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Yet another antenna question

Discussion in 'General Technical Questions and Answers' started by KI4DAC, Mar 26, 2009.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    I don't know the answer, Jim. How many new hams really care about technical truths vs technical falsehoods, old wives' tales, and myths? Should we take a poll? I would ask the question a different way.

    How many new hams don't care to improve their technical knowledge and expertise beyond the level of a CB operator?

    "The Amateur's Code" in the front of The ARRL Handbook says:
    "The Radio Amateur is: PROGRESSIVE...with knowledge abreast of science, ..."

    Next time someone posts an assertion like: "A tuner at the transmitter has zero effect at the antenna.", are you going to agree with him?

    With all due respect, I was once a new ham in 1953 and that is definitely NOT "all I needed to know". For the record, I am philosophically opposed to the dumbing down of amateur radio.
     
    Last edited: Apr 14, 2009
  2. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    I never would put it like that, and sure would not agree. I just didn't think about it carefully enough. Regarding source impedance, somehow I pictured the antenna as the source (which it would be, in the case of receiving). It just never occurred to me that it would be useful to disconnect the feedline from the antenna proper, and make such a measurement, but I do see what you and Steve were getting at, and I agree 100%.


    I agree, but there is such a thing as information overload. My philosophy is, get them on the air with sufficient knowledge to properly match the antenna to the rig, and further learning will follow, owing to their curiosity about the wonder of radio. As you can see, I am still learning, and loving it.

    73,

    Jim
     
  3. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Hate to sound picayune here, but by "a match at all points," do you mean to say, that feeding a full-wave center-fed antenna with RG-8 coax, that a tuner down in the shack would somehow provide a match for the 50 ohm coax up there at the high-Z point at the center of the antenna?
     
  4. W5DXP

    W5DXP Ham Member QRZ Page

    And I am afraid that further learning will never occur and amateur radio will be reduced to roger-beeps and "10-4's, gud buddy". IMO, it is already happening.
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    In a lossless system, the answer is definitely "YES". That's why newcomers need to understand real world transmission line losses, something that you would rather that they ignore since "all they need to know is how to twist the knobs on an antenna tuner". You seem to be shooting down your own argument.
     
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Of course!

    Ignoring losses in the coax, the tuner will cause the source impedance at the end of the coax to be a conjugate match to the very high feedpoint impedance of the antenna. If the antenna impedance is 3000+j100 the source impedance will be 3000-j100.

    If you doubt this, ask yourself what other value the source impedance might be. It can't be 50 ohms because that would result in only a small fraction of the available power "transferring" into the high impedance antenna - about 6% in the above example. And besides, if it were 50 ohms it would mean the tuner was having no effect!

    Perhaps now you're beginning to see the wonder of this stuff, and why newcomers should be introduced to it :)

    73,
    Steve
     
    Last edited: Apr 15, 2009
  7. W5DXP

    W5DXP Ham Member QRZ Page

    This might be a good time to point out that a very lossy transmission line destroys the conjugate match at the antenna feedpoint, i.e. the conjugate matching theorem assumes a lossless system. In a low loss system, there is close to a conjugate match at the antenna feedpoint. Information on conjugate matching is available at:

    http://www.w2du.com/Appendix09.pdf
     
  8. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Reality check

    I'm quite sure that most newcomers have long since QSYed away from this discussion. But, no doubt, those who may be designers of matching networks for directional antenna arrays,and the like, are still with us.

    In my opinion, to enjoy the hobby of Amateur Radio, a newcomer needs to learn a few basic facts. Among these, is the fact that feeding a Hi-Z antenna with coax will not produce satisfactory results, no matter what is done down in the shack. The best "antenna tuner" money can buy, may be able to reduce the indicated SWR on that piece of coax from rig to tuner, but can do nothing to change the SWR and consequent losses on the feedline on up to the antenna.


    73,

    Jim
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    Simply add one more small fact, i.e. ~1/4WL of ladder-line will pretty much alleviate that problem.
     
  10. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page


    Now that's what I think we can all agree
    is useful information for a new ham.


    73 es gud DX,

    Jim
     
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