Using Ladder Line Length to Achieve Minimum SWR

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5DXP, Nov 7, 2018.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    From some of the statements and questions about my favorite no-tuner matching system, I gather that some people don't understand the benefit of being able to change the length of the ladder line to achieve minimum SWR. Fortunately, one doesn't have to understand how it works - just that it does work.

    Before continuing with my no-tuner 39 foot rotatable dipole for 30m-6m, maybe a matching example would help. The 39 ft dipole wire is resonant on 12 MHz but that is irrelevant. It's equivalent to a 1/2WL dipole on 30m and 20m and behaves like a EDZ on 10m where it is 1.2WL long. It is fed with 48 feet of "300" ohm ladder line plus whatever selectable length of loops are switched in by the Ladder Line Length Selector (LLLS). For this illustration, we will switch in zero to eight feet so the feedline will vary from 48 feet to 56 feet.

    The following graphic illustrates what happens between 12m and 6m when we vary the length of the feedline. To avoid making the graph too busy, the measurements use only the even lengths but odd lengths are also possible. We should choose 7 ft for 12m, zero ft for 10m, and any length for 6m depending on what part of the band we are on. Please note that as we step from zero to seven feet on the LLLS, the same dip in SWR moves from 10m to 12m. A selection of two feet will result in minimum SWR on the CB band.

    Please feel free to comment and/or ask questions about this method of matching.
    12m-6m39.png
     
    AG5CK, AI3V, K1ZJH and 4 others like this.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Is the 1:1 Choke Really required ?

    I never used one, And it worked fine without the extra loss.
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    Pretend that you are the RF current emerging from the inside braid of the coax and you have your choice of flowing into 50 ohms on the ladder line wire or flowing into 1/2 ohm back down the outside of the coax braid to system ground. Which path would you choose? Unfortunately, the real world RF current has no choice but to obey the laws of physics.

    It is good RF engineering practice to install a balun at every BALanced to UNbalanced junction.
     
    KA2RRK and KD6RF like this.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    OK.

    I guess I have been doing it wrong for 40 years. :eek:

    And I missed the length of coax that you are using.

    Matching coax to ladder line using a 1:1 balun is a mystery to me.

    Thank You.
     
    Last edited: Nov 8, 2018
  5. W5DXP

    W5DXP Ham Member QRZ Page

    I bring my ladder line into the shack through a piece of plexiglass mounted in the window. So the only coax I use are coax jumpers.

    It is not "matching" in the usual sense of the word. A 1:1 balun is an attempt to maintain differential current balance both on the parallel line and inside the coax, i.e. to keep the two differential currents equal in magnitude and 180 degrees out of phase with each other.
     
    KA9JLM likes this.
  6. KK5JY

    KK5JY Ham Member QRZ Page

    That's why I don't like using either term (1:1 or balun) to describe a coaxial feedline choke. The only way to understand proper feedline choking is to imagine the coax cable as three conductors, one of them in series with a single-wire choke.
     
    N8CMQ and AG5CK like this.
  7. AG5CK

    AG5CK Ham Member QRZ Page

    Coax length is not critical. He is using the ladder line to tune the system close enough to 50 ohms where the coax meets the ladder line. A common mode choke of some kind is a good idea.

    I prefer to build a balanced tuner vs switching out ladder line jumpers. This gives me the ability to get a perfect match anywhere on almost any ban.......HF band.

    I can also use the length of ladder line I need to reach from point A to B instead of having to work around the lengths that work for someone else.

    The homebrew tuner method can also allow you to work lower frequencies with a short dipole. Being a city dweller I have a 70 ft doublet and its performance is comparable to a resonant dipole on 75m. Some people are dumbfounded when I tell them about the antenna. Some think I'm a liar and others suspect witchcraft.

    I've loaded it up on 160 and made ssb contacts. Ft8 is a walk in the park. The bandwidth is about 3kc wide but I can tune up and down the band, so that's ok.

    Because of the high current involved in feeding a short antenna everything has to be heavy duty. Store bought stuff will fail or convert most of your signal into heat. My ladder line and part of the doublet are made from 8 awg.

    Both ways can work well. We're all in different situations and have different needs.
     
    WD4IGX, W2VW, N2EY and 1 other person like this.
  8. AI3V

    AI3V Ham Member QRZ Page

    I liked your old posts where you showed the Smith chart of just the dipole, and showed how a quarter wavelength line transformed the impedance to 50.

    Perhaps a word on what frequency/dipole length/ matching line length won't tune will help the non believers.

    Rege
     
  9. KI7AAR

    KI7AAR Premium Subscriber QRZ Page

    And perhaps how to apply this to 600 ohm ladder line....
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    This might make it a bit easier to see the effect of changing the length of the Twin-Lead on the different bands. The model is a 39ft horizontal dipole, elevated 50ft. The only variable is the length L of 360 Ohm, Vf=0.93 (Wireman #553) Twin-Lead.

    I plot the SWR relative to 50 Ohms looking into the TwinLead from the Balun on various bands vs the length of the Twin-Lead. Not shown is 12m, but the SWR minimum occurs at 64 ft of Twin-Lead. Note that to get a match on all bands from 30 to 10m, the total length of Twin-Lead ranges from 54.4ft to 66ft. This can be accomplished with a fixed length of 54.4ft, plus a variable length that hits specific values from zero to ~11ft.

    cec.png
     
    K7TRF likes this.

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