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To be an efficient radiator, it must be resonant at operating frequency?

Discussion in 'General Technical Questions and Answers' started by KB7UXE, Aug 6, 2009.

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  1. KB7UXE

    KB7UXE Ham Member QRZ Page

    Here is a question from QRZ ham radio trivia:

    If an antenna is to be an efficient radiator, it must be resonant at the operating frequency

    True
    False

    Answer 2, "False" is Correct! As long as you can obtain a proper match to the antenna through a matching network power will flow and the antenna will radiate efficiently no matter how far off resonance it's physical length may be.


    Can someone help me understand this question....
    I understand that if the radio "sees" a good match it will have full output,
    wether it's an antenna or a wet fish on the kitchin counter.
    but how in the world does a matching network cause the antenna to be efficient radiator ?

    It's my understanding a matching network changes what the radio sees looking out.
    So I believe this question to be incorrect, in that a non-resonant antenna will not resonate
    as well as a resonant antenna.
    therefore I beleve it to be true that "If an antenna is to be a more efficient radiator,
    it must be resonant at the operating frequency"

    So a matching network will cause the radio have a full output, but still into a poor match,
    and non-efficient radiator.

    let me try this again:
    2 antennas, no tuner. ant A is cut for 3.5mhz, ant B is cut for 4mhz.
    If I operate at 3.5mhz, ant A will radiate more efficiently than ant B.

    I see this as: To be an efficient radiator, ant must be resonant at operating frequency.

    I know that my radio will "fold back" power with poor swr.
    A tuner will fool the radio to make it think it sees a good match, and thus have full power into a poor match, and poor radiator.



    What do you think ? help me understand...

    Dan kb7uxe.
     
  2. KB9BVN

    KB9BVN Ham Member QRZ Page

    Dan...to answer your question, I think we must first define the word "efficient".

    Perhaps the question ought to be: "Will it radiate?"
     
  3. AI3V

    AI3V Ham Member QRZ Page

    [Tina Turner]"What's resonance got to do with it?[Tina Turner]:D

    To be an efficient radiator the ratio of radiation resistance to the antennas copper loss and ground loss has to be high.

    Resonance has nothing to do with it.

    A dipole is efficient because the radiation resistance is much higher than it's copper loss.

    A vertical may have a very poor ground (High resistance), causing poor efficiency.

    A very small loop, say a 1' diameter 22swg on 160m, will have a very low radiation resistance compared to the copper loss. Again causing poor efficiency.

    Resonance, and antenna efficiency are apples and oranges.

    Rege
     
  4. KL7AJ

    KL7AJ XML Subscriber QRZ Page


    No
    No
    No

    As we have elaborated on here in the past, there isn't an A.M. broadcast tower in 1000 that's self resonant! It's the CURRENT times the Radiation Resistance that determines the radiated power. No power is lost in a reactance.

    Eric
     
  5. KB7UXE

    KB7UXE Ham Member QRZ Page

    I think the questions says ""if the ant is resonant"""
    then it says the tuner makes the ant resonant...
    so I guess if you brake down the question, or simplify it,
    ""is a non-resonant ant as efficient as a resonant antenna? ""
    A tuner does nothing to change the antenna,
    it only changes what the radio sees..

    It just seems like miss-information being passed to learning hams.
     
  6. K0CMH

    K0CMH Ham Member QRZ Page

    I think they confused the anwer by putting the tuner statement in the answer. As stated here already, the definition used by Hams for antenna efficiency really has nothing to do with resonance.

    An antenna is "efficient" due to "the ratio of radiation resistance to the antennas copper loss and ground loss has to be high." [from previous reply] Such an antenna may or may not have an input impedance near the radio's output impedance. As the poster indicated, the tuner will provide an improved power flow to the radiator by establishing a favorable impedance for the radio's output to work with.

    If this is a question/answer in the pool, I am suprised it made it "as is". I would say it is at best confusing to someone who actually studied antenna theory. If it is an answer found in one of the study books, shame on them.
     
    Last edited: Aug 6, 2009
  7. K6ABZ

    K6ABZ Ham Member QRZ Page

    I've found quite a few badly worded questions in the pool. Sometimes, I think that whoever wrote the questions is more intent on tricking the testees than in actually assessing their knowledge.
     
  8. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Tricking the testes indeed!


    Actually antenna efficiency is the radiation resistance over the loss resistances from WHATEVER source.....wire ohmic losses, ground losses, dielectric or otherwise.

    Although a self resonant antenna is easier to ANALYZE, it's generally just as effective to resonate it with a lumped network than to trim it to resonance. It's the SYSTEM resonance that matters, as so clearly described and re-described, and re-re-described in "My Feedline Tunes My Antenna" by Byron goodman, W1DX. A classic article that is the primary authority on the matter.

    eric
     
  9. K4SAV

    K4SAV Ham Member QRZ Page

    The answer to the question as stated is false, however the explanation of the answer, as given, leaves room for confusion. As usual with antennas you need careful wording to avoid confusion, especially when trying to explain this to anyone with limited antenna experience. The answer didn't say where the matching network was located and it didn't state that a low loss matching network was used. If the matching network is located at the shack end of a long feedline, then it is unclear if the antenna will radiate well or not because of feedline loss. Same situation with a lossy matching network, a lossy network may not leave much power to radiate. Of course the original question did not address other parts of the antenna system, so it can be assumed that those parts can be anything that is required. It is the explanation that needs some work.

    To help Dan understand this, consider the case of a matching network that has zero loss and either no feedline or a feedline with zero loss, and no resistive losses in the antenna. (Although this is physically impossible, it is possible to have insignificantly small losses.) Then once you make the transmitter happy with a good match for what it is expecting to see, all the power delivered by the transmitter has to go into the antenna since there is no other place for it to go. That power will all be radiated since there is no other place for it to be dissipated.

    One other point: Resonance says that the antenna impedance has zero reactance and looks totally resistive. The value of that resistive part may be almost anything depending on the antenna. That may not provide a match for a transmitter or a feedline, so in many cases you may need a matching network even though the antenna is resonant (where is not a good match for a 50 ohm transmitter).

    Jerry, K4SAV
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    My 40m dipole, cut for 7.000 MHz, is perfectly resonant on 20m, at 14.000 MHz.

    I fed it with 100' of coax and absolutely nobody can hear me, no matter what I do. But my antenna analyzer, right at the antenna feedpoint, indicates "X=0.0."

    Isn't that perfectly resonant?

    Signed,

    No signal in Missouri
    Or anywhere else.:(
     
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