Formula to calculat e Feedline length

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AB2UI, Jul 3, 2012.

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  1. AB2UI

    AB2UI Guest

    I have searched in a number of places and I am seeing answers that contradict each other. What I am looking for is a simple formula to calculate the optimum length of 300 ohm ladder line to a dipole. I have seen some sites saying the feed-line should be a 1/2 wavelength of your dipole (not sure how to calculate that if the dipole is used for multiple bands) then I read a website that said that the feed-line length must never be equal to 1/2 wavelength of the frequency the dipole is cut for.

    So, with that said, can anyone please provide me with a formula so I can redo the feed-line length's to my two dipoles and get the optimal length so it is an easy match for my tuner and I do not have huge losses in the cable.

    At this point my ladder line terminates into two baluns outside of my house then there is a 50' or so run of coax to my radio. I am thinking of running the ladder-line through the sill of my basement window and run the ladder line through the basement, through a hole in the floor and then place the balun within a foot or two from my rig.

    Also, if someone can provide me with a formula to calculate loss in the feed-line that can be used for both ladder line and coax. It's OK, if the formulas are a bit complex as long as all the variables to enter into the formula are clearly defined.

    This would be a huge help to me.

    P.S. If I'm only putting out 100 watts and I do end up running the ladder line along the rafters in the roof of my antenna, will I have any issues of RF interference inside of the house? This has always been my main concern and is the reason I always left he baluns outside and ran coax into the house to the rig.


  2. PD7MAA

    PD7MAA Ham Member QRZ Page

    There is a French prog. named Levy that showes the antenna length and the feedline in cm.
    Works fine
    Shows also the radiation patern.One dipole leg is on the left so the feeder is on the right side.

    have fun
    20m dipole 971cm feedline.jpg
    Last edited: Jul 3, 2012
  3. N9XV

    N9XV Ham Member QRZ Page

    The idea of "optimum length" is based on the principle of a 1/2-wave length of transmission line's ability to "mirror" the antenna impedance back to the input end of the line at whatever the resonant frequency is. For a multiband dipole fed with a single transmission line, the "usefulness" of this principle falls away because the impedance of the antenna will range according to the frequency applied to it.

    You said your feedline terminates into two baluns, and then to a single run of coax? Why two baluns?

    The "usefulness" of ladder line does come into play here because at HF ladder line is for all "practical" purposes lossless.

    With coax, you can determine the matched feedline loss. Then take into account additional loss due to swr. Bam! there's a snapshot of what the coax is doing. Do the same thing for the ladder line, and there's a snapshot of what the ladder line is doing. All of this is only relative to a specified frequency! For a resonant antenna operating at a single specified frequency, the calculations aren't too difficult.

    Thankfully, W2DU has done a spectacular job of laying it out for us,




    Find anything this gentleman ever wrote and study it dutifully!!!

    For any of this to truly "mean" ANYTHING, you would need to know the impedance of your antenna. The only real way to know that is with an analyzer. The antenna analyzer looks at the phase angle between voltage & current at the point where it's inserted. Which is why a 1/2-wavelength transmission line can be useful when analyzing the antenna, it will mirror the antenna impedance back to the analyzer and allow you "harness" the magnitude of antenna impedance. It's resistive & reactive component and the associated phase angle. Of course this all changes with frequency. Eventually you reach a point (when changing the frequency) where the 1/2-wavelength line is no longer a 1/2-wavelength and it's ability to mirror the impedance falls away! Also, too many multiples of a 1/2-wavelength line will mush or "washout" the mirroring effect. I've modeled this in TLdetails and it seems around 10 multiples or so, the mirroring effect is just about nil.

    There are some really great transmission line programs out there, and the one I like best is from AC6LA,


    with his program you can enter exact antenna impedance (your analyzed value), and choose a multitude of feedline and shave lengths down to fractions of an inch and impedances down to within an ohm etc. It's really worth the time investment to get comfortable with this program as if your using a simple calculator.

    The fact that ladder line (parallel feedlines etc) is nearly lossless at HF, allows it to more efficiently re-radiate the reflected energy due to the SWR. The ladder line allows you to "approach" the condition of NO losses due to additional losses in the feedline. ALL of the reflected energy WOULD be re-radiated if not for the losses in the feedline as the energy travels back and forth between antenna and transmitter.
    SWR does absolutely no harm to the transmitter, it's the phase angle or "de-tuning" of the final amplifier stage. The PA stage tries to deliver too much current when not enough voltage is there to produce it!

    For a multiband antenna, you COULD have multiple "ideal lengths" for multiple frequencies Vs multiple impedances corresponding to all of this, but as you can see - it;s simply not practical or necessary. Just use the ladderline & balun as you are. Be sure your using a good broadband balun. The tuner will tune it, add coffee and enjoy!

    Running ladder line along the rafters shouldn't be an issue as long as they are not near metal. I would still "stand them off" if you can to minimize humidity & moisture effects. Long term heat effects might be a bigger concern. My underside of my roof deck (upper attic area near rafters) easily reaches 130 to 140 degrees F in the peak of summer! A few years of that would truly put the ladder line to the test insulation wise anyway!

    Last edited: Jul 3, 2012
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Does anyone know which of the various TL programs has a good model for 300 Ohm ladderline?

    Steve G3TXQ
  5. N9XV

    N9XV Ham Member QRZ Page

    TLdetails does, and you can even create your own feedline.
  6. W5DXP

    W5DXP Ham Member QRZ Page

    The work that I have done on this subject is based on parallel feedlines (twinlead, ladder-line, and open-wire). I used EZNEC to generate the information and the feedlines in EZNEC are assumed to be lossless. The optimum feedpoint impedance is assumed to be at a current maximum, low impedance point, which is not necessarily the lowest loss point but is the point that will give the lowest SWR without a tuner. Lengths that are close to that point will usually work with internal autotuners. Here is that graph generated from EZNEC based on the length in WL of the dipole and feedline:

    As one can see, the relationship is not linear. I came up with a curve-fitted formula for this relationship and it is available in BASIC for download on this web page.

    The feedline wavelength formula, curve-fitted to EZNEC results is:

    fedlinwl = .25 - (TAN(2.5 * (dipwl - 1))) / 12.02

    Where the dipole wavelength (dipwl) is normalized between 0.5WL and 1.5WL.

    Also available on that page is a graphic, IMAXGRAF.EXE, that will allow one to change the length of the dipole while displaying the optimum lengths of feedline up to 150' for single frequencies in each phone band. That DOS program will run under XP but requires DOSBox for later versions of Windows. I have updated that program for my own use. The new update calculates the optimum lengths for both band edges so the display is somewhat busy with twice the number of dots. For what its worth, the harder to read but with more information version is available at:

    If anyone chooses to use this version, let me know what you think. Here's an example of the IMAXGRAF.EXE display screen for a ZS6BKW 90' antenna. Note the grouping of current maximum points around a ladder-line length of 40'.

    Attached Files:

  7. K8JD

    K8JD Ham Member QRZ Page

    According to this diagram My 90 ft dipole with 50 ft of 300 ohm line (length in package I had) should work best on 15M but I never have been able to make a contact on 15M, it works best, for me, on 40/30/20M ! WHY?
    If anyone chooses to use this version, let me know what you think. Here's an example of the IMAXGRAF.EXE display screen for a ZS6BKW 90' antenna. Note the grouping of current maximum points around a ladder-line length of 40'.[/QUOTE]
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    I don't see 300 Ohm ladderline as an option in my version 2.0 of TLDetails; I see "Generic 300 ohm tubular", but that's a quite different "animal".

    Yes I can enter a bespoke cable, but where are the parameters for 300 Ohm ladderline? I've not been able to find any.

    Steve G3TXQ
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I measured the Zo of various samples of 300 twin lead. The stuff is not well controlled. The Zo I measured (determined) was from 230 ohms to close to 300 ohms. Maybe this is the reason why TL has no characteristics for generic 300 Ohm line. The worst case was "300" ohm line from JSC. It is pretty tough but is was at about 230 ohms. I am using it since last November and it is has held up well scrapping against tree branches as it has a thick ribbon of insulation and heavier guage stranded copper. I use the pulse gen, scope and small variable resistor method to determine Zo. I found it to be the best method as I don't need to worry about baluns and their impact on measurements when using unbalanced test equipment (my equip is ancient bridges of various types). 73, Pete
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Google the MUF (maximum useable frequency). We have been in the low of the sunspot cycle headed for a high so 21 MHz, in the recent past, has been above the MUF. But the best is yet to come.
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