Calculating coax loss with connectors?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Oct 13, 2017.

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  1. K3RW

    K3RW Ham Member QRZ Page

    After getting some decent antennas, I've been looking to get off my RG-8X coax and into something better. I didn't know how lossy it was until I looked at some dB figures. The longer runs certainly don't help me either. And I'm looking at getting something decent for my VHF/UHF rigs, since I'm about deaf on 432 running RG-8X.

    So I can understand this theory, I'll say hypothetically, a certain type of coax has a 3dB loss at 100' at 30mhz per a chart.

    If it has 2 UHF type connectors on it, and its 100', what is its loss now? Do I just add in 2x the insertion loss of a UHF connector?

    I'm also looking at insertion losses. The rig, to the ATU, to the window pass through (both sides) to another coax, perhaps a barrel connector to another coax, to a balun; add in an amp, here's 2 more connections. So, running an amp on the long coax through the window setup, that's a bunch of connections and loss potential. I'm supposing the loss I'm experiencing as also on the RX end. So cutting out losses I think will help somewhat. Ideas? I'm working on the antenna end, but have recently discovered I had 3 bad coaxes after I made a couple of changes and it would not tune at all (on one, center to center had no continuity, center to shield had continuity on another one).

    I have to wonder how much loss the tuner itself puts into the equation.

    Only thing I can think of is to limit the amount of connections. On VHF I just raise the window and run directly to the antenna. Normally I don't run the amp so I disconnect those 2 from the equation. But I'm not hearing as well as I think I could, and perhaps the losses are some part of that.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    You should not mess up a good UHF antenna using 8X.

    RG6 would work better.

    I normally add .25 db loss per connector.

    Have Fun.
    K3RW likes this.
  3. K3RW

    K3RW Ham Member QRZ Page

    Agreed. RG6 is looking like my best option, budget-wise. I can go with LMR on the VHF stuff.

    I'm wondering how impacted my receive side is by coax loss. Already using compromise antennas, I can only do so much, but using a just lousy coax on long runs probably is hurting 2dB+ over better options on some frequencies, and even worse when I connect tons of things together. If 3dB is a double in signal, jeez, I can imagine on a good day I'm cutting the rx side in half.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Loss is closer to 8db at 432 with 100 ft of RG8X.
    K3RW likes this.
  5. AI3V

    AI3V Ham Member QRZ Page

    I used to make coax connectors.

    In a coax connector factory.

    On a computer controlled lathe that I programmed.

    And repaired when it broke.

    Then I tested the connectors on a test set that cost as much as a Cadillac.


    The insertion loss of a properly installed connector is so small you won't be able to measure it.

    WA7PRC, NL7W, W4IOA and 3 others like this.
  6. K3RW

    K3RW Ham Member QRZ Page

    I've been warned about using cheap connectors. Its tough for me to know if I should get 'this one' or 'avoid these'.

    Any recommendations?
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Below 222 MHz, good UHF connectors have no measurable loss. Amphenol 83-1SP is a good connector choice for a PL-259.

    At 432 MHz, a UHF connector does introduce a little bit of loss; to measure it is complex and what I've done is use several of them in series, using very short lengths of RG-214/U (like 2") as a M-M series string, connected to double female barrel (PL-258 from Amphenol), then another 2" section of RG-214/U with PL-259s on each end, etc, until 8-10 connectors and a requisite number of "barrel" (double female) adapters, and then measure overall end-to-end loss of the whole mess. With 8 connectors in use, I can measure about 2 dB at 432 MHz that way, but remember that's eight PL-259s and three PL-258 adapters all in series; so that's a total of 11 hardware items plus about 8" of RG-214/U coax. I take the 2 dB and divide it by 11 to estimate the loss of each component.

    It's not much, but at least it's measurable that way. At "HF" (below 30 MHz) even with a series string of connectors like that, I can't measure any loss to speak of -- indicating loss "per connector" is roughly zero.

    I generally use type N connectors above 222 MHz anyway, and they are better; there's still resistive loss and it can be measured, but the impedance is more consistent, so "mismatch" loss is minimized.

    Avoid import connectors or adapters (a lot of junk comes from Asia), avoid "long" bulkhead adapters, and "elbow" (right angle) adapters unless you know a lot about them.

    Most "RG6" type coax around today is very high quality and made for cable TV/satellite systems, with as many as four shields and a very tough jacket. However it's also designed to be used with only F crimp connectors, where the connector center "pin" is actually the center conductor of the RG6, which is copperclad steel and very stiff. The outer conductor (braid and foil) isn't solderable. If using "TV" RG6, I'd use good double-crimp F connectors and high quality adapters to transition to UHF type; there are some out there that are pretty good.
    WA7PRC and NL7W like this.
  9. K8JD

    K8JD Ham Member QRZ Page

    Using RG8X.
    I have been using L O N G runs of RG8x to my 160, 80 and 40M antennas out into the woods behind my house.
    150-300 ft.
    I think coax losses on those bands is not a problem when the atmospheric noise on 160 can be S9+ in the summer.
    The price for that coax in bulk was a big factor :D

    I still make plenty of contacts.:rolleyes:
    When there is not much lightning storm noise I have less noise from the houses and power lines on my street with the antennas where they are.
    I would NEVER use that kind of coax for bands above 40M ! :eek:

  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Losses below 10 MHz aren't much of an issue unless your antennas are severely mismatched (like VSWR >10:1); however RG8X is frail stuff and critters chew through it easily. Even stepping on it deforms it because it's so soft.

    I use RG8X...for jumpers in the shack.:)

    All my coax runs here are 165' to 215' long (tower's on the other end of the house from the shack, but that's really where it had to go) and on HF through six meters I use RG-214/U. It's not super low-loss, but it's okay and very tough. Critters try to chew it and I think they break their teeth.:p On VHF-UHF I use only LMR400 or 1/2" hardline.

    (RG-214/U is like RG-213/U but it's double shielded, which makes it also fits PL-259s more snugly so they really get a deep bite into the jacket, water sealing it well. And best of all, there's miles and miles of it around that's military surplus "NOS" on spools at all the surplus shops, so I can often get it for lower cost than RG-213/U.)
    NL7W likes this.

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