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Getting antenna to radiate

Discussion in 'General Electronics Discussion' started by GhostLoveScore, Jan 23, 2017.

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  1. GhostLoveScore

    GhostLoveScore

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    Nov 27, 2016
    Hi everyone

    I have a test circuit, oscillator and emitter follower.
    [​IMG]

    This is related to this thread
    https://www.electronicspoint.com/threads/help-me-calculate-output-impedance.282132/

    The problem is this. Antenna is simple ground plane antenna cut to a quarter wavelength. If I solder it to emitter of emitter follower the signal is OK. But if I solder a piece of coax, RG58 and try to get antenna to radiate that way, it doesn't work. The signal is very very weak.

    What I think is happening here: I didn't match impedance of emitter follower (output impedance) and antenna. When I connect a coax that has 50 ohms impedance there is large mismatch and no power can get through the coax to the antenna.

    [​IMG]
    It's visible here that SWR of 6:1 causes 50% power loss. In my case it's very easy to have even bigger impedance mismatch.

    Do you agree this is the case?

    The problem is that right now I am making RF circuits without having an oscilloscope. If I had oscilloscope there is a way to measure output impedance of the amp. LC matching network, is useless if I don't know impedance of both amplifier and the antenna. What do you suggest I should do?
     
    Last edited: Jan 23, 2017
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I presume you connect the braid to the same location you connect the ground plane of your antenna.
     
  3. GhostLoveScore

    GhostLoveScore

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    5
    Nov 27, 2016
    On the circuit, braid of the coax is connected to ground (power supply's negative terminal). On the antenna side, braid is connected to ground plane.

    One option that I am considering is adding common emitter amplifier with 50 ohms (47 ohms) resistor at collector. Would that work?
     
    Last edited: Jan 23, 2017
  4. davenn

    davenn Moderator

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    redesign the output stage ... it doesnt contain any tuned circuits etc .... there is nothing there that even hints that it should be generating RF

    what frequency do you think your oscillator stage is generating ?
    have you confirmed that with a frequency counter ?

    it's going to be VERY difficult for you to work on RF circuits without the proper test gear

    have you studied any other RF oscillator/transmitter circuits to get an idea of what
    path you should be following ?
     
  5. GhostLoveScore

    GhostLoveScore

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    Nov 27, 2016
    Davenn, the circuit is working on 145MHz, I confirmed it with counter and RTL SDR. It produces carrier wave.

    I read a lot regarding how radio works, oscillators, mixers, filters, amplifiers... but output stage amplifiers are still a mystery to me. How to get actual EM wave out from the antenna - I still don't have a clear picture on how it's done.

    Other circuits that I studied are simple FM bugs that don't have any output matching circuits. Like my own circuit they usually have antenna soldered to output amplifier's collector.

    So what you are saying is that I need tuned amplifier for the output instead of emitter follower that I have now?

    I understand that I need proper gear and I'm planing on buying oscilloscope in the next month or two. But until then I just want to make this work, it doesn't matter how much power the circuit will radiate, I would just like to see that it does.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    One thing you have here is a class A amplifier. Most RF amplifiers are class C.

    Taking the output from the emitter will limit the voltage swing from your amplifier. If you have a tuned circuit as yout collector load the output voltage swing can exceed the power supply voltage.

    145MHz is right in the middle of the 2m amateur radio band. You'll upset people (and possibly gain unwanted attention from the authorities) if you transmit here.

    The advantage is that it is right in the middle of the 2m amateur radio band. If you take the time to get a license, and study the plethora of information available for this band, you'll be well equipped to do this better and legally. You'll probably also find that you need to have your oscillator pretty darn stable and you may want to use a crystal to achieve this.

    How are you planning to modulate this carrier?
     
    davenn likes this.
  7. GhostLoveScore

    GhostLoveScore

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    Nov 27, 2016
    I do have ham license. Unfortunately that means nothing when it comes down to designing and building circuits. You only learn regulations.

    I am not thinking about modulating for now. Short emissions of unmodulated carrier are allowed by law here in Croatia. This is small amount of power. Signals are not going anywhere. And not to mention that 2m band is basically empty 95% of the time.
    I am very careful not to interfere with somebody, but if my hand held 2m transceiver barely receives it at 10-20m, nobody else will receive it.
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello
    Please draw a picture or take a photo of your antenna connected using the coax cable to your PCB.
    Thanks
    Adam
     
  9. GhostLoveScore

    GhostLoveScore

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    Nov 27, 2016
    This is roughly how I soldered it. Sorry for the bad quality.
    schematics.png
     
  10. davenn

    davenn Moderator

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    OK .... I repeat ... without the gear you are NOT going to be able to tell where your Osc is operating, if at all

    Yes....
    and also ... FM bug ccts on the www are generally of a dreadful design, not wise to follow most of them

    BUY a copy of the ARRL handbook ... has lots of circuit info and examples

    NOTE strongly Steve's opening comment.....

    for FM yes ! :) they only go to class AB for linear operation when using other modes like SSB


    the problem with that is ... what you are just hearing on the 2m receiver may just be a harmonic
    The strongest signal way be in a totally different band and upsetting some one

    The big problem is that you have no frequency control and any hand or other object movement anywhere near your oscillator will have the frequency wavering all over the place

    get you oscillator redesigned and working correctly is your first job

    again ... a freq counter is ABSOLUTELY essential
     
    Last edited: Jan 23, 2017
  11. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Thank you for your drawing, it is not clear how you have connected your antenna at the far end. What are the thin blue lines? For maximum power transfer you will need to match the feed line to your circuit, you may also need to match the antenna, which will depend on the size of your other plane area and the power you are transmitting.

    Thanks
    Adam
     
  12. GhostLoveScore

    GhostLoveScore

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    Nov 27, 2016
    Adam - sorry, I thought it would be clear. It's this type of ground plane antenna

    [​IMG]

    Thin blue lines are radials

    Davenn - I mentioned that I have RTL SDR stick. I can clearly see the frequency at which it works and harmonics of my oscillator. And I do have frequency counter. I'll check out ARRL handbook.
     
  13. davenn

    davenn Moderator

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    here's just one example circuit for you
    there are others on the net

    [​IMG]
     
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  14. davenn

    davenn Moderator

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    OK a good start :)

    get your Osc sorted out first and go from there
     
  15. Arouse1973

    Arouse1973 Adam

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    Ah ok I did wonder if that's what it was, how is it connected to the vertical antenna?
    Adam
     
  16. GhostLoveScore

    GhostLoveScore

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    Nov 27, 2016
    Center conductor on vertical part. There is small copper board and a hole in it. Center conductor goes through. Braid is soldered to copper and radials are soldered to it.

    Davenn, thanks for the schematic. I'll try it. Meanwhile, instead of using emitter follower, I made common emitter amplifier and tapped antenna directly from collector. And it works now more or less. I can use coax. Without antenna on its end, there is 20-25 dB smaller signal than with the antenna. I guess that means antenna works. And that's good enough for me. I'll hold on with the experiments until I get oscilloscope.
     
  17. davenn

    davenn Moderator

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    just make sure that the vertical isn't shorted out to the radials on your copper board

    No Probs ..... just start with the oscillator section up to as far as the filter, L2 and the two trimmer caps below it and get that working
    don't connect the final transistor .... don't even worry about that for a start

    ALSO VERY IMPORTANT
    and I cannot stress this enough ..... DONT build this using one of those push in white breadboard units .... these components MUST be soldered and have the shortest lead lengths as practical


    D
     
    Arouse1973 likes this.
  18. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ok so it looks like it is as mentioned by others a matching issue, I just wanted to check nothing else you had done was causing an issue. Do you have any means of measuring the return loss?
    Thanks
    Adam
     
  19. GhostLoveScore

    GhostLoveScore

    71
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    Nov 27, 2016
    :) That would be horrible circuit if built on that breadboard. I solder everything on a big copper PCB ugly style. Good side is that I can get very short wires.

    Adam - I have SWR meter for my ham radio, but it only measures from 1W up. I am nowhere near 1W. It's matching issue. I said that instead of emitter follower I built common emitter amplifier. With 1.2k in collector, output impedance should be 1.2K. So I tried building Pi matching network from 1.2k to 50 ohms using this site as guide http://electronicdesign.com/communications/back-basics-impedance-matching-part-3. But it doesn't work near as good as soldering directly to collector. At least now I get the same signal strength using coax and ground plane antenna as when soldered directly to the collector. Thanks guys for help. When I buy oscilloscope I'll try to improve this circuit and build Davenn's circuit.
     
  20. davenn

    davenn Moderator

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    you would be surprised to se how often people on this forum and on other actually do that with RF oscillators and then wonder why it doesn't work
     
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