Microwave helix as a surface coil antenna

JOURNAL OF MAGNETIC

RESONANCE

Microwave

63,383-387 (1985)

Helix as a Surface Coil Antenna

M. D. PACE AND A. D. BRITT~ Chemistry Division Code 6120, Naval Research Laboratory. Washington, D.C. 20375-5000 Received December 6, 1984

Electron paramagnetic resonance spectroscopy at x band (9.5 X lo3 MHz) generally requires a cavity resonator having a high quality factor (Q). Commonly used EPR cavities include the TE roz rectangular cavity and the TMllo cylindrical cavity. The application of EPR as an imaging technique has pointed out a need for detectors which are better suited for clinical use than is the cavity resonator. Resonant devices which differ from a cavity include modified cavity resonators, lumped circuit resonators, spherical mirror resonators, slow-wave resonators, and surface coils (Z-4). Surface coils such as the flat-loop antenna (operating in the 1% 2.0 GHz range) have been developed and demonstrated to detect nitroxyl spin-label samples at remote distances and positions from the antenna (5). We have used a second type of wire antenna, the helical antenna (or helix), to detect EPR signals from samples physically separated in space from the end of the helix. A strong paramagnetic sample such as DPPH (2,2-diphenyl-l-picrylhydrazyl) was used to record a spectrum as far as 16 cm from the end of the helix. This is possible because the helix produces a focussed radiation pattern along its axial direction which extends beyond the end loop of the helix. The helical resonator which we tested was constructed for x-band operation based upon design specifications as reported by several authors (6-10). The diagram in Fig. 1 describes the typical parameters used for the helix. The device itself is of simple construction consisting of a coiled wire (usually copper; sometimes plated with gold or silver for better conductivity), but the design parameters (i.e., loop diameter, loop spacing, and pitch angle) are critically important for proper performance. Two sources were used for the copper ribbon: No. 20 gauge wire was flattened by drawing through a pressto produce a ribbon with approximate thickness and width of 26 X 39 mils (the wire was flattened for a more uniform conduction path) and bare copper ribbon with the same dimensions was purchased from MWS Precision Wire, Chatsworth, California. The wire was hand wound onto a drill rod to construct a helix with 30 turns, a diameter of 4.4 mm (ca. 0.17 in.) and a pitch angle of ca. 7”. The helix was coupled to the waveguide by uncoiling the first turn to form a straight wire along the axial direction of the helix and inserting the helix through a 7 mm diameter port in the longer edge of the waveguide (similar to the assembly t Recently deceased. 383

0022-2364185 $3.00 Copyiigbt Q 1985 by Academic Press.Inc. All rights of reproduction in any form reserved.

384

NOTES

2a

= Diameter

P =Gap

spacing=

c =Circumference w =Pitch d = Wire L = Length

angle=

= 4.4mm 0.9

mm

=

1.37cm

70

-5

diameter=O.gmm = ca.

6cm

major

axis I

FIG. 1. Typical design specifications used to construct the x-band helix which was tested.

described by Pearlman et al. (8)). This coupling procedure produced a “match” to the helix at three frequencies in the range 9-10 X lo3 MHz (i.e., 9.1, 9.4, and 9.7 X lo3 MHz). More elaborate coupling schemes are described in several references below. The coupling produced a tuning mode shown in Fig. 2. From this plot it is possible to estimate a value of the quality factor Q. This value is estimated as the resonant frequency f divided by the bandwidth at half power f’. Thus, Q =f” = 574 for this helix. This value is typical of that for most of the helices used in these experiments. The phase matching at the resonant frequencies was maximized by means of an adjustable short located just after the waveguide port used for coupling to the helix.

FIG. 2. The tuning mode for the helix is shown. The bandwidth is 17 MHz. The Q value is ca. 570.

385

NOTES

Spectra can be recorded with the sample either inside or outside of the helix. The experimental setup which is used to routinely record spectra consists of a glass tube which fits inside the helix. The helix then slips into a larger diameter Pyrex tube which is mechanically held to the waveguide by a mounting ring with screw fasteners. Many schemes are possible. This seemed easiest to assemble. Helmholtz modulation coils are supported separately from the helix. In this case they were attached to the magnet pole faces. This allowed for easy sample exchange. The sensitivity inside the helix was measured with a sample of 4-amino-2,2,6,6tetramethylpiperidine- 1-oxy (tempamine; a nitroxide spin probe) in carbon tetrachloride. Spectra of samples from stepwise dilutions of a concentrated stock solution are shown in Fig. 3. Figures 3a-d show how the spectrum intensity decreasedwith the gain and power held constant. In Fig. 3e the same sample as in Fig. 3d was recorded using a higher gain and power setting. Figures 3f-g show two additional dilutions recorded using higher instrument settings. The spectrum in Fig. 3g shows of

number

sDins

molar

relative

concentration

2.1x10

8.1x10

2.9x10

1.1x10

1.1x10

8.2x10

17

18

18

18

18

15

4.7x10

1.8x10

8.8x10

2.4x10

2.4x10

1.4x10

-3

-3

-4

-4

5.0x10-=

5.0x10

power

xl

20mw

xl

20mw

xl

20mw

xl

20mw

x2.5

50mw

x2.5

50mw

x2.5

50mw

x.25

50mw

-4

-4

microwave

gain

-5

FIG. 3. First-derivative EPR spectra of an 0.07 ml solution of tempamine nitroxyl radical in CC& were recorded using stepwise dilutions of a stock solution. The practical limit of sensitivity is 5 X 10e5 M.

386

NOTES TABLE 1 Relative Signal Intensities of a Remotely Located Paramagnetic Sample” Signal intensity b

Distance’ (cm)

Relative gain

Power (mw)

1.0 0.75 0.30

3 6 9

Xl XI Xl

200 200 200

’ 1.6 mg of DPPH; ca. lo’* spins. b The peak-to-peak intensity of a single-line first-derivative spectrum normalized to the most intense value. c The distance separation along the major axis between the end of the helix and the DPPH sample.

a practical limit of detection to be 2.2 X lOI spins or approximately 5 X 10e5 M. This spectrum is recorded using a scan time of 200 s and a time constant of 50 ms at a modulation amplitude of 0.5 G. With more extreme settings of gain and time constant lower concentrations can be detected. The same sample as in Fig. 3g was recorded using a TM,,,, cavity. This spectrum is shown in Fig. 3h for comparison. Figures 3g-h indicate an improvement in signal/noise ratio of 40/l for the TM major axis

FIG. 4. The far-field axial radiation mode of the helix described in Fig. 1 was calculated using equations from Refs. (II) and (12). (Number of turns, N = 30; loop separation, S = 0.196 cm; wavelength = 3 cm.)

cavity over the helix. This is not surprising since the (2 value of the cavity resonator is 3000 compared to a value of 500 for a helix. If a paramagnetic sample is very concentrated it is not necessary to have the sample located inside the helix to record an EPR signal. Table 1 shows the results of an experiment which measured the signal intensity as function of distance separation between the end of the helix and the paramagnetic sample (a 1.6 mg sample of DPPH; ca. lo’* spins). The sample was held in the center of the pole pieces (Ho) and was located coincident with the major axis direction of the helix. The helix was positioned away from the sample by using progressively shorter pieces of waveguide to connect with the microwave bridge. At distance separations of 3, 6, and 9 cm there was no problem detecting the signal. At a distance separation of 16 cm the signal was still detectable, but required a considerable increase in gain. A practical lower limit to the sensitivity at a distance separation of 3 cm was measured by using stepwise dilutions of DPPH in CCL. The practical lower limit was determined to be 1 X 10” spins. The detection of a remotely located paramagnetic sample is possible because the near-field and far-field axial radiation modes extend beyond the end loop of the helix. Approximate equations describing the geometric shape of these modes are presented by several authors (II, 12). By using such equations we calculated the approximate far-field axial mode pattern for this helix as shown in Fig. 4. The shape agreeswith our qualitative experimental measurements of the axial radiation pattern during actual operation of the helix. REFERENCES I. 2. 3. 4. 5.

6. 7.

8. 9. IO. Il. 12.

CHAMEL, R. CHICAULT, AND Y. MERLE D’AUBIGN~, J. P~Ju. E 9, 87 (1976). FRONCISZ AND J. S. HYDE, J. Magn. Reson. 47, 515 (1982). BEVERINI, S. MARCHETTI, AND F. STRUMIA, J. Phys. E 10, 1072 (1977). S~HMALBEIN, A, WITI’E, R. RYDER, AND G. LAUKIEN, Rev. Sci. Instrum. 43, 1664 (1972). FUJII AND L. J. BERLINER, “Development of Surface Coils for EPR Imaging”, 16th Southeastern Magnetic Resonance Conference, University of Kentucky, 1984. (Abstract) F. VOLINO, F. CSAKVARY, AND P. SERVOZ-GAVIN, Rev. Sci. Instrum. 39, 1660 (1968). R. H. WEBB, Rev. Sci. Instrum. 33, 732 (1962). M. R. PEARLMAN AND R. H. WEBB, Rev. Sci. Instrum. 38, 1264 (1967). J. C. COLLINGW~~D AND J. W. WHITE, J. Sci. Instrum. 44, 509 (1967). J. D. KRAUSS, “Antennas,” p. 137, McGraw-Hill, New York, 1950. W. L. STUTZMAN AND G. A. THIELE, “Antenna Theory and Design,” p. 260, Wiley, New York, 1981. C. A. BALANIS, “Antenna Theory: Analysis and Design,” p. 385, Harper & Row, New York, 1982.

M. W. N. D. H.