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Dissimilar metals in coax; alternatives

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N0IOP, Oct 29, 2018.

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  1. N0IOP

    N0IOP Ham Member QRZ Page

    I had always thought that the LMR-xxx cable from Times Microwave was some of the best available while still being cost-effective. Reading an article on repeater construction, there was a problem described involving corrosion between the aluminum foil shield and the plated copper braid. This condition is said to be particularly problematic in duplex applications where it can lead to receive noise.

    I have three questions:
    1. Is this a real problem or just one guy who had trouble?
    2. Is the problem limited to repeaters or is it more general?
    3. What is the best alternative family of coax to use? If plain old RG/xxx, what sources have high-quality RG that meets the mil specs, at fair prices, in quantities that make sense for individual projects?
  2. KW4GT

    KW4GT Ham Member QRZ Page

    Remember - if moisture can't get inside the jacket you'll never have a problem. Galvanic corrosion occurs when dissimilar metals come in contact IN THE PRESENCE OF AN ELECTROLYTE.
    N0IOP, W2AAT and NH7RO like this.
  3. NH7RO

    NH7RO Ham Member QRZ Page

    When I lived in KH6 I found that some of my older runs of LMR-400 had verdegris on the braid and I always figured iy was due to moisture ingress rather than galvanic corrosion---but I must admit that that does make a lot of sense. Ithink in my case it was due to the high humidity coupled with age and less careful construction (I always use 3>1 heatshrink nowadays and am really careful with assembling my PL-259s).

    Sometimes I could just cut a few feet off and the braid would be fine but in others it was a total loss so it went to the dump.

    I like LMR-400 and am sticking with it. I suppose a good alternative would be Amphenol RG-6 if proper matching via half-wave multiples is employed, or DXE-MAX400 for a direct replacement.


    N0IOP likes this.
  4. K7JEM

    K7JEM Ham Member QRZ Page

    1. Yes, but it varies with installation. I have some repeater sites that use LMR400 and they work OK. But I have seen the issue occur.

    2. It is pretty much limited to uses where duplex signals are happening. If you have a repeater that uses two antennas, both fed with LMR400, but split in distance, you won't see the problem because the level is so small. But on a single cable, it can be an issue. In simplex there is no problem.

    3. RG213/214 or Heliax type cables.
    KX4O and N0IOP like this.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    LMR-400 (not UF, but the solid-core 400) has one additional problem I've seen which exhibits at higher transmit power levels: That is, the core conductor is copperclad aluminum, and if when stripping the cable for connector installation you "score" the conductor through the copper cladding down to the aluminum (oops) and then install the connector, over time at power the aluminum literally disintegrates until you have an open connection.

    In fact, this can be accelerated purposely for demonstration purposes and it's pretty exciting!

    Pays to be careful with the stuff.

    RG-214/U is about as well shielded as LMR400 and is more flexible, but with somewhat higher attenuation at HF and measurably more at VHF. For a mil-spec cable that's lower loss, I use RG-217/U. Reason is, it's mil-spec and extremely rugged; all copper conductors, no aluminum or other materials; it's still flexible enough; has about the same loss as LMR400 but is much "tougher;" and it's usually available NOS surplus from surplus outlets like APEX Electronics locally. It accepts N and DIN connectors and other types made for it, but does not accept standard PL-259s (too large O.D.). There is a "speciality" PL-259 made for it (AMPH P/N 182124) but they're expensive.

    The RF Connection has RG-217/U at $218'/100 feet but I've bought it for $75/100' at Apex when they have it (walk-in, not shipped). Very good stuff.
    N0IOP likes this.
  6. KX4O

    KX4O Ham Member QRZ Page

    It's worth adding that many radio sites adhering to R56 or equiv. disallow the use of LMR-xxx even if it isn't a problem for a repeater operating duplex or perhaps an APRS digi operating simplex since PIM mixing products might well be made from combinations of "any" site rf emissions, within any coax, in the vicinity. This is akin to problems caused from rusty bolts, corroded fencing, old guy wires, aging connectors, etc. Lightning protectors have their issues as well. With proper coax readily available to anyone, site operators don't want to borrow trouble.

    Use at home is likely just fine (I do) unless you are running a multi-op mega contest site, but then you have money to buy better coax than LMR anyway.
    N0IOP likes this.

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