Does changing the coax length effect SWR at the rig?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Oct 29, 2019.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    For the pictured test case at 14.1MHz, does changing coax length L effect SWR seen by the rig?

    Yes or No?

  2. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Maybe, but not by much.

    1. Loss in coax will improve a mismatched SWR as seen at the radio end. Loss will generally increase with increasing coax length.
    2. Depending on the routing of the coax, it may couple somewhat with the dipole and this may affect SWR. The portion of the coax which is perpendicular to the dipole should not couple with the antenna, but the portion that runs parallel to the dipole will couple a bit, if it is close to the antenna.
  3. ND6M

    ND6M Ham Member QRZ Page

    If it does, then there are other issues.
  4. SM0GLD

    SM0GLD Ham Member QRZ Page

    It will just change the phase, rotating around the same swr circle
    KD0CAC and W6KCS like this.
  5. KV6O

    KV6O Ham Member QRZ Page

    Losses make a bad match look better.
    AK5B and K7JEM like this.
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    That's what I would have said, until I saw this:
    puz1.gif puzrjx.gif puzsmith.gif
    No tricks. Not caused by CM current on the coax. The only variable of the simulation is the length of coax L.

    The loss in the coax clearly reduces the SWR peaks and increases the SWR nulls as the coax gets longer. You can clearly see that in the Smith chart as the points spiral inward.

    The reason that the SWR circle is not centered on the Smith chart origin as expected is because a feedline is reactive and has always been capable of storing and giving back energy. The antenna is also reactive. When combined together, there is a energy interchange between the feedline and the antenna elements.

    This makes the feedline/antenna a "system", and it has to be analyzed as a "system", not as is typically done when using a Smith Chart; find the impedance of the dipole (in isolation), then think of the feedline purely as a "impedance transformer". The analysis has to consider the effect of the feedline on the antenna at the same time that the feedline transforms its impedance.

    AutoEz/EzNec has no trouble computing the R and jX and SWR at the bottom end of the feedline system, which the text book Smith Chart method completely misses.

    Remember when hams used to laugh at CBers that claimed that they could improve the SWR by cutting the feedline. Who is laughing now?
    KD0CAC, AK5B, AI3V and 2 others like this.
  7. W6KCS

    W6KCS Ham Member QRZ Page

    It seems like this would only happen if the bottom of the feedline was terminated with something besides the line's impedance. Wouldn't there need to be a reflection at the bottom of the line for the above to happen? This is interesting, thanks for posting it. I've never seen the above effect when playing with network analyzers and line-stretchers, but not to say it doesn't happen.
  8. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    What does EZNEC report for the dipole feedpoint impedance?
  9. N3DT

    N3DT Ham Member QRZ Page

    Now stick a piece of 75Ω coax on that antenna and do the same plot. And yes, what is the R±jX at the antenna port?
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I just ran a few EZNEC models of a 50 foot high 33 foot long dipole fed with varying lengths of coax with RG8x properties based on Belden's data sheet and I'm not seeing anything like your plots above. I see the expected reduction in SWR with coax length based on loss and very small variations based on length alone (for instance if coax loss is artificially set very low) but I'm not seeing anything like the SWR excursions shown above. Are you sure your model only varies coax length and doesn't also vary things like antenna height?

    For instance looking at RG8x feed line lengths near zero feet, 7 feet and 20 feet which are big variations on your charts above I see:
    Coax length = 0.001 feet

    Coax length equals 7 feet:

    Coax length equals 20 feet:

    Yes there's a general downward trend in SWR as you'd expect but there aren't large excursions like your charts show. Something must be different between our models to show such different results.
    Last edited: Oct 30, 2019
    ND6M and KD0CAC like this.

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