FM RADIO ANTENNA AMPLIFIER

[palesha]

v tried to assemble fm radio antenna amplifier ( 88~108 Mhz) published in popular electronics. But unable to get it working. Anybody hv assembled it? or suggest some good circuit.

Added after 2 minutes:

here is the circuit

 

[IanP]

There is nothing wrong with this circuit, however, to make this amplifier working you need some experience in RF circuits ..

Maybe you should first try to build an amplifier that doesn't require any tunning and doesn't use inductors ..
Have a look at this 20dB wide-band VHF amplifier - picture below; for full circuit description go to: https://www.electronics-lab.com/projects/rf/009/index.html

Regards,
IanP

 

[biff44]

Well, the tuned input and output circuits can be a problem, but they have a very important function--to reject out of band signals that could overload your front end (CB radio, Ham, TV stations, etc).

The first thing I would do is to see if the transistor is working properly. Put a voltmeter on the collector of Q1. It should read in the 4 to 8 volt DC range. If it reads near zero or near 12 volts, then the transistor is either blown out, connected wrong, or you need to adjust the values of the resistor R3. Once the transistor is operating properly for its DC bias point, then it is capable of amplifying RF signals.

Tuning the coils is harder. For one thing, you could just put your input at C2, and your output at C4, and disconnect the other parts, and it should kindof work.

To tune the coils up you will need a signal source in the FM band range, and some way to measure the output (like an oscilloscope). Set the signal source to the center of the FM band and very low output power (maybe 20 mV), and adjust TR1 for the biggest amplifier output. Then do the same to TR2. If you can get this to work, the next step is to sweep the output over the entire FM band and get the output power to be flatest by simultaneously adjusting TR1 and TR2.

BTW, the reason I suspect that the transistor DC bias point is off is that the method of bias in this design is not very good. Transistor DC gains vary all over the place, and a good circuit should take that into account. There is some "negative feedback" provided by R3, but a much better circuit would have a resistor between the emitter and ground. The resistor value would be on the order of 200 ohms, and in parallel there would be a capacitor of greater than 15000 pF. That would kill the DC gain of the amplifier, but preserve the RF gain of the amplifier--allowing for a much more stable operating point.

 

[E-design]

If the amplifier is because you have bad FM reception you will need a tuned amp like biff44 said. For optimal performance a 2 stage amp is much better than a single stage because the first stage can be matched for minimum noise figure and some gain, and the last mainly for gain.

 

[palesha]

Thanks 4 ur help. I found one FM booster on the same website. But the luanguage is unknown. I just want the details of coils. Rest i can understand.
Regarding the VHF circuit, it must be good. But how it can go 500Mhz when the device BF197 ft is just 550 Mhz. Is it better to use something like 2SC 2570A ft. 5000Mhz. instead of BF197.
Kindly comment

Added after 4 minutes:

Link to the FM booster webpage:
h**p://www.electronics-lab.com/projects/rf/022/index.html