RF Safety
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| May 15th 2021, 14:41 | |
wb4rtp@mindspring.comTotal Topics: 0 Total Posts: 0 |
The May/June 2021 issue of QEX, in the article on page 3, "QO-100: Working the First Ham Satellite in Geostationary Orbit", included the following editorial comment: "Words of warning: do not stand in front of the dish while transmitting. Even with 5W or less from the upconverter/amplifier, the dish with 20 or 40 dBi of gain will result in an EIRP of hundreds or even thousands of watts of microwave energy. This can cause blindness and other serious harm." In my opinion, this statement is wrong on several details. First, antenna gain does not create energy. 5W in to an antenna is still 5W radiated. Effective Isotropic Radiated Power is the effect of the power density concentration relative to an isotropic radiator and only applies in the far field. The actual power in the beamspot is still the same or less than the actual power into the antenna (the difference is antenna losses, atmospheric absorption, sidelobes, etc.). Standing right in front of the dish, the power density is the feed power dispersed over the area of the dish. The bigger the dish, the lower the power density. For example, a 25 cm dish has an area of about 2000cm2, so 5W will result in power density right in front of the dish of 2.5mW per cm2, which is below the microwave long term exposure limit of 10mW per cm2. This is how microwave near field exposure should be calculated. I like to keep in mind that 10mW per cm2 is the same as 100W per m2, and this is 1/10th sunlight. Also, eye damage usually not a problem below 100mW per cm2, which is 1000W per m2, the same as sunlight. I would like to see a correction included in a future issue of QEX. |
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