Field Measurements

Radiation Hazards exist where ever any transmitter is radiating into any space. Just because an antenna is outdoors does not diminish the amount of radiation leaving the antenna.

In order to measure the radiation out of an antenna we use an isotropic probe and calibrated meter.

It's worth pointing out to those of you who don't know that this type of radiation is not nuclear. It is radio frequency radiation (RF) and as such is totally different. The effect it has on the human body is difficult to say as no long term studies have yet concluded any meaningful data.

The safest way to work with RF radiation is to avoid it. Never stand in front of any radiating antenna. Even a mobile phone radiates power and should not be over used. the generally accepted view is that a small amount of exposure has no lasting effect and I have often been told that a family could have a picnic every day for a year in front of a VSAT before any damage would occur.

Remain behind antennas and there is hopefully no problem, use your common sense.

Using our probe, on a long stick if necessary, place the head of the probe at the focal point of the antenna and slowly move it away from the antenna along the axis of the beam. The highest reading on the meter is the maximum power per cm squared. The recommended safe level is 5 mW per cm squared. I recommend 1 mW to be on the safe side. When I started in this industry the safe level was 10 mW and has since been reduced.

For a particular antenna the power will fall below the safe level after an unsafe distance has been exceeded. This means that a secure area can be set up to prevent access inside this distance from the front of the antenna.

For satellite antennae that point skyward this distance is reduced because the beam is travelling up and away from the ground. The antennae that point along the ground such as line of sight antenna do not have this reduction.

Radars are often overlooked in this matter and infact use much higher power levels. They are usually pulse power systems transmitting very high power levels for only a fraction of a second but repeatedly. These systems are still dangerous and should not be approached too closley.

Precautions

The safest thing to do is make sure that no body has physical access to the immediate fron of the antenna no matter which way it points. Use a physical barrier to prevent this access upto a distance from the antenna determined by a radiation hazard inspector with his meter and probe.

These distances can be as much as 30 m from the antenna. How many times have you seen that done? Every transmitting terminal should have a certificate which not only details the type of transmitter, operating frequency and power but also the safe distance that should be kept in front of the antenna.

Fence off the area and there is no risk to anyone.