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Does feed line length matter?

Discussion in 'CB Antennas' started by Marconi, Sep 13, 2019 at 5:46 PM.

  1. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    The links below are discussions, in part talking about coax length and the idea for making a feed line invisible. There is a thread below "Make Your Coax Invisible" that got me to thinking, but I'm not suggesting anything about amplifiers.



    In the CB world I've heard many suggest the feed line length does not mater, period.

    I've also heard folks say the feed line length does not matter, and then adding, if your match is off...re-tune your antenna.

    I've also heard many say to make your feed line as short as possible, period.

    I read manufactures recommend specific feed line lengths and multiples for their antenna kits.

    My old antenna mentor use to tell me to keep the feed line length at random lengths and avoid problems. He also said never make your feed line length a resonant length at your frequency of choice. In my limited understanding I never could determine his reasons for saying this...except for the idea that feed line length did not matter, if the antenna was tuned perfectly to a resistive match and was resonant. Back in those days for me, I never considered CMC an issue I could deal with.

    I don't recall ever hearing things demonstrated and described as noted in the link below. I guess only these newer analyzers make such considerations possible for the average operator. This was revealing for me.






     
    #1 Marconi, Sep 13, 2019 at 5:46 PM
    Last edited: Sep 13, 2019 at 6:42 PM
    Shadetree Mechanic likes this.

  2. hotrod

    hotrod Well-Known Member

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    marconi .ive always went with shortest run from point A to point B. jumpers included. 3ft . is this right? i dont know but its worked for me for years.
     
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  3. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Hotrod, I can't answer your question, "...is this right" regarding 3' foot jumpers and the shortest coax idea we hear claimed a lot. This guy gives us a formula to follow for the right coax length in the situation he presents, but he is talking about 160 meters maybe, very low frequencies at least.

    This is not about CB antennas, but the ideas demonstrated may be informative when we hear folks making claims and talking generally about using such devices for better control of your match and/or minimizing CMC's whether CB or Ham.

    Apparently this is more about working most of the HF bands without using a tuner, so you don't have to go changing your setups.
     
  4. The DB

    The DB Sr. Member

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    Have you watched the video 543-dallas posted in that thread? Its been a while since I watched it myself, but did he not cover any of your questions?


    The DB
     
  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    It does not matter on any radio service as long as the antenna is properly matched to the feedline.

    Exactly right. The feedline length does not matter. If the match is off the antenna needs to be re-tuned and/or matched properly.

    Again all true but only because you want to keep losses as low as possible. Why waste watts in an extra 50 feet of unnecessary cable?

    Some of those antenna have common mode current issues or are poorly designed with no current return path and rely on the coax cable to function as part of the antenna. Poor design basically.

    [/quote]
     
    binrat, Tallman, Beetle and 3 others like this.
  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    In my everyday experiences, Whether co-phased setups for "dual whips" on a semi, or a simple re-tune of an older Antenna Specialists (remember the Zebra?).In these cases - any of them with the "smashed coax" or kinked and then twisted line - usually means replacement of the WHOLE line and it's exact length. Not just a barrel connector here or there like patchwork - there is a considerable amount of loss per connection "joint" - even pre-molded co-phasing harnesses are the better replacement for the older OEM stuff - its' a pain to route but the results are more OEM than to try and fix with additional patching cords or lengths.

    I've seen trucks come into the lot with their co-phase harnesses chopped and clamped into two PL-259s' onto a Tee fitting and then onto a length of coax to reach the radio...yes, it does affect the radios' ability to see a true SWR.

    One thing they forget ...

    You can see that on a simple field strength meter.

    You can see the losses - then the replacement coax and it's (considerable) field strength difference and the restored receive.

    How about SWR? Well that may be a different story...

    I guess to answer the question properly is - how much of a loss are you willing to take in a performance hit in order to meet the radios' own desire to run at a 1:1 SWR versus a 1:1.7 SWR?

    So, to provide an answer, you'll need to know what kind of beast you're dealing with, Greed, Envy or just plain Bastard - types. Each has it's own personality and working set of values. Knowing that the Wilson, or the AS or even the R/S types that have a matching network on the base of the coil - the coax you'll need to use to make it work is the only way to restore that antennas original performance - by length and types ( RG-58 or RG-8x - as examples) dues to the nature of the radiational parameters of the beast itself.

    Some "loaded" whips are for the 1/4 wave into a stubby shorter design, others are 1/2 wave into a coax length for balun matching and you get that kind of radiator pattern - onto the 5/8th Wavelength and it's own headaches for matching into an acceptable SWR back at the other end. - did I forget to mention location?

    Location - Location - Location...

    Hey come'on guys, did you ever read a Book written by a guy a long time ago that called himself "Luke"?

    The Audience Responds...
    "You mean This...?"
    CoolHandLuke.jpg
    ---A Feminine SIGH is heard......

    NO! NO! NO!

    I mean this Luke, located in this book....
    HolyBible.jpg
    It comes in several different translations
    - should be one for your own Alma Mater - of your Dura Mater...
    Luke8-16.jpg
    Where you mount your antenna is just as important as the antenna type and your radio - are you using a Cobra 19+ to get thru the commute? Or are you into shooting skip and wondering what he looks' like...

    You can be good with your setup and just run a low SWR, but disregarding location, with that comes some of what's been discussed here already, Common Mode Currents - you get more (in problems) and produce less (in signal) - if you don't follow what the good book sez...

    This one is more helpful and contains more information that pertains to the topics at hand...
    ARRLHandbook.jpg
    Common Mode
    Counterpoise...
    Stuff like that...
    It's in there...
    Because of you don't pay attention to details - you can get hurt...

    Things like RF burns, Lightning and the Girl Next door...

    Even this...
    OOPS.jpg
    Because you ignored her pleas; asking for help, to stop the noise she would hear on her radio she listens to at night, that's by her bed.

    So did her Mom...she did make a plea or two..to the Police...

    So you know now, what may happen...if you don't pay attention to LOCATION, for all the effort you do to make the match a 1:1 or provide the lowest SWR and you find yourself with tons of coax acting as patch-cords to make your Hi-Po-Galaxy Radio of a Miracle ...
    MarvinTheMartian.jpg
    (Sorry, couldn't Resist...)

    More of a pain and tripping hazard than it's worth...

    So when it all boils down - you arrive to the Antenna - it's own needs, to provide a low-SWR onto the other issues of it's location and the length of coax you need to run in order to connect to it...

    So some books describe a "balun" while others say, you install this antenna and leave it's coax alone - what are they getting at?

    Balundefinition.jpg

    It means that if you use the antenna (Whip, Mount and its Coax cable) - keep it together AS A SYSTEM - and add your proper coax "patch cord" if needed to make it to the radios location - DO NOT REMOVE OR CUT OFF COAX that system uses - else you'll be back at square one.

    • Can't do that? Well, we're Sorry to hear about all that - then get another type of brand of antenna then. Why? Because you simply cannot remove the coax on these types of ANTENNA SYSTEMS - they use that "cable" as coax and as a BALUN - your radio will see a 1:1 SWR or it's lowest SWR at that PL-259 connector - ok? It's that simple. IT may sound cheap - or copycat - or poorly designed - you decide that phrase - but you still have an Antenna System that will use -REQUIRE- the SPECIFIC length of coax it uses, to match the load and whip to work right as in - low SWR, not necessarily Field Strength...

    Well, once you have an antenna (we are talking SYSTEMS' here) that uses that kind of setup - antenna (being it's whip and mounting with it's own internal loading and matching - key words here) the coax it uses per instructions from the manual of the antenna and your radio. MEANING: - if the end of that coax shows a 1:1 SWR where you've mounted that antenna and you find it's acceptable in performance - run accordingly.

    But if it doesn't, or you can't, then you shouldn't use that antenna (it's construction - not it's type) and you'll have to find other ways, brands and/or locations (did I mention location?) will have to be tried in order to obtain what you want in performance. Anything less you run the risks of performance hits - see above...
     
    #6 Handy Andy, Sep 14, 2019 at 3:43 PM
    Last edited: Sep 14, 2019 at 3:50 PM
  7. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    "So let it be written, so let it be done!"
    The shortest is the best to eliminate receive loss of weak signals.


    Andy.....Geezus!
     
    #7 Tallman, Sep 14, 2019 at 5:45 PM
    Last edited: Sep 14, 2019 at 5:53 PM
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  8. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Consider that I'm only talking here about the video, "Building An Inverted Dipole," starting at about 8 minutes to about 10 minutes.

    Here we see Kevin N6VLF talking about the math = L' needed when adding the feed line for this simulated Inverted V at 3.9 MHz with a near perfect match. His math shows the feed line length needs to be 83' feet with a 0.66 VF, or multiples of this length in order to be transparent or invisible as he notes.

    Is Kevin's math right and does the feed line length matter in this purposed setup, or is this just more BS?
     
    #8 Marconi, Sep 14, 2019 at 7:48 PM
    Last edited: Sep 14, 2019 at 7:53 PM
  9. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Seems I saw it long ago. I saw the link to the video in another thread, but I don't recall anybody discussing any meaningful details so I just read on. I don't think I made any comments in that thread.

    The video was interesting with the lights at different intensities going on at various feed line points. Base on my work in modeling I had a very vague idea of what could be going on with the light along the FL. Frankly, I didn't get much out of the demonstration or the talk.

    DB, I wish I knew more, so what specifically do you think I missed?
     
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  10. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    For what it's worth, here are 2 Eznec models in Free Space modeled similar to the Inverted V discussed in the video. Included in the images are the Transmission Line modeling details and the Source Data - Feed Point information.

    1. is a model of the IV at 3.9MHz with a feed line 83' foot long with a VF = 0.66% as noted in the video referred to above.

    2. is the same antenna as #1 above, but it has a 50' foot feed line with a VF = 0.66% as is also noted in the video.

    Sorry for the poor images of the Wires and Transmission Line reports. The PDF file will allow you to zoom in closer for a better view.

    DB, this is a pretty simple model to make, why don't you do it and see what you get when adding different lengths of feed line. I may have missed something important to understand.
     

    Attached Files:

    #10 Marconi, Sep 14, 2019 at 10:13 PM
    Last edited: Sep 14, 2019 at 10:19 PM
  11. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Didn't see the video so just answering your question based on what you said. Yes his math is correct. A half wave at 80m is approximately 126 feet. An ELECTRICAL half wavelength of coax cable with a Vf of 66% is about 83 feet and since a half wave of coax will simply repeat the impedance the antenna exhibits back to the TX the coax appears "transparent" as far as impedance transformation is concerned. Certain lengths of coax can be used as a conjugate match to match two unequal impedances and is required in SPECIAL cases but never in normal everday one-antenna systems. I use a power divider consisting of a half wavelength of 35 ohm impedance feedline to match a pair of 50 ohm yagis phased on 2m to feed a long length of 50 ohm feedline to the shack. You can either use two parallel lengths of 75 ohm cable or a single piece of 35 ohm cable if you can find it. I made it from a piece of half inch copper pipe and a piece of square aluminum tubing of the proper size so that the resulting impedance was about 35 ohms. the ONLY time coax needs to be a special length is when using it transform impedances and NEVER when using a single antenna. In that case the single antenna should be corrected to a nominal 50 ohms.
     
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  12. The DB

    The DB Sr. Member

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    I was the one who posted it before. It is a very thorough video. While it has also been a while since I've seen it as well, I was wondering if your question wasn't answered within?


    The DB
     
  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Earlier I had the feeling I had seen the video on "standing up for standing waves" but I seemed to recall it was done by some other man. I did a search for it, but nothing showed up for me. I did see some video link you posted from AT & T on coax however.
     

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