should be restricted, as even with cryostat cooled liquid helium

temperatures a significant deteriorating performance load will be present (Appl. Opt. 41 (2002) 6543). We present a planar

antenna for millimeter band produced by lithographical methods only, which, without lens concentrator or a horn, has

convenient beamwidth about F=3–F=4: The radiation and impedance characteristics of the antenna were obtained from a

moment-method calculation. The measured beammap of the antenna prototype is presented in this paper.

r 2003 Elsevier B.V. All rights reserved.

PACS: 07.57.k

Keywords: Slot antennas; CMB; Polarization; TES bolometers

1. Dual polarization, single band and single

polarization wideband antennas

In our calculations we use the method describedin Ref. [1], which treats the substrate as semi-infinite dielectric half-space. This can be a goodapproximation if an antireflection coating isapplied and its bandwidth includes the antenna’sintended operating frequency range [2].The design of a dual polarization antenna is

shown in Fig. 1. This antenna is too small to havesufficiently narrow beam in the air, and is shownhere as an illustration. The feed line is shifted tothe side to reduce the antenna impedance. Becauseof this assymetry, a small cross-polarization signal

onding author.

ddress: alexey.goldin@jpl.nasa.gov (A. Goldin).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.11.342

is excited. Due to offset feed line this antenna has asignificant inductive impedance, which is tuned outby a stub capacitor (Fig. 3). The feed network wassimulated using supermix library [3].A single polarization wideband antenna is built

using long slots with multiple taps across thewhole area of an antenna. The periods, both alongthe slot and perpendicular to it, should be less thanl=

ffiffie

pfor the shortest wavelength in a band to

avoid exciting surface waves (Fig. 2).The impedance of this antenna is changing

relatively slowly, which allows the use of the sameantenna at frequencies from 100 to 400 GHz: Theantenna has an impedance shown in Fig. 4. Thebeam is F=3:3 wide at 100 GHz and is gettingmore narrow at higher frequencies, reducing theilluminated spot on the telescope dish inverselyproportional to the frequency. The size of the

d.

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Fig. 1. Reduced two polarization antenna, single band.

Fig. 2. Reduced wideband antenna, single polarization.

Fig. 3. Dual polarization antenna—self and mutual (vertical to

horizontal slot) impedance of the periodic slot antenna, shown

in Fig. 1. The taps are offset 223 mm from the center. The

period is 700 mm; width 10 mm; length 550 mm:

Fig. 4. Single polarization wideband antenna impedance. The

taps and slots are spaced at 155 mm; for a total of 64 slots, 64

feeds in each.

A. Goldin et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 390–392 391

beam on the sky stays roughly the same due todiffraction, not achieving the diffraction limit athigher frequencies, since the whole area of the dishis not used.Following the discussion in Ref. [1], we can use

the calculated electric field distribution in a slot tofind a radiation pattern. The beammaps of bothantenna designs are very similar, except that thedual polarization antenna has higher cross-polar-ization due to the asymmetry of the taps, notexceeding �38 dB according to our calculations.

2. Test results

To date, we have built a simple prototype of thesingle polarization antenna. The antenna shown in

Fig. 5 was built on a 380 mm thick silicon substratewith a dielectric constant of 11.9. There are 16slots, spaced at 620 mm; each fed by 16 feedlines also with a period of 620 mm: The groundplane and microstrip layer are 3000 (A thick

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Fig. 5. Single polarization antenna prototype.

Fig. 6. Measured and predicted antenna beammap at 110 GHz:The shaded beammap represents measured data, the dashed

line—calculations. No lenses or horns were used. The contour

spacing is 3 dB:

A. Goldin et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 390–392392

superconducting niobium. The dielectric layer is4000 (A SiO with a dielectric constant of 5.6.

The microstrip feed network, after passingthrough a filter, is connected to a SIS junctionwith a critical current 10 kA=cm2 and an areaof 2 mm2: The chip with the antenna and SISjunction is housed in a mixer block lookingthrough a 40 mm thick mylar window. Theroom-temperature IR radiation is blocked by aZotefoam window heat sunk to 77 K: A1 mm quartz plate with a dielectric constant of4.3 is used as an antireflection coating. Thislayer thickness is approximately 3

4of a wave-

length at 110 GHz for a normal incident wave(Fig. 6).The signal detected by the SIS junction is

amplified by an MMIC low-noise amplifier pro-duced by Agilent Technologies. The source micro-wave signal generator, also built by Agilent, ismodulated at 30 MHz and the signal is detected bySR830 radio frequency lock-in amplifier, byStanford Research Systems.

Acknowledgements

We gratefully acknowledge the generous sup-port of Alex Lidow, Caltech Trustee. Additionalsupport for this work was provided by NASAGrant NA5-10317. Alexey Goldin is supported byan NRC Associateship stipend.

References

[1] J. Zmuidzinas, H.G. Leduc, IEEE Trans. Microwave

Theory Tech. 40 (1992) 1797.

[2] J.J. Bock, M.J. Griffin, W.K. Gear, Appl. Opt. 41 (31)

(2002) 6543.

[3] J. Ward, F. Rice, G. Chattopadhay, J. Zmuidzinas, in:

Proceedings, 10th International Symposium on Space

Terrahertz Technology, University of Virginia, Virginia,

March, 1999.