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

    Better to say that the antenna system may radiate more efficiently. Non resonance does not effect the efficiency of the antenna itself in any meaningful way.

    But SWR also includes the effect of antenna losses. If you tune a 1/4 wavelength, ground mounted vertical for minimum SWR50 and you have a poor radial system, you have not tuned the antenna for it's best efficiency. In fact, if you achieve an SWR50 of 1, your antenna may be only 68% efficient whereas an SWR50 of 1.4 may indicate nearly 100% antenna efficiency.

    These issues highlight why it is more insightful to monitor complex impedance (R and X) than SWR.

    - Glenn W9IQ
  2. KE0NSK

    KE0NSK Ham Member QRZ Page

    If you see 1:1 on a quarter wave vertical, changing frequency until you hit 1.4:1 will make it more efficient? I really don't think you believe this, so I'm just going to let you be.
  3. W9IQ

    W9IQ Ham Member QRZ Page

    The answer lies in the formal definition of antenna efficiency:

    Effeciency = Rr/(Rr+Rloss)
    Along with the realization that the real component of the feedpoint impedance is Rr+Rloss.

    Then recall that the radiation resistance, Rr, of an ideal 1/4 wavelength antenna is ~35 ohms. The SWR50 would be ~1.4 for the ideal antenna with 100% efficiency. But if you introduce ground losses of 15 ohms, say by removing or shortening radials, the feedpoint impedance is 50 ohms which registers as an SWR50 of 1 but the antenna is only ~70% efficient.

    This clearly makes the point that an SWR dip is not indicative of the most efficient antenna configuration.

    - Glenn W9IQ[/sub]


    I appreciate the info. But for sake of minimizing things, what if the antenna had no feedline between itself and the Tx, or, the feedline varibale was a constant in the analyzing. Does the characteristics of the Tx output stage impact VSWR? As example, I measure a simple 500MHz 1/2wave omni on a VNA and I see a VSWR graph across 200kHz of bandwidth. Should I expect to see that same VSWR after connecting the antenna directly to the Tx? Surely some insertion variable there, but my question is more about Tx output stage. If I had 20 different Tx's from various manufacturers, would they all have same VSWR? In other words, if the Tx spec sheet said 50Ω and the actual output stage was 47Ω, there's a mismatch there if not connected to a 47Ω antenna.
  5. W9IQ

    W9IQ Ham Member QRZ Page

    Since there is no transmission line in you example, there is no SWR. Instead, the issue is matching impedances. The transmitter has been specified to output a certain power into a specified impedance, generally 50 ohms. If the load (antenna) impedance is not 50 ohms, the transmitter will generally put out less power. At some point, too high or too low of an impedance will cause the transmitter to dramatically cut power in order to protect its output finals from failure due to excessive voltage or current.

    Transmitters do not have an SWR. Instead, SWR is caused by the load (antenna) impedance not matching the characteristic impedance of the coax or other feedline. So SWR has nothing to do with the transmitter. The feedline will generally have more losses due to SWR and the transmitter may put out less power due to the SWR causing an impedance mismatch for the transmitter.

    - Glenn W9IQ
    Last edited: Oct 8, 2021
    KE0NSK likes this.
  6. KE0NSK

    KE0NSK Ham Member QRZ Page

    I never said that the swr dip indicates the best antenna, just the best that antenna can do as is. There is only 1 frequency where a 1/4 wave over perfect ground will measure 1.38:1, and that's at the dip, and also where it is most efficient. So even you trying to argue against it actually confirmed it by measuring the VSWR at the dip yourself.

    If you are claiming I said low VSWR indicates efficiency, you are mistaken.


    I not saying the Tx itself has an SWR, need to at least attach an antenna, which may be 50Ω but it's impedance is not identical to a 50Ω pure resistance. If the antenna is say 45Ω at some frequency and has R I and C in the antenna, there is no VSWR between Tx and antenna?
  8. W9IQ

    W9IQ Ham Member QRZ Page

    If the antenna impedance does not match the coax impedance (typically 50 ohms), there will be an SWR on the coax. Under this condition, there is no spot along the length of the coax where the impedance is 50 ohms. Instead, there are an infinite number of impedances that are anything other than 50 ohms. These essentially repeat in a predictable pattern every 1/2 wavelength of coax.

    The effect on the transmitter is that it sees an impedance, "looking into" the coax, that is not 50 ohms. The higher the SWR, generally the greater possible excursion from 50 ohms. Because the transmitter does not see its desired 50 ohm load, its output power is reduced.

    - Glenn W9IQ
  9. W9IQ

    W9IQ Ham Member QRZ Page

    An SWR dip doesn't say anything about the performance of the antenna itself. It does indicate a point where the transmission line loss will likely be at its lowest and where the transmitter will likely put out maximum power (presuming no ATU). Under these conditions, we could say that the antenna will radiate the greatest power or that the antenna system is most efficient.

    Perhaps that is what you meant to say?

    - Glenn W9IQ
  10. KE0NSK

    KE0NSK Ham Member QRZ Page

    No, because some of that isn't true, but its probably as close as you'd ever come to admitting my statement is true.

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