A surprising phenomenon related to the FM antenna length.

Discussion in 'Wireless & RF Design' started by aromring, Jul 26, 2016.

  1. aromring

    Thread Starter New Member

    Jul 22, 2015
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    I bought the following toy transmitter to play with:
    http://store.qkits.com/electronic-kits/radio-frequency/3v-fm-transmitter-kit.html
    The description and schematic is in the attached k7.pdf file. It came with 160 cm of thin, multi-strand, insulated wire as a monopole antenna. I assembled it as instructed and it was working fine across all the 88-108 MHz range.
    But, me being me, I could not resist tinkering with it. :) The instructions state:
    "Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively."
    Well, I cut the antenna down to 75 cm expecting better gain and ... the oscillator stopped oscillating! The attached WAVE2.png shows the right BC547's emitter voltage. It shows the oscillator tries to oscillate but is quickly damped down. I tuned the trimmer all the way down to lowest capacitances and the oscillator started working at about 135 MHz and higher; see WAVE1.png. And again, when I lower the frequency below 135 MHz, the transmitter stops oscillating.
    I disconnected the oscilloscope probe and monitored the FM signal with my SDR, confirming the same phenomenon.
    I am perplexed. Why is this happening? Should't the quarter-length monopole resonate quite fine? Why this dramatic dampening effect? What is the theoretical explanation for this? I will be grateful for any insight from RF experts. Thank you.

    P.S. I am posting to this forum for its expertise. This is not about FM transmitter design; I was just playing with this simple circuit and noticed this scientific curiosity. Please don't send me advices on how to design advanced FM transmitters; you would be wasting your ink... :)
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    These simple one-transistor oscillators / transmitters are always very unstable.

    Every bit of parasitic capacitance in the board, antenna and the layout in general contributes to the circuit.
    Pruning the antenna changes the overall circuit, there is really no isolation between the circuit and antenna.

    One transistor FM transmitters are fun to play with, but almost useless if one expects it to function as more than just a curiosity.
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    387
    A half wave antenna has a much higher impedance than a quarter wave. The antenna connects directly to the tuned circuit of the transmitter so the low impedance of a quarter wave antenna imposes much greater loading on that tuned circuit reducing its effective Q value. Ultimately it can reduce the loop gain sufficiently to prevent oscillation.
     
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  4. aromring

    Thread Starter New Member

    Jul 22, 2015
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    Thank you very much. How to calculate an impedance of a monopole antenna? What would be an equivalent circuit? Is there a way to counter the lower antenna impedance on the circuit side? Higher battery voltage? Should I replace the 22 pF isolation capacitor with a smaller one? Later, I rolled the whole 75 cm antenna around a pencil, effectively creating an air coil, which allowed me to lower the boundary frequency to about 115 MHz, but I still cannot transmit in the 88-108 range.
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  6. Willen

    Member

    Nov 13, 2015
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    Simply cutting down the antenna even more like 30cm would decrease overloading problem. To decrease frequency, you can adjust three things:
    -compress your coil (L)
    -Increase the pf value of the trimmed capacitor to around 45pf
    -If still one working, decrease the battery voltage little bit
    -Adding some 5 to 10pf capacitor in parallel with trimmed capacitor or inductor would decrease the frequency even more.

    That is pretty normal if there's an antenna connected to an VHF oscillator. If you want to get little bit stable output, you could pick the output from 2nd round of the coil from top (Taping). Or from the emitter of the transistor OSC.

    A monopole antenna (1/4 wave length) has typically 20 to 25 ohms of impedance. It means you are adding a 20 ohms resistor from collector's capacitor to ground, which is almost stopping oscillator, in your case at least. You could add another transistor as output boofer for BETTER stability and greater range too.

    It's little strange that you have better oscilloscope and seems better understanding of electronics too, in some case (you do not looking like just a hobbyist). But why you are spending $12 for such thing? Feeling lazy to make one yourself? You could make it easily since you have oscilloscope. :)
     
    Last edited: Jul 26, 2016
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  7. aromring

    Thread Starter New Member

    Jul 22, 2015
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    Willen, thank you very much for excellent suggestions and explanations!
    But I did make my own TX! See the attached pic: it's alongside the QK7 unit.
    If you are interested in the full story, I've made my own transmitter first, then I bought QK7 for comparison. Mine works OK with 8-inch straight, but its range is only ~50 feet. I tried to increase the range by affixing a lambda/4 monopole, but it overloaded the oscillator. Hence, I tried the same thing with QK7 and got the same overloading problem.
    Now, I omitted my TX from the original posting because it would introduce too many variables which would invite irrelevant responses like "Your circuit is wrong", etc. Thus I limited my posting only to the widely known and standard QK7 design. Makes sense?
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    A 1/4 wave antenna has a low feedpoint impedance, while a 1/2 wave antenna has a very high feedpoint impedance (if end fed). So the 1/4 antenna is loading down the oscillator a lot more.
     
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  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I beg to differ. Antenna design is one of the most disciplined and predictable aspects of all electronics. It only SEEMS like black magic because so few want to take the time to learn the stuff.

    Eric
     
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  10. aromring

    Thread Starter New Member

    Jul 22, 2015
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    Excellent answer. How does one calculate an impedance of a monopole antenna? What would be the equivalent circuit?
     
  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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  12. BobaMosfet

    Active Member

    Jul 1, 2009
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    I see 75cm above, but shouldn't that be 85cm?

    Quarter wavelength is 85cm. Half is 170cm.
     
  13. aromring

    Thread Starter New Member

    Jul 22, 2015
    20
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    0.25 * (3.0E+8 m/s) / (1.0E+8 Hz) = 0.75 m = 75 cm
     
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