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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. W9IQ

    W9IQ Ham Member QRZ Page

    Reactance, in itself, does not cause losses.

    We only need to look at the impedance of a few lossless, free space antennas to see that the feedpoint reactance of an antenna has nothing to do with the efficiency of the antenna.

    1. A lossless, freespace, center fed, 1/2 λ dipole has an impedance of 73 + j42.5 ohms
    2. A lossless, freespace, 1/4 λ monopole has an impedance of 36.5 + j21.25 ohms.
    Note these antennas are by definition lossless yet they have a reactive component to their feedpoint impedance.

    Furthermore, you can change the location of the feedpoint of a 1/2 λ center fed dipole to find a feedpoint impedance of ~100-j100 ohms. Clearly the losses of a center fed dipole and an off center fed dipole should be the same. Yet if the reactive part of the impedance is a proxy for antenna loss, then the OCF antenna would seem to have more loss - simply nonsense.

    - Glenn W9IQ
     
  2. KE0NSK

    KE0NSK Ham Member QRZ Page

    I'm not sure what you are looking for. Can you quote where anyone made the statement that reactance itself caused losses? Just say where you think the "ghost watts" disappear to. If you don't know, you may want to rethink your ideas.
     
  3. W9IQ

    W9IQ Ham Member QRZ Page

    I don't know from where you get the term "ghost watts". There is no such thing. Try describing it instead of labeling it.

    You made the comment;

    Do you mean that Z, since it is larger than R when there is reactance, is a mismatch that wastes transmitter watts heating up the surrounding air?

    - Glenn W9IQ
     
    Last edited: Oct 13, 2021
  4. KE0NSK

    KE0NSK Ham Member QRZ Page

    I never made that comment, so yeah.. this is awkward
     
  5. W9IQ

    W9IQ Ham Member QRZ Page

    - Glenn W9IQ
     
  6. KE0NSK

    KE0NSK Ham Member QRZ Page

    You claim you can add reactance to a resonant load (which we agree will increase VSWR on the feedline) and the antenna will become more efficient. Lets say you have a quarter wave vertical antenna system that the VSWR dip is at 1.4:1. If you add reactance until the VSWR is say 1.6:1, there is more power reflected by the antenna at 1.6:1, agreed? If 1.6:1 makes the antenna more efficient, why is more power being reflected? Where are whose watts going if they aren't being radiated by the antenna and aren't being reflected back? They are disappearing watts in your theory, not mine. You need to name them.
     
  7. W9IQ

    W9IQ Ham Member QRZ Page

    No, we do not agree.

    In post 48, I gave you a real world example where adding reactance to the feedpoint impedance of a resonant load reduces the reflection coefficient to zero. This means that the addition of reactance in this example eliminated the standing wave - the feedline only has a traveling wave. The VSWR and the ISWR are both 1 (or 1:1 if you prefer). The VSWR and the ISWR have decreased with the addition of reactance to the feedpoint.

    The concept of reflections (that gives rise to a standing wave) and the concept of antenna efficiency are distinct and separate concepts. They do not directly impact one another nor do they interact.

    If the antenna impedance is not the same as the transmission line impedance, some of the source power will be reflected back to the source. This reflected power never "entered" the antenna. Depending upon the source impedance, this reflected power may be once again reflected to the antenna or it may be lost (never to return to the antenna). But this does not mean the antenna is any less efficient. It simply means that less power has made it into the antenna. The antenna will still radiate the same percent of power that it actually receives.

    Antenna efficiency is affected only by resistive losses in the antenna itself (technically within the near field of the antenna itself). Any resistive losses cause power in the antenna to be dissipated as heat instead of being radiated as RF energy. The formula for antenna efficiency does not need to take in the reactance of the antenna feedpoint because it has no effect on antenna efficiency.

    This is not to say that reflections do not cause losses. Typically the losses occur in the transmission line (although there are cases where reflections reduce the losses in the transmission line). So when thinking about reflections and efficiency, it is better so say that the antenna system efficiency generally decreases when SWR rises. The "antenna system" is thought of as the antenna, its feedline and all matching and interconnecting elements.

    - Glenn W9IQ
     
    Last edited: Oct 13, 2021
  8. KE0NSK

    KE0NSK Ham Member QRZ Page

    You are comparing apples and oranges. You are hyper focused in the antenna feed point, and completely disregard the properties of the feedline except for return losses. Just because you can measure something does not mean that measurement is valid or relevant.

    "Any measurements that contradict this indicate that either the measuring equipment or the technique (or both) are in error"
     
  9. W9IQ

    W9IQ Ham Member QRZ Page

    What aspects of transmission lines would you like to add to this discussion that negates the validity of post 48? I am more than happy to discuss any aspect of transmission lines.

    - Glenn W9IQ
     
  10. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    I use traveling-wave type antennas.

    Most Radio Amateurs don't know that the key concept of traveling-wave type antennas like a Vee Beam, Rhombic, or Beverage is that the current and voltage levels are the same everywhere along the antenna conductors, so no standing waves on the antenna itself, the input impedance and radiation pattarn are constant. These antennas therefore have the distinct advantage of working over very wide frequency ranges.

    On traveling-wave type antennas, just match it correctly to your feed line type and don't worry about all of this.

    Antennas are a fun part of amateur radio.

    73 from,
    The K0UO " Rhombic Antenna Farm" 2 miles of wire in the Air & On the Air daily
     
    K9AXN and KI4ZUQ like this.

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