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. G3TXQ

    G3TXQ Ham Member QRZ Page

    This is a common misconception. If we ignore the problem of how to get power into the antenna, a small antenna is very nearly as effective at radiating a signal as a "full-size" one.

    For example, on 20m the proportion of the applied power radiated by a 3ft dipole constructed of #14 wire is only 0.6dB down on the proportion radiated by a half-wave dipole. The small antenna radiates almost as effectively as the large antenna.

    The vast majority of the losses come in matching and feeding the low radiation resistance and high reactance; 0.4-j5000 in this example.

    Edit: Cecil, we "overlapped"!

    Steve G3TXQ
     
  2. N4CD

    N4CD Ham Member QRZ Page

    still not resonant.

     
  3. WA4OTD

    WA4OTD Ham Member QRZ Page

    To Steve and Cecil;

    Interesting, but with the transmatch we have matched for maximium power.

    Let me try this example as it would be easy to do:
    If I am on 10 meter with a perfect dipole I load up to 100W and can talk to a station so many miles away at some signal strength. If I put a 3 foot antenna next to this dipole and use transmatch to match my radio to the 3 foot antenna this will radiate as well as a dipole? I would have guessed not, but would like to understand! This would be easy to do.

    I haven't looked at the link yet, I will.

    Thanks
    Leroy
    WA4OTD
     
  4. N4CD

    N4CD Ham Member QRZ Page

    "Now, here's the counter question: If you match your antenna system using an antenna tuner, will that make it receive better as well?
    2 points for the correct answer."

    Yes...but it might really not matter. What matters is on reception is usually signal to noise. if you have a noisy vertical, if you have 3dB loss because of impedance mismatch, it won't make a difference if you have so much noise that the signal to noise is not affected.

    If you have a weak signal condition, with near zero noise, then obviously a 3dB or 20dB mismatch loss will be a major factor. For HF, most of the time you won't notice 3dB difference or 10 dB difference on 40 or 80 or 160. If you have a quiet location, then you will see a loss of weak signal performance.

    same as having more loss in the coax.

    That is why folks use Beverage antennas on receive on 160 - for better S/N perforamnce.....and those beverages don't radiate worth a dang...you use a vertical to radiate....efficiently......and an antenna optimized for s/n for reception -
     
  5. N4CD

    N4CD Ham Member QRZ Page

    "That's the ARRL Antenna Book definition of antenna efficiency: "the fraction of applied power that is actually radiated"."

    It's more than that....

    You want the power radiated in the directions you want.

    It does no good to radiate full power directly down, or at a 45 deg up angle if your goal is to work DX, even if you have a 100% efficient antenna.

    You want two things - power delivered to the radiator(s) and with power being radiated primarily in the direction(s) wanted - and that includes radiation angle obviously.
     
  6. N4CD

    N4CD Ham Member QRZ Page

    "ny antenna will be efficient when it presents a 50 ohm load to the transmitter, regardless of the actual performance. An antenna is efficient (radiates all its energy) when the impedance is exactly that of the transmitter. This usually happens at the resonant frequency, but you can make the antenna more efficient at any frequency by using a matching network."

    Get yourself a tube transmitter with a good wide range Pi-network and you'll find your statement false. It matters not if your antenna impedance is 50 ohms or 600 ohms with a good pi-network output. And no, you are not matching your tube to 50 ohs anywhere. Likely 8000 ohms to whatever your antenna feedline impedance.

    And no, an antenna does not radiate maximum power when its 'impedance' matches that of the transmitter. It radiates maximum power when the output stage of the transmitter can deliver maximum power to the antenna, regardless of the impedance involved You can have a wide range of impedance depending upon frequency and "Q" of the antenna. An antenna 'tuner' is merely a wide range impedance matching device.

    Most solid state transceivers now do have a fixed 50 ohm output looking for a nice resistive 50ohm load.

    A dummy load fails to be efficient because it radiates no power in the directions desired.

    For practical communications, it always comes down to getting power delivered in the direction wanted (at the right radiation angle). Or received from the right direction.

    That means you minimize your ground losses and your 'copper losses'.
     
  7. N4CD

    N4CD Ham Member QRZ Page

    matching

    You can take a loop antenna. Let us say it is resonant...at the frequency you want.

    it has a 100 ohm feed point, nominally.

    You put your 'antenna tuner' in this case, a 2:1 simple RF transformer, and you have 'matched' your transmitter (solid state, likely) to the antenna so you get maximum power. Otherwise, your transmitter barfs at the SWR and won't put out full power.

    It had nothing to do with 'resonating' the antenna. It has everything to do with getting power delivered to the antenna.

    If you built yourself a balanced tuner, with a 50ohm balun at the input, all it would be doing is matching impedance to get maximum power delivered.

    If you had a 100 ohm dummy load, your 'antenna tuner' would have nothing to do with 'resonating' the antenna......
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::If it's non-resonant, better to have one with inductive reactance than capacitive.

    Think about why that is, and it all comes together.:)
     
  9. WA4OTD

    WA4OTD Ham Member QRZ Page

    I like the T network tuner simulator! I see what the simulator says and agree with the results just doesn't feel intuitive that such significantly shorter dipoles radiate so well.

    Maybe I haven't llooked at tuners enough seems like tuner loss would be a key parameter.

    Great discussion, Thanks
    Leroy

     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::It's the most important parameter when dealing with mismatched antennas.

    I can try putting a kilowatt into a 3' long 20m dipole but my biggest tuner, an Ameritron ATR-30 (rated 3kW PEP) would not handle that; and if it did, the calculated tuner loss is over 17 dB, which would reduce my signal by a factor of more than 40:1.

    WB2WIK/6
     
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