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. K9ZMD

    K9ZMD Premium Subscriber QRZ Page

    There is a real ambiguity in the question wording that needs to be cleared away. Until then, both answers are correct, depending on how you look at the question.

    Reader A naively takes the word "antenna" to be just a simple length of wire, and then locks onto the word "resonance". He then perceives the question to mean the following:
    If a simple wire is to be an efficient radiator, it must be cut to a resonant length for the operating frequency.
    Given the reader's misconception, that statement is TRUE. It will remain TRUE to the reader until he recognizes that an antenna is more than just a simple length of wire, and also that a non-resonant length of wire can radiate efficiently when a matching device is part of the antenna system.

    Reader B recognizes that any practical "antenna" is really an antenna system, so he believes the question is asking the following:
    If an antenna system is to be an efficient radiator, it must be resonant at the operating frequency.
    Given this reader's mindset, the answer is TRUE. As many of the responses have already pointed out, a non-resonant length of wire can radiate efficiently if the system is tuned to resonance.

    Reader C fully appreciates that any practical antenna is really an antenna system, and also knows that any length of wire can radiate efficiently when the system is tuned to resonance. His mind then intuitively concludes that the original author could only have meant to ask the following:
    If an antenna (system) is to be an efficient radiator, it must be cut to resonance at the operating frequency.
    That reader will know, with dead certainty, that the correct answer to the question is FALSE, and that is borne out by the original author's explanation.

    Could the guy who wrote the question have done a better job? Sure! He was a "Reader C"-type who knew just what he meant to ask; he just didn't ask it that way. Did the guy deliberately play with words to trick the readers? Your guess is as good as mine, but I see no evidence of it.

    So, what do you think? Based on his comments in this thread, does Dan (the OP) fit into one of the three "Reader" categories?
     
  2. N4CD

    N4CD Ham Member QRZ Page

    Resonant Antennas

    Wow...what a bunch of wrong answers.

    All that is necessary to have an efficient antenna is one you can deliver the power to, and one that radiates the power in the direction(s) that are useful.

    Likely nearly everyone hear has seen or used a 5/8th wave vertical on 2M or 440FM. It is 'not resonant'. If you try to connect your 50 ohm coax to the base, you will find a 'bad match'. If you match the impedance at the base, the antenna will be an 'efficient' radiator. (if mounted on a ground plane of large enough area). So, right off the bat, we have demonstrated the antenna doesn't have to be 'resonant'.

    You can take a loop antenna on 160M. It is resonant there. You can use it on 80M and other bands. The pattern will shift and the radiation angle will shift depending upon frequency. It will not be 50 ohms or 100 ohms at most frequencies. Is it 'an efficient' radiator? Likely - as long as you don't have nulls in the direction you want to talk. Could you tune it to a non-mulitple of 160M. Probably - and it will work.

    All that is required is having the 'radiating element' radiate power in the direction you want.

    There are tens of thousands of hams using short verticals with matching networks at the base. They work. Are they 'resonant' antennas. No way. Can you get them to be 'efficient radiators'. On many frequencies, yes - if you have a good matching system. a 3/8ths or 5/8ths vertical is 'not resonant' but will work.

    On the other hand, a full wavelength vertical will probably be crappy. Even if you can match it, the radiation pattern will be all wrong and you won't have an 'efficient' antenna to talk to anyone.

    So, the question is correct and the answer correct. Resonances is not a prerequisite of an 'efficient antenna'. Getting power to the radiating element(s) - including the ground system - and having them radiate in the desired directions - pattern - are the most important factors.

    Heck, a G5RV isn't resonant anywhere and people swear by them. Most of the time, the radiation pattern is a real mess, but folks make contacts. The power goes somewhere.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::What you might be missing is that it's also resonant on all harmonics.


    ::But that direction needs to change, depending who you want to contact. Ditto the radiation angle.

    ::To make it work, you tune it to resonance with an antenna tuner. That's what they're for.

    ::Guess you don't know much about a G5RV: It's resonant as a 3/2-WL doublet on 14 MHz.

    WB2WIK/6
     
  4. KB7UXE

    KB7UXE Ham Member QRZ Page

    well, it is like all kinds of good information about coax, tuners antenna systems etc. All good information. but again, kinda bouncing off in all directions kinda like getting the first two words out then having someone run off with a close but unrelated answer. maybe the question was just too simple.(?)

    Let me see if I have this correct, the following I believe to be true and If I've compiled my information correctly:
    "effective radiation" is the ability to "conduct rf into the ether".
    So even if the swr is greater than 1:1, it can still be an effective radiator.
    so it can also be said:
    Just because it has a an swr of 1:1 does not mean it can effectively radiate the signal out. ( like a dummy load ) ...

    ok, i got this much.. now the hard part:
    how did a tuner make my antenna radiate better?
    isn't the ability of a particular antenna to radiate rf kind of a fixed unit, influenced by construction and design?

    I believe a tuner makes my radio happy, it sees a 1:1 swr.
    But what did it change on my antenna ?



    Thank you everyone for all the great information.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::What it changes, and the part you're missing, is it changes the antenna system. Unless your transmitter is located at your antenna terminals, your antenna is not your antenna system. The system includes the antenna and its transmission line. The tuner changes that a lot, with its ability to turn a non-resonant system into a resonant one.

    I guess you're just not getting it.

    WB2WIK/6
     
  6. KB7UXE

    KB7UXE Ham Member QRZ Page

    Dan fits into Reader catagory D, on an endless search for understanding of all things electrical and mechanical, the never ending quest.
    Thats what gave us the SR71 and the nuclear bomb.
    check out the photo :
    http://en.wikipedia.org/wiki/Alfred_J._Gross
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    I explained that in an earlier posting. The tuner achieves a near-conjugate match which tends to cancel the reactance throughout the system including at the antenna feedpoint. The tuner actually changes the impedance at the antenna feedpoint looking back down the transmission line toward the tuner. If you don't believe it, take your MFJ-259B and prove it for yourself.

    The antenna tuner achieves the conditions necessary for maximum available power transfer from the source to the antenna. The antenna radiates best when maximum available power is delivered to the antenna.
     
  8. KB7UXE

    KB7UXE Ham Member QRZ Page

    question did not specify antenna system.
    it directly only stated ANTENNA.
    I do agree, yes,, a tuner is a valid and effective part of an antenna system.
    and it did not state transmission line or tuner. The question was about the antenna. THE ANTENNA...
    but thast ok, I think I figured it out. the antenna swr does not have to be 1:1 to radiate well. and just because the swr IS 1:1 doesn't mean it will radiate well. However this can be augmented and compensated by adding grounding systems, tuners and other stuff.

    Thanks Dan.
     
  9. KB7UXE

    KB7UXE Ham Member QRZ Page

    I see, a tuner didn't make my antenna radiate better, it just got more power to the antenna to radiate.

    but then I already knew that....
     
  10. KM3F

    KM3F Ham Member QRZ Page

    The example of a 5/8 wave not being resonant forgets that if a base loading coil is used, that (coil) is resonated with the radiator element to the operating frequency "first", then the feed point tap is made on the same loading coil to match the feedline.
    So it is a resonant system, and there is impedence matching going on as well.
    Not much different than using a regular antenna tuner except the tuner and feedline in combination do the rest of the matching.
    Also this is supported by the fact the feedline length becomes sensitive to length proving it is part of the tuned system by being length sensitive.
    The antenna tuner compensates for the length, otherwise it's length would have to be trimmed to do what the antenna tuner can do by adjustment.
     
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