FM transmitter 3W

Description: This package contains the schematic for an FM transmitter with an output power of 3 to 3.5 W, operating within a frequency range of 90 to 110 MHz. Although the stability is relatively acceptable, a Phase-Locked Loop (PLL) can be integrated into this circuit. The circuit was originally constructed for a friend, who utilized it alongside the BLY88 amplifier to achieve a total output power of 20 W (for that schematic, refer to the LINBLY88.EXE file). According to the notes on the original schematic, it functioned effectively with a Standing Wave Ratio (SWR) of 1:1.05, which is typical for the antenna used. While the circuit was not designed for stereo broadcasting, this should not present any issues. It is noted that the component numbering may appear disorganized due to the OrCAD Annotate software.

Components/Hints: A PCB layout is not available; however, the circuit was previously assembled on a standard RF-testing breadboard, which features copper on one side. Connections should be made between both sides. To ensure optimal performance, the transmitter should be housed in an RF-proof enclosure, with high-quality connectors and cables, and appropriate shielding between different stages. Transistors Q1 and Q5 require heat sinks for cooling, and the case pin of Q4 should be connected to ground.

Adjustments: Capacitor C24 is designated for frequency adjustment, while the other trimmers should be calibrated to achieve maximum output power with minimal SWR and input current.

Component list for the 3 - 3.5 W FM transmitter (Type 1):

Resistors:
- R1: 10 kΩ
- R2: 22 kΩ
- R3: 22 kΩ
- R4: 10 kΩ
- R5: 3.9 kΩ
- R6: 680 Ω
- R7: 150 Ω
- R8: 100 Ω
- R9: 68 Ω
- R10: 6.8 kΩ
- R11: 680 Ω
- R12: 100 Ω
- R13: 3.9 kΩ
- R14: 10 kΩ
- R15: 10 kΩ

Capacitors:
- C1: 4.7 pF ceramic
- C2: 100 nF ceramic
- C3: 100 nF ceramic
- C4: 100 nF ceramic
- C5: 100 nF ceramic
- C6: 10 nF ceramic
- C7: 100 nF ceramic
- C8: 60 pF trimmer
- C9: 10 nF ceramic
- C10: 10 nF ceramic
- C11: 100 nF ceramic
- C12: 100 nF ceramic
- C13: 82 pF ceramic
- C14: 60 pF trimmer
- C15: 27 pF ceramic
- C16: 22 pF ceramic
- C17: 10 µF/25V
- C18: 33 pF ceramic
- C19: 18 pF ceramic
- C20: 12 pF ceramic
- C21: 40 pF trimmer
- C22: 40 pF trimmer
- C23: 40 pF trimmer
- C24: 40 pF trimmer
- C25: 5 pF ceramic

Inductors:
- L1: 5 turns, diameter 6 mm, 1 mm CuAg, spaced 1 mm
- L2: 6-hole Ferroxcube wideband HF choke (5 turns)
- L3: 6-hole Ferroxcube wideband HF choke (5 turns)
- L4: 1.5 turns, diameter 6 mm, 1 mm CuAg, spaced 1 mm
- L5: 6-hole Ferroxcube wideband HF choke (5 turns)
- L6: 1.5 turns, diameter 6 mm, 1 mm CuAg, spaced 1 mm
- L7: 6-hole Ferroxcube wideband HF choke (5 turns)
- L8: 1.5 turns, diameter 6 mm, 1 mm CuAg, spaced 1 mm
- L9: 6-hole Ferroxcube wideband HF choke (5 turns)
- L10: 8 turns, diameter 5 mm, 1 mm CuAg, spaced 1 mm

Transistors:
- Q1: 2N3866
- Q2: 2N2219A
- Q3: BF115
- Q4: 2N2219A
- Q5: 2N3553

Others:
- D1: BB405, BB102, or equivalent (most varicaps with C = 2-20 pF will suffice)
- U1: uA78L08, uA7809, uA7810, or equivalent (MC78LXX, UC78LXX, etc.)

This FM transmitter circuit is designed to provide a reliable and effective means of transmission within the specified frequency range. Proper assembly and adjustments are critical to achieving optimal performance and stability.This package contains the schematic for an FM transmitter with 3 to 3.5 W output power, and can be used between 90 and 110 Mhz. Although the stability Although the stability isn't so bad, a PLL can be used on this circuit. This is a circuit that I've build a few years ago for a friend, who used it in combination with the BLY88 amplifier to obtain 20 W output power (for that schematic, get the LINBLY88.EXE file).

From the notes that I made at the original schematic, it worked fine with a SWR of 1 : 1.05 (quite normal at my place with my antenna). He didn't use it for stereo broadcasting, but that shouldn't be a problem. I don't have a PCB-layout for it, but as I remember I've made it on a normal RF-testing breadboard (with one side copper).

Make some connections between the two sides. Build the transmitter in a RF-proof casing, use good connectors and cable, make a shielding between the different stages, and be aware of all the other RF-rules of building. Q1 and Q5 should be cooled with a heatsink. The case-pin of Q4 should be grounded. Adjustments: C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.