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Coax Enquiry???

Discussion in 'CB Antennas' started by barefootindian, Jun 3, 2012.

  1. barefootindian

    barefootindian Supporting Member

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    Hi All, Has anyone used a type of coax called LMR240. I was going to get a roll of Mini RG 8 and was wondering if it was generally a solid enough performer for cb transmissions. So I went on the web and clicked on a forum from eHam net, and the subject was a question which was 'Which is better RG213 'v' RG8 Mini'? and I read down the screen and one contributer mentioned a coax called LMR240. I then went on a search for same and found that it is a 50omh coax with an attenuation of 1.3 or 1.7 or so. It's claimed to have a lower loss dB wise than RG8 mini. Does anyone know about or used this type of coax in their set-up. How has it performed for them???


     

  2. Robb

    Robb Yup

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    Sure, I use LMR-240 for jumpers, mobile coax, and even my Ham rig antenna for 10 meter thru 80 meter transmit and receive range.

    Coax receive and transmit loss over a 100 ft length will change vastly depending on what frequency you are using it for regardless of what coax is used. Better coax - like LMR-240 - will have significantly less loss than cheaper coax. In this case, you are using it for 27mhz. While the losses are not as bad as it would be if you were to use on on 54mhz/6 meter band; coax losses begin to go up asymptotically from ~25mhz and above.

    If you are going to use more than 100 ft in length; then using quality coax becomes a need more than a want. Is the LMR better than the best mini 8? Yes . . .
     
    1 person likes this.
  3. Mudfoot

    Mudfoot Elmer

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    Good stuff, especially the armored stuff they use in aircraft.
     
  4. barefootindian

    barefootindian Supporting Member

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    Thanks again Robb and m42duster, I now have to find a supplier over this side of the pond.It dosen't appear to be as popular as say RG's 58 or 213 but that said I'll have a trawl and see where I go. Perhaps a marine radio supplier might be a good place to start.As regards the armored LMR 240 that m42duster mentions.....Is it armored on the inside of the coax or is it run through a small diemeter pipe like might be found on the motor of a domestic fridge?? Anyway Thanks again for your quick reply folks...
     
  5. DXman

    DXman Yes, that's 3100 degrees F. Nine yrs of hard work.

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    Same here on the LMR 240, good stuff!
     
  6. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    yes the LMR 240 is good quality coax. i use it in my mobile and it does a great job.

    however, just be carefull not to pinch it, or get it cought in anything as it has a foam dielectric. it dosen't hold up as well to abuse as the poly dielectric coax does.....such as rg213 and the like.
     
  7. barefootindian

    barefootindian Supporting Member

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    Thanks DXman and bandit35. It's on the shoping list after those answers. I wasn't so sure about the rg8mini ,though it seems to be the recomendation from my local contacts and price was the main factor for them. However for myself it's not price - but concern that I dont give any TVI to my neighbours and I dont mind spending a bit more on a low-loss coax.I have to get approx 60 meters of a run to where I can put up my Delta -Loop Antenna which is higher than where I had it before. The run would go past my neighbours house and I wasn't sure if TVI was going to be a problem or not. I also have a 30mhz low-pass filter that would help eliminate any such like problems hopefully. Its just that we all need our neighbours-- know what I mean...... Thanks again folks.
     
  8. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    that and the low pass should help.....but keep in mind, that nothing will elminate it completely if the radio and or amplifier( if you choose to use one) are generating harmonics that the antenna isn't designed to radiate.
     
  9. barefootindian

    barefootindian Supporting Member

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    Hi bandit35, I wasn't thinking of running any real high power through the delta-loop. I intend to run it barefoot at first and if I get any distance around the locale say thirty forty miles radius then I'll be happy, really really happy.hOPEFULLY the harmonics will be tame enough not to cause any hassle.I'll give it a go and if the results are bad I'll try to rectify them,but maybe it'll all be ok.I've a few weeks before I can get the coax dollarwise so I'll have to wait and see and when I do FINALLY have the set-up that I would like to have I'll post the results with pics.......
     
  10. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    sounds good, im interested on how you make out. good luck
     
  11. Robb

    Robb Yup

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    Harmonics are primarily a product of the radio you use and how you use it. If you want less harmonics; then be sure that you have less than 100% modulation on AM, make sure the AM limiter hasn't been removed/compromised, the 54mhz trap hasn't been disabled, don't overdrive any amp being used, and use a high-quality Low Pass filter after the radio/amp. That should make the output fairly clean - if all is done right - IMO . . .
     
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  12. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    "the 54mhz trap hasn't been disabled"

    just out of curiosity, how would one diasble such a trap?
     
  13. Robb

    Robb Yup

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    I believe that all FCC accepted radios have a circuit to block the 2nd harmonic of 27mhz; which would be 54mhz (2 X 27 = 54). 54mhz is the 6m Ham band. So these FCC accepted radios use chokes and capacitors to make a kind of a filter to keep those harmonics down to an acceptable level.

    If you have opened up a radio and seen some copper coils near the final output and seen then flattened down; then that 54mhz trap has been disabled. Some guys have the idea that because they see more power output of the radio on a meter once they spread or flatten these coils down - that the radio is actually putting more power out. Actually, they are NOT. Power meters are dumb and don't recognize the fact that it is seeing 54mhz power coming through; not just 27mhz power. So, the golden screwdriver that does this can tell his customer 'Your radio can now do 25 watts instead of 18 watts'. Because the meter is reading the 27mhz AND 54mhz at the same time. Doesn't mean that you are hearing those watts on 27mhz; but the meter will say so.
    "Bad tech; no soup for you . . ."

    The other harmonic distortion is made by having more than 100% modulation. One cannot have more than 100% of anything. So, that level that goes above 100% modulation ends up as adjacent channel 'splatter' or channel 'bleedover'. Removing or altering the AM limiter will give up to 140% modulation; but guess where the rest of that extra 40% modulation is going? You guessed it -bleedover!

    Now what happens when you are running a radio that has all of those 'mods' in to an amp? You've guessed right if you said that the amp is now amplifying all of that crap out to the antenna. Overdriving a linear will not only eventually cause the amp to fail; but it will also sound like crap while it is also giving off amplified 54mhz noise.

    Ever wonder why Hams are so ticked off at CBers?
    Well; that will do for a start.

    At least, that is the way I understand it and that does make sense.
    I hope I wrote it all down correctly...
     
    #13 Robb, Jun 9, 2012
    Last edited: Jun 9, 2012
    1 person likes this.
  14. barefootindian

    barefootindian Supporting Member

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    Well now, I haven't opened up my radio and got out my screw-driver to it. Thare ain't no way that I'm gonna mess with whats inside cos I'm not a techy in any way nor would I pretend to be. There are real techs out there and if I need one then I'll go to them.I got one of the anytone 5555's and its a Chinese copy of the KPO and others with the same basic format.Its all tiny computer technology in there, so no messin about with it. Anyways the harmonics that I was afraid of causing hassle with was 27mhz x 2 = 54mhz so Robb you were bang on the button so to speak and what I was afraid of was bleeding all over this freq.However I have seen a list of the tv freqs and if my maths is correct then I wont be causing any problems. Years ago ( about 35 or so) one tv chanell was affected by TVI problems and that was broadcast on 108mhz I think, so unless one had a tvi filter back then it was best to limit ones time on the cb.......
     
  15. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    i played around with low pass filters years ago before cable came around and the small dish satellite tv when there used to be more complaints of tvi. i found in my experence you have better luck with just doing a mild tune up and keep the modulation a little below 100% was good enough. i agree with robb on this point.

    i have seen that for those that want the loud, banging, swinging audio.....and then run a amplifier, especially a class "c" device...there isn't anything that will be much of a help.
     

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