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Coax Loss vs Connector Loss

Discussion in 'Amateur Radio News' started by VK6FLAB, Mar 15, 2018.

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    jim.jpeg Foundations of Amateur Radio - Episode 145

    A question that comes up regularly is one about loss, specifically loss in the coax and connectors between your radio and your antenna. The general wisdom is that better coax gives you better results and more connectors is bad.

    Anything with double joiners, or such like is really bad.

    So, essentially we've been taught that we should have the shortest coax possible with as few connectors as possible.

    Pretty fair and reasonable, right?

    During the week I was introduced to a video made by Jim W6LG. Jim has a YouTube channel going back a couple of years with about a 100 videos.

    One video is loosely called Jim measures the loss in coax connectors and 100 foot of RG8X. In case you're wondering, 100 foot is 30m and 48cm of coax. I know this because the United States of America despite appearances to the contrary is actually metric, they defined the inch as being 2.54cm back in February of 1964. Other than driving on the wrong side of the road, they're not too strange and they talk on the air, a lot, so there's that.

    Back to Jim.

    He rummaged through his bits box, the one you have, the one that every amateur has, and if you don't then you clearly need to spend some time being with an Elmer and learning the ropes. Jim pulled out 30 odd connectors, SO239 and PL259 by the looks of things and daisy chained them all together. Jim has been around the block a few times and he has connectors going back to World War 2, so he really did find the bottom of his box to make his video.

    Anyway, he rigged up a testing tool to compare a single connector to 30 connectors. Measuring the difference, showing pretty graphs, lines and scales, the whole bit.

    He even compared 20m to 6m and tested both extensively and even re-did the tests with a kilowatt.

    Then as icing on the cake, you know the one, with a cherry on top, whipped cream on the side, he did the same test with the 30 odd meters of RG8X coax.

    I could leave you hanging here and let you go and find Jim's video, but that wouldn't be fair if you're currently in the middle of your commute to work like several people I know, so I'll share the outcome, but if you get the chance, the 5 minutes of your life that you'll spend with Jim are worth every second.

    So, what was the outcome of Jim's test you ask?

    Surprisingly, there was no discernible difference between one connector and 30 connectors in-line, not at 14 MHz, not at 50 MHz, not at 50 microvolts and not at 1 kilowatt, about 223 and a half million microvolts.

    Using RG8X coax, which sits about halfway between RG58 and RG213 in terms off loss, there was however 22% loss at 14 MHz and 40% at 50 MHz.

    Does make me wonder if it's the coax manufacturers who have been telling us to buy more coax rather than join two bits of coax together with a connector.

    Might have to do that test myself. Better go and start digging through my bits box.

    I'm Onno VK6FLAB

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    TA2OXY, W6ERM, N9PBD and 5 others like this.
  2. AC6RM

    AC6RM Ham Member QRZ Page

    Is there a link to this video?


    N2II likes this.
  3. KP4SX

    KP4SX Premium Subscriber QRZ Page

    I'd like to see the same test done with 30 segments of cable spliced together :)
  4. WA9WSJ

    WA9WSJ Ham Member QRZ Page

    Hi Bob,

    I did a little research on his call and found it here...

    73 Tom wa9wsj
    KJ6CE, KC4DJQ and W6ERM like this.
  5. W1YW

    W1YW Ham Member QRZ Page

    Actually a good video, with some caveats....

    The losses with increasing SWR are NOT insertion losses; they are mismatch losses.

    I very much doubt the measurements are better than 0.25 dB RMS. So giving percentages to a unit percent is not correct. You don't have that degree of precision OR accuracy.

    Connectors are reasonably lossless. The problem arises when they loosen and/or oxidize (which is inevitable). Then you --may--suffer from intermod, which can raise the noise floor. Each additional connector decreases the mean time between failures, and increases the probability of increased intermod noise. That's why people don't use 30 connectors.

    UHF connectors are good into low UHF. Then you use N's, for example.

    I measure all my coax losses with a VNA (S12) with a short piece of hardline as a normalization to zero loss.

    Best to Jim:)
    Last edited: Mar 16, 2018
    KR3DX, KA0HCP, K0PIR and 3 others like this.
  6. K8ERV

    K8ERV Ham Member QRZ Page

    I am at a loss to understand all this.

    TOM K8ERV Montrose Colo
    W1SRR, W6ERM, WB2JIX and 2 others like this.
  7. W1YW

    W1YW Ham Member QRZ Page

    He's trying to show you that connectors are pretty much lossless (when clean and tight) and that coax losses become a problem once you go to VHF.

    So most coax is fine at HF--within given power ratings.
    AD6FR, NU4R, KG4BFR and 2 others like this.
  8. KT0DD

    KT0DD Ham Member QRZ Page

    RG 8X is a poor choice for long runs anyway. RG 8U or LMR 400 type coax is much better.
    AG7JN, KR3DX, KK5R and 4 others like this.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Vanna says coax connectors are virtually lossless if well installed and used within their frequency range.

    She'll turn over a letter on this, shortly.
    W1SRR and WA7PRC like this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, they are.

    Interestingly, "UHF" connectors (Amphenol 83-1SP) have slightly less insertion loss at 450 MHz than type N (UG-21D/U) connectors do.

    I've measured both many times.

    But going above 500 MHz, Type N, as well as C, BNC-TNC-SMA etc. are a better match and lower loss.
    NL7W, KK5R, W1YW and 1 other person like this.

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