What effect does a tuner have at the antenna?

Discussion in 'General Technical Questions and Answers' started by W5DXP, Jun 4, 2008.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    All of us have read that the only thing a tuner does is make the transmitter happy. If we couldn't make measurements at the transmitter and could only make measurements at the antenna (using ideal instruments including field strength meters), what would we measure?
     
  2. N2RJ

    N2RJ Ham Member QRZ Page

    Radiated power?
     
  3. WD8RWI

    WD8RWI Premium Subscriber QRZ Page

    The tuner has absolutely no effect at the antenna. The job of the tuner is to "tune out" the reactive portion of the antenna impedance so that the radio is feeding in to a purely resistive load, hopefully somewhere around 50 ohms.
     
  4. W4INF

    W4INF Ham Member QRZ Page

    If the antenna needs a tuner, then it does NOT matter if the tuner is at the rig or at the antenna, it is still matching the antenna. Wouldnt this sound correct?
     
  5. WA4ILH

    WA4ILH Subscriber QRZ Page

    only if the transmission line was infinitely short. <grin>
    Tom WA4ILH
     
  6. W5DXP

    W5DXP Ham Member QRZ Page

    This is what I was hoping someone would say - and that is the current popular myth. Here are the facts: Assuming the antenna feedpoint impedance is constant: using nothing but a forward power or forward current reading at the antenna feedpoint, the tuner can be adjusted for maximum radiated power without worrying about whether the transmitter is happy or not.

    55 years ago, my Elmer worried about radiated power, not about whether his transmitter was happy or not. Good grief!
     
  7. KL7AJ

    KL7AJ XML Subscriber QRZ Page


    Actually, in the broadcast business, measurements made at the antenna are the only ones that count. That's why it's required to have an R.F. ammeter at the feedpoint of the antenna, where the known radiation resistance is.

    eric
     
  8. N8CPA

    N8CPA Ham Member QRZ Page

    That's where the ideal transmatch would be, at the feedpoint of the antenna. In the shack, at the rig, it tunes to the entire system, antenna plus feedline of whatever length, a single component matched to the signal source impedance.
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    What??? There's no FCC requirement to make your transmitter happy??? Shame, shame on you! "No unhappy transmitter" is like "no child left behind". How can you blatantly deny unhappy transmitters their individual rights???
     
  10. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    There's a very vague rule about "good engineering practice" and they'd probably raise an eyebrow if you were running 40% efficient, even if the radiated power was correct. And most broadcast transmitters had pretty wide range Pi networks anyway...so they were usually pretty happy no matter what sort of load they had. By the way, I got to work on one of the last direct fed "slant wire feed" broadcast stations, KFAR, before they got all modern and conventional. :(... They had this wonderful link coupled output network that went right to the antenna. Of course, it was installed before coax was invented.


    eric
     
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