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GL-01 — Guidelines for the Measurement of Radio Frequency Fields at Frequencies from 3 KHz to 300 GHz

Appendix 1

Section A - Procedures for Measuring the Levels of RF Energy at and in the Vicinity of FM/Digital Radio, VHF/UHF/Digital TV and MDS Transmitting Sites

1. Introduction

This part of the appendix covers transmitting facilities involving FM Radio (88-108 MHz), Digital Radio (1452-1492 MHz), VHF, UHF, Digital TV in the bands 54-72 MHz, 76-88 MHz, 174-216 MHz, 470-806 MHz and MDS operations in 2596-2686 MHz. Field measurements should be done by qualified personnel who have a good understanding of broadcasting operations and of broadcasting facilities. The following procedures apply to all classes of undertakings, including low-power stations.

2. Related Documents

Pertinent documents on the subject of non-ionizing radiation (NIR) are listed below:

3. Measurement Zone

3.1 Estimating Appropriate Measurement Distances

The maximum radial distance(s) to which measurements should be undertaken at an antenna site should be estimated by first performing a theoretical analysis of the power density contributions of each transmitter. This analysis should take into account the permissible power density contribution at each frequency in use, the vertical and horizontal location of each antenna on the supporting structure(s), azimuth and elevation patterns of each antenna, the type(s) of modulation, the polarization of the radiating antenna(s), the terrain elevations near the site and the maximum ERP of each transmitting facility.

Power density prediction programs, such as 'HIFIELD', may be used to estimate the critical distances within which locations exceeding the Safety Code 6 levels may occur. It should be noted that most programs of this type have built-in assumptions and safety margins and the calculated maximum measurement distances may be somewhat conservative.

When taking radiation patterns into account in calculations relating to estimating maximum measurement distances, appropriate allowances should be made for variations due to the antenna support structure(s). The use of Expansion (E) and Quadrature (Q) allowance factors, as envisaged in 'HIFIELD', may be appropriate, depending upon the antenna.

3.2 Layout of Measurement Points

The layout of measurement points depends on the system under consideration. Measurements should be taken along at least eight equally spaced radials, extending from a central reference point at the site (e.g. the base of the tower) to the maximum assessment distance, as determined in Section 3.1 above. If measurements are to be made at specific individual points rather than continuously, the distance between measurement points should be no greater than two metres.

The number of radials may have to be increased, and/or the maximum distance from the central reference point may have to be extended if readings suggest that additional measurements should be taken in order to ensure compliance with the Safety Code 6 limits at all locations on or near the site, where public access is possible.

4. Measurement

4.1 Method of Measurement

For single-station sites, measurements can be made using an appropriately calibrated device of either the broadband or single frequency type (see Section 3 of GL-01).

For multi-station sites, the use of a broadband total power density-measuring device is appropriate. A spectrum analyzer may be used to determine all the sources of emission received at the site, and:

  • In the absence of contributors from below 30 MHz and above 300 MHz, measurement could be done using a device having a"flat" amplitude-frequency response.
  • In the presence of contributors below 30 MHz and above 300 MHz, the instrument should be a "weighted response" device exhibiting the recommended Safety Code 6 curve.

The surveyor's "scan" of each measurement point should follow the suggestions discussed in Section 4.2 (of the main document). Normally, these are done holding the probe away from the body, with no other object present within a few metres from the surveyor. The body of the surveyor should not lie in the path of the signal measured, neither in front of, nor behind, the probe. To better illustrate this, assuming the surveyor faces the source of the signal measured, his/her arm should be stretched out sideways to hold the probe that should preferably, in turn, be pointed towards the signal source.

In those instances where a tripod is used, the tripod should be non-metallic to avoid any disruptive effect. The resonance of such a device can fall near the frequencies involved and the parasitic disturbance can substantially disrupt the local field.

If analog TV station(s) are involved and where there is a hot spot exceeding 75% of the permissible power density, both the peak and the average readings, over a period of one (1) minute, should be recorded.

More closely spaced measurements are required near potentially reflective objects such as walls, fences etc. whether or not these are located along the radials chosen. Measurements closer than 20 cm from an object are not considered valid.

4.2 Measurement Report

A test report should include the following data:

  • a general description of the site and the transmitting facility;
  • a statement of compliance/non-compliance to the Safety Code 6 limits;
  • highlight any measured values in excess of 50% of the applicable limit and the location of these points (or zones); and
  • provide a description of the means by which the applicant will prevent general access to, or warn of, locations and "hot spots" exceeding the appropriate recommended limits.

Section B - Procedures for Measuring the Levels of RF Energy at and in the Vicinity of AM Radio Transmitting Sites

1. Introduction

This part of the appendix covers AM Radio Broadcasting Service in the band 525 to 1705 kHz. Field measurements should be taken by qualified personnel who have a good understanding of broadcasting operations and applications. The same procedures also apply to low-power AM stations.

2. Related Documents

Pertinent documents on the subject of Non-Ionizing Radiation (NIR) are listed below:

3. Measurement Zone

3.1 Distance Calculation

Due to the distances between the radiators (towers) in AM arrays, each tower must be assessed separately. Alternatively, for each tower, a practical radial distance, where measurement can begin and proceed inward therefrom, can be established using Table 1 (the same table as in Appendix 2 of BPR-1 issued by the Department). The distances in the table were derived using the Numeric Electromagnetic Code (NEC) program, as applied to linear radiators. The model assumes the worst-case distances from single AM towers. In-between distances can be estimated by linear interpolation of the two closest listed distances. If the Table method is used, the measurement zone for each tower should be determined using the relevant input power at its base. While this is only an approximate method, it will be sufficiently accurate in most cases. When in doubt, for low-power towers, a minimum measurement radius of 5 metres is suggested.

3.2 Layout of Measurement Points

The Safety Code 6 limits will be found to lie along a locus generally circular or slightly egg-shaped around the foot of each tower. For a detailed measurement, a minimum of four readings should be taken along each radial for each tower, moving inwards from the maximum measurement radius. In general, however, only the 'hottest' tower (i.e. the one with the most current, needs to be considered). The calculated measurement radius may have to be extended if readings at the starting point already exceed the recommended general public Safety Code 6 limits.

4. Measurements

4.1 E field, H field and Power Density Measurements

In general, the measurement zones for AM stations lie in the near and reactive field of the transmitted RF energy and therefore both E field and H field measurements are required. For single-station sites, measurements can be made using an appropriately calibrated device of either the broadband or single frequency type (see Section 3 of GL-01).

For multi-station sites the following should be considered:

4.1.1 Dual AM sites. As the Safety Code 6 limit in the AM Band becomes frequency dependent at 1 MHz, the simplest approach in determining the distance is to use a power value corresponding to the total for the two stations and using the highest frequency to determine the Safety Code 6 permissible limit. Otherwise, the individual contributions would have to be determined and added (square of each field) to determine compliance, a procedure which requires turning off each station in turn during measurements.

4.1.2 Shared AM and FM sites. The difference in weighting is substantial and the percentage contribution of each station may need to be determined by switching off, in turn, each facility. The VHF facility's contribution would have to be established over the area of concern using, preferably, a power density instrument having a "weighted response" exhibiting the recommended Safety Code 6 curve. The sum of the respective AM and FM contributions should be determined to establish the presence of zones exceeding 100% of the Safety Code 6 limit.

The surveyor's "scan" of each measurement point should follow the suggestions discussed in Section 4.2 (of GL-01). Normally, these are done holding the probe away from the body, with no other object present within a few metres from the surveyor. The body of the surveyor should not lie in the path of the signal measured, neither in front of, nor behind, the probe. To better illustrate this, assuming the surveyor faces the source of the signal measured, his/her arm should be stretched out sideways to hold the probe that should preferably, in turn, be pointed towards the signal source.

In those instances where a tripod is used, the tripod should be non-metallic to avoid any disruptive effect. The field disrupting effect on the local field, can invalidate the measurements.

More closely spaced measurements are required near potentially reflective objects such as walls, fences, etc. whether or not these are located along the radials chosen. Measurements closer than 20 cm from an object are not considered valid.

4.2 Induced Current

Induced current measurements should be done at the calculated distances from the Safety Code 6 limits. A minimum of four (4) measurements should be taken at this distance from the base of the tower(s). Measurement points should be chosen in locations where the highest RF Energy have been recorded or expected.

The actual measurements are done using a properly calibrated induced current meter with a broadband human-equivalent antenna (approx. 1.75 metres). If a broadband human-equivalent antenna is not readily available and a person is substituted, be sure that the subject's front/back is in line with the tower.

4.3 Contact Current

Contact current measurements may be required at nearbyFootnote 2 conductive structures (objects/buildings/fences and guy-wires etc.).

Suitable contact current meters with appropriate grounding plates or clamps must be used for this measurement. It is also important to ensure that the detector probe or connector makes a good contact with the test object.

4.4 Measurement Report

A test report should include the following data:

  • a general description of the site and the transmitting facility;
  • a statement of compliance/non-compliance to the Safety Code 6 limits;
  • highlight any measured values in excess of 50% of the applicable limit and the location of these points (or zones);
  • provide a description of the means by which the applicant will prevent general access to, or warn of, locations and "hot spots" exceeding the appropriate recommended limits; and
  • address specifically those zones close to metal and potentially reflective objects.
Table to Predict the Location of Various Exposure Contours for AM Undertakings
Electric Field Strength (V/m) Magnetic Field Strength (A/m) Tower base input power (kW)
50 25 10 5 2.5 1.0 0.5 0.25 0.10
25 0.06 109 83 60 47 37 27 22 18 13
50 0.13 65 51 37 29 23 18 14 11 8
75 0.19 49 38 28 23 18 13 11 8 6
100 0.25 40 31 23 19 15 11 9 7 5
150 0.38 30 24 18 15 11 8 6 5 4
200 0.5 25 20 15 12 9 7 5 4 3
280 0.74 21 17 12 10 7 5 4 3 2
300 0.75 20 16 11 9 7 5 4 3 < 2
400 1.00 16 13 9 7 6 4 3 < 2 < 2
500 1.25 14 11 8 6 5 3 3 < 2 < 2
750 1.88 11 8 6 5 4 3 < 2 < 2 < 2
1000 2.50 9 7 5  4 3 < 2 < 2 < 2 < 2

Table 1: Distances (in metres) at which fields from AM undertakings are predicted to fall below various field strength levels (from OST Bulletin no. 65).

Note: This table can be used for all AM frequencies and tower heights. The entries in this table apply to both electric field strength and the corresponding magnetic field strength (assuming a free-space impedance equal to 377 Ω).

Footnotes

  1. Footnote 2 This must be assessed on a case-by-case basis. The distance within which measurement may be required will be a function of the size and orientation of the conductive structure(s), their distance from the radiators and the power of the station.
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