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. K8JD

    K8JD Ham Member QRZ Page

    This is an easy one to answer !
    Sure X=0 but a full wave dipole (yours on 20M) has Z= many thousand ohms, making a horrible mismatch to your 50 ohm coax.
    On the other hand, a halfwve Dipole (yours on 40M) Z is closer to 50 ohms and you no longer have mismatch losses. :eek::eek:
     
  2. KC8VWM

    KC8VWM Ham Member QRZ Page

    I would bid higher but I came to realize it's not possible to achieve more than 100% loss.
     
    Last edited: Aug 10, 2009
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Back at Texas A&M (during the dark ages) we labeled that high resistance, zero reactance condition as "anti-resonance".
     
  4. K0CMH

    K0CMH Ham Member QRZ Page

    "I honestly don't know anyone else who has suffered any damage from RF exposure. Certainly not in the HF spectrum.

    Has anyone?"


    Well, not as a Ham, but yes as a military op. The old '70s vintage RTTY rig ran 500 watts AM around 5 MHz. We had the option of a 15 foot copper whip for mobile use. One day the SWR was wacky on the 15 foot whip and we could not get a match. I told my fellow operator: DO NOT, UNDER ANY CIRCUMSTANCES, KEY THE RIG UNTIL I GET BACK IN THE HUT.

    I checked if the wip was loose by seeing if it would tighten up by hand.

    Yep, you know what happened -- a nice red streak in the palm of my hand that lasted for about three weeks -- kind of like a super sunburn.

    Stupid on me, I should have shut down the rig and disconnected the coax before I went outside. But that was back in the tube days, and it would have taken a long time to get the rig back in operation. But no excuse for stupid.
     
  5. W2VW

    W2VW XML Subscriber QRZ Page

    Yes. So can anyone else on here.
     
  6. K0CMH

    K0CMH Ham Member QRZ Page

    So, in summary, it appears that a non-resonant antenna can be an acceptable radiator, provided one can get the power to it. Just about any metal thing will radiate, but there may be to much loss in obtaining a "match" for it to be practicable.

    The non-mathmatical way of looking at resonance in an antenna is the condition where the input impedance of the antenna is close enough to the output impedance of the system feeding it, so that an acceptable amount of power is transferred.

    The use of a "tuner" (aka match box, ATU, matching unit, impedance matching unit, etc., etc.) is to make the output impedance as close as reasonable to the antenna input impedance. Some times the tuner can do it and sometimes it can't -- it depends on how much mismatch the tuner can handle.


    Please correct me if I am wrong.
     
  7. VK7DR

    VK7DR Ham Member QRZ Page

    Another typical dumb antenna thread.

    To be an efficient radiator on any given frequency, the antenna (a) must be the whole of, or (b) must form a significant part of a resonant circuit.
    All other frequency dependant variables flow from this.
    Period.

    (a) equals resonant antenna, matched feedline, matched transmitter output.
    (b) equals random wire, unmatched feedline, tuner to transmitter output.
    (b) is always less efficient than (a) as the feedline and the tuner form part of the antenna system generating feedline loss and poor radiation characteristics caused by standing wave effects.

    Efficiency and Resonance are directly related. Radiation resistance is a valid term only at the characteristic resonant frequencies of the antenna.
    ie, a quarter wave vertical has a RR of 36 ohms. Why? Because it is a quarter wave antenna - ie, resonant at the full wavelength frequency.

    Corollary of this is that a random length antenna will be only be an efficient radiator provided it forms part of a resonant circuit - ie, its radiation resistance is utilised to greatest effect.

    To suggest that any random length of conductor can be an efficient radiator on any given frequency is to say nothing at all. Of course it can, provided all the variables it presents to the problem of radiating power at that particular frequency are dealt with.
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Not "always". 100 feet of matched line RG8x has a loss of 0.924 dB on 10 MHz.

    100 feet of matched line RG213 has a loss of 0.597 dB on 10 MHz.

    100 feet of open-wire line with an SWR of 12:1 has a loss of 0.342 dB on 10 MHz.

    There is a current maximum point on the open-wire line where the impedance is 600/12 = 50 ohms so no tuner is needed.

    http://www.w5dxp.com/notuner.htm
     
  9. VK7DR

    VK7DR Ham Member QRZ Page

    Understand what you are saying, but in my condition (b) (which is the way it seems a lot of hams set up their random length (ie, non resonant) antennas) the coax line is rarely if ever matched to the antenna - as it must be matched at its characteristic impedance and the antenna only presents this match at one frequency - which in the case of a dipole or vertical not cut for the operating frequency, is never the operating frequency. There is also the problem of balanced to unbalanced line, but we all know how this is overcome.

    (And how often is it overcome by using a "4:1 Balun") :confused:

    The inefficiencies in the system can certainly be "tuned to a minimum" by a tuner, but the mismatch between the characteristic impedance of the cable and the antenna remains. (Using a 4:1 balun might, just might, produce the happy chance of a match off resonance. But still only at one frequency.)
    Only when a "tuner" (whatever it might need to be in the situation) is between the antenna proper and the coax, is this condition overcome.

    Open wire line is another matter. I use it only. There is no tuner between the link coupled tank circuit of my amp and the line feeding the antenna. Reason - none is necessary.
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Sorry, I didn't realize that you were talking about coax only. If "coax" is added to your statement, I agree with what you said:

    (a) equals resonant antenna, matched (coax) feedline, matched transmitter output.
    (b) equals random wire, unmatched (coax) feedline, tuner to transmitter output.
    (b) is always less efficient than (a) as the (coax) feedline and the tuner form part of the antenna system generating feedline loss and poor radiation characteristics caused by standing wave effects.
     
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