Is it important to calculate the length of the coaxial cable?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by YY5RM, Sep 16, 2017.

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  1. YY5RM

    YY5RM Ham Member QRZ Page

    Greetings Colleagues. A topic of discussion and controversies commonly among radio amateurs, which has now been eradicated thanks to the knowledge of the theory, is that of the physical lengths to be used in the coaxial cables of our antenna systems. There are several opinions and arguments supported on theoretical bases, there is enough technical information on the subject, with a high level of engineering and where the non-dependence of the length to be used in coaxial cables is demonstrated. However when we install radio stations we get with seemingly out of logic situations ( Most noticeable in short wavelength HF ), equally indicated in theory and easy to analyze, not taken into account by many experts, but that make us dependent of the length in the line, we do not notice it and in many cases we do not recognize it. Because not all readers are knowledgeable about the topic, this article explains in a simple way what typically not considered by many experts, as well as the advantages of using coaxial lines of resonant lengths, only for SWR lower than 2.0, characteristic impedances of 50 ohms and typical propagation speeds on the market. In most studies and articles on physical lengths of transmission lines they use confusing terminology for beginners, generalizing ideal systems and impedance balances only for a specific frequency, without taking into account the displacement of the original resonance frequency of the antenna and its behavior in the spectrum, these being the main reasons and themes of this article. Opportunities will use a familiar language comprehensible, with its explanations due to the beginning or end of the article. Hoping to be useful and serve as the initial tool for optimizing our radio stations.

    Link to download the PDF file:

    If you would like to provide some information to add to this article, please contact us via email: , ,

    This and other articles I have written, are free to use and the latest versions are always available in or entering ( YY5RM in the search ).

    N6BQC and WB6OZD like this.
  2. WB6OZD

    WB6OZD Ham Member QRZ Page

    Well, I know what my reading material is for tonight!

    As a fairly new HAM, I thank you in advance for this. I'll come back and comment after I've digested it.
  3. AG6QR

    AG6QR Premium Subscriber QRZ Page

    It might or might not matter.

    When the coax and load are matched to one another (for us, that's usually a 50 ohm coax and 50 ohm load), and when there is no common-mode current, the coax length makes no difference.

    If the coax is radiating because there is common mode current on it, then its length matters tremendously. I can't think of a situation where the length of a radiator is unimportant.
  4. PA1ZP

    PA1ZP Ham Member QRZ Page


    I have built over 200 antennas.
    If your coaxcable length needs to be tuned, your antenna system is wrong.
    Length can be important when used in stacking devices, you can think of in stacking beams or verticals or vertical phased arrays. as transformation stub in a balun for folded dipole etc.

    But as from the antenna system to the rig, if the length matters your system is wrong.
    Never even considered to look at the coaxial length, shorter is better, it needs to be long enough to connect the antenna system to the rig thats it no more no less, and less coax is better.
    I even apply this with my ladderline fed antennas, if my tuner can not match the balanced antenna at the ladderline feedpoint, I need to improve the tuner.
    And again the ladderline is there to connect the balanced antenna system to the balanced tuner.
    We do sometimes use a tuned ladderline in zepp antennas to make a J-antenna coaxial fed out of an vertical or horizontal placed half wave end-fed antenna.

    Then we apply a shorted 1/4 wave length to make a J-antenna out of this half wave.
    We connect the coax at the 50 ohm point in the 1/4 wave shorted ladderline stub and were back to 50 ohm, apply a good 1 .1 current balun at this point and never look at the coaxial length to the TX, this coax needs to be long enough to get to the TX thats all.
    But again this stub is part of the actual antenna system itself this falls under the chapter that the coax balun for a folded dipole is in or the gamma match on a beam, its part of the antenna and not of the feedline.

    If you realy need to tune the antenna with the feedline , wrong design .
    If you SWR gets better with longer coax cables this often means high losses.

    73 Jos
    ND6M and K6BRN like this.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    The length of coax maters most on 11 meters.
    ND6M, KQ0J, WA8FOZ and 3 others like this.
  6. AA7EJ

    AA7EJ Ham Member QRZ Page

    Before you overeat on the lengthy and sometime irrelevant article, check out the basic of transmission line.

    As Jos pointed out - OP seems to mix transmission line function as access to antenna with transmission line property - length at frequency of interest - as an impedance transformer.

    I am not sure why "transmission line resonance" term is even used.

    73 Shirley
    WD4IGX likes this.
  7. AA7EJ

    AA7EJ Ham Member QRZ Page

    Correct FIRST sentence in your article. It gives a bad impression about the rest of your work.

    Coaxial cable in NOT 50 Ohms ,
    coaxial cable CHARACTERISTIC IMPEDANCE is Z0 ( and can be 50 Ohms).

    Had anybody checked / edited your article?
    My native language is not English , but even I can see some "weird" stuff.

    73 Shirley
  8. YY5RM

    YY5RM Ham Member QRZ Page

    Hi Colleagues.
    Unfortunately most experts often think it is due to impedance imbalance. The real cause is due to the displacement of the original resonant frequency of the antenna (seen from the end of the transmitter), this shifts the minimum SWR to another frequency, in turn shifting all the bandwidth.
    If they apply the Smith Chart, the answers will be equal in each displaced bandwidth, for this reason they do not understand the theory "The impedance and frequency of resonance can vary when modifying the physical length of the line, but the SWR remains constant"
    Note in this example VHF antenna 146MHz, calculated smaller than the resonant length. The minimum SWR and resonance frequency are approximately 151 MHz. Any length of coaxial cable that adds + jX 20.28 can make it resonate at 146MHz, regardless of impedance imbalance.

    I suggest reading the PDF file completely, in it are all your answers.

    Once they have read it, they can check it on their antennas, verifying the bandwidth and frequency of minimum SWR. Then cut into the coaxial cable 2 fragments 1/8 ƛ. In each clipping measure to what frequency the minimum SWR and bandwidth are shifted.

    Last edited: Sep 16, 2017
  9. YY5RM

    YY5RM Ham Member QRZ Page

    At the beginning of the article is your answer "Only for 50 ohm systems"

    The end of the article also specifies "characteristic impedance"

    Sorry for any bad translation into the English language, the important thing about the file is its didactic content

    The file is written for beginners and experts.

    Last edited: Sep 16, 2017
  10. YY5RM

    YY5RM Ham Member QRZ Page

    ① The resonance frequency is not always the same as the minimum SWR frequency.
    ② Resonance: jX = 0, (By adding XC or XL from the coaxial cable, the resonance frequency can be shifted to almost all frequencies in the table above). This makes the antenna look longer or smaller.

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