3W Wireless Power Transfer at 5V Over 2-4m: Frequency, MOSFETs, Antenna, Filters?

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#1 21666487 13 Feb 2013 12:01

Sthitapragyan Pattanayak Sthitapragyan Patta...

Anonymous

Post #1
21666487 13 Feb 2013 12:01

I need to design a wireless power transmitter and receiver, so that at least 3 W (at 5 V) power is received at receiver section. Transmitter and receiver can be at maximum distance of 2 to 4 m. The transmitting and receiving antenna cant be more than 1 m. My idea is to generate a high frequency wave, and amplify it using *MOSFET* connected in *cascade*. Then improve the power by *darlington power transistors* and transmit it using an antenna. Rectify it in receiver by using Schottky diodes.

Please help me to choose.
# Appropriate operating frequency (please don't suggest microwave)
# Appropriate MOSFET and darlington transistors
# Appropriate antenna design (may be directive or non directive)
# Passive filter in receiver end

Can I increase the power strength of signal, simply by using high power amplifiers and coupling it to antenna by using transformer?

I know that radiated power is proportional to square of antenna current, which is again a function of radiation resistance. entire things goes more and more complicated in this way. Please help me to design such a transmitter (efficiency can be anything).
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#2 21666488 13 Feb 2013 13:07

Steve Lawson Steve Lawson

Anonymous

Post #2
21666488 13 Feb 2013 13:07

* Why do you need 3W to traverse 2 to 4 meters? Depending on frequency and terrain, 3W can travel quite a distance, especially if the transmitter [TX] and receiver [RX] can be in fixed locations -- if you have line-of-sight between TX and RX, then with a focused antenna system you can reach quite a distance with 3W. But then, I'm assuming you want to use this for communication or data transfer. So...

* What is the actual purpose or application?

* Not sure in what country you want to run this device, but you probably should check local ordinances for radiated power.

* Frequency choice will likely have much to do with laws governing the RF spectrum in your area.

Check out this site for ideas (and even legal implications):

http://www.arrl.org/radio-technology-topics
#3 21666489 13 Feb 2013 20:43

Rodney Green Rodney Green

Anonymous

Post #3
21666489 13 Feb 2013 20:43

A transmitter for power transmission is likely to need a directional antenna.
The use of MOSFETS is excellent and 5 watts is not a problem either. A single band transmitter will be best and perhaps a crystal oscillator to keep it stable. There is dont forget, the legal requirement for a license for it as it can disrupt other services. Now you mentioned darlington transistors. These are not suitable for RF. A good start would be to have a transistor crystal oscillator, and buffer amp followed by a single ended class C MOSFET amplifier. I suggest a PD55008 by ST. If you need more information pleasse ask.
You did not mention frequency range. That will determine what components are needed
Are you trying to duplicate some of Teslas experiments?.
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#4 21666490 13 Feb 2013 23:47

Sthitapragyan Pattanayak Sthitapragyan Patta...

Anonymous

Post #4
21666490 13 Feb 2013 23:47

the receiver is mobile with in a arena of 3*3 square meter area. my actual application is to run a autonomous line follower (which has two 1w motors) by wireless power transfer. i am experimenting this in an area which has neither any public establishments nor any other radio transmitter installed. i can also mount more antenna around the arena for better coverage. it would be better if it cover more distance, but the minimum requirement is 3 meters.

but i want to know the relation between frequency and received power.
#5 21666491 14 Feb 2013 00:41

Sthitapragyan Pattanayak Sthitapragyan Patta...

Anonymous

Post #5
21666491 14 Feb 2013 00:41

i would design the circuit on a *perforated board(vero board)*. thus the wire *stray capacitance * would *limit the higher frequency* use. i know that E=hf. higher frequency means higher energy. but does it mean that higher frequency transmits higher power? i exactly wants the *relation between frequency used and power transmitted*. sir please specify the components required, and few details of power amplifer used. whether i would choose a Marconi type antenna or a yagi( parabolic antenna construction is difficult)? my actual application is to run a autonomous line follower (which has two 1w motors) by wireless power transfer. i am experimenting this in an area which has neither any public establishments nor any other radio transmitter installed.
#6 21666492 14 Feb 2013 00:44

Sthitapragyan Pattanayak Sthitapragyan Patta...

Anonymous

Post #6
21666492 14 Feb 2013 00:44

please choose any frequency so that it would not be affected by wire stray capacitance and interectrode capacitance.
#7 21666493 14 Feb 2013 00:51

Mark Harrington Mark Harrington

Anonymous

Post #7
21666493 14 Feb 2013 00:51

Try the ARRL amatuer radio handbook You can still purchase these believe it or not Or amatuer radio sites
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#8 21666494 16 Feb 2013 11:19

sachin Kothari sachin Kothari

Anonymous

Post #8
21666494 16 Feb 2013 11:19

I would like you to read some thing more about Testla experiment as people have mentioned
and What you are looking for the amount of wireless power transfer depends lot of parameters
and Also you have to go thru regulation requirement as well
Friis equation will give you a better idea for what you need to do

http://en.wikipedia.org/wiki/Friis_transmission_equation

This is a basic idea for what you need to select for in terms of frequency , antenna and distance

I hope this helps
#9 21666495 16 Feb 2013 14:29

Steve Lawson Steve Lawson

Anonymous

Post #9
21666495 16 Feb 2013 14:29

Consider that wireless energy transmission has possible health issues. Consider the implications of the electrical system in your body exposed to medium to high density RF energy -- is there no affect? Consider that even subtle affects can be significant. Not to mention the cooking power of RF -- have you ever touched the antenna out of a 60W 40M CW transmitter while it is keyed? I have -- burnings to the bone!!
#10 21666496 16 Feb 2013 17:16

Mark Harrington Mark Harrington

Anonymous

Post #10
21666496 16 Feb 2013 17:16

Here is a very good link which has good explanation, theory y plus practical experiments

http://www.powerlabs.org/plasmaglobes.htm

Looks like fun : _ The days i can remember which made learning interesting with good group involvement and kept class attention instead of the boring put you to sleep lectures

Here is another link for you if you like that type of science

Also interesting
http://www.coilgun.info/levitation/home.htm

or

http://www.harrington.force9.co.uk/SciencePhy...er/High%20Frequency%20Induction%20Heating.htm

If you want more of those type of links goto my home page at this address

http://www.harrington.force9.co.uk/SciencePhysics/Science.htm

Or this link stuff you dont see in class rooms anymore

http://www.coe.ufrj.br/~acmq/electrostatic.html

How to build these magnificant machines

Or you might like this

http://makeprojects.com/Project/Simple+Van+de+Graaff+Generator/2072/1#.UR6syHr6Bak

Real fun
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#11 21666497 16 Feb 2013 17:57

Mark Harrington Mark Harrington

Anonymous

Post #11
21666497 16 Feb 2013 17:57

There is also this site as well quite good

http://www.mos.org/live-presentations/lightning

http://en.wikipedia.org/wiki/Van_de_Graaff_generator

This is where you can buy the belts from i would have thought Indian rubber would be quite good as well for this and a couple of dog bowls tied together to form the dome

Any way theres another link for you
http://www.ensalco.com/urabelt/vandegraffinfo.asp
#12 21666498 16 Feb 2013 18:04

Mark Harrington Mark Harrington

Anonymous

Post #12
21666498 16 Feb 2013 18:04

And some more Tesla sites with Jacobs ladder etc

http://www.tesla-coil.com/
#13 21666499 01 Apr 2014 19:52

Hadley Lockington Hadley Lockington

Anonymous

Post #13
21666499 01 Apr 2014 19:52

These guys claim to have the safest wireless charging platforms:

"http://powerbyproxi.com/wireless-charging/(Power by Proxi)":http://powerbyproxi.com/wireless-charging/

Many would consider the ICNIRP guidelines as a reference to the limits of RF energy humans can be exposed to before it becoming harmful.

"http://www.icnirp.de/PubEMF.htm(ICBIRP)":http://www.icnirp.de/PubEMF.htm

Although they don't seem to go into specifics in the realm of wireless power, this doesn't mean that wireless systems operating under these guidelines would be unsafe.

Electromagnetic fields are considered to be at the lower end of the exposure spectrum.
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Topic summary

✨ The discussion focuses on designing a wireless power transfer system capable of delivering at least 3 W at 5 V over a distance of 2 to 4 meters, with transmitter and receiver antennas limited to 1 meter in size. The user plans to generate a high-frequency signal amplified by cascaded MOSFETs and Darlington power transistors, transmitting via an antenna and rectifying at the receiver with Schottky diodes. Key considerations include selecting an appropriate operating frequency (excluding microwave bands), suitable MOSFETs and Darlington transistors, antenna design (directive or non-directive), and passive filtering at the receiver. Responses emphasize the importance of frequency choice based on regulatory constraints and stray capacitance effects, recommending stable single-band transmitters with crystal oscillators and class C MOSFET amplifiers (e.g., PD55008 by ST). Darlington transistors are discouraged for RF amplification. Directional antennas are advised for power transmission to improve efficiency and range. The Friis transmission equation is suggested for estimating link budget and power requirements. Safety concerns regarding RF exposure and legal regulations are highlighted. Additional resources include amateur radio handbooks and links to Tesla coil and induction heating experiments. The user’s application involves powering a mobile autonomous line follower robot within a 3x3 m arena, requiring stable wireless power delivery without interference. Increasing transmitted power via high-power amplifiers and transformer coupling is questioned, with emphasis on antenna current and radiation resistance affecting radiated power.

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]