FAQ
TL;DR: For ~5 W RF stages, "Darlington transistors are not suitable for RF"; use a stable single-band chain and a directional antenna to deliver short‑range power. [Elektroda, Rodney Green, post #21666489]
Why it matters: This FAQ helps hobbyists design safer, lawful, short‑range wireless power links for small robots and sensors.
Quick Facts
- Regulatory check comes first; transmitting RF power may require a license in your country. [Elektroda, Rodney Green, post #21666489]
- Link budgeting starts with the Friis transmission equation for frequency, antenna gain, and distance trade‑offs. [Elektroda, sachin Kothari, post #21666494]
- Health and burn hazards exist with dense RF fields; treat open antennas like hot surfaces. [Elektroda, Steve Lawson, post #21666495]
- Target use case here: mobile RX in a 3×3 m arena powering two ~1 W motors. [Elektroda, Sthitapragyan Pattanayak, post #21666490]
- Perfboard stray capacitance limits high‑frequency builds; keep layouts short and simple. [Elektroda, Sthitapragyan Pattanayak, post #21666491]
Can 3 W at 5 V really work over 2–4 meters?
Yes, with line‑of‑sight and focused antennas, a few watts can span several meters and more. Fixed TX/RX helps. A directional antenna concentrates energy toward the receiver and reduces wasted radiation. This thread’s context is short range, so beamwidth and placement matter more than raw power. “Depending on frequency and terrain, 3 W can travel quite a distance,” especially with focused antennas. [Elektroda, Steve Lawson, post #21666488]
What RF chain should I build for a simple power transmitter?
Use a single‑band crystal oscillator for stability, a buffer, and a single‑ended class‑C MOSFET power amplifier feeding a directional antenna. Avoid Darlington RF stages. One suggested RF MOSFET is the ST PD55008. Keep it to one frequency to simplify matching and filtering. Add a stable power supply and heat sinking. This approach minimizes drift while delivering several watts efficiently. [Elektroda, Rodney Green, post #21666489]
Are Darlington transistors okay for RF power?
No. Darlingtons add excess gain and capacitance at RF and are inefficient at several megahertz and above. As one expert put it, “These are not suitable for RF.” Instead, use an RF‑rated MOSFET in class C after a buffered crystal oscillator. This reduces loss and eases matching. [Elektroda, Rodney Green, post #21666489]
How do I choose frequency without using microwaves?
Pick a single non‑microwave band that suits legal constraints and your layout. Use a crystal for stability. Then size the antenna and matching network for that band. Keep leads short to reduce parasitics. The forum guidance emphasizes a single‑band, crystal‑controlled approach with a class‑C MOSFET PA and directional antenna for efficiency. [Elektroda, Rodney Green, post #21666489]
What’s the relation between frequency and received power?
Use the Friis equation for free‑space links. It relates received power to transmit power, antenna gains, wavelength (frequency), and distance. Higher gain antennas can offset path loss. For short links, alignment and polarization matter. Start with Friis to estimate feasibility, then add system losses and rectifier efficiency. [Elektroda, sachin Kothari, post #21666494]
Is it legal to blast RF power in a lab or field?
Not automatically. Radiated power and spectrum use are regulated. You may need a license even on private land. Check your country’s RF rules and stay within permitted bands and power. Plan for spurious emission control with filters and proper antennas to avoid interference. [Elektroda, Steve Lawson, post #21666488]
What antenna type should I start with for power transfer?
Use a directional antenna to concentrate energy. Yagi or other directive forms improve link margin for short ranges. The thread suggests a directional approach for power transmission, paired with a stable single‑band transmitter and class‑C MOSFET PA. Align TX and RX carefully for best results. [Elektroda, Rodney Green, post #21666489]
Can I just use a high‑power amplifier and a transformer to the antenna?
Skip the transformer‑to‑antenna idea. Build a proper RF chain: oscillator, buffer, class‑C MOSFET PA, then impedance‑matched feed to a directional antenna. Avoid Darlington power stages at RF. This approach is simpler to match and more efficient for several‑watt outputs. [Elektroda, Rodney Green, post #21666489]
How do I power a small line‑follower robot wirelessly?
Budget for about 2 W mechanical (two 1 W motors) plus conversion losses. Plan coverage over a 3×3 m arena. Use directed RF and keep the receiver rectifier efficient, such as with Schottky diodes. Test at fixed points, then add antennas for coverage uniformity. [Elektroda, Sthitapragyan Pattanayak, post #21666490]
Is perfboard okay for RF prototyping?
Perfboard works at lower RF only. Stray capacitance and long leads limit higher frequency operation and can cause oscillations or loss. Keep traces short, ground well, and consider compact layouts. This is an edge‑case failure source many hit when they push frequency upward. [Elektroda, Sthitapragyan Pattanayak, post #21666491]
Any safety concerns when moving a few watts of RF around?
Yes. Dense RF fields can burn skin and damage tissue. Treat energized antennas as hazardous. One contributor described severe burns from a 60 W HF antenna touch. Maintain distance, enclosures, and interlocks, and avoid unnecessary exposure near the beam. [Elektroda, Steve Lawson, post #21666495]
Where can I read practical RF basics and legal tips?
Consult amateur radio resources recommended in the thread. They cover antennas, matching, stability, and compliance basics. Start with ARRL’s radio technology topics for foundational knowledge and guidance on spectrum use and power limits. [Elektroda, Steve Lawson, post #21666488]
What is the Friis transmission equation, in plain terms?
It gives received power from transmit power after accounting for distance, frequency (via wavelength), and antenna gains. Use it to size antennas and estimate margin before building. It guides frequency selection and helps predict how distance impacts received power. [Elektroda, sachin Kothari, post #21666494]
How do I prototype a basic short‑range RF power link?
- Build a crystal‑controlled oscillator and buffer for a single band.
- Add a single‑ended class‑C MOSFET PA (e.g., PD55008) with matching network.
- Feed a directional antenna; align to the receiver and test rectifier efficiency.
[Elektroda, Rodney Green, post #21666489]
What receiver front‑end and rectifier should I try first?
Use a tuned receive antenna and a low‑loss matching network into a Schottky diode rectifier. Keep wiring short and components sized for your band. Validate DC output under realistic spacing and orientation. The OP planned Schottky rectification, which is a solid starting point. [Elektroda, Sthitapragyan Pattanayak, post #21666487]
How can multiple antennas help in a 3×3 m arena?
Strategically placed transmit antennas can improve coverage and reduce dead zones. Fixed geometry helps alignment. Start with a single directional link, map received DC, then add antennas where the robot dips below its power threshold. Maintain compliance and avoid overlapping beams that cause hotspots. [Elektroda, Steve Lawson, post #21666488]
