Your question is a very good one. This is an area of concern for many people, and a question we receive on a fairly regular basis. Please allow me to provide you with some data on our Radars that will hopefully help clarify subject item:
The misconception lies in the fact most people look at the "Peak Power" output of the radar and think that is what is being radiated at them when they are within the beam pattern. However, that’s not the case. Radar’s radiated power has a vertical component which is directional and focused into a beam that is approximately 20 degrees wide (+/- degrees from the antenna’s horizontal plane). This means that the radar’s absolute power is radiated within this sector. Energy outside of this vertical Beam width is suppressed.
Radar’s radiated power has a horizontal component that is directional and – depending on the system configuration and the specific radar antenna – ranges from .75 degrees to 6.2 degrees in width. RF energy outside of this horizontal Beam width is suppressed.
The energy power density of a transmitter diminishes with the square of the distance from the transmitter. This simply means that the energy levels from the transmitter drop exponentially as distance increases.
Marine Radars employ timing circuits which turn the transmitter “off” and “on” up to 2100 times a second during normal operation. During this sequence, the transmitter is only keyed for a total of approximately 1 microsecond during its transmit cycle; the remainder of the cycle the transmitter is idle (not transmitting) while the receiver is on and waiting for the transmitted energy to travel to and from surrounding targets.
Radars are rated by their Peak Power Rating. This Peak Power rating identifies the maximum radiated energy that is transmitted during the 1 microsecond interval – or one-millionth of a second – that the Radar is actually transmitting.
The Federal Communications Commission also rates Radars according to their “Average Output Power.” The Average Radiated Power of a Radar can be calculated by taking into account the fact that a Radar is only transmitting for a very short period of time during its transmit/receive cycle. We can calculate the Average Power of Radar by using the following formula:
Pavg = Ppk x pw x prt
Pavg = Average Power
Ppk = Peak Power
pw = Pulse Width
prt = Pulse Repetition Time
As an example, please consider one of our most popular Radars: The Furuno 1835 Radar. This Radar has a Peak Power rating of 4 kW (4000 Watts), it has a pulse width of .8 microseconds in long range (.0000008 Seconds), and its Pulse Repetition Time in long range is 600 Pulses per Second.
Using the formula above, we can calculate the 1835’s average Output Power:
4000 x .0000008 x 600 = 1.92 Watts
Peak Power Pulse Width PRT Average Power Output
The average radiated Power from our 1835 (4KW) is actually less than 2 Watts!
It may be interesting to note that the RF energy levels from VHF and SSB transmitters can be 100 to 1000 times greater than the average radiated power from a 4 kW Radar. For example: Marine VHF-FM transceivers are rated at 25 Watts Output Power. When you key the mike on a VHF-FM Transceiver, the Radio transmits 25 Watts of continuous RF energy. (Approximately 1200% greater average power output than the 1835 Radar).
The environmental hazards from such a low-power system are normally insignificant. We do not, of course, recommend looking into radar that is mounted directly in front of you a few feet away on the same horizontal plane. We include a “radiation hazard” note in our Operator’s Manual that identifies what we consider the safe distance on most of our units when Radar is transmitting. In example we generally use withdraw to a power level of 10 W/m² ; the unit of measurement is Watts Per Meter Squared (W/m²), and the specification for our popular 4KW radars (all of them) is 1.65 meters for 10 W/m²; this is normally thought of as a worst-case scenario.
All Radars sold in the United States comply with Federal safety standards concerning RF radiation hazards.
Although the US has not set a safety distance standard for marine radars, we would rather be on the safe side.
Here are the suggested withdraw distances from one of our commercial 25Kw radar, using antennas of different length. (Per Furuno operator's manual for the FAR28X7 series) Please note: the larger the antenna results in the power is dispersed over a larger area and shorter withdraw distances.
Distance to 10W/m² point.
4 foot antenna 11.20 meters
6 foot antenna 8.60 meters
8 foot antenna 5.80 meters
S-band radar withdrawal distance would be approx the same as X-band.
Ideally, if you can stay out of the radar beam entirely, you are better off, but we all know this isn't possible in most applications.
Overall, you are advised to stay out of the withdraw area when a radar is transmitting.