FAQ
TL;DR: L2 in the 4 W FM transmitter is an RF choke; it "chokes off the AC" while passing DC, and you wind fine wire around a 1 MΩ resistor used as the coil form. [Elektroda, David Adams, post #21665015]
Why it matters: Builders asking how to fix hum or instability learn how to make and place L2 correctly; this FAQ is for hobbyists replicating the 4 W FM project and troubleshooting its RF choke.
Quick Facts
- L2 is the RF choke that looks like a DC short but blocks RF feedback into the supply. [Elektroda, David Adams, post #21665015]
- The 1 MΩ resistor serves as a physical coil form for L2; its resistance is not the RF element. [Elektroda, Steve Lawson, post #21665016]
- Use fine magnet wire; scrape the enamel and solder each end to a resistor lead. [Elektroda, Steve Lawson, post #21665017]
- Real-world build note: T2 ran hot; a heatsink and fan were added to keep power stable. [Elektroda, Randy Dively, post #21665024]
- One user intended operation on “dead channels” for local comms during 12/21/2012 contingency. [Elektroda, Randy Dively, post #21665023]
What does L2 (RFC) actually do in the 4 W FM transmitter?
L2 is the radio-frequency choke on the DC rail. It offers low impedance to DC but high impedance to RF, preventing the RF signal from feeding back into the power supply. As one expert put it, "Chokes off the AC." This stabilizes the transmitter and reduces hum. [Elektroda, David Adams, post #21665015]
How do I wind and connect L2 around a 1 MΩ resistor?
Use fine magnet wire. Solder one wire end to one resistor lead. Wind tight, adjacent turns along the resistor body. Scrape enamel off the free end and solder it to the other lead to complete the coil. This creates the choke on a compact form. [Elektroda, Steve Lawson, post #21665017]
How many turns should L2 have?
The shared build notes did not state a turn count. One builder guessed the number of turns, completed the coil, and reported the transmitter worked strongly afterward. If unsure, start with moderate turns and adjust by testing supply-borne RF. [Elektroda, Randy Dively, post #21665024]
Why use a 1 MΩ resistor—does its resistance value affect L2?
Here, the resistor is just a convenient coil form. The wire you wind around it becomes the inductor. The 1 MΩ value is incidental and not the radio-frequency element of L2 in this construction method. [Elektroda, Steve Lawson, post #21665016]
What wire type should I use for L2, and any prep tips?
Use fine insulated (enamelled) magnet wire. After winding, scrape the enamel from both ends so solder wets the copper. Then solder each end to the resistor leads to make solid electrical connections. Keep turns neat to maintain inductance. [Elektroda, Steve Lawson, post #21665017]
What’s the difference between an RF choke and a regular inductor here?
Functionally similar parts, different roles. In this transmitter, the choke is placed in the DC feed to look like a short to DC but an open to RF, isolating stages from supply feedback. That placement and purpose define it as an RF choke. [Elektroda, David Adams, post #21665015]
My output transistor T2 overheats—what should I do?
A builder reported significant heat and solved it by adding a heatsink and a small fan on T2. Expect heat rise at 4 W RF output, especially during continuous duty. "Add thermal management early" to avoid drift or failure. [Elektroda, Randy Dively, post #21665024]
Where can I find the original 4 W FM transmitter project link mentioned?
The thread shares the build link as the DIY Electronic Projects page for the 4 W FM transmitter (p121-4W-FM-Transmitter). Use that reference to review the schematic and parts list. [Elektroda, Randy Dively, post #21665019]
Is analog FM being replaced by digital—should I still build this?
One participant argued that digital methods and phase-locked designs are the forward path, suggesting analog FM is fading. If your goal is learning RF basics or legacy receivers, this build still teaches fundamentals. For modern systems, study digital modulation too. [Elektroda, Mark Harrington, post #21665022]
Can I transmit on “dead channels” for emergencies?
A user planned to use dead channels for neighborhood communication on December 21, 2012. This thread does not provide legal guidance. Always verify local regulations and licensing before transmitting, even for emergency preparation. [Elektroda, Randy Dively, post #21665023]
What does RFC stand for, and why is it critical here?
RFC stands for Radio Frequency Choke. It prevents RF from traveling back through the power line, which can cause oscillations or hum. Without it, supply-borne RF can destabilize the transmitter and radiate unintentionally. [Elektroda, David Adams, post #21665015]
Quick how-to: How do I build L2 in three steps?
- Tin one resistor lead and solder one end of fine magnet wire to it.
- Wind tight, even turns along the resistor body.
- Scrape enamel from the free end and solder it to the other lead to finish the choke. [Elektroda, Steve Lawson, post #21665017]
