Is 450 ohm twinlead better

Discussion in 'General Technical Questions and Answers' started by K0CMH, Mar 12, 2008.

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  1. AC0FP

    AC0FP Ham Member QRZ Page

    I am relying on the window line to be a transformer, so that my antenna tuner can match transformed impedance even if there is a +/- j component.
  2. W5DXP

    W5DXP Ham Member QRZ Page

    Here's how I did it to the extent that I don't even need a tuner.
  3. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Nice job, basically a modified G5RV..
  4. K0CMH

    K0CMH Ham Member QRZ Page

    Unfortunately, I have discovered yet another hole in my "radio knowledge base".

    I realize that I had heard all the "things" about transmission lines, and have actually used some of them. One example, using a quarter wavelength of twin lead as a way to "transformer impedance" for my end fed zepp.

    But I now realize I do not completely know WHY or HOW these things happen. Example, why does a half wavelength of transmission line terminate with an input impedance of whatever the load is (or what it is connected to)?

    Maybe that would be better in another thread. But I will first try to find the answers in my references.

    Ugh, I am never going to get the Extra test taken.

    (two of my references are the ARRL antenna book and the ARRL handbook)
    Last edited: Mar 13, 2008
  5. AB1GA

    AB1GA Ham Member QRZ Page


    Take a look at the Wikipedia entry for "transmission line". There you'll find an equation for the input impedance of a lossless transmission line with a given characteristic impedance Z0 and a load impedance ZL.

    Even if you don't want to bother with the math and physics of the derivation, this equation can teach you a lot. Try substituting different values for Zin, ZL and the line length in wavelengths and see how it behaves. You can spend a whole afternoon messing about with it, learning something new every step of the way.

    For ZL, try inserting 0, infinity, and Z0
    For Z0 try inserting ZL
    For line length, try multiples of 30 and 45 degrees, up to 360 degrees.

    Have fun!!
  6. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    Take a look at a half wavelength of transmission line, any transmission line. The current and voltage are the same at both ends of the line so the impedance is the same. On a quarter wavelength feedline the current and voltage are shifted in phase by 90 degrees so if the line sees a very low current and high voltage at one end ( high Z) it will see just the opposite at the other (low Z).

    By the way, this is how matching stubs work. You can cut a length of transmission line to provide however many degrees of shift you need to provide a match. A lot of times this is done by using a quarter wave matching section with a moveable slider. All your doing is changing the ratio of voltage to current to come up with some desired ZO...
    Last edited: Mar 13, 2008
  7. K0CMH

    K0CMH Ham Member QRZ Page


    Yes, in fact, I had just had a conversation with a good friend who explained it to me pretty much like that.

    The light bulb has come on and I now understand the "electronics" of how this works.

    Yes, I now understand that at a quarter wavelength, we have the maximum amount of voltage put into this feed line that can act like a capacitor and/or inductor, thus the maximum change in that "component" of our system. At the half wavelength, we are back to zero voltage, thus the feed line stops acting like a component in the circuit.

    I know this is not percisely, exactly the best way to explain it, but that is how my feble brain works. Very mechanically. Things need to be mechanical for me to truley understand it.

    Thank you all for your help.

    Also: No I understand why the new mobile 2 meter antenna is not working so well when I installed it. I just took a length of coax that made sure I had enough to make the run. Now I will be re-doing that so that I have a half wavelength, or mulitple of half wavelength, so that my 50 ohm output matches the 50 (hopefully) ohm input of the antenna. I had noticed that my receive seemed to come up to a considerably higher level when I put a SWR meter (with two short jumpers) in the feed line. I guess I got close to that half wavelength multiple, while the length of the feedline alone is closer to that quarter wavelength multiple. This will be a great experiment.
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Believe it or not, learning the basics of the Smith Chart is the easiest way I know to understand what is going on in a transmission line with reflections. If those transmission line equations leave you cold, try a graphical approach using a Smith Chart. Learn how to normalize and plot impedances - then learn about SWR circles.
  9. VK2TIL

    VK2TIL Ham Member QRZ Page


    There are quite a few transmission-line calculators on the 'net.

    TLDetails is a good one;

    It has a Smith Chart in it; you can see how the plot changes as you vary the length of a tx line with SWR on it.
  10. W4DNR

    W4DNR Ham Member QRZ Page

    Forget the impedance of the balanced line.

    Run enough of it ( forget 1/4 and 1/2 wave transmission theory )
    to connect your antenna to a high quality balanced tuner at the shack.

    If the tuner will match it..... Operate.

    Too many times we can get involved with transmission line theory that works on limited frequency applications. Properly designed, a transmission line can act like an antenna matching device on one band, but for multiple band applications with unknown transmission line lengths......... use a tuner and make contacts.

    WA4NPL Don
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