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Antenna Analyze, or System Analyze?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by CHUCKSTEIN, Oct 8, 2021.

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  1. KM3F

    KM3F Ham Member QRZ Page

    Low VSWR at the Tx end of feed line does not indicate best antenna efficiency.
    Why: There would be no VSWR if the antenna accepted ALL the power, minus losses, fed to it's terminals.
    Efficiency it at the antenna end of the coax, you cannot affect it from the other end of the feed line.
    You cannot correct the antenna Impedance from the Tx end of the coax, with any amount of matching.
    You cannot change the element lengths, the element diameters or any other parameter from the Tx end.
    Obtaining a match at the Tx end is for max power transfer out of the radio into the feed line.
    What happens up the feed line can only be reflected back and does not change Impedance at the antenna or it's efficiency (to any significant degree).
    A change of Impedance does not = efficiency, they are not one and the same, not even close.
    VSWR does not = efficiency and not even close.
     
    Last edited: Oct 9, 2021
  2. W9XMT

    W9XMT Ham Member QRZ Page

    True... and when that tuner is correctly designed and adjusted, the load impedance at its input terminals presented to a 50 ±j0 Ω transmission line leading back to the transmitter provides an antenna system load SWR(50) of very nearly 1.00... :1 to that transmitter — regardless of the fact that the radiator itself may be non-resonant (j ≠ 0) on that frequency.

    As an aside, most broadcast (and ham) transmitters do not have an internal source impedance of 50 +j0 Ω. If they did, then 50% of the output power they produce would be dissipated as heat within the transmitter, and their output efficiency never could exceed 50% even if the final r-f stage had a Pout/Pin conversion efficiency of 100%.

    Typically their effective source impedance is very low, and when presented with a 50 +j0 Ω output load, the transmitter delivers nearly all of the r-f power it generates into that load.

    Modern AM broadcast transmitters have Pout/Pin PA efficiencies of better than 90% into an SWR(50) load Z of ~1.5:1 or less.

    R. Fry, CPBE
     
  3. KI4ZUQ

    KI4ZUQ Ham Member QRZ Page

    I think there must be a reason why WA7ARK went silent after post #4....

    I will go back and digest the content of post #2.....
     
  4. W9IQ

    W9IQ Ham Member QRZ Page

    Hopefully it was one of the correct paragraphs. His writings are plagued with some critical errors / omissions.

    - Glenn W9IQ
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    I think of modern HF transmitters as a low-output impedance Voltage Source followed by a VSWR sensor, configured such that if the voltage output from the "reflected" side of the VSWR sensor exceeds a preset limit corresponding to about a SWR=2, then the RF voltage out of the transmitter final stage is reduced to prevent the voltage output from the "reflected" side of the VSWR sensor from going any higher.

    A 100W HF transmitter would put out ~71Vrms (200Vpp) into 50 + j0 Ohms. Less voltage if the Swr limit is exceeded. The VSWR protection feature protects the finals if the load impedance is too low, like a short, or if the load impedance is too high, like leaving the antenna disconnected. Top get full rated power, you only need enough "matching" between the transmitter to antenna to get the Swr<~2.
     
    KI4ZUQ likes this.
  6. CHUCKSTEIN

    CHUCKSTEIN QRZ Member

    This is kinda what I am asking about.
    TX-1 with feedline-B and Antenna-X has some characteristics at Tx and at antenna.
    Now, take same feedline-B and Antenna-X and connect it to various Tx's. Do all the characteristics stay the same, or will they change because the Tx changed?
     
  7. KX4OM

    KX4OM Ham Member QRZ Page

    You might get some better insights if you were to read "Antenna Physics", 2nd Edition.
     
  8. W9IQ

    W9IQ Ham Member QRZ Page

    Essentially nothing changes when you change the transmitter. One may put out a little more or less power but not to any meaningful degree.

    Many transmitters have built-in matching circuits (antenna tuners). The typical built-in version can compensate for an SWR up to 3. Some might have more range which can be helpful at times. The published specs for the radio will make this clear.

    - Glenn W9IQ
     
  9. KE0NSK

    KE0NSK Ham Member QRZ Page

    I've read some other stuff, but I am fond of Maxwells work.
     
  10. G0JUR

    G0JUR Ham Member QRZ Page

    No.
     

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