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. EI6GXB

    EI6GXB Ham Member QRZ Page

    I reckon :

    Power transfer loss = 20log (Power at feedpoint/Power at TxRx).
    The less that is, the more efficient in POWER TRANSFER. The transfer loss is minimised by matching the antenna SYSTEM AS A WHOLE (including feedline) to the transmitter. All the available power makes it to the radiating element if the feedline is of negligible loss. If it ISN'T (ie. coax), then a horrid SWR at the antenna, no matter how 'perfectly tuned' at the TxRx end will be far less than efficient, as multiple standing waves and losses will be incurred down the feedline.

    Radiation efficiency = 20log (Power within 1m of radiating element/power at TxRx).
    Use a field-strength-meter. You'd have to be extremely close to nullify the inverse-square law of electromagnetic wave propagation.

    The latter is the one I'd personally go for...say field measurement right in the middle of a loop antenna. If my loop is resonant and thus efficient (because of dimensions/harmonics/sympathetic resonances, etc.) and attached to my TxRx through a feeline of negligible loss (<1dB) then I should be seeing pretty damned close to what is being transmitted at the TxRx end!

    Last edited: Aug 11, 2009
  2. WA4OTD

    WA4OTD Ham Member QRZ Page

    Thanks Steve, core and valuable concept. Something to put in the permanent grey matter.

  3. K0CMH

    K0CMH Ham Member QRZ Page

    Calling it "another dumb antenna question" seems pretty dumb.

    there is so much mis-information out there that new Hams get fed, it is good that a thread like this happens every so often.

    There has been a lot of very good exchange here. Some of the "wives tales" have been dispelled. Correct answers have been given on levels for both the very experienced/educated and for those not so well versed in the theories and mathmatics.

    With the lack of good elmers these days, where else is someone going to get some straightforward answers at a level they can understand?

    About the only thing I could suggest is that the elmer page may also be a great place for this thread.
  4. KB7UXE

    KB7UXE Ham Member QRZ Page

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


    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.

    No mention of coax, no mention of transmit power or anything else.
    Does the ant need to be resonant to be an efficiant radiator?
    nothing about antenna system, nothing about coax, the question involves two factors only: 1:frequency 2: RF radiation..
    No mention of antenna gain, coax loss, solar conductivity of nuclear flux through out the universe etc.
    The answer threw me, seemed like a trick question,
    as with the old question:
    If you were in the desert, where woud you sleep? under the tree? on the camel, or near the pond?
    answer: I'd turn the corner and sleep in the hotel...

    But I did learn something..
    you will get totaly unrelated answeres for even the simplest questions.

    I love ham radio.....
  5. W5DXP

    W5DXP Ham Member QRZ Page

    It's easy to see why.

    A 130 foot 50 ohm dipole used on 80m and fed with open-wire feedline will have an SWR of 600/50 = 12:1 and radiate efficiently.

    A 130 foot 7200 ohm dipole used on 40m and fed with open-wire feedline will have an SWR of 7200/600 = 12:1 and radiate efficiently.

    Same SWR whether feeding a 50 ohm antenna or a 7200 ohm antenna. That's the beauty of open-wire feedline.
  6. W2VW

    W2VW XML Subscriber QRZ Page

    As well as posters taking anything said to extremes to prove some point.

    Just use the noise blanker here.
  7. K0CMH

    K0CMH Ham Member QRZ Page

    I think you actually got a lot of good and useful information. The problem with the final product of radiation is that there are a lot of different variables involved.

    So lets look at your original questions individually.

    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.

    Yes, this is true, but is not the complete story. A "tuner" will attempt to perform the function of making the entire system (radio output, transmission line, antenna, and any other items such as baluns) allow as much energy as possible to flow from the output of the radio into the antenna. All the parts of this "system" make up a circuit. There are a number of points where impedances can change. When the changes can be electronically adjusted to minimize loss, then the most power can make it to the antenna radiators.

    but how in the world does a matching network cause the antenna to be efficient radiator ?

    The trick here is that just about any antenna CAN be "an efficient radiator", provided sufficient power enters the radiating parts. As described above, that is what the tuner tries to do. The use of the term "efficient radiator" is something of a trick part of the question. I believe the author of the question was trying to determine if the person understands the difference between "resonance" of the antenna itself and power transfer to the antenna.

    It's my understanding a matching network changes what the radio sees looking out.

    Yes, this is a function of a tuner, but only one of the functions. This allows the power from the output stage of the radio to at least make it into the circuitry of the tuner. Now the tuner has to do another function, that is to pass that power on to the feed line and into the antenna.

    So I believe this question to be incorrect, in that a non-resonant antenna will not resonate
    as well as a resonant antenna.

    Well, I take this to be unfamiliarity with the strict definition of "resonace". However, yes, of course a non-resonant antenna will not have the attributes of a resonant antenna. The verb, resonate, is not really used in this application. This imples phasing, which is another whole subject that adds to the ability of power transfer between parts of a circuit or between individual circuits.

    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"

    As stated, this is not true. Again, we have to look at the intent of the author of the question. Was the author trying to see if the person understands the difference between resonance and the ability to transfer power, and how the transfer of power may be enabled.

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

    Not accurate as stated. The tuner (or matching network) will have performed it job correctly when it establishes sufficiently close "matches" so that a resonable amount of the available power will flow through the entire system. When this is done, then all the "poor matches" in the sytem have been modified and power flows. Remember, the tuner is effecting the "matching" throughout the system. In the simplest form, the "system" consists of the output of the radio, the tuner, the transmission lines and the antenna.

    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.

    In one of the latest posts, a person describes how the feed line can perform the function of a tuner. In other words, the feed line became the part of the system that allowed the power to flow from the radio into the antenna. If the same amount of power arrives at both antennas, then we most likely would see them both having about the same "radiating efficiency". Again, it is not so much the antenna, but how much power is getting into the antenna. Your example is a good one for when a tuner may not be capable of performing its function. To get all the parts "matched", it may be either economically silly to try to make one with big enough parts to handle the differnce impedances in the system, or the tuner may simply eat up to much of the power in making the match (which was also previously posted here).

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

    I hope by now it is clear that this is incorrect.

    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.

    This term about "fooling the radio" is part of the wives tales and is really inaccurate. The tuner fools nothing. The tuner provides an impedance match so that the maximum power can flow out of the radio, given the specifics of the system's components. "Given the system components" means the various impedance differencence encountered between the output of the radio and the radiating elements. Again, if the tuner is capable of handling the impedance difference, there will be no more "poor matches" and as stated a few times before, the antenna is not necessarily a poor radiator when power is put into it.

    I have tried to put this into "non-mathmatical, non-theory" terms. Many of the posters here have made correct statements, but many of them apply to just one or two parts of the entire picture. When we put all these posts together, we get a really good understanding of the ins and outs of the question.

    I believe the question was really trying to see if the person taking the test is operating from the mis-information and wives tales floating around about antennas and resonance, or if they understand the concepts and functions of resonance, raidating efficiency and power transfer ("matching"). The question was composed with the commonly mis-defined and misused terms about antennas. The bottom line is that not one of these terms determines the strength of the signal leaving an antenna. They all must work together, and be adjusted together, so that power does arrive at the radiating elements.

    Hope this helps.
  8. K0CMH

    K0CMH Ham Member QRZ Page

    OK, that last one was pretty long, but I forgot to say that in most cases, a tuner gets "the best compromise". Usually it does not allow all of the power to flow into the antenna, but maximizes the amount of power that does.

    As an example, without a tuner in the system, an antenna at a specific frequency may only receive 10 percent of a 100 watt output. The tuner may increase this to only 70 percent, but that is such a great increase, it appears to the operator to be working very well. Each doubling of power is the same as 3 decibles. So from 10 to 20 watts is 3, from 20 to 40 is another 3 and from 40 to 70 watts is somewhere around 1.8 decibles or so. Total increase with the tuner would be approximately 7.8 decibles. That is a lot of increase.
  9. KB7UXE

    KB7UXE Ham Member QRZ Page

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


    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.
  10. VK7DR

    VK7DR Ham Member QRZ Page

    Simply a matter of which way you look at it.
    Is the matching network to be considered as part of the antenna or not?
    If not, then you are using a network to create and efficient transfer between two differing load values.
    If it is, then you are matching the whole antenna system to a specific impedance for the efficient transfer of power.

    Either way, taken as a whole the end result is a resonant system at the operating frequency.

    My point that the original question was "dumb' is because an "antenna" is not an element disconnected from a source. To be an antenna it must radiate. The whole question of antenna efficiency relates directly to operating frequency. Without such connection the point is of no consequence. IE, a three foot dipole is 98% efficient at 160mhz but only 1.85% efficient at 14Mhz. Reason - not resonant at that frequency. And even if correctly "matched" (resonated as part of an "antenna system") the efficiency remains 1.85%. That is, it would take over 13 times more power input to radiate the same amount of power as a 20metre dipole.

    So the question should really have been "the radiating element of an antenna needs to be of resonant length at the operating frequency to be an efficient radiator."

    Answer: No - but it had better not be too far short.
    Last edited: Aug 12, 2009
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