Resonant Antenna Qs

Discussion in 'Wireless & RF Design' started by Art, Jul 20, 2015.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    Hi Guys,
    A difficult couple of questions to explain.
    Say I have a loop antenna that is DC short circuit,
    cut or tuned with a capacitor to resonance at f,
    and in the other hand I have a clear carrier at f.
    f might as well be 10MHz but could be anything.

    Now say I take the carrier at f, and perfectly null every tenth peak, so now measured over one second the new f is 900kHz, but it's got a flat spot after every 9 peaks, and it's peaks
    are still aligned with the original 10MHz carrier.

    Q1: Am I still holding a resonant antenna?

    Q2: if the answer was yes, what if a team of monkeys select another random quantity and location of peaks to perfectly null, and the result f is maybe 812kHz over a one second interval,
    but the peaks left are still aligned with the original 10MHz wave.

    Cheers, Art.
  2. BR-549

    Well-Known Member

    Sep 22, 2013
    Is the loop a DC short circuit, or does is have a capacitor in series?
    A diagram would help.

    You labeled the resonant frequency with small f. Is the other hand have an external field? You labeled that with a small f also.

    Which f are you nullifying?

    How do you null the tenth peak, without nullifying the ninth and eleventh peak?

    May we see the circuit for this?

  3. Wendy


    Mar 24, 2008
    Antennas at resonance are generally just a chunk of wire at a specific length. So is a loop antenna.

    I used to build 2.4GHz downconverters, they used 2 pound tin coffee cans because the physical dimensions happened to be close to resonance. An antenna that is not quite resonant is either capacitive or inductive to that frequency, but that is an artifact of length.

    Antennas can also be antiresonant. As I recall it is very close to a trig tan function.
  4. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    Hi Guys,
    Thanks for the replies.
    I should have said nulling a cycle rather than a peak.
    The situation is hypothetical at the moment so there is no circuit,
    but it's just an antenna.
    A loop antenna could be a Jpole, QFH,
    Or wire loop, or ferrite loopstick.
    They are all DC short circuit.

    One cycle of a carrier could be eliminated at the transmitter's oscillator by having another circuit short it.
    So some cycles would be missing, but there would still be flat space between the adjacent cycles.
    This would be easy to do at lower frequencies.
    By f, I mean frequency.
  5. InspectorGadget

    Active Member

    Nov 5, 2010
    You still have a resonant antenna at the carrier frequency. You're just amplitude-modulating it and creating a beat frequency.
  6. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    This was old and I forgot about it, but the question was more theory, than applicable to any antenna I have.

    As frequency increases, more energy is required, and a circuit usually drops voltage of it’s peaks as f increases.
    The rise and fall times of peaks also change, so even a single pulse of, say a 10MHz wave, has a characteristic of a 10MHz wave,
    even if you could only observe one single pulse.
    So my question is more about.. say you have that one pulse... it should still be considered a single 10MHz pulse?

    and more importantly... a transformer or antenna tuned to 10MHz, should be ranged to pass that particular pulse.
  7. KL7AJ

    Senior Member

    Nov 4, 2008
    Yes....a resonant antenna can have, in theory, an infinite number of harmonic frequencies. However, because an antenna typically has some "fatness" the harmonics may not physically be precise multiples of the frequency, especially at very high orders. For the third or fifth harmonic, a typical thin-wire antenna comes very close to theory
  8. KL7AJ

    Senior Member

    Nov 4, 2008
    I might want to add....and I've written about this numerous times in many amateur radio publications.....there is absolutely nothing sacred about a self resonant antenna as far as its radiation characteristics. Look at all the countless A.M. broadcast towers around the country. Not a ONE of them is self resonant! They simply bolt together the number of 20 (or sometimes 60 foot) sections that come CLOSEST to resonance...and then use matching networks to do the rest. (And even if some odd A.M. station started out with a resonant tower...over the past 7 decades or so, the majority of them have changed frequencies!) Radiation efficiency is determined by radiation RESISTANCE only...and don't ever forget it!

    nsaspook likes this.
  9. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    Interesting.. I’ll try not to forget it :D
    Does the same apply to pulse transformers?
    If I wind a pulse toroid and send a pulse, it will certainly output a decaying ring at a particular frequency.
    Now is that ringing going to be stronger or longer duration if I send a pulse of the same characteristic in the first place?
    ie. the pulse I send has the same rise and fall time as the ringing the toroid outputs.