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. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::Well, if you actually understood that, then you've answered your own question.

    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.

    WB2WIK/6
     
  2. KC8VWM

    KC8VWM Ham Member QRZ Page


    Therefore, it would be better to place said antenna tuner at the antenna feed point and not necessarily at the transmitter itself.

    My Best.
     
  3. WA4OTD

    WA4OTD Ham Member QRZ Page

    I think dipoles are resonant on odd harmonics. 1/2 wave 40M is 3/2 wave on 15M but doesn't work on 20M. 80M doesn't work on 40M, etc. Note this thread on topic: http://forums.qrz.com/showthread.php?t=179752

    For match it is simple, you get max power transfer when your 50 ohm transmitter is matched to a 50 ohm resistive load. A transmatch does this. There is a second match in an antenna of the actual radiating element to air. If the antenna is 5 foot dipole on 40M you can perfectly match that with good transmatch, but it will not radiate well and the remaining power that has been matched to this 5 foot dipole goes to heat, ie the dummy load scenario below. Perfectly matched but 50 ohm resistor in a can has poor match to air and all power is dissipated into the resistor.

    I agree the correct answer is false but you cannot make the statement no matter how far off resonance the antenna is. The very short antenna will have high heat losses from the radiating element to air mismatch.
     
  4. WA4OTD

    WA4OTD Ham Member QRZ Page

    Yes, matched source and load always delivers max power transfer. For any specific antenna matched will always deliver most received power over unmatched antenna. Right?

     
  5. AH6K

    AH6K Ham Member QRZ Page

    Receive better? The primary factor in reception is signal to noise ratio. Most receivers have plenty of front end gain so matching the antenna will have little noticable effect on being able to hear the signal above the noise. Impedance matching will improve the signal strength but also the strength of the noise leaving the S/N ratio unchanged.

    But--- Resonating the antenna system can help reduce adjacent channel noise by narrowing the bandwith of the receivers first RF stage and thus would reduce noise. So - Yes, when the matching is done along with resonating through the use of a tuner.

    Matching impedances can be done with a broadband transformer but this will not improve S/N ratio. Resonating the antenna system will provide another stage of selectivity and will reduce adjacent channel noise.

    Resonating and matching are not the same thing. Ideally, we want both in both transmitting and receiving. By resonating, I mean resonating the antenna system (including the feedline) not the antenna itself.
     
    Last edited: Aug 7, 2009
  6. KB7UXE

    KB7UXE Ham Member QRZ Page

    My experiance has been:
    say for example ( hf) I hear a signel thats S9 with no tuner .
    I can bring the tuner on line and with the tuner properly tuned
    the signal will be S9 also. ( with tuner not properly tuned the signal is less. )

    Dan.
    as to answering my own question, yes I did, and stated it several time , but I guess no one noticed. :cool:
    and the answer is: an antenna does not have to be resonant to be an efficiant radiator. ( not antenna system )
    like yard sprinklers, some do a better job of spraying water than others.
    But a tuner will compensate for poor swr.
    A 50ohm resistor will be a 1:1 match, but a very poor radiator.
    As to antenna systems, no one thing makes a good radio station.
    Lots of little things do. Paying attention to connections, coax legnths and size, antenna design for space provided,
    and a really big amp makes a coat hanger radiate really well. ( and you can arc weld with it too. )

    :cool:
     
    Last edited: Aug 7, 2009
  7. W1VT

    W1VT Ham Member QRZ Page

    How many of you are using receivers with a good 50 ohm input impedance? My 6M receiver has a lossless feedback amplifier for good SWR, but many do not. One way to find out is to use a manual tuner--can you increase the S-meter reading by re-adjusting the tuner after the tuner has been optimized for transmit?
     
  8. W2VW

    W2VW XML Subscriber QRZ Page

    What is the radiation resistance of your dummy load??
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    I don't buy that:)

    As you hint in your first paragraph, efficiency depends on what your objective is. If my dummy load dissipates as heat 99W of every 100W applied, and radiates 1W as RF energy, it's 99% efficient if my objective is to generate heat, or 1% efficient if my objective is to radiate RF.

    I would think that most hams want their antenna to radiate as RF as much of the applied power as possible. Therefore efficiency has EVERYTHING to do with radiation.

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

    Steve G3TXQ
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    The original question was, "... how does putting a tuner inline make my antenna better?" I assumed that question was about the antenna system.

    An antenna tuner doesn't increase the radiation efficiency of an antenna but it can increase the absolute magnitude of radiated power to a maximum available value. I would certainly say that makes the antenna system considerably "better".

    A Z0-match at the antenna tuner supplies an approximately equal magnitude of mismatch gain which offsets the mismatch loss at the antenna feedpoint, i.e. it redistributes most of the reflected energy back toward the antenna.

    In a station with a 100 watt transmitter, when the SWR is 5.8:1 between the tuner and the antenna, the power incident upon the antenna feedpoint is very close to 200 watts - double the power supplied by the transmitter.

    If anyone is interested in the technical details, they are covered in an article on my web page:

    http://www.w5dxp.com/energy.htm
     
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