Microwave transmitters

This is a hypertext version of an article I posted to the UNE mailing list une-forum in February 1998. At the time there was a debate about the positioning of a new mobile phone tower in Armidale.

From: David Bromage <dbromage@metz.une.edu.au>

To: une-forum@metz.une.edu.au

Subject: Re: UNE-forum: Microwaves

Date: Thu, 26 Feb 1998 11:49:30 +1100 (EST)

This isn't a scientific forum so I won't go into too much detail. more on mobile phones and satellite ground stations after the bit about microwave ovens. References at the end.

Microwaves are produced inside the oven by an electron tube called a magnetron. The microwaves bounce back and forth within the metal interior until they are absorbed by the water molecules in the food. The absorption of energy by the water molecules causes them to vibrate, which in turn produces heat which cooks the food. Since microwaves are electromagnetic waves, as is visible light, they do not make the food "radioactive".

Mains electricity cycles at 50Hz, AM radio is in the region of 1MHz, FM radio is in the region of 100MHz, microwave ovens are 2450MHz and mobile phones are in the region of 825 to 890MHz (AMPS/analogue) and 890-960MHz (GMS/digital).

On satellite ground stations:

Satellite ground stations use higher power levels than point-to-point microwave comminications towers dur do the distances involved. However, as with microwave point-to-point antennas, the diameter of the beam used to transmit the radiation is narrow and highly directional. It is unlikely that anybody would access the main beam. You'd have to be sitting in the dish or flying too close to the station. Radiation levels on the ground will vary, depending upon the angle of inclination of the antenna, the antenna pattern itself and the intensity of the transmitted signal. It's also worth remembering that not all ground stations are transmitters.

On mobile phones:

The allowable general public exposure limit in AS2772.1 for the frequencies used by mobile phone services is 0.2 mW/cm-sq. This is lower than the ANSI/IEEE [1], ICNIRP [2] and NCRP [3] exposure standards. There is a 10-fold safety margin for occupational exposure and an additional 5-fold safety margin for continuous exposure of the general public. The result is a highly conservative public exposure standard that is set at a level that is below 2% of the level where potentially hazardous biological effects have actually been observed.

On mobile phone towers:

A mobile phone base station antenna, mounted 20 m off the ground and operated at the maximum possible intensity, might produce a power density as high as 0.02 mW/cm2 on the ground near the antenna site; but ground level power densities will more often be in the 0.0001 to 0.002 mW/cm-sq range. These power densities are far below all the safety standards, and the standards themselves are set far below the level where potentially hazardous effects have been seen.

Close to the base of the antenna site, the power density may be greater at elevations above the base of the antenna site (for example, at the second floor of a building or on a hill) but the carrier is required by law to ensure that the general public exposure limits are not exceeded. Power densities are lower inside buildings anyway.

The Bioelectromagnetic Society has conducted numerous studies on the effects of non-ionizing radiation. No induction or promotion of brain cancer was found in rats that were exposed for their entire lifetime (starting at conception) to radiowaves at intensities above the ANSI standard [4], radiowave exposure at intensities above the ANSI standard did not cause DNA damage in mammalian cells [5], radiowaves had no effect on the growth of human leukemia cells [6], and ideband radiowave exposure did not cause birth defects in rats [7].


[1] IEEE Standards Coordinating Committee 28 on Non-Ionizing Radiation Hazards: Standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 400 GHz (ANSI/IEEE C95.1-1991), The Institute of Electrical and Electronics Engineers, New York, 1992.

[2] International Commission on Non-Ionizing Radiation Protection: Electromagnetic fields (300 kHz to 300 GHz). Environmental Health Criteria 137. World Health Organization, 1993.

[3] National Council on Radiation Protection and Measurements: Biological effects and exposure criteria for radiofrequency electromagnetic fields. NCRP Report No. 86, 1986.

[4] WR Adey, CV Byus et al: Brain Tumor Incidence in Rats Chronically Exposed to Digital Cellular Telephone Fields in an Initiation-Promotion Model. BEMS, Victoria, June 1996.

[5] RS Malapaya, EW Ahern et al: Measurement of DNA Damage by the Alkaline Comet Assay after in vitro Exposure to 835 MHz Electromagnetic Radiation. BEMS, Victoria, June 1996.

[6] R Fitzner, E Langer et al: Long-Term Influence of High Frequency Electromagnetic Fields on Growth Behavior of HL60 Cells to Investigate Cancer Promoting Effects. BEMS, Victoria, June 1996.

[7] MP Dooley, BL Cobb et al: Development of Rat Embryos Exposed to an Ultrawideband Electromagnetic Field. BEMS, Victoria, June 1996.



And this is a reply to my posting.

From: "Hans van Leeuwen" <jvanlee@metz.une.edu.au>

To: une-forum@metz.une.edu.au

Subject: Fw: UNE-forum: Microwaves

Date: Thu, 26 Feb 1998 15:05:54 +1100

I would like to compliment David Bromage on a very well presented short literature review. Very interesting - and extremely appropriate to the debate. It is the type of info that needs to go out to the Armidale community.