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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. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Double no.

    I've never seen SWR change as a result of changing the emitter if everything else remains the same.

    However "the measurement" might change! That's due to changing ground paths. It's a reason a hand-held antenna analyzer rarely provides the same data as using a grounded transmitter, even though they should both indicate exactly the same thing.

    This does not imply the SWR actually changes; the measurement does.
     
    KI4ZUQ likes this.
  2. KX4OM

    KX4OM Ham Member QRZ Page

    "Antenna Physics" will cover stuff from the other Maxwell. James Clerk Maxwell.

    Ted, KX4OM
     
    WE4E likes this.
  3. CHUCKSTEIN

    CHUCKSTEIN QRZ Member

    So when there's a Z mismatch you get a SW? Is there a SW when antenna Z=60Ω but Tx wants to see 50Ω?

    What if Z of antenna is 50Ω but is heavy in XL or XC ? Will a heavy XL send back some power? Is this equivalent to a SW?

    Tx output stages, typically now-a-days transistors, have XL and XC characteristics per Tx, which electrically "add" to the rest of the XL and XC in the antenna system.
     
  4. KM3F

    KM3F Ham Member QRZ Page

    Chuck; if using a Tuner, it cancels out the Xl and Xc by offering the reverse signed values besides making a 50 ohm R match left for the radio.
    Doing a Manual tune may not make it perfect but close enough.
    As stated before in my rely, adjusting with an analyzer or VNA is the only way you get to 'see' a near perfect cancellation of j and the Imp R value for the radio.
    Said another way, the normal way we use a Tuner cannot indicate it unless your were luck with the settings.
    Matter of fact, if you were to look at the Phase display on the NANOVNA you would see the crossover point as the VNA sweeps the frequency. Trying to get it perfect at the Zero crossing is almost impossible because the rate of change with F is so fast in that area.
    .
     
  5. KI4ZUQ

    KI4ZUQ Ham Member QRZ Page

    When only R exists, current and voltage are "in phase," meaning when voltage increases so does current. With either + or - jX they are not in phase. So then

    Z = R +/- jX

    which really graphically describes a right triangle, the long side is Z. Which is bigger than just R and is a mismatch that wastes transmitter watts heating up the surrounding air.

    The "tuner" or antenna coupler minimizes the mismatch to the transmitter.
     
  6. W9IQ

    W9IQ Ham Member QRZ Page

    For a given resistance any increase in reactance will increase the reflection coefficient which is the metric for returned (not used by the antenna) voltage and current. SWR is based on this reflection coefficient reflection coefficient.

    Note that the voltage and current returned due to the reflection coefficient has nothing to do with the efficiency of the antenna since the only thing that reduces antenna efficiency is resistive losses, not reactance. Instead, the reflected current generally causes increased losses in the transmission line. The standing wave that is developed on the transmission line may result in an impedance mismatch for the transmitter if counter meadure, such as an antenna tuner, are not deployed.

    - Glenn W9IQ
     
  7. KE0NSK

    KE0NSK Ham Member QRZ Page

    Walt maxwells reflections tells us

    "Any reactance added to an already resonant (resistive) load of any value for the purpose of com-pensation to reduce the reflection on the line feeding the load will, instead, only increase or worsen the reflection. It is for this reason, although contrary to the teaching of several writers, that the lowest feed-line SWR occurs at the self-resonant frequency of the radiating element it feeds, completely independ-ent of feed-line length. Any measurements that con-tradict this indicate that either the measuring equip-ment or the technique (or both) are in error"

    I'm still waiting for someone to state where that power is going if less is being reflected. If it's not being radiated, they must be ghost watts!
     
  8. W9IQ

    W9IQ Ham Member QRZ Page

    There is an example of where Walt starts to go wrong either by oversimplification, sloppy writing or failure to consider real world conditions.

    It isn't a matter of resonance, or non-resonance, of the load that increases or decreases the reflection coefficient (which is a proxy for SWR, Return Loss, etc.). It is only a matter of the relationship of the characteristic impedance of the transmission line to the impedance of the load. This is an important distinction as all real world transmission lines have a reactive component to their characteristic impedance and this varies, just as the real portion of the impedance and the velocity factor vary, by frequency.

    For example, we say that Belden 9913 coax cable is 50 ohms. But that is simply an approximation. At 7 MHz, its actual impedance is 50.326 -j 0.376 ohms. So to have no reflection, i.e. a reflection coefficient of 0, the load impedance (antenna feedpoint impedance in this discussion) must be 50.326 -j 0.376 ohms. Notice that this a load with 0.376 ohms of capacitive reactance and not a resonant load. If the load were resonant at 50.326 -j 0 ohms, there would be a non-zero reflection coefficient and therefore standing waves on the transmission line. Then adding a series capacitive reactance of 0.376 ohms would reduce the reflection coefficient to 0 which is in direct conflict with Walt's assertion. Generally speaking, the higher the matched loss of the transmission line, the larger the reactive portion of its characteristic impedance at a given frequency.

    - Glenn W9IQ
     
    Last edited: Oct 13, 2021
  9. WE4E

    WE4E Premium Subscriber QRZ Page

    The Pythagorean Theorem at work.
     
  10. KE0NSK

    KE0NSK Ham Member QRZ Page

    We both know what your explanation indicates.

    You climbed onto the chair and slipped the noose around your neck, of course I'm going to call your bluff and kick the chair. 73
     

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