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Higher swr from DIY coax?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K5GHM, Jun 6, 2021.

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  1. K5GHM

    K5GHM XML Subscriber QRZ Page

    Icom 7300, MYANT EFHW 1080 inverted L, 50' rg8x generic feed to bulkhead, generic rg8x patch from bulkhead to 7300.

    I rarely have any swr to worry about. Never anything approaching 1.5. Been very happy. Life is good.

    So I decided to try my hand at rolling my own coax for future projects and convenience. Built 4 separate patch cables each about 4-5 ft. Bought generic crimp-on pl259's from ebay. Bought some Davis RF RG8X at HRO and a handful of their generic connectors. Watched youtube instructions carefully from several sources. Practiced. Took my time, followed instructions to the letter and was very careful to getting it right. Each cable passed the continuity test on the multimeter. Time to test. Hooked up to 7300 and each of the four I made shows an increase in swr of at least 1.5 or greater on most bands. :mad: All four.

    So I hooked the original patch back up. Same as before. SWR barely discernible.

    Frustrating. Am I missing something here?
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Assuming your cables are actually fine, you changed the transmission line length by adding one or more of them.

    Unless the terminating load is absolutely perfect (which is almost impossible), the transmission line tunes the antenna and changing its length will change the indicated VSWR at the source end of the cable. A small change like 4-5 feet shouldn't create much change on a low frequency band like 80 meters, but could create quite a change on a higher frequency band like 10/12/15 meters.
     
    K7JOE likes this.
  3. N4UFO

    N4UFO Ham Member QRZ Page

    I knew CB operators back in the 70s that would snip pieces off their coax and redo the connector until they got the SWR down to 1:1. They actually thought they were improving things and not just fooling the meter. :rolleyes:
     
  4. N8RKD

    N8RKD Ham Member QRZ Page

    What are your DC resistances with one end open, shorted, and into a dummy load?
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's not just fooling the meter, it's changing the system load impedance. On a band like 2 meters, this can often be accomplished by adjusting line length only a couple of inches if the load is mismatched.
     
    K7JOE, WB5YUZ and AG5CK like this.
  6. W9IQ

    W9IQ Ham Member QRZ Page

    A short change in the length of coax, by itself, does not change SWR unless there are compounding factors. Longer lengths of coax with some loss is what reduces SWR. This lower SWR comes at the expense of more power being converted to heat.

    There are a couple possibilities:

    A quick DC continuity test may not show a half ohm or so of shield to connector resistance in your new cables that can raise havoc with the SWR.

    Another possibility is that your original cable is the one with the problem (more loss) and so your homemade, less lossy cables show a higher SWR.

    Given the antenna type, another possibility is that you may have common mode current on the outer braid of the coax. In this situation, the outer braid of the coax is part of the radiating and receiving antenna. Changes to the length of the coax cable (outer braid) affects the antenna SWR because the change in length is "tuning" the antenna.

    - Glenn W9IQ
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Except...sure it does. The only time coax length won't change SWR at the source is if the load is nearly perfect.

    SWR "on the transmission line" does not change with length (other than to be reduced by loss), but SWR at the source terminal does unless the line length is zero.

    The only compounding factor required is a load mismatch at the end of the line. Antennas are never perfectly matched at more than one frequency.
     
  8. W9IQ

    W9IQ Ham Member QRZ Page

    It sounds like you are mixing up input impedance and SWR.

    When you make small changes in the length of the coax, the input impedance changes. But the new impedance represents exactly the same SWR. The SWR will only appreciably change if you introduce more or less loss by changing the length of the cable.

    The input impedance will only change as described if the load impedance does not match the characteristic impedance of the coax (i.e. a load reflection coefficient <> 0).

    - Glenn W9IQ
     
    Last edited: Jun 6, 2021
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    I agree with Glen.

    Here is the Swr50 looking into T metres of Belden 9913 (RG-8/U, Zo=50.25 Ohms) if the far end is terminated into a 100 Ohm resistor, which makes the nominal coax Swr = ~2. T ranges from 0 to 50m in 1m steps.

    Plotted as a Smith chart. The Swr50 looking into the coax starts at 2 and stays near the Swr=2 (green) circle. It spirals inward to a lower Swr only due to loss in the coax:

    upload_2021-6-6_15-7-14.png

    When plotted as Swr50 vs coax length T, the slight ripple is because the coax Zo is not perfectly = 50.0 Ohms. The downward average slope is due to coax loss.
    upload_2021-6-6_15-10-5.png

    There is no point cutting coax to change SWR!

    As Glen said, you maybe confused because R and jX looking into the coax does change (a lot) as a function of variable T:
    upload_2021-6-6_15-16-53.png

    upload_2021-6-6_15-17-59.png

    There might be a point to cutting coax to change R or jX
     
  10. AG5CK

    AG5CK Premium Subscriber QRZ Page

    What about Hamz doing this with balanced feedline? Are they just fooling a meter or is there something to it?
     

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