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how a FM radio antenna amplifier works

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ZackJ205

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I am a EE major and I have a project on a FM radio antenna amplifier but i have no idea how it works and i cant find anything on the internet about it..and i really need some help. The website and the diagram that the instructor gave me is **broken link removed**
 
Why would you use an "amplified antenna" vs just a quarter-wavelength mono-pole or a half-wave dipole?
 
I hate to say it but im having troble with all of it. My instructor gave me that diagram and said you will have an oral presentation and tell how it works and what it does. I learned most of the componets like the inductors and resistors in the class before and i am lost in this course. I would really appreciate all the help you could give me.
 
ZackJ205,

I hate to say it but im having troble with all of it. My instructor gave me that diagram and said you will have an oral presentation and tell how it works and what it does. I learned most of the componets like the inductors and resistors in the class before and i am lost in this course. I would really appreciate all the help you could give me.

It appears you need more intensive and sustained help than any of us can give you. Our time and space here is limited, as is the scope of what we can present to you. You must have had some instruction at the facility where you are studying. What about the resources there? Why are you so far behind? It appears you need some remedial study beyond the capabilities of this forum.

Ratch
 
No im not behind. In the instructions it says the purpose of this project is to take this circuit/system that you have little or no experience (in my case no experience) and become an expert and be able to answer the colleagues questions. I cant find anything on the internet about it to help me.
 
Start by asking under what conditions you would need a preamplifier between an antenna and the input of a receiver?
i.e., what problem is the preamplifier trying to solve?

Hint: There are hundreds of millions of receivers out there that do not use or need such a preamplifier. What would you have to do to an antenna to make adding such an amplifier necessary?
 
The amp is not the best design but here are some details.

Input assumed to be low impedance, about 75 ohm coax from antenna.

The two 5.6 pF are impedance matching caps (although likely incorrect impedance transformation). L1 inductor is about 95nH requiring about 26 pF to resonate at 98 MHz. The tapping at 1 turn actually increases the input matching losses but it reduces the effects of input impedance variation on detuning input resonant tank circuit.

The DC biasing of the transistor is based on the 27k base feedback and 1k + 270 ohm collector resistors.

Icollector = beta * Ibase

Ic = beta * (Vcollector - Vbe) / R1 = beta * (Vc - 0.6)/27k

Ic = (12 volts - Vc)/ (R2 + R3) = (12 -Vc) / 1270

Solve simultaneous equations for last two items. Ic = beta * (Vc - 0.6)/27k = (12 -Vc) / 1270

Biggest issue I see is the collector voltage will be low resulting in limited dynamic range (amp subject to overloading) with potential of bad intermodulation interference.

From an AC perspective, the 27k provides a little (very little) AC feedback on the amp. This would lower the input and output impedance of the device a little bit.

On output, the bypass cap C3 forces R2 (270 ohms) to be a low impedance output load. Primary purpose is to keep amp stable (prevent it oscillating).

The output tank coil, L2, is about 60 nH requiring parallel resonating capacitance of about 44 pF to resonate at 98 MHz. The two 10 pF just jack up the Q of the output tank and provide about 20 pF of additional capacitive loading on L2. They provide no impedance matching for taking the <270 ohms device output impedance to 75 ohm low impedance output from circuit.

All in All, not a very good circuit. (hope your instructor did not design it.)
 
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In a major city, the simple FM amplifier is absolutely useless since it will be severely overloaded by all the local radio and TV stations.
Every half-decent FM radio already has an FM RF amplifier with AGC that automatically reduces the gain if there is a strong local station in its input bandpass.

I think your project was designed a few hundred years ago when the nearest FM station was hundreds of miles away. Tell your teacher.
 
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