FAQ
TL;DR: A 12 pF capacitor with its own leads can resonate near 400 MHz; "it will resonate at about 400 MHz" when the leads act as an inductor. [Elektroda, Chris Bezuidenhout, post #21666999]
Why it matters: This FAQ helps beginners turn vague ideas about “capturing RF” into practical tuning, detection, and power-harvesting steps for real circuits.
Quick Facts
- RF = Radio Frequency; used for wireless communication systems. [Elektroda, Chris Bezuidenhout, post #21666995]
- A capacitor is two conductive plates separated by a dielectric; it stores electric field energy. [Elektroda, Chris Bezuidenhout, post #21666995]
- A basic tuned circuit is an inductor and capacitor, typically in parallel. [Elektroda, Chris Bezuidenhout, post #21666995]
- Rectified RF can charge a storage capacitor up to approx. the RF RMS value, if the signal is strong. [Elektroda, Chris Bezuidenhout, post #21666998]
- A lone capacitor does not "capture" RF; use LC resonance or a detector stage. [Elektroda, Rodney Green, post #21666992]
What does “capture RF with a capacitor” actually mean?
People usually mean one of two things: tune to an RF frequency with an LC resonant circuit, or detect and store RF energy as DC using a diode and capacitor. Clarify your goal before choosing components or topology. [Elektroda, Steve Lawson, post #21667000]
Can a capacitor alone capture an RF signal?
No. A capacitor passes AC and cannot capture RF by itself. You need a resonance with an inductor or a detector using a diode and a storage capacitor. As one expert put it, “ac capacitor cannot ‘capture an RF signal’ per se.” [Elektroda, Rodney Green, post #21666992]
How do I tune to a specific RF frequency with a capacitor?
Form an LC tank. Select C and L so their resonance equals the target frequency. At resonance, impedance peaks (parallel) or dips (series), concentrating RF energy at that frequency for further detection or amplification. [Elektroda, Chris Bezuidenhout, post #21666995]
What is a varicap (varactor) diode and why is it mentioned in radios?
A varicap is a diode whose junction capacitance varies with reverse bias. Designers use it in tuners to sweep LC resonance electronically for AM/FM. It’s the capacitor element in a voltage-controlled resonant circuit. [Elektroda, Floy Viola, post #21666991]
How does an LC tank circuit work in RF?
An inductor and capacitor exchange energy between magnetic and electric fields. At resonance, reactive currents cancel and the circuit selects or sustains signals near that frequency. Shielding makes a "tank" in a metal enclosure. [Elektroda, Chris Bezuidenhout, post #21666995]
How can I detect RF and get DC using a capacitor?
Use a diode to rectify the RF, then a larger capacitor to smooth it. The storage capacitor can charge up toward the RF RMS level, given sufficient input strength. This DC can power ultra-low-power stages. [Elektroda, Chris Bezuidenhout, post #21666998]
Quick how-to: build a simple RF detector
- Make an LC tuned circuit for your target band.
- Feed it to a diode rectifier.
- Add a larger capacitor after the diode to store the detected DC and measure it.
[Elektroda, Chris Bezuidenhout, post #21666994]
Can rectified RF really power a circuit?
Yes, in special cases. One contributor built a regenerative receiver powered by the received RF itself. Expect tiny power; headphones-level loads are typical. Strong signals and efficient detection are essential. [Elektroda, Chris Bezuidenhout, post #21667001]
Do disc capacitor leads act like an inductor at RF?
Yes. Lead inductance plus the capacitor can create unintended resonance. Example: connecting both leads of a 12 pF capacitor can resonate around 400 MHz, determined by geometry and value. [Elektroda, Chris Bezuidenhout, post #21666999]
What’s the failure case if my signal is weak?
Your storage capacitor may never reach a useful DC voltage. Energy harvesting requires strong fields or close coupling. Detection remains possible, but power output collapses with weak input. [Elektroda, Chris Bezuidenhout, post #21666994]
What’s the difference between superheterodyne and regenerative in this context?
They are distinct receiver architectures. The thread references a regenerative design that ran from received RF power, underscoring extreme efficiency at the detector and RF front end. [Elektroda, Chris Bezuidenhout, post #21667001]
What is RF, in simple terms?
RF means Radio Frequency. It covers alternating signals used for wireless transmission and reception in radios and related electronics. [Elektroda, Chris Bezuidenhout, post #21666995]
What is an inductor, practically speaking?
An inductor is a conductive coil (or equivalent structure) that stores energy in a magnetic field. It is essential for RF tuning, filters, and oscillators. [Elektroda, Chris Bezuidenhout, post #21666995]
Do I need modulation to detect and store RF energy?
No. Unmodulated RF can produce a steadier DC after rectification, which simplifies measurement or ultra-low-power operation. Modulation is not required for energy capture. [Elektroda, Chris Bezuidenhout, post #21667001]
Why do some answers insist on clarifying the word “capture”?
Capture could mean demodulate data, harvest energy, or contain fields in a resonant cavity. Define your goal to avoid confusion and pick the right circuit. [Elektroda, Steve Lawson, post #21667000]
