Answer to Question #11158 Submitted to "Ask the Experts"

My husband and I are considering buying an apartment on the fourth floor of a building in New York City. We have two small children.

The buildings adjacent to the potential apartment and across the way are all about the same size and many of them have satellite dishes on their roofs. Some of the satellite dishes are small and look like DIRECTV or othr television-related devices. I read on your site that these types of devices are not dangerous because they do not emit any radiofrequency (RF) radiation. However, the building right next door has a larger-looking satellite dish on its roof and it is very close to the bedrooms in the apartment we are looking to buy. Is it possible to know by looking at the satellite if it is only a receiving-type dish or if it also emits RF radiation? Can I send a picture of the satellite dish to someone who would know what kind of satellite it is? Is it dangerous to have our bedrooms so close to a large satellite dish?

Also, I'm wondering if it is possible to hire someone to check the RF radiation levels on the block or in the apartment to make sure that they are normal. Do you know of anyone in the New York area who does this kind of work?

The short answer to your question is that you should have no concern for exposure in the apartment you are considering. The smaller "dishes" you mention most likely are, as you say, used for satellite TV service, e.g., DISH TV. These antennas, which require a clear unobstructed line-of-site path to the satellite, do not transmit electromagnetic energy, nor do they change the strength of the ambient satellite signals.

The larger antenna you mention on the roof of an adjoining building might also be used for satellite TV or for other purposes, e.g., satellite data communications, terrestrial point-to-point data transmission. In these cases, the antennas could both receive and transmit electromagnetic energy. However, if the antenna does transmit electromagnetic energy, there should be no concern for exposure at locations outside the main beam of the antenna. There are several reasons why this is so. For example, as indicated above, antennas used for satellite communication or point-to-point data transmission depend on a clear unobstructed line-of-sight path between the transmitting and receiving antennas. In addition, "dish" (parabolic reflector) antennas transmit energy in a very narrow collimated beam, similar to that of a searchlight. Depending on the frequency of the signal and size of the antenna, the beam divergence (the amount the beam spreads) is of the order of one degree, or less. Almost all of the transmitted energy is concentrated in this collimated beam—the small amount of energy transmitted in off-axis directions is typically thousands of times lower than that in the main beam and far below the ambient levels associated with other sources, e.g., over-the-air radio and TV broadcast, cellular-radio base stations, WiFi, and the myriad of radiofrequency sources in our environment. Unless the antenna of concern is pointed directly at the apartment of interest, in which case the purpose of the system would be defeated, exposure to the small amount of energy transmitted in directions off-axis should be of no concern.

This conclusion is based on antenna theory and measurements carried out over the years on similar antennas by the US Environmental Protection Agency, Bell Laboratories, and others. For example, a number of years ago Bell Laboratories carried out measurements on the roof of the NBC building in New York City. The purpose was to document exposure in normally accessible locations associated with several "dish" antennas mounted on the roof (used for satellite communications). Instruments (spectrum analyzers and calibrated antennas) were used that could distinguish between the signals transmitted from the dish antennas and those from other sources. In all cases, the signals from the dish antennas were far below the general ambient background—the dominating signals were associated with FM broadcast. The same was true for measurements made in the vicinity of much larger antennas (10–20 m diameter) used for satellite communications and smaller antennas (2–4 m diameter) used for point-to-point data transmission.

Antennas, such as the one you mention on an adjacent building, are specifically designed to maximize the power in the inaccessible main beam of the antenna and minimize that in other directions (which could interfere with other systems). Although the larger antennas look impressive, and may look threatening to some, exposure to energy off axis is minimal and far below the ambient radiofrequency background. Because of the low power at which these systems operate (a few watts), even exposure in the main beam is usually below the safety limits.

Regarding measurements, these antennas have been studied for decades. The electromagnetic fields at off-axis locations close to the antenna are usually below the ambient background levels and should not require measurement.

The bottom line is that in order for the system to function, the antenna of concern on the roof of the adjacent building must be directed toward the receiving antenna. Because of the narrow beam of such antennas, orientation is critical, as is the requirement for a clear, unobstructed path between antennas. Thus the energy cannot be directed in the direction of the apartment of interest. Energy directed in directions other than along the main beam is minimal and usually far below the ambient radiofrequency background.

Ron C. Petersen