Electromagnetic radiation exposure: assessment against ACA mandated limits

Transcription

1 Electromagnetic radiation exposure: assessment against ACA mandated limits General radio services (operating above 0 MHz) (Edition May 0) Disclaimer Unless otherwise specified, the information contained in these guidelines is intended as a guide only and should not be relied upon as legal or technical advice or regarded as a substitute for legal or technical advice in individual cases. Opinions contained in these guidelines do not necessarily reflect the opinions of ACMA. It is recommended that independent specialist advice be sought before relying upon the information contained in these guidelines.

2 Introduction This information booklet has been prepared to enable licensees and/or operators of radiocommunications services not covered by a service specific booklet to make a simple assessment of their facilities against electromagnetic radiation (EMR) exposure limits mandated by the Australian Communications Authority (ACA). The charts contained within allow a determination of the distance from the antenna at which the exposure limit is met. There is no obligation on licensees to use this booklet and other theoretical assessment methods are acceptable to the ACA provided they can be traced back to the equations given in Appendix B of AS 77. Radiofrequency radiation: principles and methods of measurement 00 khz to 0 GHz. This booklet covers transmitters operating above 0 MHz. Applicable Conditions These charts assume a single antenna (fed by any number of transmitters) no interference from adjacent transmitting antennas no reflections from significant metal structures in front of the antenna. If these conditions do not apply detailed analysis (beyond the scope of this booklet) or physical measurement is required. Near-field correction Close to an antenna, in what is known as the near-field, the gain of the antenna may be less than the farfield gain specified by the antenna manufacturer. This effect is frequency and antenna dependent. Due to the broad frequency range that radiocommunications services operate over and the large variety of antenna types available this booklet does not provide near field correction charts. Making the assessment Three steps are required to assess a particular installation:. Ascertain antenna type. Determine EIRP. Determine the distance from the antenna at which EMR exposure limits are met. Step : Ascertain antenna type Radiocommunications installations may use use omni-directional antennas such as dipoles or directional antennas such as yagis and panels. Parabolic (dish) antennas are sometimes used as part of a service for communication between sites. This booklet does not cover this type of antenna. Instead, refer to the companion booklet on fixed links. Omni-directional antennas Monopole or stacked dipole: Energy is transmitted equally in a horizontal direction but not above or below the antenna. Directional antennas Yagi: Side view Top view Majority of energy transmitted in front of the antenna. Very small amount transmitted in other directions. Page of 9

3 Panel or corner reflector: Energy transmitted in front of the antenna. No significant energy transmitted to the back or sides or directly above or below the antenna. Side view Top view Step : Calculate EIRP from power and gain Before determining the safety distance from the antenna, the maximum equivalent isotropically radiated power (EIRP) must be determined. Some radiocommunications licenses may specify the EIRP. If this is the case, use that value and skip this step. Chart provides a method for determining the EIRP for an antenna from the power fed to the antenna and the antenna gain. Gain: the gain of the antenna will be provided by the antenna manufacturer. It may be expressed in either dbd or dbi. If the antenna gain is expressed in dbi, it will need to be converted to dbd for use with Chart. This is done by subtracting.5 from the dbi figure. For example, 8.5 dbi is equivalent to 6 dbd. Power: The power figure is Chart is the power, in watts (W) input to the antenna. The power can be the known transmitter output power or that specified on the radiocommunications licence. When more than one transmitter feeds the antenna, the power of all transmitters should be added together and the sum used for Chart. A range of equipment, for example combiners and filters, can cause signal loss between the transmitter and the antenna. The manufacturer of the equipment may provide a db figure for these losses. If this is the case, subtract this figure from the antenna gain. If the value of the losses is unknown the safety distance determined in step three will simply be overestimated. Use of Chart Using a straight edge, for example a ruler, draw a line between the appropriate power figure on the left and the appropriate gain figure on the right. The EIRP can be estimated from the point where the line intersects the EIRP column. For example, if power is 0 W and antenna gain dbd, the EIRP column is intersected at approximately 650 W (the actual value is 656 W). If in doubt as to the precise value, use the next highest division. Page of 9

4 Example use of Chart : Power (W) EIRP (W) Gain (dbd) Alternatively, the gain may be calculated using the following equation: EIRP = P G where P is the power fed into the antenna and G is the linear gain of the antenna determined by the following equation: dbd+.5 G =. Page of 9

5 Power (W) EIRP (W) Gain (dbd) Chart Page 5 of 9

6 Step : Determining the distance from the antenna at which EMR exposure limits are met Once the EIRP of the antenna is known, the distance at which exposure limits are met can be determined. Charts and provide a method for determining this distance. The charts are broken up into two categories:. (): the area directly in front of the antenna is known as the boresight. For the purpose of the charts in this booklet, the boresight has been defined as the area ±5 both horizontally and vertically in the intended directon of radiated energy. Refer to figures.. Off axis (): the area between 5 and 90 from directly in front of the antenna both horizontally is as off-axis. Refer to figures. Panel/corner reflector side view Figure Panel/corner reflector top view Yagi side/top view Figure Page 6 of 9

7 Omnidirectional side view Figure Omnidirectional top view The appropriate chart(s) for determining the distance at which EMR exposure limits are met must next be selected. Figures indicate which charts may be used for various areas around the antenna. Once the appropriate chart(s) have been selected draw a horizontal line across the chart from the appropriate EIRP value until it meets the curve. From this point draw a vertical line down to the bottom of the chart to intersect the distance axis. The value at this point is the distance from the antenna at which EMR exposure limits are met. For example, if an antenna has an EIRP of 500 W, indicates the safety distance is approximately. metres in front of the antenna. is used to find the appropriate distance off-axis. For an EIRP of 500 W the EMR exposure limit is met approximately. metres above, below and to the sides of the antenna. Once the distance at which EMR exposure limits are met is determined, ensure that adequate access restrictions are put in place to ensure that members of the public cannot enter areas around the antenna where exposure limits are exceeded. Example use of : - no near-field correction EIRP (W) Distance (m) Page 7 of 9

8 - no near-field correction EIRP (W) Distance from antenna (m) Page 8 of 9

9 Off-axis - no near-field correction EIRP (W) Distance from antenna (m) Page 9 of 9