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FM antenna amplifier circuit

2015-01-04 00:42  
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Use this fm antenna amplifier in areas where the signal reception of FM stations is too bad.
This circuit is just the right thing to do it. It is designed in such a way that the supply current powering it flows through coaxial cable.
With this technique, an extra cable to power the fm antenna amplifier is unnecessary. The RF signal and the DC current supplying the amplifier use the cable simultaneously. Check another FM antenna booster circuit diagram.

FM antenna amplifier circuit schematic

fm antenna amplifier circuit schematic

The RF signal is however prevented by LC filters from flowing into the power supply. The amplifier works with either 50 ohm or 75 ohm antenna. It has a gain of 25 … 30 dB. MOSFETs are used to avoid the problem of crossmodulation or intermodulation. The circuit is divided into two parts. The first part is the active aplifier circuit which is normally installed very near the antenna. The second part is the power supply circuit which supplies current to the amplifier through the coax cable.

After thefm antenna amplifier circuitis constructed, the power supply should be connected to it. You must then test the DC voltage in the cable. This must be between 15.5 and 36.6 volts. Then test the voltage between T5 collector and ground while adjusting the supply voltage. It must move from 3 to 24 volts. The emitter of T2 should be approx. 11.4 volts. If the voltage at R4 is between 0.7 and 2 volts, then the MOSFET is functioning properly.

Important points to follow in constructing the circuit:
- The circuit must be constructed in a double sided PCB
- T1 must be shielded
- The source terminal of T1 must be soldered directly into the copper plate.
- C4 terminals must be as short as possible
- Coil terminals must be as short as possible
- The antenna must be conneted directly to L1
- T2 must be heatsinked

Power supply for the FM antenna amplifier
fm antenna amplifier supply
FM antenna amplifier active components and coils values

T1 = 3N225, 3SK85, 3SK87T2 = 2SD781, 2SD1177, 2SD1684, MJE243, MJE244T3 = 2SA970, 2SA1136, 2SA1137T4, T5 = 2SC3622, 2SC3234, 2SC3248L1 = 9 turns 0.7mm, tap at 1st turn from the groundL2 = 6/3 turns (A&B) 0.5mm on Amidon T37-12L3 = 9 turns 0.7mm, tap at 3rd turn from the ground
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