¼ wavelength matching section versus balun

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AA7EJ, Mar 28, 2012.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    Anybody has a real practical experience in using ¼ wavelength “matching section” versus using toroid balun?
    Assuming both are single band usage devices and balun is having the advantage of being able to transform the balanced antenna to unbalanced source, what else is there to set them apart?
     
  2. AD4J

    AD4J Ham Member QRZ Page

    What do you intend to match? Generally, the purpose of a balun is to balance the current (or voltage), though it can secondarily transform impedance. The 1/4 wavelength matching section transforms the impedance by the square of the ratio of the load to the matching section's characteristic impedance.

    For example, an 800 + j0 load fed with a quarter wavelength of 200 ohm line has a ratio of 800/200 = 4. The square of 4 is 16, so the impedance at the non-load end of the line is 800/16 = 50 ohms.

    Jim AD4J
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    A properly designed balun can reduce common mode problems.
     
  4. W9GB

    W9GB Ham Member QRZ Page

    The Butternut/Bencher HF-6 & HF-9 multi-band vertical antennas use a 75 ohm matching section (RG-11/U) for its 20 meter operation.

    w9gb
     
    Last edited: Mar 29, 2012
  5. W9DTC

    W9DTC Ham Member QRZ Page

    I was given an old Butternut to use, but the 20m matching section had been rebuilt improperly (hastily at the wrong length I believe). I initially tuned the antenna up without it, and then having cut a new tuned matching section re-measured SWR and impedence on all bands. The matching section improved SWR and impedence on all bands the antenna is designed for, especially 20m (of course).

    I configured my matching section in a loop. As I understand a current choke (or 1:1 balun) can be built by wrapping coax around an air core... so that is basically what I did with my length of RG-6 ( which had to be cut to a different length than RG-11 as the velocity factor is different). Anyhow, if I'm not mistaken (and I really could be!!!), my matching section is also an air core balun. :) Anyhow, I don't have any common mode problems with the coiled matching section in place.

    Good luck!
     
  6. AA7EJ

    AA7EJ Ham Member QRZ Page

    Thanks for all the replies.
    It seems that my statement of “ transform a balanced antenna to unbalanced source “ by itself is not that important or descriptive when it comes to balun.
    As pointed out , the key should be the ability of balun to reduce the common mode current when coaxial feed line is used.

    Personally, I like the combination of ¼ wavelength matching section with choke “balun”.
    It may not be mechanically great, but it is definitely less parts - KISS approach.

    Little OT.
    I checked few articles about the “common mode” and was surprised that one of them stated that it is a “problem “ even when open wire “ladder line” feed is used. Does not seems right.
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Certainly a common-mode signal can exist on ladder-line. However, unlike coax where the common-mode signal travels on the outside surface of the braid independent of the differential-mode signal, on ladderline it exists as an imbalance between the two line currents.

    Steve G3TXQ
     
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    I have a used Butternut HF9 , it is mounted about 20' in the air .
    It was missing the 75ohm matching cable , all the wires at the top of the antenna and during cleaning up and tuning one of the door-nob caps broke .
    During my research I could only find info on the DX-Engineering site on the dimensions of the CPK kit [ a counter-poise for elevated use ] built one of those .
    The manual says to remove the matching network 75ohm cable , and also the 80m coil for the elevated / CPK kit .
    I do not remember , but must have read about using a 1:1 current / choke balun , I ended up using one of Balun Designs .
    It seems like I must have messed up , because with a 100 watt and using a Palstar AD2K tuner and my old Kenwood TS-850Sat , I am competitive with some of the other stations on nets and contesting .
    Now to get that used HY Gain HY tower up and play with that .
    Find stuff cheap and experimenting in ham radio :)
     
  9. KA0GKT

    KA0GKT Ham Member QRZ Page

    A 1/4 wave section works well; however unless you are rolling youe own transmission line (a bit of a trick when it comes to coaxial transmission line), getting the proper impedance line for the section can be tough.

    Matching can be done with lumped constants or with setions other than 1/4-wave. Check out some of the good texts regarding smith charts online and in league publications. The Smith chart might be hard to get to know, but it can be your friend!
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    When I was in college I ran an off-center-fed dipole fed with 300 ohm TV twinlead. It had serious common-mode problems. Anything that unbalances the currents will result in common-mode problems.
     
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