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).
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.
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.
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.