Radiation, Microwave Ovens, and SARBy Bob Hawkins and Laurence Gray,

3-May-2019

One of the conveniences of the galley in C-GRSC was the microwave oven. It seems so unremarkable now that one wonders why I would mention it at all. By the early 1980’s, the microwave oven had inculcated the modern kitchen even though the basic concept had been demonstrated in the 1933 World’s Fair1 some 50 years earlier. Before we got the ‘microwave2’ on board, the only conveniences in the galley were a couple of ‘hot’ cups and a coffee maker that appeared shortly after takeoff. Having this device on board made a huge difference to the aircrews3, particularly in long transits.

Current safety regulations specify that the leakage flux4 levels for microwave ovens5 should be less than 5 mW/cm2. How does this compare with the radiation levels from the CCRS Synthetic Aperture Radars (SARs)? The typical microwave oven operates at about 2.45 GHz, to be outside communication bands, and has powers up to about 1 kW6. Hopefully almost all that power gets used for cooking and little leaks out.

If the humble microwave oven could need safety regulations, what about a whole aircraft filled with microwave gear? The Synthetic Aperture Radars (SARs) on the Convair-580 operated at X- and C-bands (9.35 and 5.30 GHz) and ranged up to about 34 kW peak radiated power7. It also had a big antenna to concentrate the energy. So what kind of fluxes could that generate on the ground? If you do the calculation, for typical acquisition heights of 20000 feet, calculation gives only 2x10-6 mW/cm2 roughly 1 millionth of

1 Microwave Oven, Wikipedia, https://en.wikipedia.org/wiki/Microwave_oven, extracted May 1, 2019.2 To give some idea of how novel the device was, I remember opening the door for trick-or-treaters about this time at Halloween to discover one kid with a box over his head that at first I thought was a TV. “No,” he explained, “I am ‘a microwave’”. Today, these devices are routinely referred to as ‘microwaves’ and we take them for granted. 3 About the same time our coffee room got a unit and there was some tentative use of it at first. Kurt Holthusen brought in an egg for lunch only to discover it was uncooked. Before too long he had successfully exploded it in the microwave oven much to his annoyance and the mirth of the others in attendance.4 Flux here means power per unit area.5 Ontario Ministry of Labour Exposure Limits, https://www.labour.gov.on.ca/english/hs/pubs/radiation/gl_radio_3.php , extracted May 1, 2019.6 Typically the leakage from such a device is less than 5 mW/cm2.7 “CCRS Airborne C/X SAR”, Minister of Supply and Services Canada, 1988, Cat No. M77-40/1988E, ISBN 0-662-16/70-x, yellow brochure, 6p.

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the allowed leakage from a microwave oven. That’s partly because it is a pulsed system so only a tiny fraction (0.21%) of the time is the radar actually on8; and partly because it is so far away that beam spreads over an enormous area of 840 square kilometers.

So what happens when you operate the radar on the ground? Things here are not so obvious and some serious measurements were undertaken to assure and protect ground personnel. As you can see from the picture below, this resulted in a 9 m distance warning painted above the main radome and a flashing light to warn people to stay back when the radar was operated on the ground. A calculation of the radiation hazard at 9 m shows 1.76 mW/cm2 while the recommended9 safe level at this frequency is about 0.92 mW/cm2.

To further reduce this risk, radiated power was reduced by a factor of 100 by redirecting the radiation path whenever a ‘wheels down’ condition was detected. For those who want to see the details see the appendix.

8 This is called duty cycle.9 See Exposure Limits, Ontario Ministry of Labour, https://www.labour.gov.on.ca/english/hs/pubs/radiation/gl_radio_3.php , extracted, May 6, 2019.

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Appendix. Detailed Calculations on Ground Flux.

The peak radiated power, Pr , from a radar is mainly directed along the direction of the main lobe of its antenna and peaks at its centre where its gain, G is the highest. This power also spreads spherically outward over the area of a sphere, 4 πR2 at range, R . The radiation flux or power per unit area, Pp/ A , is then given by:

Pp/ A=PRG4 πR2

The average flux, ¿P pA

>¿for a pulsed system takes into account how long the radar is actually on by

considering the pulse repetition frequency and the width of the pulse. The duty cycle factor is given by

Dc=τ× f PRF

where τ is the pulse width, and f is the pulse repetition frequency or PRF, of the radar. The average power is then given by

¿Pp/ A≥Dc×Pp /A

To protect ground personnel, a special protection circuit automatically places a 20 dB (factor of 100) attenuation in the transmitter path whenever the landing gear were deployed and a ‘wheels down’ condition was detected. All these factors are summarized in Table 1 below.

Finally the health hazard flux, H seems to depend on the radar frequency, f and is given as8:

H=0.002619 f 0.6834

Table 1. Microwave Radiation Hazard Determination for the Convair-580

Parameter Value UnitsPp 34 kWf 5.3 GHz

G 24 dB

f PRF 300 Hzτ 7 μsDc .0021

Pp/ A at R=9 m 839 mW/cm2

¿ Pp/ A>¿ at R=¿9 m 1.76 mW/cm2

Wheels down Attenuation 20 dB¿ Pp/ A>¿ Wheels down at 9m 0.176 mW/cm2

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H 0.92 mW/cm2

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