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How to build 3W FM Transmitter

2016-04-06 01:05  
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This is the schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz. 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. 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).

Circuit diagram

3W FM Transmitter-Circuit diagram


R1,R4,R14,R15 10K 1/4W ResistorR2,R3 22K 1/4W ResistorR5,R13 3.9K 1/4W ResistorR6,R11 680 Ohm 1/4W ResistorR7 150 Ohm 1/4W ResistorR8,R12 100 Ohm 1/4W ResistorR9 68 Ohm 1/4W ResistorR10 6.8K 1/4W ResistorC1 4.7pF Ceramic Disc CapacitorC2,C3,C4,C5,C7,C11,C12 100nF Ceramic Disc CapacitorC6,C9,C10 10nF Ceramic Disc CapacitorC8,C14 60pF Trimmer CapacitorC13 82pF Ceramic Disc CapacitorC15 27pF Ceramic Disc CapacitorC16 22pF Ceramic Disc CapacitorC17 10uF 25V Electrolytic CapacitorC18 33pF Ceramic Disc CapacitorC19 18pF Ceramic Disc CapacitorC20 12pF Ceramic Disc CapacitorC21,C22,C23,C24 40pF Trimmer CapacitorC25 5pF Ceramic Disc CapacitorL1 5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mmL2,L3,L5,L7,L9 6-hole Ferroxcube Wide band HF Choke (5 WDG)L4,L6,L8 1.5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mmL10 8 WDG, Dia 5 mm, 1 mm CuAg, Space 1 mmD1 BB405 or BB102 or equal (most varicaps with C = 2-20 pF [approx.] will do)Q1 2N3866Q2,Q4 2N2219AQ3 BF115Q5 2N3553U1 7810 RegulatorMIC Electret MicrophoneMISC PC Board, Wire For Antenna, Heatsinks


Email Rae XL Tkacik with questions, comments, etc.The circuit has been tested 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 heat sink. The case-pin of Q4 should be grounded.C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.Local laws in some states, provinces or countries may prohibit the operation of this transmitter. Check with the local authorities.

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