1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice
Dismiss Notice
This forum does not allow a single user to have more than one username. If anyone wants to change their username contact an admin and it will be done. Multiple accounts belonging to the same member will be deleted without warning.

Cutting to length

Discussion in 'General CB Services Discussion' started by Cody Dixson, Mar 8, 2021.

  1. Cody Dixson

    Cody Dixson Active Member

    Joined:
    May 3, 2020
    Messages:
    107
    Likes Received:
    61
    Watched an interesting video about coax jumper lengths ect. Now before everyone gets wild and starts picking sides my main question is what equation is used to figure coax length. I'm in the process of turning current system on my cb in the semi from a single to dual antenna and i want to figure out how to calculate the length for each 75 ohm peice of feed line to get a good match and then a way to calculate what I'm guessing is the "multiples" of a resonate length. Hoping to shorten my runs without using 18ft

    Any info is always greatly appreciated thank you!


     

  2. Slowmover

    Slowmover Elmer

    Joined:
    Feb 17, 2015
    Messages:
    4,300
    Likes Received:
    6,369
    9’ or 18’ each.

    My Peterbilt had 6’. Got stretched and popped apart.

    More is less of a problem:

    1). Routing (plus attachment to body)
    2). Need for CMC Choke or Isolator at antenna feed point (you might decide to wrap a toroid or two).
    3). Yet another filter of one type or another at the radio.

    In some installations, a filter at every quarter wave-length can be a suggested noise solution. 9’ is a handy number. You just won’t be able to do much more than one toroid with it (depends on diameter of coax wrap of toroid).

    With 18’ legs I was able to wrap two (2) toroids into each at ends. Maybe 6’ left over to install with no excess. 3.25” diameter coils thru FT240-31 and -61 toroids.

    Decide what you want to do and run a piece of 3/8” stiff cotton rope as substitute.

    .
     
  3. Cody Dixson

    Cody Dixson Active Member

    Joined:
    May 3, 2020
    Messages:
    107
    Likes Received:
    61
    @Slowmover thanks for clearing that up for me ive been confused on if the length helps or is just there after reading a previous comment on another post about tuning the harness. the length of 50 ohm coax can be any length? Or would cuting it to length like some people talk about or resonate length be better? I watched a video where a tec took an peice of wire with one end and an antenna Analyzers and tuned the jummer untill it has a flat match. So if I sound crazy this is where my questions have stirred from
     
  4. Slowmover

    Slowmover Elmer

    Joined:
    Feb 17, 2015
    Messages:
    4,300
    Likes Received:
    6,369
    Co-phase needs exact length match. Past that is (on my part) conjecture.

    Reading around this site
    will be of help on the question of coax length.

    My take is that wavelength multiples are easiest as Total Coax Length isn’t of much concern in mobile install (past becoming part of a noise-related problem).

    My concern is that it’s easy to cut TOTAL length too short.

    When I get home from this month-long trip there’ll be a new 18’-leg cophase harness awaiting me to install in the Peterbilt. Higher quality cable plus ends plus assembly.

    I’ll be installing feedpoint antenna filters at outer ends and have a few devices at the other before it reaches the radio. I’ll wrap excess into a Figure-8 and place into the console at what seems the best locations.

    IOW, the truck determines what matters (install-related problems) in what’s crucial. 9’ or 18’ legs aren’t hard to deal with as I see it.

    If you read around far enough you’ll also see that 18’ legs “help”, overall (Firestik). I’m not going that far, but cut to length just isn’t important enough in mobile. Fitting it to the vehicle is the priority.
     
    #4 Slowmover, Mar 9, 2021
    Last edited: Mar 9, 2021
    Shadetree Mechanic likes this.
  5. Cody Dixson

    Cody Dixson Active Member

    Joined:
    May 3, 2020
    Messages:
    107
    Likes Received:
    61
    @Slowmover 10-4 10-4 !
    I'm hoping to take 2 equal lengths of rg6 with f type connectors to a t connectors then a short jumper of rg8x to the radio

    I'll honestly have to go with 18 ft legs after thinking about how it's going to enter the truck.
    The jumper of 50 ohm coax from T connector to the radio is my concern. Not that it's a massive concern but from what I've seen it's better to tune jumpers. Because alot of people have told me whatever length works. But would tuning the jumper lower swrs and increase power output efficiency.
     
    Slowmover likes this.
  6. Slowmover

    Slowmover Elmer

    Joined:
    Feb 17, 2015
    Messages:
    4,300
    Likes Received:
    6,369

    The details you note are of interest to me also, but it’s past my ability (and interest) at present.

    My opinion about big truck installs is that inefficient antenna mounts (ground) is where the larger problem (the real problem) lays.

    I assume use of a dual final radio (35-watts) or greater.

    A guy could crank major wattage, but even as it seems to get him OUT, he’s also bathing in RF inside the cab.

    IOW, expect to hit limits early with each aspect of a system using typical CB truck antennas.

    I’d imagine tuning the coax is a help, but is it enough to change the outcome?

    What test after what formula to use? (There've been some heated, albeit funny, “discussions” on the subject).

    I don’t want to sound discouraging on a subject in which I can’t help (coax length tuning), my only concern was that the coax harness be long enough for actual routing and any accessory additions.

    .
     
  7. M0GVZ

    M0GVZ Sr. Member

    Joined:
    Oct 18, 2011
    Messages:
    1,714
    Likes Received:
    1,409
    None because it's all bullshit. There is only one correct length, the length required to get from A to B by the route you wish to take.

    However if you're co-phasing then yes it's critical.

    You need to know the velocity factor of the coax for a start as that alters the length. Coax typically comes with two velocity factors, either 66 or 85. If you're making a jumper that needs to be a specific wavelength out of 66 it'll end up shorter than one made out of 85.
     
    Cody Dixson likes this.

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!