RF Radiation Exposure - Determining Compliance

Radio World, 9 January 1991

by Harold Hallikainen

Several months ago, I discovered in the FCC Regulatory Agenda (as published in the Federal Register, 55 FR 17093) that the FCC was looking at modifying the rules regarding the distance between an operator and the transmitter or the operator and a remote control (or extension meters). Currently, the operator must be able to observe the transmitter indications from the duty position. A remote control down the hall is not permissable. In speaking with Commission staff, I found that they were looking into authorizing the operator to be up to 100 meters from the transmitter or monitors. This is similar to the original extension meter rules that did not require extension of antenna monitor indications (the current rules do require this). Such a rule modification would eliminate the need for extension meters and make station design more flexible. A Notice of Proposed Rulemaking was scheduled for June 1990, but no action has been taken yet. I understand this is part of an overall reconsideration of operator requirements and remote control.

Last month we looked at some of the rationale for the RF radiation exposure limits of ANSI C95.1-1982. The FCC has published OST Bulletin 65 to help you determine compliance with the ANSI guideline. You can get a copy of OST65 by calling NTIS at 800 336 4700. The publication costs $17 plus handling (another $3).

Determining Compliance

Probably the best way to determine compliance with the FCC requirements is by making measurements in all areas that humans will occupy. The FCC will, however, accept predictions. For simplicity, the predictions are generally a "worst case" analysis, so any errors in the modeling of the system result in the actual field strength being below that predicted. OST Bulletin 65 has several charts to aid in evaluating compliance for AM and FM stations. The OST 65 limits take into account radiation reflection by the surface of the earth, the radiation pattern of typical antennae (not isotropic). The ground reflection increases the field strengths by up to 100% (by creating standing waves). Since some of the power is absorbed instead of being reflected, the EPA assumes the field strengths will increase 60% due to ground reflection, resulting in a power density increase (proportional to field strength squared) of 156%. A typical FM antenna is designed to radiate the maximum power density towards the audience (assumed to be on the horizon unless beam tilt is used). This reduces "downward" radiation below that of an isotropic source. In tests around FM broadcast facilities, the actual power density was less than that predicted by the EPA model, demonstrating it is a worst case model, but not as restrictive as an isotropic model would be.

AM Antenna Radiation Limits

For AM stations, the NEC (Numerical Electromagnetic Cod) model for linear antennae was used. Independent of frequency or tower height, both the electric and magnetic field strength are predicted to meet ANSI specifications 12 meters from a tower being fed with 50 KW. If the power is decreased by a factor of four (to 12.5 KW), the ANSI limit distance would decrease by a factor of two (to 6 meters), since the power density is inversely proportional to the square of the distance. However, as we get closer to the base of the antenna, the electric to magnetic field proportion changes, so the "square root rule" does not work exactly. Towers with high voltage at the base (those close to 0.5 wavelenghts long) have a high electric field at the base. The limits are:
Station Power (KW) Distance to ANSI Limit (meters)
<=0.5 <0.2
1.0 3
2.5 4
5.0 5
10 7
25 9
50 12

For AM directional antennae, a worst case analysis can be accomplished by assuming that the full transmitter power is going into a single unspecified antenna element. For example, a 5 KW 3 tower DA would have fences restricting access 5 meters from each tower base. Assuming the full power is driving each tower allows us to consider only the radiation from that one tower and ignore the radiation from the other towers, simplifying the analysis. If we wanted a more exact analysis, we'd have to sum the fields from each tower, taking into consideration the power into each tower, the distance from it and the phase relationship with the fields from the other towers.

OST 65 also includes graphs for various tower heights (0.1, 0.25 and 0.5 wavelength) relating the maximum expected electric and magnetic field strength for 1 kilowatt input at various distances from the tower, measured 2 meters above the ground. These can be used to predict the field strengths at various distances from the tower. If other than 1 kilowatt is used, the field strengths should be multiplied by the square root of the power in kilowatts. These charts only go to 100 V/M and 0.25 A/M while the ANSI limits are 632 V/M and 1.58 A/M.

These charts can be used to determine where to put fences and signs to restrict access to areas where the field strengths exceed the ANSI limits.

Transmitting Equipment Radiation

The above discussion has been restricted to radiation from the antenna. The transmitting equipment (transmitter and phasing equipment) also radiate. OST65 does not make any predictions as to radiation from anything other than the antenna. I've spoken to transmitter manufacturers, phasor manufacturers, consulting engineers and the FCC regarding radiation from other sources. It's been quite interesting ("awsome" to use today's terminology).

Bob Weirapher at Harris said that most recent transmitters radiate less than 80 dB below what would be radiated by an isotropic radiator. If we make a few assumptions (such as use the far field equivalent), we can calcualte the power density 1 meter from the transmitter. 80 dB below 50 KW is 500 uW. 500 uW spread over the surface area of a 1 meter radius sphere would give us 39.8 microwatts per square meter or 3.98 nanowatts per square centimeter, well below the 900 mW per square centimeter for AM and the 1 mW pwer square centimeter for FM.

Ben Dawson and Jim Hatfield of Hatfield & Dawson said that stations may have excessive leakage from transmitters with glass windows. They also said the distance limits in OST65 are very conservative, which is to be expected, since they are "worst case". They have found that radiation from a tower generally exceeds that from the coupling unit at the base of the tower, so a fence at the OST65 suggested distance should be adequate to meet the ANSI guidelines for radiation from both the tower and the coupling unit. Sealed phasor cabinets have generally met the ANSI spec. Open panel phasors or those with painted doors (preventing a good RF seal) should be measured.

Radiation from equipment is a very interesting (awsome) subject. It's fairly logical that putting a grounded metal cage (a Faraday cage) around a source of an electric field should "stop" the field. We could perhaps visualize the electric field generator as one plate of a capacitor and the electric field "receiver" a second plate of a capacitor. Putting an AC voltage on one plate of the capacitor will cause an AC voltage to appear on the other plate (depending upon plate area, plate spacing, dielectric constant, etc.). If, however, we put a grounded plate between these two plates, we now have two capacitors in series with the junction of the two grounded. We should get no voltage to ground on the "receiver" plate.

Shielding of AC Magnetic Field

Magnetic radiation, however, is amazing (more awsome). A conductive but nonmagnetic shield (permeability close to that of air) will have no effect on a DC field. However, with an AC magnetic field, Lenz's law states that the changing field will cause a current in the conductor that will generate a field that opposes the change in field. If, for example, the magnetic flux is increasing at 1 weber per second, a current would be induced in a perfect conductor. This current would generate an opposing magnetic field, also increasing at 1 weber per second. On the "outside" of the shield (away from the generator), this opposing field cancels the originating field, resulting in no AC field outside the shield. Inside the shielded enclosure, the polarity of the field is the same as that created by the generator, but is "travelling" away from the shield. This creates a magnetic field that is twice the magnitude of the incident wave at the shield. The "incident" and "reflected" waves are propogating in opposite directions. Adding these waves results in "standing waves" where the field strength of the magnetic and electric field strengths take on minimum and maximum values depending upon where in the shielded box the measurement is made.

Next month we'll try to use Smith charts in a nontraditional manner. We'll see if we can model a chunk of aluminum (the side of the phasor cabinet) as a lossy transmission line with a low characteristic impedance, causing the electromagnetic wave to be reflected inside the phasor.


Harold Hallikainen is president of Hallikainen and Friends, a manufacturer of transmitter control and telemetry systems. He also teaches electronics at Cuesta College, San Luis Obispo. He can be reached at 805 541 0200.