Microwave radiation indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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Household microwave ovens, which have become widespread, are certified by KVK manufacturers as safe for the user. And indeed, the level of external microwave radiation of the vast majority of them when they are released from the plant is much lower than the maximum permissible level according to sanitary standards. However, after prolonged use, the fastening of the door of the heating chamber of the furnace weakens, gaps appear between it and the body. Contribute to the growth of radiation and food residues sticking to the place of contact of the door with the body. Finally, radiation can increase dramatically after unskilled repairs.
The proposed simple indicator is impossible, of course, to accurately assess the degree of compliance of the microwave oven with sanitary standards. However, it will help to detect the increased level of radiation in a timely manner.
The diagram of the indicator is shown in the figure.
Diode VD1 serves as a detector of the signal induced on its outputs by microwave radiation. The inductance and capacitance of the twisted-pair wires connecting the diode to the rest of the device are quite sufficient to filter the high-frequency components of the detected signal. Its envelope is enhanced by a cascade on the op-amp DA1.1. The use of the LM324 microcircuit (it can be replaced by K1401UD2, KR1435UD2 and others similar), capable of operating at an input voltage close to the potential of the negative power bus, made it possible to get rid of zero bias circuits and greatly simplify the sensor circuit.
The magnetron of the microwave oven generates bursts of oscillations that follow with a frequency of 50 Hz, the pulses at the output of the op-amp have the same frequency. If their amplitude exceeds a certain threshold value, the HL1 LED flashes, connected to the output of the op-amp through a follower on the transistor VT1. The device is powered by a battery of galvanic cells GB1 with a voltage of 9 V - "Krona" or the like. Button SB1 - power switch.
When checking a household microwave oven, the detector is in the near zone of its radiation. The field here is characterized by irregularity and the absence of a flat wave front. In this situation, an electrically short (significantly less than the wavelength) antenna is optimal for the indicator. For this reason, the conclusions of the VD1 diode itself were used as the antenna. It is fixed in the body of a ballpoint pen and is connected to the indicator itself by a two-wire flexible cord.
It must be said that the proposed indicator is very broadband. It is sensitive not only to the fundamental (frequency - 2450 MHz, wavelength - approximately 12 cm), but also to the higher harmonics of the magnetron radiation, up to the fourth and fifth (frequencies, respectively, 9800 and 12250 MHz). Higher harmonics are always present in the spectrum and often make a significant contribution to the total leakage. A microwave oven that emits higher harmonics can seriously interfere with satellite TV reception.
A variety of detector and mixing microwave diodes are suitable as VD1, designed to operate at frequencies above 1000 MHz. For each of them, it is useful to choose the optimal load resistance (resistor R1), which provides the highest sensitivity. Good results were shown by the DZB diode. Tests on a stand equipped with a G4-79 generator with a horn antenna and an MZ-54 power meter showed that the HL1 LED starts to glow at a power flux density of 2 ... 3 μW / cm2.
Almost the same sensitivity was obtained with a mixing diode D405. However, over time, its detector properties degrade, and at low frequency or direct current, checking, for example, a diode with an ohmmeter, this cannot be noticed.
According to American standards, a power flux density of 2,5 μW/cm2 is allowed at a distance of 70 cm from the furnace, and at a distance of 5 cm it should not be more than 1 mW/cm2. Domestic standards require that this indicator never exceed 10 μW / cm2. In practice, the indicator LED flashes at a distance of 5 ... 10 cm from a working furnace.
Author: A. Kontsevich, St. Petersburg
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