5W Amplifier needed for 2-20 MHz range

Discussion in 'General Electronics Chat' started by zero_coke, Feb 18, 2013.

  1. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Anyone know any amplifier kit I can get that will deliver 5W from approx. 2 - 20 MHz? Most likely I'm going to use it at 10 MHz and the reason I need this is to drive my 140W 2-50MHz amplifier because it needs a 5W input signal to get it working.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    How are you planning to legally emit those power levels in that frequency range and into what kind of antenna? What is your plan for dealing with the interference you will be causing to licensed services.

    You seem to feel that the RF spectrum is your exclusive domain to do with as you please. You can go pound sand as far as I am concerned.
     
  3. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Why would there be any interference? It's only a magnetic field, and it's at resonance too so it interacts very weakly with anything that is off-resonance. I don't understand why there would be any interference with the exception of something at the exact same frequency in a short range being coupled to my transmitting coil. You're thinking of an antenna radiating E and H field at this frequency. It isn't the case. It's not an antenna doing that. It's a coil that just sets up a magnetic field around itself. And its strength decreases with r^3 of the distance anyway so it's not going to interact with anything more than a few meters at max.
     
  4. #12

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    Nov 30, 2010
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    140 watts into a "transmitting" coil? Planning on cooking something? A bit of induction heating?

    Please clarify.
     
  5. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Sure. I'm pumping out 140 watts into a 10 turn copper pipe that has an impedance of nearly ~50 ohms at 10 MHz. I need to setup a large and strong oscillating magnetic field at resonance to allow my receiver to pick it up from an appreciable distance (0.5-2 meters away, hopefully).
     
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  6. #12

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    You're over my head now but I got enough information on the board to avoid giving people the wrong impression.

    Personally, I think needing 140 watts to be detected at 6 feet of distance is way too much, but again, you're over my head at this point. (I suck at RF.)
     
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  7. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Yeah weird things happen in RF domain. We've done some simulations and whatnot to see what kind of currents and voltages we're getting because I need have some sort of estimate of what magnitudes are running before I build this monster.
     
    Last edited: Feb 18, 2013
  8. Papabravo

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    Feb 24, 2006
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    Because 140 Watts of RF power into a load will either radiate "around the world" so to speak or be reflected back to the source with possibly disastrous results. I think you'd better spend some time learning of the possible effects of your experiments on aircraft and public safety communications. You're going to have a great deal of time to reflect on this from the inside of a jail cell.

    BTW 18 watts was good enough to work 5X8C in Uganda on the 20M band two nights ago.
     
  9. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Relax man, I'm not going to go to jail. I'm not interfering with anything. I only have a magnetic field setup with the electric field confined to the plates of the capacitors. Dumping 140W into a coil is not going to cause this mass chaos that you depict. Its just going to setup a strong magnetic field for my experiment. A magnetic field, especially one oscillating a very specific frequency, will not affect anything except an object within the vicinity of the transmitting coil AND who has a natrual frequency to that of the resonators'. I appreciate your concerns and I do value it, however, I just don't see how it applies this specific case. If it were an issue, MIT would've run into big trouble since they pumped over 400W into a self-resonating oscillator which had the electric field running freely (due to the spacings and windings of the transmitting coil).
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The experiment at MIT was also carried out inside a Faraday cage, so no RF could escape.

    When you are using a linear amp, it's not just going to pick the 10Mhz signal you want, it will amplify EVERYTHING in the bandwidth of the amp, sending it to a higher power amp, which again, amplifies EVERYTHING in the bandwidth, and all the things you have connected become antennas.

    If you have a room that has a grounded mesh cage on all 6 sides, then it would be "ok to work on", if you don't, it's a lot bigger issue than you apparently realize.
     
  11. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    I don't see no faraday cage during their demonstrations. They've clearly demonstrated this lighting up a whole TV wirelessly at a TED conference as well as numereous other fairs like CES. I don't understand why everything in the bandwidth of the amplifier would be amplified. Suppose what you said is true, then how in the world do radio towers work? They pump out 100kW easily, how is it they don't amplify everything else in their so called bandwidth?

    Don't get me wrong guys, I'm all for learning and RF is not exactly a place to start but this project requires it so I have to. Anyways, please enlighten me on why all the frequencies will be amplified when I've designed this to just oscillate at high power at a certain specific frequency.
     
  12. Papabravo

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    Feb 24, 2006
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    Making an oscillator at a single frequency is very nearly an impossible task. Any non-linear operation will generate spurious signals, spurs for short, at frequencies above and below the frequency of interest. In well designed equipment filters are used to suppress the spurious radiation. If the output of an amplifier producing significant power is matched to the load then a significant fraction of that power will be radiated into the surroundings. If the surroundings include a grounded shield on six sides then the radiation will be dissipated in the shield. If the transmitter and the load are not matched, then a significant amount of the power delivered to the load will be reflected back to the final stage of the amplifier. If the reflection is in phase with the outgoing signal the output stage could be destroyed with smoke and flame. In either case if you don't know what you are doing you're a danger to yourself and others and I advise you to quit while you're ahead and still in one piece.

    You really really don't want to suffer an RF burn. They're nasty and take a very long time to heal.
     
  13. #12

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    Oh yeah. That part, I know about.
     
  14. thatoneguy

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    Feb 19, 2009
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    Have you ever been in the transmitter room of a tower? Each oscillator for the different frequencies is VERY tightly controlled, stubs and other filters are added to block all frequencies outside of the licensed bandwidth for each transmitter. The temp is controlled, usually by air conditioning, even in wintertime, due to the power of the Final tubes, which light up the room.

    It is difficult to make a signal with very high Q (a few kHz bandwidth), at a specified frequency, first, you need to know that frequency isn't being used for anything else (Time standards used to broadcast on 5Mhz, 10Mhz, 15Mhz and several others so Ham operators had a handy clock source). The radio band is rather full at the lower end (< 1Ghz).

    The FCC needs to provide a provisional license for experiments where power may be radiated. They WILL actually verify your license by walking around with field strength meters.

    YOU would be held responsible if that amount of power into a linear amp had a harmonic that forced a heart attack in a neighbor 3 doors down who had a pacemaker. That's not a joke. Sending broadband at 100+W is often referred to as "Modulating enough power so a light bulb could read the signal".

    If you do not have the equipment to show what your low frequency circuit is oscillating at (if it is a variable R/C Circuit, all bets are off), you shouldn't be working with RF. Please read the AARL Intro books for becoming a HAM operator.

    I am aware you are trying to replicate a test, but remember, the tests you've seen were conducted by engineers with all the test equipment required to ensure negligent signals didn't exist. If you aren't an engineer, or do not have the required test equipment to perform a small scale test, you are a essentially a danger to yourself and others, not to mention a criminal in all Western countries.

    This isn't an insult to you directly. You are asking questions about some very basic concepts, and expecting to replicate results shown in a video or written about in a paper. Those reports leave out 99% of the actual "Work" it takes to set up a high power resonance system.

    I'd suggest taking a look at the the "wireless charging pads" that Energizer and Duracell make. See how they work, they get good efficiency at a distance of 1-2cm. Try replicating that before trying to send a hundred watts over 3 feet.
     
    Last edited: Feb 21, 2013
  15. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
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    Ok thanks for the heads up. Yes, I just realized that when I did my prototype (which just consisted of a signal generator + driving coil and a receiving coil with led attached to it) I was able to light up the LED at many frequencies, not just the resonant frequency. The prototype was designed to resonate at 150 KHz but the LED would light up starting at 100 KHz all the way up to 180 KHz which is quite a wide bandwidth. However, it was brightest at 150 KHz which is what it should do, however, it also *worked* at all the frequencies from 100-180 KHz. I guess the amplifier will amplify the signal the same, and just because the receiver is set to pick it up at 10 MHz doesn't mean something at, say, 5 MHz won't be able to as well.

    Something is inconsistent here and doesn't make sense. Tens of experiments on wireless power transfer were conducted (all over youtube, demonstration at fairs like CES, TED conferences, etc) and they did high power stuff like lighting up a TV and 60W lightbulbs. To be able to light up a TV or a 60W lightbulb you have to pump out at least 300W of power because their supposed efficiency is around -40% at best case scenario. My question is, how do you do this without interfering or causing harm?

    Currently I'm looking at RF exposure limits set by our government (see here: http://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf01904.html#s4)

    When I use the maximum magnetic field strength listed in the table (2.19/f for 1-10 MHz) and rewrite the magnetic field due to a solenoid equation and solve for current using the above maximum field strength limit, the current I'm allowed to circulate in my driving coil is in the order of mA and I just don't see how these guys could've lit up a TV using mA at these frequencies. Can someone shed some light on this please, and if you've already have, then I'm sorry, I'm slow.
     
  16. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    RF power is in Watts. 100W can be 1 volt at 100 amps, or 100V at 1 amp.

    Often, high voltage at low currents for power, the antenna increases "effective radiated power" (ERP)

    The receiving antenna needs to match in plane/orientation and size for optimal reception.

    The receiving side may only get 60W in, but at a low voltage and high current, or vice versa (haven't looked into it much, post links to the technical and tech-ish Youtube if you have them). That's where a switching power supply shines. Watts in = 90% Watts Out (efficiency loss). However, low current high voltage can be transformed to medium voltage, low current. Same for high currrent low voltage to medium voltage medium current. These are also NOT easy to design efficiently, especially when working with a 100Mhz input.

    Listen CAREFULLY at 1:13 in
     
    Last edited: Feb 21, 2013
  17. thatoneguy

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  18. Externet

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    zero_coke... Private message sent.
     
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  19. zero_coke

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    Apr 22, 2009
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    Yes, we are aware of the significant harmonic levels present with using a square wave over a sine wave so we're not using a square wave. However, that doesn't mean the sine wave doesn't have harmonics either. We're using a low pass filter that came with our 140W amplifier to suppress those harmonics depending on the cut off frequency of choice. Thanks for the video, it describes the phenomena nicely.
     
    Last edited: Feb 23, 2013
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