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Tuning 102" Whip?

Discussion in 'CB Antennas' started by buickid, Jun 3, 2013.

  1. purenrg

    purenrg Member

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    Marconi, I cant believe your asking this ? How have you ever tuned a antenna with out knowing what was at the feed point, without using the 1/2 wave method ?:confused:


     

  2. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    pruenrg, if I asked somebody a specific question could you give me the question or a link?

    Can we agree that if we want to tune an antenna, it is very likely the antenna is not tuned and maybe new construction, the load impedance at the feed point is likely not perfect, and likely shows us just about any values for the match to start with?

    So to answer your question, under these circumstances I never know what the condition is at the feed point, but I am going to try and change something if can, trying to find that perfect match...if possible. I'm not looking for the wrong results unless that can help. I'm looking for the perfect match, and when I find it...the length of the line won't matter.

    Can we also agree that the rule that governs the 1/2 wavelength theory is based on what happens at a point 180* degrees BACK from the feed point of a perfectly matched load, where we can see indicated R=50, X=0, Z=50? Will you also agree in this case, a random feed line will show the same perfect results at any point along the line?

    The bold print indicates a distinction that I suggest are generally ignored in considering the use of an electrical 1/2 wave tuned line for tuning...as having special features and providing special information. I think you will find similar claims in Reflections II, Maxwell's book on transmissions, among others. I don't have a link at the moment, but if I find it, I'll post it.

    And, if this is true so far, and we cut a coax at some desired special tuned length, and we connect it to this unknown and mismatched load...we can expect the results to show us some stragne results...simply because we don't have a perfect match.

    IMO, this conditions is due to feed line transformation, and considering the rule above is based on the load being perfect...we will not get any good information in this process and what significance is the values we see anyway. As soon as we make some guess as to change the matching device, the values change again, maybe worse or maybe better. We are no better off than we woud be using a random line at this point.

    To be fair, at this point I am just trying to get lucky and make the right move, and if the rule is true you can do the same using a random length of line just about as easy as you can with a tuned line. This is because neither line will provide helpful information or guidance to what to do next, and as line length effects go you are only within a 1/4 wavelength, 90* degrees either way from your perfect match...if a perfect match is possible, and with a feed line attached to an end fed vertical...that ain't likely either.

    How will you ever know if a perfect match is truly possible with a newly constructed antenna? The only real way to know for sure what is going on at the feed point of any antenna is to check it at the feed point, and thus you remove possibly the worst variable attached to that area, mainly the feed line.

    You can test these effects simply by changing the feed line length, and if you see your results change, then the feed line is effecting the match, and those results may be close and don't matter much, but they are not perfect like the theory is based on. Plus the more the mismatch the more this is evident.

    The main truth of this is in the rule too: if you have a perfect match then any length will indicate the same load impedance results at any point along a line of any length, excepting maybe for a small amount of wire resistance as your line gets longer.

    If you have questions please ask, maybe I can explain better. This should be a simple idea, and most guys I talk to agree with you...the idea is simple and straight forward, just cut the coax to the desired frequency and it will show you correctly what the match is at the other end of the actural load. I would ask right here, even if you were lucky and the results were as noted above...what do you do with this information, what does it tell you. All it can possibly indicate is that the match is not what you wanted at this point. It might be worth writting down somewhere and compare it to another setting you might try. Most guys can't even tell which way they have to go with their matcher...that is why tuning instructions suggest we go both ways to test. Even then many won't likely get the implications.

    I just don't agree and think most guys ignore the theory as stated, what happens if the load is not perfect. IMO most antenna loads are not perfect and are at best maybe close, even though guy's claim they see a perfect SWR.
     
    #47 Marconi, Jun 11, 2013
    Last edited: Jun 11, 2013
  3. The Jerk

    The Jerk Active Member

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    The newer ones I've seen are crimped on...
     
  4. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    ? ok,... take two exactly the same meters, put one @ the feedpoint, and the other 1/2wl back , they "should" (more or less) read the same.

    now, change the 1/2 wl feedline to some other random value.
    does the meter @ the feedpoint change value?.... No
    does the other meter change value?..... perhaps, perhaps not.

    if the remote meter changes, then there are other problems with the antenna/coax, and the swr "appears" to have changed.

    the real reason to use the 1/2wl concept is to be able to REPEAT the value @ the feedpoint to a remote location for monitoring/testing purposes.

    bottom line: feedlines (unless they are designed to be used as tuning sections) will not affect the actual vswr, but, they may affect a randomly placed vswr meter READING.
     
  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    If you know how to use an antenna analyzer and understand the readings it gives you, you can determine whether the antenna presents a capacitive load or an inductive load and from that info you can determine whether the antenna needs to be longer or shorter. If you REALLY know antenna theory you can compute what value of reactance is required to offset the antenna reactance and use that info to build a tuning network for use at the base of the antenna. Not at all uncommon to select component values based on calculations that were done using info from an antenna analyzer and simply install the tuning network into the system and be VERY close to a perfect match requiring only a slight variation in component values to correct for stray capacitance/inductance. It's not something for the faint of heart or someone that is just beginning to understand antenna theory but it is indeed something that can be done with info gathered from an analyzer rather than just write it down as a reference. 99.9% wouldn't do that or hardly even know how to do it but nevertheless it is something that CAN be done with the info.

    FWIW when dealing with AM antennas we always used a Delta OIB (operating impedance bridge) to take measurements. The OIB measures antenna impedance under normal operating power for even more accurate results. This info is used to either keep track of changes, retune the antenna system, or to build an antenna tuning network from scratch.

    Delta Electronics, Inc. Model OIB-1 and OIB-3
     
    #50 Captain Kilowatt, Jun 11, 2013
    Last edited: Jun 11, 2013
    2 people like this.
  6. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    I think I could agree in part #6, if the load showed a perfect match, and there was 0 reactance for sure. I might even agree with your idea for the setup you suggest with a possible mismatch, where you suggest we would see the match duplicate 1/2 wave back from the feed point. However, loads are not always perfect and this complicates how long such a line should be, and for sure if we have to use math, and measure that length at the transmitter end of the system. The antenna load will surely determine where this 1/2 wavelength back point is, and every other 1/2 wave point in the line, but unless the match is perfect we can only guess. Maybe math will be close, but it is still a guess what that length really is...when the load is a mismatch.

    If the match makes no difference in this situation, then I'm dead wrong with my idea presented here. 6, I know your idea here would not be easy to conduct, but have you actually tested it with two similar meter, one at the feed point, and another 1/2 wave back...with a random load at the feed point? I admit this would be easy with a perfect match however, so don't go there with me.

    What if for example your load was say 50 ohms of resistance, and the reactance was 30 ohms, and Z = 38 ohms? There would surely be an imbalance of currents not resonate and not matched, similar to what a mismatched antenna might be. In such a case we would have common mode currents on the outside of the shield. Even if the line was electrically tuned at some desired frequency, it would likely be tuned at some other frequency from what we were seeing as noted in my example above. When that happens the match will change with every change in line length, and then how will your tuned 1/2 wave line deal with that? IMO, it will probably be just like a random length line.

    You guys can test this effect on your own rigs without going out of your house, just by adding a 3' - 6' jumper at the transmitter end of you setup. If the SWR changes, then your antenna is not perfectly matched, no matter if it shows a perfect SWR or not.

    I agree once you get your setup real close to a reasonable match or almost perfect, and you wish to monitor that precise matching value remotely, you could cut a line to the exact 1/2 wave or multiple for that precise match, and not only watch it, but you might even see it maintain showing the same values. We know the theory is true.

    However, just being close or almost perfect, you will likely have a similar result using a random length line. In this case, due to some slight mismatch you may see a small net value difference in the matching results, but again if the theory is true that value will remain just as steady, and just about as close as you would see with the tuned line. Because, the theory also says a perfect match will also indicate a mirror image of the matching results all points along a random line, except for some small loss due to resistance with longer lines.

    We just can't get away from the fact the theory is based on a perfect load, where all values along the line are equal, random or a tune 1/2 wave.

    I was considering most guys talk about such tuned electrical 1/2 wave lines for use in tuning. I know guys also use these same lines or multiples as a working feed line to monitor, and the same would hold true if they were lucky and made their cut precise to the 1/2 wave point, but there can be other negative factors to consider in working a tuned 1/2 wave or multiple feed line.

    #6, we can come at this from two different directions, but we may never find common ground for any agreement...you considering a tuned working feed line connected to a good to perfect match, and me considering tuning an antenna with an unknown match using a tuned 1/2 wavelength line.

    What you are carefully watching out for with your working line, would be the same thing I'm describing as a caution to consider in trying to use such a line for tuning. You would be looking for bad results as an alert if something went askew, and I'm just suggesting that is what you will see if you're trying to tune and antenna with a unknown mismatch that is not even close is exactly the same thing. It might not be consider bad, but it is just about as useless for information on tuning.

    See guys this stuff seems simple, but it can get complicated fast if you try to play simple with the theory in every situation.
     
  7. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Yep CK, you're right, but I my VA1 shows the sign and the value of X and I'm not always sure it is right, because sometimes I have to go the other way. Figuring this out using a 259B should be really simple too, but it could be really confusing before one really gets the idea. I've done a few videos using my VA1, why don't you help us out and give us some video tips?

    I suspect a lot of guys would love to be able to figure this all out simple...like you describe. However, I image most probably use their 259b for nothing, but an SWR meter. Most probably don't even bother to try and use it for more and better understanding.
     
    #52 Marconi, Jun 11, 2013
    Last edited: Jun 12, 2013
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  8. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    #6, presents a good idea to test here. It is not exactly a simple as my idea of adding a shot piece of coax to the transmitter end of your antenna system, to see what effect it has on your SWR match, but it could be done.

    If any of you guys follow up on these two ideas, why don't you post your results and tell what you see?
     
  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Marconi, I have a couple questions for you. Hookedon6 has presented some facts here, which I have agreed with in other threads etc, and yet you still seem to disbelieve some things that are widely known in engineering circles as being FACT. Why is this? Specifically talking about where you said "I think I could agree in part #6, if the load showed a perfect match, and there was 0 reactance for sure. I might even agree with your idea for the setup you suggest with a possible mismatch, where you suggest we would see the match duplicate 1/2 wave back from the feed point." You seem to be having a hard time excepting fact.

    Also what does load impedance have to do with determining a 1/2 wavelength of transmission line? You suggested this when you said " However, loads are not always perfect and this complicates how long such a line should be, and for sure if we have to use math, and measure that length at the transmitter end of the system. The antenna load will surely determine where this 1/2 wavelength back point is, and every other 1/2 wave point in the line, but unless the match is perfect we can only guess. Maybe math will be close, but it is still a guess what that length really is...when the load is a mismatch."

    Length is determined by frequency and not impedance.

    "See guys this stuff seems simple, but it can get complicated fast if you try to play simple with the theory in every situation. "

    It sure can get complicated fast especially when you don't know about what it is you are talking about and start throwing accepted fact out the window and look to substitute your own findings instead.


    Last question. just how accurate are you trying to get? You keep saying that no load is perfect and you don't think a perfect match can ever be obtained. I used to run an FM transmitter at 22,000 watts forward with 5 watts reflected for an SWR of 1.03:1. while not "perfect" perhaps to you I considered it about as good as it was going to get and considered it perfect. Trying to obtain anything better is bordering on the obsessive-compulsive. There are perfect numbers and then there are perfectly practical numbers.

    If I am wrong on any of this forgive me. I have had 5 hours sleep in the last 48 hours. :sleep: Things take a little longer to sink in. :cry:
     
    2 people like this.
  10. Lil'Yeshua

    Lil'Yeshua .......

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    Tune it for the freqs or freq you want
     
  11. Robb

    Robb Yup

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    How would one use a tuning stub for a 1/4w whip on a car?
     
  12. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Don't you mean WHY would one use a tuning stub for a 1/4w whip on a car? :D
     
    3 people like this.
  13. purenrg

    purenrg Member

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    Why would you need to do this w/a 102" whip ?

    You would only need to do this, if you were moving away from the current range of frequency's to a different range of frequency's, and still using the same antenna length whip?
     
  14. hookedon6

    hookedon6 W9WDX Amateur Radio Club Member

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    tuning stubs are no more "frequency agile" than a 102" whip.

    a "tuning stub" and a "trap" are two different animals**Jump_im**
     
  15. purenrg

    purenrg Member

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    I ? I'm using the term stub as that of a loading devise, to obtain a match.
     

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