Yet another antenna question

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

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    Jim,

    Comments such as that don't change the technical facts.

    To state as you did in posting #86, that the source impedance seen at the antenna end of the line remains the same, is just plain wrong. The concept may be difficult for the newcomer, but that's no reason to mislead them.

    Steve
     
  2. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    ''Curiouser and Curiouser''

    I'm sure you're familar with the phrase,
    "Close enough" only counts in horshoes and handgrenades." As an IEEE member, I would hardly expect wuch an imprecise characterization.

    This endless debate; IMHO, mainly over semantics,
    brings to mind a passage from Lewis Carrol's
    Through the Looking Glass.
    Humpty Dumpty is speaking:

    "When I say a word, it means
    exactly what I choose it to mean.
    Nothing more, nothing less."


    Another way of saying that the maƗimization of radiation is accomplished by the use of a matching device between the transmitter and antenna system (including feeders). No, matching is not the entire purpose, nor
    is it to "maximize the radiation of available RF energy from the antenna."

    Let's take this semantics debate a step further:

    Is the "entire purpose" of said matching not to effect the desired response in a distant receiver, whose output is matched to an acoustical transducer, rippling and rappelling throughout the synaps of the some Himalayan Ham's left hemisphere cerebral system, causing his motor reflexes to write your call into his log, alongside a "5 by nine plusity-plus" report?

    So, yes, the nobel antenna tuner
    (matchbox) does all this and more.

    But as to its function in the antenna system, all a new ham needs to know is that amatching network or tuner, will match his or her rig to the antenna, but cannot overcome deficiencies such as lossy feedline, or an inefficient antenna proper.

    73, my friend, and hope to talk to you on the air sometime. And that goes for Steve, and others here, too, including the originator of this post, Tim. We could have a pretty decent discussion. And likely some breakers with opinions of their own.

    Jim
     
    Last edited: Apr 13, 2009
  3. W5DXP

    W5DXP Ham Member QRZ Page

    There you go again using that key word "all". That is obviously not all a new ham needs to know or the amateur exams would contain only one question about antennas. It is certainly not all I needed to know when I was a new ham 55 years ago. As a matter of fact, I still don't know all I need to know. FYI, most statements containing the words "all", "none", "never", "always", etc are false.

    And that's your latest statement on the subject which at least doesn't imply support for any technical myth. Compare your above statement with this previous one.

    The key word in that sentence is "all". You have admitted that is NOT all an antenna tuner does so I am satisfied that you are not accidentally or purposefully trying to promote a myth. Unfortunately, it is hard to convince one who refuses to admit a mistake that one indeed made a mistake in one's choice of words so I am going to quit trying.
     
  4. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Cecil, you are absolutely correct. I made a poor choice of words there, for sure. My webtv box is too slow; and I don't have the time to go over every word, as I would if I were writing a QST article.


    I do see your point,and I apologize for the inconvenience. And I thank you for helping me disprove a statement whose author I can't recall. What he said was, something like "There are no semantics or opinions, when it comes to antennas." (No time to chase down who posted that, but it's in the thread somewhere.

    Hey, when did you first get on the air?
    My first license, WN9JEF, was issued on August 3, 1954. I've always been fascinated with antennas, and it sure helped with my broadcast enginering career. I understand the concepts of calculus, but hated the drudgery. On the job, I could always get things into the ballpark on intuition, and then cut-and-try for the tweaking.

    For a long time, I didn't own an SWR bridge. Just tuned my matchbox for maximum "neonicity"--seat-of-the-pants engineering, I admit. I recently purchased a surplus B&W dummy load/SWR bridge. It allows me to dip the SWR while feeding a 50 ohm dummy load, with just a sample--a tenth of a watt or so to the bridge (and on to the antenna).

    I'm using a turnstile on 80, feeding it with 4-wire open line. I was astonished to observe a sometimes 20 db or more difference in received signal strength between left and right-handed rotation. And equally better signal reports transmitting with the opposite sense. I was going to name this the W9JEF effect, but a search led me to this forum and KL7AJ. Eric and other hams in Alaska have been taking advantage of the X and O-modes for some time. He has a thread or two on the very subject, and I describe my turnstile in my qrz callsign bio. Being 55 years a ham, I agree; you are so right about "'all' you need to know."


    73, and best of DX.

    Jim
     
  5. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Let me say this about that . . .


    Steve, I went back and here's what I think you refer to: (Originally Posted by W9JEF):

    Would you also agree that impedance at the antenna end of the line remains the same? Then what does the matching network do besides match impedances?

    You then asked whether I meant souce or load impedance, and I replied BOTH.

    I never used the term souce impedance, those are your words.

    Okay, say an antenna has an impedance of 72 ohms at its terminals. This is what I maintain will remain unchanged by a matching device, or anything else you do at the shack end of the feedline (short of physically yanking the whole thing down). I hope this clears that up.


    73 and Pe*ce,

    Jim
     
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    Jim,

    OK I'm happy to let this rest.

    But do you see that, when you stated the impedance at the antenna end of the line remained the same, and I asked whether you meant the source impedance or the load impedance, and you answered both, I assumed you thought the source impedance remained the same.

    I now understand that you are agreeing that the tuner does alter the source impedance at the antenna.


    73,
    Steve
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    Almost anyone can prove it for himself by following these experimental steps.
    1. Adjust the tuner so that the system is matched.
    2. Disconnect the transmitter and connect a 50 ohm dummy load across the input to the tuner. This emulates the transmitter's output impedance.
    3. Disconnect the antenna and, using an antenna analyzer, measure the impedance looking back down the feedline at the antenna feedpoint.
    4. In a low-loss system, that impedance will be close to the conjugate of the antenna feedpoint impedance, e.g. 72+j2 ohms if the antenna feedpoint impedance is 72-j2 ohms.
    5. Detune the tuner and watch what happens to the impedance looking back down the feedline at the antenna feedpoint. The impedance will change proving that a tuner has an effect on the impedance at the antenna feedpoint looking back down the feedline.

    It is possible to perform a mathematical network analysis to prove that the above is true. I will perform such an analysis and post it on my web page.
     
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Here's a practical example of what Cecil is saying:

    Tx---Tuner (Low pass LC)---300 ohm feedline (0.175 wavelength)---Antenna (100ft doublet at 30ft)

    Antenna impedance from EZNEC: 25-j500
    Load impedance looking into feedline: 6.5+j21
    Tuner components: Series L=1.5uH, Parallel C (load end)=2,400pF
    Impedance looking into Tuner: 50+j0

    Now place a 50 ohm load across the Tuner input.

    Impedance looking back into the tuner output terminals: 6.5-j21
    Impedance looking back into the feedline from the antenna end: 25+j500

    Notice that there is a conjugate match at both ends of the system and at the intermediate points. Very easy to check this for yourself on a Smith Chart !

    Steve
     
  9. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    Back for a 'final'

    Cecil, this is all very interesting, to me (a seasoned ham) and I appreciate knowing all of this. But how many new hams, whose priorities be just getting on the air, really care about whether a tuner has an effect on the impedance at the antenna feedpoint looking back down the feedline?

    Having been Chief Engineer at directional radio stations, I have never had to deal with that. (If I had, I guess I could have called in a consultant such as yourself.)

    I have put up countless ham antennas, including a 5/8 sloper on 80, off a 700 foot broadcast tower. Using only 100 watts, I just tuned for maximum "neonicity," and was heard 5 by nine in Australia, and other DX. But you are right about the source impedance looking back down the line. It just never crossed my mind, as I was too busy working the DX.

    I will take your word for it, as I'm not about to disconnect my feedline from the antenna. Each of my 4 wire feeders, and its respective turnstile leg are a continuous length of wire, as I have learned not to have any outdoor splices (if I can help it).

    Upon reflection (pun intended), regarding the key word "all" (here we go with the semantics again), I should have been more clear in the writing of the restrictive clause. What I should have said was: "All a new ham needs to know about the function of an antenna tuner in the antenna system as a whole, is, that it matches the rig to the shack end of the antenna feedline. Function as opposed to effects rippling throughpout the system, which you (and Steve) have expanded on so eloquently. Semantically speaking, "My bad" --but I think most new hams got the gist of what I was trying to say.

    73,

    Jim
     
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Jim,

    I suspect I'm beginning to sound like a "dog with a bone" on this one :)

    I just think it's a shame to teach a new ham something that is erroneous in the interests of simplicity. This results in a serious "hurdle" when they want to learn a little more about antenna systems.

    What is so difficult about teaching them from the outset that a tuner provides a match at all points in the antenna system, including providing the designed load for the transmitter?

    Steve
     
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