Volumetric 1.9-GHz Fields in a Hospital Corridor: Electromagnetic Compatibility Implications

Don Davis ' 2 4 ' ' Bernard Sega1"2i374i5 Dino M M a r t u c ~ i ' ~ ~ ~ ~ ' ~ Tomas JF P a ~ l a s e k ~ ' ~

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Departments of Otolaryngology I , Electrical & Computer Engineering, Biomedical Engineerin2 McGill University4, SMBD-Jewish General Hospital', Concordia University6

Montreal, Canada, H3T lE2

Abstract: The current drafi of the next medical-equipment EMC standard, IEC 60601-1-2, will recommend usage of free- space minimal separations (between RF sources of given power and medical devices of given immunity) to minimize EM1 malfunction of medical equipment. We have previously reported that such separations were usell in most hospital corridors, but such reports were based on mid-corridor measurements. We now describe preliminary three- dimensional 1.9-GHz extensions of these reports. We found that mid-corridor path loss was less than that at corridor walls. Path loss near floors was much less then that at higher locations. Because medical devices are rarely placed on the floor, our previously reported minimum-separation recommendations are still likely to apply at most corridor locations.

INTRODUCTION Increased usage of wireless communication is required to improve the efficiency and effectiveness of healthcare delivery. The implementation of such increased usage must take place without increased incidence of medical-device malfunction due to electromagnetic interference (EMI). The current draft [l] of IEC 60601-1-2, a medical-equipment collateral standard for electromagnetic compatibility (EMC), proposes that EMC can be fostered by specifying minimal separations, chosen so that medical devices will not be illuminated by fields above their specified immunity levels. Such minimal separations are obtained assuming the approximate validity of free-space propagation within healthcare environments. We have previously reported [2] that using such separations within hospital corridors, an obviously non-free-space environment, would in most cases prevent RF- source fields from exceeding immunity-levels of medical devices. However, these conclusions were based on measurements made along the centerlines of corridors. We now describe a preliminary study to evaluate if such conclusions are valid at other corridor locations.

METHODS Fields were generated by an un-modulated 1.898-GHz transmitter (HP616A) connected to a 14-cm monopole antenna, which was directly attached to the transmitter case, approximating a ground plane. Fields were received by an adjustable dipole (Electromechanics 3 121 C-db4) connected to a spectnim analyzer (Anritsu MS2601B). The 16-cm receiving

antenna was approximately calibrated in a large (22m x 15 m x 5 m) room, by appropriately comparing its response to a second calibrated antenna.

Fields were measured within a single 48-meter hospital corridor (1.9 m wide by 2.4 m high) along 12 linear paths, each having a different height and location (See Fig 1). The corridor had clay-block walls, glass-block corridor ends, with reinforced concrete floors and ceilings. A gyp-rock and tile sub ceiling hung from the concrete one. Metal lockers lined one side of about a third of the corridor.

I I I 3001 I'

Figure 1: Cross-section of hospital corridor and - measurement paths. Diamonds show locations of measurements made close to the floor at a 0.3-m height. Boxes and stars show locations of measurements made at higher locations, 1.2 m above floor, and 2.07-111 above floor (near tiled sub-ceiling), respectively.

The transmit antenna was located at one end of the corridor at the centerline. The corridor was cleared of people, but furnishings (e.g., chairs) were left within. Both transmit and

0-7803-6569-0/01/$10.00 0 2001 IEEE

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receive antennas were vertically polarized. Fields were space estimates. In all but one case, path loss as less than that measured at 1 -m intervals using a programmable free-moving robotic positioning system. The relatively low 1-m sampling resolution was known to yield acceptable path-loss estimates

predicted from free-space propagation.

I [3]. The robotic positioning system'was dksigned to provide high-accuracy positioning, with minimal perturbation of fields.

Path loss was estimated by using linear regression to fit measured data with the relationship:

where E was the electric field, b was the y-intercept, n was the path-loss exponent, and r was the transmitter-receiver separation. Note that n = 1 for free-space propagation, and that Equation 1 estimates the expected value of electric field strength as a function of transmitter-receiver separation.

E = b -t n log(r), (1)

RESULTS

Figure 2 compares measurements made along the 12 linear paths (Fig 2A: 1.2- and 2.07-m heights; Fig 2B: 0.3-m height) with predicted free-space fields (bold straight line) that would radiate from an ideal isotropic source having the same location and power level as the actual transmitter. Measured fields were highly variable, being within 10 dB of free-space

A

+ Ht120DlW + Ht207D 60 - Ht207D 92 + Ht207D160

-60 1 oo 10'

Separation (m)

I \ predictions in some paths, and deviating by up to 20 db in other paths.

-20-

For measurements at 1.2- and 2.07-m heights (Fig. 2A), fields - clustered around free-space predictions, becoming more $ variable when fkther from the transmitter. For measurements 8 at 0.3 m (Fig 2B), fields tended to vary around a constant level 5 for the entire length of the corridor. Thus surprisingly, fields f close to the concrete floor tended to be independent of Z4-

E ii

transmitter-receiver separation.

B

Measured fields exhibited another less obvious pattern: Fields closer to the corridor walls attenuated more rapidly with separation than those closer to the middle of the comdor.

Using linear regression to fit straight lines to the measurement data permitted such patterns to be seen more clearly (Fig. 3). Most lines fitted to data obtained at higher heights (filled symbols) had slopes similar to free-space predictions (heavy line), although fields fell more rapidly at some locations than others. Lines fitted to data obtained near the floor had near- zero slopes.

Computing path-loss exponents (i.e., slopes of lines fitted to data) helped characterize overall comdor-field behavior. Figure 4 shows how path-loss exponents varied with measurement-path height and location. For the higher-height measurements (1.2 m (circles); 2.07 m (Xs)), path loss at the center of the corridor was less (smaller negative exponents) than path loss at the walls (i.e., larger negative exponents) Path loss close to the floor (0.3 m (triangles)) was less (smallest negative exponents) than that at 1.2 and 2.07 m. When close to the source, fields tended to be lower than free

-+ Ht30D130 -e- Ht30D160

-60 lo" 10'

Separation (m)

Figure 2: Comparison of measured fields with free-space predictions. A: Higher locations. Solid symbols show measurements made 1.2 meters above floor (circles: 0.6 m from left wall: diamonds: 0.77 m from left: inverted triangle: 0.92 m from leff; upright triangle: 1.3 m from left; rightward triangle: 1.6 m from left). Hatched symbols show measurements made 2.07 m above floor (star: 0.6 m from left wall; X: 0.92 m from left; +: 1.6 m from left). B: Lower tbcations. HoIlow symbols show measurements made 0.3 m above floor (Upright triangle: 0.77 m from left wall; leftward triangle: 0.92 m from left; rightward triangle: 1.3 m from left; inverted triangle: 1.6 meters from left). In both A and B, free-space field predictions are shown by the bold straight line without symbols.

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t Ht12oD 92 4 M 120D 130

J-~gz~, , , , , , , , , , ,

-c HtXnDlGO

1 oo 1 0' Separation (rn)

Figure 3: Straight lines fitted to measurements. Symbols are defined in Fig. 2.

Position (m)

Figure 4: Pass-loss parameters associated with higher measurement paths (circles: 1.2 m above floor; Xs: 2.07 m), and with those near floor (triangles: 0.6 m above floor).

DISCUSSION

Generalfield behavior

This is the first volumetric study of 1.9-GHz field behavior in hospital corridors. Fields were estimated for a source located at the center of one end of a concrete and clay-block corridor. Fields tended to be larger in the middle of the Corridor than at the walls. Also, fields tended to decline more rapidly at the

walls than at the corridor center. This yielded path loss exponents that tended to be larger (i.e., more negative) at the walls than those at the middle of the corridor. However, to a first approximation, fields at 1-2 m heights tended to decline at rates that were similar but somewhat smaller, than those in free space.

In contrast, fields near the floor tended to remain constant along the entire 48-m corridor, leading to path-loss exponents that were close to zero. Near the source, fields near the floor were about 10 dB less than those at 1-2 m heights. At progressively larger source-receiver separations, fields near the floor tended to stay constant, while those at 1-2 m tended to fall to levels less than those at observed close to the floor. Like those at higher heights, fields near the floor tended to decline more rapidly at the walls then at the corridor center.

Methodology

As this was only a preliminary study, we did not attempt to implement the very-time-consuming task of completely characterizing volumetric field behavior, even though we had access to a robotic positioning system. Typically, fields were only sampled at I-m intervals along the corridor length and height, and at 0.15- to 0.8-m intervals across the corridor. Thus, such low spatial-resolution sampling intervals usually spanned several wavelengths of the 1.9-GHz transmitter frequency, clearly violating the Nyquist sampling criterion. However, such under-sampling of spatial fields still could be used to determine low-spatial-frequency trends of field behavior. We have previously compared [3] path-loss parameters based on both high- and low-resolution data and showed that path-loss estimates based on similar low- resolution data approximated those based on high-resolution data. This result was presumably obtained because the 48-m- length straight line fitted to the data obviously had very-low frequency spatial components. Also, the consistency of the cross-corridor observation, that path-loss exponents became more negative as the walls were approached, suggests that this result will be confirmed using higher resolution data. For these reasons, our low-resolution study was deemed an acceptable initial study of field variation as a function of volumetric position within hospital comdors.

Usage of Minimal Separations

The next medical-equipment EMC standard [I], IEC 60601-1- 2, will require that manuals of all new medical-devices tell the medical-device user to maintain minimal separations from RF sources of given power, such separations being estimated assuming the validity of free-space propagation. Initial reactions are that that such recommendations cannot be appropriate because buildings are not free-space. Surprisingly, we have demonstrated that such recommendations are often appropriate [4, 51.

For most cases in the current study (Fig. 2), we found that free-space field predictions were within f 10 dE3 of measured

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fields when within about 4 m from the RF source, but such predictions tended to tinderestimate measured fields when futher from the source. Since fields from commonly used lower-power RF sources fall below medical equipment immunity levels at such larger distances anyway, EMC is no longer an issue at these separations. However, caution would be required if higher-power sources were used.

Observations that, in some cases, measured fields exceeded free-space predictions by say 10 dl3, is not a practical concern because such predictions neglect all antenna losses. When these are included, actual radiated fields fall by a roughly 10 dB factor so that fiee-space predictions provide a useful upper limit for radiated fields for distances less than about 4 m.

In terms of hospital policy, the use of minimal-separation- based EMC guidelines is acceptable for low power sources. The reduced path loss near the floor was balanced by the fact that field strength received at the floor was lower than fields measured at greater heights above the floor. It should also be noted that the likelihood of having medical devices located within 0.3 m of the floor is relatively small as most devices are placed so as to be within easy reach of the medical staff.

In the case of higher power sources, minimal-separution-based guidelines should be used with more caution. For higher- power sources, or for simultaneous usage of several lower power sources, an additional safety term, extra distance, could be added in order to minimize EM1 potential. If this were not possible, then it would be preferable to define zones [e.g., 41 within which specified medical devices and transmitters would not be used simultaneously.

Mechanism Speculation

The mechanism or mechanisms responsible for observed differences in path-loss estimates are uncertain. One possibility is that guiding of waves in comdors might yield different propagation modes that are sensitive to comdor location. However, one of the most striking features of our results was the constancy of field levels near the corridor floor. Such observations suggest the possibility of surface- wave propagation along the floor boundary. Noting that our RF source was vertically polarized, further study is required to determine how these effects might be polarization dependent. Other factors might affect propagation along other paths, such as the cavity between the ceiling panel and the floor slab above. The effect of transmitter locations on propagation behavior also needs examination (e.g., transmitter near floor or ceiling). Extensions of preliminary simulations [e.g., 61 would be appropriate.

SUMMARY

This preliminary study has described volumetric behavior of 1.9-GHz field propagation due to a source transmitting at the center of a comdor. The following observations were made:

1.

2.

3.

4.

5.

Further

Fields near the comdor central axis tended to be highest, and tended to decline slower with separation than those in free-space. Fields near the walls tended to be less than those near the central axis, and tended to decline at a free-space rate. Fields near the corridor floor tended to remain constant regardless of separation from the source. In most cases of interest, minimal separations to promote EMC were largest in the middle of the corridor. Thus, previously reported minimum- separation guidelines, estimated on the basis of mid- corridor propagation measurements, are likely to apply at most corridor locations, except those near the floor when high-power sources are used. The use of minimal-separations guidelines proposed in the upcoming IEC 606061-1-2 standard is appropriate for reducing the EM1 risk associated with lower-power RF sources.

work is required to better understand propagation mechanisms affecting field behavior in comdors, and to clarify mechanisms responsible for such behavior. In addition, further clarification is required of the geometries, construction materials and furnishings that influence such behavior.

REFERENCES

[l] IEC 60601-1-2 (200X), Medical electrical equipment- Part 1 : General requirements for safety-2. Collateral Standard: Electromagnetic compatibility-Requirements and tests. In preparation. (Presently IEC 62A/32 I/CD).

[2] Davis D, Segal B, Cinquino A, Hoege K, Mastrocola R, Pavlasek T Electromagnetic compatibility in hospital corridors. Proc 1999 IEEE Int Symp Electromagnetic Compatibility: 268-272. Seattle, WA. 1999

[3] Davis D, Segal B, Chu D, Trueman C, Pavlasek T Effect of spatial-sampling resolution on electromagnetic path-loss & interference-potential estimates in hospital corridors. Proc 2000 Can Med Biol Eng SOC 26: 46-47.2000

[4] Davis D, Segal B, Pavlasek T Electromagnetic- interference risk statistical quantitization when using free- space minimal-separations between wireless sources & medical devices. Proc. Can Med Biol Eng SOC 25: 126-7.'1999

[SI Davis D, Segal B, Trueman CW, Calzadilla R, Pavlasek T Measurement of indoor propagation at 850 MHz and 1.9 GHz in hospital corridors. Proc 2000 IEEE-APS Conference on Antennas & Propagation for Wireless Communication, p. 77- 80.2000

161 Trueman CW, Davis D, Segal B. Ray Optical Simulation of Indoor Corridor Propagation at 850 and 1900 MHz. Proc 2000 IEEE-APS Conference on Antennas & Propagation for Wireless Communication, p. 81-84.2000

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