o3-mini-high
In this experiment, it is investigated whether a simple arrangement consisting of an LED and a germanium diode1[A diode made of the semiconductor material germanium with low forward voltage] in a microwave can absorb energy from high-frequency electromagnetic radiation and convert it into light. In doing so, a so-called LECTENNA (Light Emitting Rectenna) is constructed. The LED is bent so that its contacts function as a receiving antenna, while a diode connected in reverse polarity rectifies the microwave radiation and supplies the LED with DC current. The physical and electrical fundamentals are explained in detail, and possible applications such as rectennas, NFC, wireless power transfer (WPT) and energy harvesting are discussed.
Null Hypothesis: An LED cannot be made to light up by microwave radiation without an external power source.
1. Introduction
Electromagnetic radiation2[Wave-like propagation of electric and magnetic fields that transport energy] is an omnipresent energy source that finds technical application in many frequency ranges. A rectenna (Rectifying Antenna) is an antenna circuit that receives electromagnetic waves and converts them into electrical energy. A special form is the LECTENNA (Light Emitting Rectenna), which uses the received energy directly to light up an LED.
In this experiment, a simple LECTENNA is built and tested in a microwave. This technique not only demonstrates the fundamental principles of high-frequency technology3[A technology field that deals with the generation, transmission, and use of electromagnetic waves in the high-frequency range above 3 MHz], but also has far-reaching applications in wireless power transfer.
2. Materials and Methods
Materials:
- 1 red or yellow LED (low forward voltage approx. 1.8V)
- 1 diode
- e.g. germanium diode OA70 or 1N34A, low forward voltage approx. 0.3V
- e.g. Schottky diode NTE112 or 1N5711
- Solder and soldering iron
- Non-conductive substrate (e.g., ceramic plate or glass dish)
- 1 glass of water-
- Microwave oven
Methodology:
- Bending of the LED: The LED pins are bent directly on the plastic housing at a right angle to serve as a dipole antenna4[A type of antenna with two opposing conductive elements that receives or transmits electromagnetic waves].
- Soldering of the diode: A germanium or Schottky diode is soldered in antiparallel to the LED. This means:
- The cathode of the diode is connected to the anode of the LED.
- The anode of the diode is connected to the cathode of the LED.
- The dipole antenna should be approx. 30 mm long on each side. If necessary, solder on some copper wire to extend it.
- Placement in the microwave: The finished LECTENNA is positioned on a non-conductive substrate.
- As a load, a glass of water is added. The microwave oven should not operate empty, as this could damage the magnetron.
- Test procedure: The microwave is briefly switched on at the lowest power setting (1-2 seconds) and the LED is observed.
3. Results
After turning on the microwave, the LED briefly flashes before being destroyed by an overvoltage. By adding a protective resistor5[A resistor used in a circuit to limit current surges and protect components from overload]s or an optimized antenna arrangement, the lifespan of the LED can be extended. This clearly demonstrates that microwave radiation can be used to generate light.
4. Discussion
4.1 Physical and Electrical Functionality
The microwave radiation in the oven has a frequency of 2.45 GHz. The angled LED pins act as a dipole antenna that receives the high-frequency electromagnetic wave. This occurs through electromagnetic induction6[The generation of an electrical voltage in a conductor by a changing magnetic field], whereby the electric field of the microwaves generates an alternating voltage in the antenna.
The electric field strength7[A measure of the force that an electric field exerts on a charge, measured in volts per meter (V/m)] of an electromagnetic wave is determined by Maxwell’s equations8[A set of four fundamental equations that describe the interaction of electric and magnetic fields]. The power P that an antenna extracts from a wave depends on the effective aperture of the antenna Aexteff and the power density9[The amount of energy transported per unit area and time, measured in watts per square meter (W/m²)] S of the wave:
Since the typical power density in a microwave is about 700-1000 W/m² and a small wire antenna only has a very limited effective area, the extractable power is only in the milliwatt range.
The germanium diode acts as a rectifier10[An electrical component that converts alternating current into direct current] and converts the alternating voltage into a pulsed direct voltage. The diode rectification occurs according to the well-known rectifier equation, which describes how alternating current is converted to direct current and thereby makes the electrical power usable for direct current circuits:
where:
- Is is the reverse saturation current11[The small, usually undesired current that flows through a diode in reverse bias],
- q is the elementary charge12[The smallest known unit of electric charge, describing the charge of a single electron or proton, approximately 1.602 × 10-19 C],
- V is the voltage across the diode,
- k is the Boltzmann constant13[A physical constant that describes the relationship between temperature and energy in a system, approximately 1.38 × 10-23 J/K] and
- T is the temperature.
The output voltage depends directly on the forward voltage of the diode, which is only about 0.3V for germanium diodes, making it ideal for an LED.
4.2 Comparison with Rectenna Technology
The LECTENNA is an example of a miniature rectenna, as used in wireless power transfer. Similar technologies are employed in RFID, NFC, and wireless charging.
- Wireless power transfer (WPT): This technology enables the transmission of electrical energy without a direct cable connection, for example in wireless charging systems for smartphones or electric vehicles, as well as in implants for medical use. It is used for wireless charging systems in smartphones and electric vehicles.
- NFC (Near Field Communication): Uses inductive coupling to transmit small amounts of energy.
- Energy Harvesting: The use of high-frequency radiation to power sensors or IoT devices.
4.3 Optimization Possibilities
- Use of a protective resistor (~470 Ω – 1 kΩ) to protect the LED from overvoltage.
- Better antenna geometry: A larger receiving antenna with a tuned wavelength (e.g., λ/4 ≈ 3 cm for 2.45 GHz) can increase energy capture.
- Parallel capacitance: A 100 pF – 1 nF ceramic capacitor could intercept voltage spikes and extend the lifespan of the LED.
Safety Instructions
- Metal objects in a microwave can cause sparks or damage. The LED arrangement should not be placed directly on metal.
- Test for a maximum of 1-2 seconds, then immediately check if the LED lights up.
- Never operate the microwave without a load to avoid damaging the magnetron.
- Do not bypass the microwave’s door safety interlock!