• Status: Approved for implementation.
• Compact, through-hole, two-transistor FM voice transmitter powered from a 9 V battery.
• Architecture follows the Art Swan / Instructables “Ultimate FM Transmitter” concept: electret-microphone common-emitter preamplifier feeding a VHF common-base LC oscillator.
• This revision improves DC bias repeatability, separates audio and RF supply filtering, uses C0G/NP0 parts in the resonant path, and capacitively couples the antenna.

• Hobbyist prototype and RF experimentation platform.
• Hand assembled and tuned with an FM receiver and preferably a frequency counter, oscilloscope with a low-capacitance probe, or SDR.
• Not production-certified hardware.

• Accept a two-wire electret microphone.
• Amplify voice-band audio with Q1.
• Frequency-modulate a tunable LC oscillator built around Q2.
• Tune across approximately the 88–108 MHz broadcast-FM region after allowing for coil, trimmer, transistor, and PCB parasitics.
• Provide a DC-isolated antenna connection.

• Two onsemi 2N3904 TO-92 transistors.
• 9 V battery input through a two-pin 2.54 mm header.
• Separate RC-filtered AUDIO_9V and RF_9V rails.
• Edge-accessible microphone, tuning capacitor, hand-wound coil, and antenna connections.
• Through-hole passives and ordinary two-layer fabrication.

Text
9 V battery
  ├─ RC filter → AUDIO_9V → electret bias → Q1 audio preamplifier
  └─ RC filter → RF_9V → Q2 LC oscillator → small coupling capacitor → antenna
                              ↑
                  amplified audio modulation

• J1 is a 2-pin battery header: pin 1 = +9 V, pin 2 = GND.
• R8/C6/C7 generate AUDIO_9V; R9/C8/C9 generate RF_9V.
• No reverse-polarity device is added because the scope prioritizes the simple two-transistor topology and compact through-hole construction; polarity is clearly documented.

• J2 is a 2-pin electret microphone header: pin 1 = MIC+, pin 2 = GND.
• R1 biases the electret from AUDIO_9V.
• C1 AC-couples the microphone to Q1.
• Q1 uses a divider-biased common-emitter stage with emitter degeneration for improved bias stability.
• Nominal design point with AUDIO_9V ≈ 8.5 V, β ≈ 100 and VBE ≈ 0.65 V: Q1 collector current ≈ 0.52 mA, emitter ≈ 0.53 V, collector ≈ 4.2 V.

• Q2 is biased at a few milliamps rather than by the original low-value base feed, reducing heating and improving repeatability.
• C3 RF-bypasses Q2 base to RF_9V while C2 injects audio modulation.
• L1 and CT1 connect from Q2 collector to RF_9V and form the parallel resonator.
• C4 provides collector-to-emitter RF feedback.
• C5 is the antenna coupling capacitor; J3 exposes ANT and GND.

• Source: alkaline or rechargeable nominal 9 V battery.
• Estimated active current: Q1 ≈ 0.5 mA, Q2 ≈ 3–5 mA, electret ≈ 0.3–0.8 mA; total approximately 5–8 mA depending on transistor gain, microphone, tuning and antenna loading.
• A typical alkaline 9 V battery should provide many hours to multiple days of intermittent operation; exact runtime depends strongly on battery chemistry and low-voltage behavior.

• Standard two-layer FR-4, ordinary through-hole vias, 1 oz copper acceptable.
• Board maximum 100 × 32 mm; target is a compact elongated battery-top form.
• All fitted components are through-hole and hand-solderable.
• Resistors may be stood vertically if the builder needs additional physical clearance, even when the footprint is a conventional axial footprint.

• None; the design contains no programmable device.

• Audio/microphone end at one board edge.
• RF oscillator, L1, CT1 and antenna at the opposite edge.
• L1–CT1–Q2 collector loop must be extremely short.
• Bottom copper is used as the principal ground return, with routing/zone geometry kept away from directly beneath the air-core coil where practical.
• Trimmer adjustment and antenna connector remain edge-accessible.

• Preserve exactly two active devices and avoid IC synthesis or added RF power stages.
• Use the Instructables/Art Swan component topology as the source baseline, but alter bias values for stable mid-supply Q1 operation and lower-current Q2 operation.
• Replace the source design’s direct antenna loading with a 4.7 pF C0G/NP0 coupling capacitor.
• Use separate RC supply filters instead of a shared unfiltered 9 V rail.

• The builder will use an onsemi-style 2N3904 pinout: viewed according to the selected footprint/datasheet, pins are 1 Emitter, 2 Base, 3 Collector. Other vendors and substitute transistor families may differ.
• CT1 is represented by a generic two-lead through-hole footprint. Its exact lead spacing and adjustment-shaft geometry depend on the user’s trimmer; flying leads may be used if it does not fit.
• L1 is represented by two plated through-hole pads. Coil lead spacing, body diameter and stand-off are builder-dependent.
• A starting L1 target is approximately 0.10–0.12 µH. A 5–30 pF trimmer plus unavoidable 3–8 pF parasitics should permit practical alignment into the desired band; actual range is hardware-dependent.
• The antenna is an external insulated wire or experimental load, not a controlled-impedance connector.

• Reconstructed the source topology and BOM from the Instructables mirror, Art Swan archive references, and the independently reproduced LEAP #467 circuit.
• Added explicit power-domain filtering, improved Q1/Q2 bias, RF-grade capacitor notes, and capacitive antenna coupling.