LMR400 or RG8 or ???

Discussion in 'General Technical Questions and Answers' started by KC2UYZ, Apr 4, 2009.

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  1. KC2UYZ

    KC2UYZ Ham Member QRZ Page

    I am starting to accumulate what I'll need to construct my antenna. I'm going to have to run approximately 100' of coax so I'd like to buy coax that will have the lowest loss. I found some online charts that suggest LMR400 or Belden 9913 as some of the better choices. I've also seen some generic LMR400 being sold online and represented as "the same as" the real LMR400.

    So...is there a recommendation considering the distance I have to run?

    Any experience with the generic LMR400 (ebay)?

    thank you
     
  2. K7JEM

    K7JEM Ham Member QRZ Page

    What's the frequency, Kenneth?
     
  3. KC2UYZ

    KC2UYZ Ham Member QRZ Page

    Ah...therein lies the rub....As a new HAM I haven't found a favorite band so I'm trying to plan for as many contingencies as I can. Tech license for now, but General in a month or so.

    thanks
     
  4. K3WRV

    K3WRV Guest

    Dear Biggziff-

    Mostly on HF, feedline losses aren't that big a deal. Pretty much anything better than RG-58 or 174 will work just fine. If you're really worried about feedline losses, try openwire / window line with a balanced tuner (no Balun), but even at QRP levels, that wouldn't be my biggest concern. For HF, save the bucks towards a tower or a beam.

    Try something. Balance your needs against the cost. On HF, $100 for feedline to feed a $10 antenna doesn't make a lot of sense in my book, but then I remember CBers who insisted on RG-8 to feed their mag mount antennas.

    Antennas are lots of fun - Put one up, as cheap as possible, and then try to make it better, and then repeat (recursive function). When you get down to bare metal and really need to get an extra dB - well tink about it, but with a OCF dipole, you're only going to do so much (which will probably be plenty!).

    On VHF, I'd spend a bit more, maybe, perhaps, but I'm using RG-58 on 2 meters. Too lazy to replace it, and my interest there is local repeaters. Good Enough generally is.
     
    Last edited by a moderator: Apr 4, 2009
  5. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    For frequencies between 1.8 and 29.7 MHZ RG-8, RG-213, or LMR-400 will all work fine. the differences in loss at 100 feet is not worth worrying about.

    Whats more important is to find coax that has a non-contaminating jacket, one that is UV resistant and won't oxidize. It will last for years. The Times LMR-400 ( good stuff) pays off if you plan to run VHF and UHF. If you really get serious about those bands then you may want to look into Heliax or solid shield air core coax.

    For now do what fit's your budget until you find your nitch.
     
  6. K7JEM

    K7JEM Ham Member QRZ Page

    Generic LMR400 is usually just a clone of the original, and meets similar specs. LMR400 would be OK for about any frequency from 2M and down. RG8 or RG213 type cables are OK for 100ft runs at HF, but get lossy at VHF and above.

    At UHF, 100 ft of any type of small cable is going to have a lot of loss. 7/8 inch Heliax (or larger) would be a better choice, but that is very pricey.

    LMR400 has a solid aluminum center conductor, and as such, it doesn't like a lot of stress or continued bending. Bury-Flex is a better choice if a lot of flexing will be anticipated.

    Joe
     
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    RG8/U "type" coaxial cables can run the gamut from very good to absolutely dismal. RG8/U has not been a valid specification for coaxial cable for a number of years. Therefore, there no longer is a "standard" which RG8/U cable has to meet. The "new" specification is RG213/U which is the equivalent of the old true RG8/U specification.

    For HF operation you are not going to see any real difference between RG8/X (smaller diameter cable than RG213/U), RG213/U, or LMR-400. Both RG8/X and RG213/U can be used in applications where the cable needs to be "flexed" (i.e. to go around a rotor) whereas LMR-400 has a solid aluminum (flashed with copper) center conductor and therefore will break in a VERY short period of time if used in an application where the cable is "flexed".

    For VHF (50 MHz, 144 MHz, and 222 MHz) LMR-400 is preferred by many because it has somewhat lower losses than RG213/U in runs up to around 70 feet. However, there are a lot of antenna installations that use RG213/U at these distances on VHF. To go around a rotor most people use RG213/U while some use LMR-400 "superflex".

    For UHF (serious "weak signal") and for anything with a run greater than around 70 feet for VHF, then the "hard line" and Heliax type of cables come into play. There is a difference between "hard line" and Heliax types of cable. "Hard line" has a solid shield that is difficult, but not impossible, to bend whereas Heliax (and Heliax "type") of cable has a corregated shield. Often "hard line" does not have a "sheath" over the outer shield whereas any Heliax (or Heliax "type") of cable that has been manufactured for the last several decades does have a "plastic" sheath over the outer shield. Back in the 1950s into the 1960s Andrew (the manufacturer of Heliax) did make Heliax that did not have the plastic sheath. But, it has been years since I have "run into" any of that type of Heliax.

    Heliax that is 1/2 inch in diameter is considered by many to be a minimum for VHF and UHF "weak signal" work and 7/8ths inch diameter Heliax (or larger) is generally preferred for 420 MHz and higher frequencies.

    All but one of my antennas are fed with Heliax (including that for 160 meters). Now my phased verticals for 40 meters use RG213/U but that was a necessity due to the fact that equal lengths of cable are required between the antennas and the phasing switch box and I have a "fair" length of cable coiled in my attic because of this. Heliax is "way" too difficult to coil for this application. For my 432 MHz yagi I use 7/8ths inch Heliax. I do admit that using Heliax for HF is an "overkill". However, over the years I have acquired quite a number of lengths of 1/2 inch and 7/8ths inch Heliax for absolutely no cost and therefore, to use a lesser quality feedline I would actually have to spend money.

    Getting back to your original question: What is the "best" feedline (taking into consideration loss versus absolute cost) is going to depend on whether or not your are going to be using the feedline for HF, VHF, or UHF.

    Glen, K9STH
     
    Last edited: Apr 4, 2009
  8. W1BAN

    W1BAN Ham Member QRZ Page


    I got some generic runs of lmr-400 off ebay and was a little concerned with quality of it. I gave a 50' length to my father to sweep on a network analyzer where he works and all specs were within what Times specs. So I would not worry about the generic stuff.
     
  9. KC2UYZ

    KC2UYZ Ham Member QRZ Page

    Thanks. Good information.
     
  10. NA0AA

    NA0AA Ham Member QRZ Page

    I'm not sure why LMR400 is so highly prized, I find it a right pain in the fanny to work with - the aluminum wire is hard to bend, the jacket is so tough it's actually a bit hard to work with...and getting that bonded foil off the dielectric is the very devils work.

    I'd only buy it for 2 meters and up myself. RG-213 of good quality is fantastic for most everything. I use RG-8X [mini-8] for end supported dipoles to reduce weight, never had any issues.
     
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