DX RX

The Doctor of DX prescribes the
answers to your problems, low and
high, near and far.

A how to of what for, by Mike(aka Billy)Graham, NE1V

Evaluating RF Exposure Part 3

Part 1 of this series published in the January, 1998 issue of the NARC Bulletin, discussed the background and general information criteria regarding Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields as defined in FCC OET Bulletin 65 and Supplement B thereto. Part 2, published in the February, 1998 issue of the NARC Bulletin, discussed specific requirements and evaluation criteria. In Part 3, we'll perform an actual evaluation, using a fictitious amateur radio station. It is also strongly recommended that each of you download the contents of OET Bulletin 65 and Supplement B, and retain copies in your files for future reference. These guidelines apply to applications for FCC Station Licenses filed on or after January 1, 1998. The Commission has also set a date of 1 September, 2000, by which time all existing facilities and devices must be in compliance with the new guidelines. Both OET Bulletin 65 and Supplement B thereto may be downloaded from: http://www.fcc.gov/oet/.

In order to better understand the process of evaluation, it may be instructive to devise a “typical” amateur radio station. Let us assume that this station consists of a standard 100-watt HF transceiver covering the 160-10 Meter bands and a 2-Meter FM mobile 50-watt rig. A linear amplifier running 1000 watts is used on 160-15 Meters. Antennas consist of a 160-80-40 Meter trap dipole, a 3-element 20-15-10 Tribander, an all-band HF vertical, and a 2- Meter Ringo. The Tribander is on a 60-foot tower located 40 feet from the operator's house, and the base of the tower is 100 feet from the closest point of the property line. The 2-Meter Ringo is atop the mast on the tower at 70 feet. The closest point of the dipole (one of the ends) is 55 feet from the operator's house and 75 feet from his neighbor's property line. The vertical is mounted on the ground and is 30 feet from the operator's house and 100 feet from the closest point of the property line. The trap vertical is used only for the WARC bands. The 2-Meter Ringo is used for occasional QSOs on the local repeater and for packet on the DX Cluster. Because our operator likes to operate all modes, including CW, RTTY and SSB, it was decided to use Tables 4a and 4b from Supplement B to OET 65, the “worst case” scenario tables. These tables, developed by Fred Maia of the W5YI Group, working in cooperation with the ARRL, can very quickly tell you if you are “in trouble” or are safe. Our operator also posted signs every 100 feet around the perimeter of his property, warning of potential RF hazards.

Tribander Evaluation

Using right-triangle calculations, the actual distance from the Tribander to the house is about 72 feet, and to the property line is about 116 feet. Based on a real antenna gain of about 6 dBi and a transmitted power of 1000 Watts, the safe distance for controlled access is 4.3 meters (14.1 feet) on 20, 6.4 meters (21 feet) on 15, and 8.9 meters (29.2 feet) on 10. Since actual distance to the op's house is 72 feet, you're safe. For uncontrolled access the safe distance is 9.6 meters (31.5 feet) on 20, 14.4 meters (47.3 feet) on 15, and 19.9 meters (65.3 feet) on 10. Again, since the distance to the closest point on the property line is 116 feet, you're safe. If the values had shown a danger, then using the time-averaging method of determining distance would have significantly improved the situation. By using “worst case”, you are absolutely sure that you are OK. Also, the values on 10 Meters were calculated at 1000W of power. If you use significantly less power at 10 Meters, again you would prove to be safe.

Loaded Dipole Evaluation

Again, the closest point of the dipole to the operator's house is 55 feet, and 75 feet to the property line. One KW is used on all three bands. Antenna gain is calculated as 3 dBi. For controlled access the safe distance is 0.6 meters (2 feet) on 160, 0.9 meters (2.95 feet) on 80, and 1.6 meters (5.25 feet) on 40. For uncontrolled access, safe distances are 1.06 meters (3.5 feet) on 160, 1.9 meters (6.25 feet) on 80, and 3.5 meters (11.5 feet) on 40. Once again, the operator can safely operate 1 KW and be well inside of FCC Guidelines.

Trap Vertical Evaluation

Once again, the trap vertical is used only on the WARC bands (250 watts maximum on 30). The trap vertical is 30 feet from the house, 250 watts are used on 30, 1 KW on 17, and 100 watts on 12. Antenna gain is calculated at 3 dBi. Looking at Table 4a, you see that figures for 100, 500, 1000, and 1500 Watt power levels are given, but not for 250W, needed to calculate safe distances at 30 Meters. In this case you must interpolate. First, let's do the figures for controlled access. 100W controlled access is 0.7 meters; 500W is 1.5 meters. To calculate the safe distance at a 250W level, estimate the approximate distance between the high and low values where the target values lies…..very roughly 250 watts is slightly less than midway between 100W and 500W. But let's use 50% anyway. Half the difference of the two published distances comes out to about 0.4 meters. Add that value to the 100W value and you derive an approximate distance value of 1.2 meters, or 3.94 feet. More than safe since the distance to the house is 30 feet. On 17 Meters, we use the 1 KW value of 3.9 meters (12.8 feet). Again we're safe. On 12 Meters, we use the 100W value of 5.3 meters (17.4 feet). Bingo! Safe again. For uncontrolled access, the values come out as follows: 2.4 meters (7.9 feet) on 30, 8.6 meters (28.2 feet) on 17, and 3.8 meters (12.5 feet) on 12. Again, we are totally safe using the pre- specified values for this antenna. So much for HF.

2-Meter Ringo Evaluation

For "worst case" evaluation of VHF/UHF antennas, we use Table 4b from Supplement B. In our case, assuming a 2-Meter Ringo with about 3 dBi gain and 50 Watts, we very quickly determine that the safe distance for controlled access is 1.4 meters (4.6 feet) and for uncontrolled access the safe distance is 3.2 meters (10.5 feet). Since the antenna is 70 feet above ground, it is readily apparent that no danger exists at those power levels.

ALWAYS ESTIMATE ON THE SIDE OF CAUTION!

For now, 73 es best DX………….de NE1V

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