MICROWAVE  TECHNOLOGY


ISM  BANDS
According to international arrangements for industrial, scientific and medical purposes, several frequency ranges are designated. These are the ISM (Industrial, Scientific, Medical) bands in the range of 915 MHz, 2,45 GHz and 5,8 GHz.
The 915 MHz band is intended exclusively for Region 2 (South ans North America). Devices operating at frequencies of 2,45 GHz and 5,8 GHz can be used on all continents.

DIVISION  OF  MATERIALS
A flat wave falling on the surface of material can be reflected, penetrate into the interior, bend or move to the other side. Depending on material, different combinations of these possibilities can be noted. Each type of material can be assigned into a specific group: conductors, insulators or  dielectrics.
The conductors include metals such as steel, aluminum, copper, brass, silver, etc. microwaves reflect from their surface, penetrating at a very small depth.
Insulators are the majority of plastics, i.e. polyethylene and gases (e.g. air). Microwaves practically do not reflect from them and pass through them without losses.
Dielectrics are, for example, water, most of the elements and chemical compounds, composites, food products, biological materials etc. Microwawes basically do not reflect from their surface, they penetrate into the interior and a small part penetrates through the dielectric.

PENETRATION  DEPTH
Microwaves penetrate into all materials, only the depth of penetration varies. The distance between the surface and the place where the microwave power is e2 times smaller is called the penetration depth. The depth at which the microwaves penetrate the materia lis strongly related to frequency. Higher frequency microwaves penetrate at smaller depths.
Incidenting on the conductor, the microwaves with frequency of 2,45 GHz penetrate to 1 - 3 µm (0,001 - 0,003 mm). The amount of energy absorbed by the conductor of microwaves is negligible. In a significant majority, microwaves are reflected from the conductor.
Through insulators microwaves "pass" practically without losses. They don't reflect from their surface. For insulators the penetration depth can be tens of meters or even kilometers.
Microwaves with frequency of 2,45 GHz penetrate the dielectrics (depending on the material) to a depth of several - several dozen cm. Most of the microwave energy is converted into heat, a small amount of microwaves passes through the material.

MICROWAVE  ENERGY
The ability of microwaves to heat materials simultaneously on their surface and in deeper layers is used. In dielectrics, the microwave field causes the rotation of dipoles and ions oscillation. The effect of this is the release of heat and the increase of material's temperature. The role of the microwaves that initiate and significantly accelerate chemical reactions is not fully explained. An interesting fact is that during the heating of water micro bubbles containing overheated steam are created, similar to the phenomenon of cavitation.
The microwave efficiency is closely related to the penetration distance. The frequency of microwaves and the thickness of the material should be selected to absorb the greatest amount of energy.
When druing with microwaves, the material is not heated intensively. Water, which in the material is in the form of droplets or small pools, under the influence of microwaves begins to evaporate. The increase in the temperature of material is, to a large extent, the result of heat dissipation of the generated steam, which escapes to the outside. Microwave drying can therefore take place even at material temperature of 50 - 60 oC. This is essential for drying temperature-sensitive materials (food, thermoplastic materials, fabrics).

SAFETY
Microwave radiation is not-ionizing. It does not cause the permanent changes in the material structure. As soon as the devices are switched off, the microwaves disappear and no residual radiation remains in the material. The effect of the microwaves is only the increase in the temperature of the matter.
Microwave devices produce a field, the value of which is defined as power density and measured in W/m2. Regulations and standards regulate the permissible power density values. Around the equipment there are determined, on the basis of measurement, the so-called zones, whose field density is within specified intervals. A division into four zones is assumed:

A properly designed, built and technically efficient device is safe for people, animals and plants. Employees and others who are in the vicinity of such device do not have to worry about their health.

DEVICE  COMPONENTS
MAGNETRON
In the vast majority of household appliances, industrial and laboratory appliances, the source of microwaves are magnetrons. Magnetron is powered by high voltage (approx. 4000 V), requires an efficient cooling system, it i salso difficult to regulate its power. Magnetrons used in the industrial systems are characterized by a longer, so-called MTBF (Mean Time Between Failures). In advanced soultions, water-cooled magnetrons are used.

POWER  SUPPLY
In traditional systems magnetrons are supplied by the MOT (Microwave Oven Transformer) - high voltage transformer. It is a proven and inexpensive solution. For large transformers, soft-start systems are used. Transformers with the so-called gap heat up strongly and require intensive cooling. With this type of power supply, smooth regulation of magnetron power is significantly impeded
Switched-mode power supplies impulsowe are characterized by low mass, small dimensions and good efficiency. They allow to smoothly adjust the magnetron power, protect the magnetron from damage and prolong its lifespan. Switched-mode power supplies allow to power magnetrons with a power of up to 3 kW.

COOLING
In portable and even stationary devices, the most common microwave systems are air-cooled. Depending on the scale of the problem and the requirements, there are used efficient fans extracting the air from the room, intake hoods sucking cool air from the outside or exhaust ducts.
Devices operating within technological lines and even free-standing stationary devices sometimes work in difficult climatic and environmental conditions (high temperature, dustiness). An alternative for air-cooling is the use of an ice water generator (chiller). Cooling water flows in a closed circuit, so it is not necessary to connect the water supply. The advantage of this solution is quiet operation, cooling capacity independent of the ambient temperature, the ability to work in a dusted room, no ventilation ducts.

MICROWAVE  SUPPORT
Due to technological reasons, additional technologies are used to increase the efficiency or the possibility of receiving a product of better quality, i.e. vacuum, IR, UV, hot air. The advantage of combining technologies is to obtain results that cannot be obtained using only microwaves. Such examples are hot-air-aided food drying, drying of plastic granules using vacuum or disinfection of water and wastewater in cooperation with UV radiation.

MEASURING  DEVICES
In applications using microwave technology, it is necessary to measure various parameters, the most often measurements are of:

  1. Microwave power density, which aims to determine acceptable values and protection against excessive radiation. A stationary device-mounted or portable metres are used for this purpose, useful for health and safety services.
  2. Temperature, to determine the value at which the process runs. Available are:
  3. Humidity, manual or laboratory for periodic measurements or stationary for continuous monitoring,
  4. Power current and total used for a given process.

AUTOMATION  AND  CONTROL
Depending on the needs and requirements of device, microwave systems are equipped with more or less advanced control, automation and visualization systems. Manual or automatic regulation is usually subject to: The data collected from the measurement sensors and the settings of the controls can be recorded and analyzed. The results are displayed in digital form or presented as tables or graphs.

TYPES  OF  DEVICES
Microwave devices can be subdivided according to the:

  1. Applications:
  2. Number of microwave sources:
  3. Construction:

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