RF amplifier and Power amplifier

Discussion in 'Wireless & RF Design' started by samrat, Jun 19, 2008.

  1. samrat

    Thread Starter Active Member

    Apr 2, 2008
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    Good-day,
    For our major project, my group wants to boost up the existing power that the school radio is running on from 4 watts to 50 watts. The radio station is using a 3 element FM radio omni antenna(dipole) with impedance of 50 ohms. My understanding is that we will have to design a separate rf amp and a separate power amp that is turnable(0-50)..The current station is operating on an exciter only..
    I am just asking for any handy tips while designing the circuit...Some handy reference would be great...:confused:..
     
    Last edited: Jun 19, 2008
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The ARRL site should be a good source for RF amps. They are a bit of a challenge for a first-time project, though. You might also Google for "linear amplifiers" to see if you can turn up something there.

    You might wish also to check your station license to make sure the power boost is legal. The regulating authorities can be very nasty to have to deal with if they think you're not obeying the rules.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I second Beenthere's recommendations - but I feel that the 2nd part needs a lot more emphasis.

    You must be absolutely certain that the station is operated in strict compliance with the license granted by the FCC. They don't generally give warnings. They'll seize equipment, slap heavy fines on the operators of the station, and it will practically take an act of Congress to reverse the damage.

    It is the station operator's responsibility to ensure compliance. "I didn't know" will NOT be an acceptable defense.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Transmitting more than 10 watts into your antenna would put you in violation of 47 USC 73.506, assuming your a Class D station, and places your school's license in jeopardy. If your transmitting 4 watts and your license allows you 10 watts, then you can build a smaller amplifier to satisfy the requirements of your license. You will have to ensure the amplifier meets FCC compliance before placing it in service at your station.

    Ensure you understand the ramifications of 47 USC 73 as it applies to your school's license. What does the Chief Operator at the station say?

    If your not in the U.S., I'm sure your country has a regulating agency like our FCC, following similiar legislated rules. Check with your regulating authority.

    Ignorance of the law is not a plausible defense.
     
    Last edited: Jun 19, 2008
  5. Gadget

    Distinguished Member

    Jan 10, 2006
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    Is that 10 watts or 10 watts EIRP as is the norm. If so, then that 4 watt transmitter/3 element yagi combo is probably already at the limit..... (even with cable losses)
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I read it as EIRP. Their license would specify.
     
  7. samrat

    Thread Starter Active Member

    Apr 2, 2008
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    Thanks for the infor guys. I live in fiji (in the South Pacific)and I am quite sure that the licence granted to the school allows it to operate on 50 watts. This is a challenge for us but with help from you guys, we will complete this in due time. By the way, I will check again with the authorities on Monday just to confirm where we stand. Will keep you guys informed with our progress and will ask for help in any difficulties arise.
    Thanx again..SR
     
  8. samrat

    Thread Starter Active Member

    Apr 2, 2008
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    As for power amplifiers, our best bet will be a Class C power amp???Pliz advice...
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    You can use a Class C amplifier if your antennna (load) is tuned to your allocated transmit frequency, and filter(s) are used to reduce sideband and harmonic emissions to within the limits allowed by the station license.

    Good quality high-power RF filters are not cheap.
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I like to know about the frequency of your transmitter.
    Most amplifiers found on the internet are made by HAM radio amateurs.
    They are working at specific frequencies, like 160 Meters , 80 meters , 40 meters, 20 meters , 15 meters , 10 meters , 6 meters , 2 meters , 70 centimters , 23 centimeters, 13 centimeters.
    See the link to an radioamateur with lots of schematics links and drawings.
    http://www.ac6v.com/

    Greetings,
    Bertus
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  12. samrat

    Thread Starter Active Member

    Apr 2, 2008
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    The frequency of our radio is 89.6MHz with max deviation of 75kHz.The range of broadcasting is not so important in my country because I have had a look at the laws and as long as we stick with th 50watts out put from power amp, we are ok. So any suggestions how to calculate the gain around the transistor or mosfet would be great.I mean, no exact values but just a dummy diagram and how to calculate the exact component values from the diagram would be awesome...we are stuck now in trying to ensure that our calculations are fool prove. Please a step by step guidance is highly appreciated.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Two ways to increase the "effective power" of a broadcast station ... add a power amplifier or use a higher gain antenna. Your country doesn't specify using the "effective power" from the antenna ... called EIRP?

    You would need an 11 dB amplifier to go from 4 to 50 W.

    What is the model of your transmitter, the transmission line, and what is the model of your antenna?

    Can you provide the link to your regulating agency's information?
     
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