Radio Frequency Heating
Why exactly "27.12 MHz"
heard 27MHz is being used !
A simple Google brings up a lot of references to 27.12MHz http://fshn.illinois.edu/food_proces...esentation.pdf
Keith
http://www.petrieltd.com/index.php/radio-frequency
Commercial 27.12 MHz RF dryers are often in the power-range up to 100kW, Big tubes and heavy inductors. It requires many truck loads with rice, to be an economical alternative compared to other drying methods. If you need to ask how to design it, forget it. That amount of power can mummify you in seconds if you not know what you are doing.
Complete 600 Watt DIY instruction: http://www.microsemi.com/index.php?o...oad&gid=123626
That amount of power is relative harmless, especially as that power level only will be radiated when load impedance is ideal.
Next step is to build that load impedance, an oven where the rice should be heated in an effective way, with low RF return loss and low loss to external environment.
If oven is less in size then a few wavelengths (~25 meter) is it near field heating which is a bit different compared to how a normal microwave oven at 2.45 GHz operates.
" The RF frequency bands used in dielectric heating centred in 27.12MHz is reserved specifically for use by Industrial, Scientific and Medical purposes
(I.S.M.) to avoid possible interference with other users of the radio spectrum (i.e. broadcasting,
satellites etc.). "
"Complete 600 Watt DIY instruction: http://www.microsemi.com/index.php?o...oad&gid=123626"
The components used in the circuit is not available in our country.
The RF Oven were going to make doesn't require that much power.
Were going to dry just .5-2kg of rice grain for experimental research purpose only. I believe 50W-100W would be enough.
But I'm having a hard time designing the rf power amplifier, from an input power of 3W to my desired output power.
For those who do not know: 27.12 MHz is the internationally recognized "free to use" frequency for industrial use like heating.
The problem with rice drying is mostly not due to the RF power generator (many radio amateurs have 100 W or higher-power transmitters) but how to design the load cell where rice is dried.
Wet rice absorbs well the RF power while dry rice does not. A vacuum-tube RF power generator can operate with badly mismatched load while semiconductor devices require a good match.
I would recommend to start with developing the load cell for rice grain and a suitable matching circuit. Only then it is time to make the RF power generator.
A simpler way to dry rice is to use a microwave oven with power control which makes sure rice will dry slowly, not boil. To protect the magnetron, locate an absorbing load in the oven next to the vessel with rice to dry. Such load can be a plastic or glass jar with wet sand, well enclosed to prevent water vapor getting into dried rice.
If you use a semiconductor PA, you could measure its output voltage (and current?*) when well matched into a load and if the output voltage (or current) exceeds this reduce the drive to the PA.
* the DC input current will do.
Frank
Most RF transmitters for HF bands do use a SWR meter and control the RF power as chuckey wrote. But the RF power amplifiers are usually completely blocked for a high SWR which unfortunately is a typical case in loads like driers.
Vacuum-tube RF generators are better in handling heavy mismatch.