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Best Moblie Antenna for OTR Truck

Discussion in 'CB Antennas' started by ctvanover, Oct 9, 2016.

  1. CTR

    CTR
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    This thread may be too fragmented to exchange information. Antennas, feed lines, and radio systems all work a certain way. They follow rules that cannot be broken and cannot be changed.

    If someone comes up with something that goes against how things really work, and this certainly happens a great deal, it is because they either are not communicating accurately or they are not measuring correctly.

    If someone picks one particular thing and talks about it and only it, so the thread doesn't diverge into a 10 different things, then I'm sure almost everyone could get on the same page. The problem is confining it to ONE thing at a time until that ONE thing is reasonably finished.

    There are seven issues I see here, although I may not have seen all of them:

    1.) Receiving measurements of an antenna system
    2.) Transmitting measurements of an antenna system
    3.) What SWR actually does and how reactance affects SWR, and how it is measured
    4.) The cause of loss
    5.) Power transfer
    6.) How an antenna radiates
    7.) How signal-to-noise is established

    Each of these could take anywhere from a few pages to a few chapters to explain. My good friend Walter Maxwell, who has now passed away, spent a great deal of his retired life trying to "unconfuse" people about SWR, but people still don't get it.

    If we could PICK ONE THING this might become a productive exchange for most people, but I don't see that happening. I don't see that happening because some people have conclusions that completely fly in the face of how things work and they are convinced basic physics and science is meaningless. I have no idea how they got there, perhaps through incorrect measurement methods, but the end result is they are where they are.

    If someone could pick ONE thing, it would be possible to have a nice discussion. Otherwise, I'm afraid this is all just unproductive wasted time.


     

  2. undertaker

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    I would like to understand SWR. I have always been told the lower the better..

    Bobby
     
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  3. CTR

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    SWR is the easy part if people don't get too caught up in wave theory they don't need or are not ready or able to understand. It still really takes a good book, like Maxwell's or even the ARRL books. I would stay away from Internet and especially YouTube.

    In a long feedline, SWR above 1:1 ratio means the feedline has voltage and current peaks and valleys, instead of a smoothly decreasing voltage and current level along the line as we move away from the signal source. Almost all feedline loss at HF is from conductor resistances. Standing waves increase the feedline loss because they increase the current at peaks. Way up in VHF or UHF, or perhaps with a wet cable, dielectric losses also become important.

    In a transmitter, in an SWR meter, or right at an antenna, SWR means the impedances are not matched to the system design impedance. It is actually different in a SWR meter or at an antenna or transmitter than the SWR meaning in a feedline, because there is no place for standing waves to "stand". This is an important point, because your SWR meter does not really measure SWR. It measures the difference in its calibrated impedance to the circuit impedance where it is connected.

    I know that can be confusing, but here is an example that proves an SWR meter or analyzer does not measure SWR. Let's say we take a 75 ohm line and terminate it in a 75 ohm pure resistance. That line, with a 50 ohm SWR meter or analyzer of any type, will measure 1.5:1 SWR anywhere along the line. It tells you the line has an SWR of 1.5:1 but the SWR is really 1:1.

    Now take the same 75 ohm line and terminate it in 50 ohms. Now the line has standing waves. If the line had zero length to the meter, SWR would be 1:1. With a perfect line and perfect meter, 1/4 wave away from the 50 ohm load, SWR would measure 2.25:1.

    We see our SWR meter tells us anywhere from 2.25:1 to 1:1 as we move along the line, yet the actual SWR is 1.5:1.

    A transmission line, if it is a lot longer than 1/4 wave, always has more loss with higher SWR. How much more loss for a given SWR depends on the actual line. It is possible to have a 10:1 SWR in a line and not have noticeably more loss in the line, but your equipment probably will not like it! Some lines have a lot more loss with SWR, if they are lossy to start with. Now here is something few people know. Electrically short transmission lines, less than 1/4 wave long, can have less loss when they are mis-terminated. This is because a mistermination can reduce line current, and at HF the dominant loss is conductor resistance. This isn't important, except to show how little most people understand SWR, loss, and SWR meters. Especially people who argue. It especially is difficult to find anyone who actually understands SWR.

    But for the transmitter, lowest SWR is almost always best. We pretty much always want the lowest possible SWR at the transmitter measured on a meter calibrated to the transmitter design impedance. We just don't want to get that low SWR by adding resistance losses, but the transmitter will certainly be operating where it was meant to be run.
     
  4. M0GVZ

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    Eh? RF is AC. The voltage and current are constantly rising and falling and have peaks and troughs along the full length of the feedline if you were to freeze it at one point in time and analyse it at multiple points along its length.
     
  5. The DB

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    I agree with these books as sources. I would suggest starting with the ARRL books, and move on to Maxwell's books/articles. In my opinion, to truly understand Maxwell's writings requires some prior knowledge on how things work. I have also, more than once, stated that the internet is not a reliable source, at least not in and of itself. There is good information out there, but not all of it is good.

    Actually, the key to his statement is the non-perfect SWR he mentioned. When you don't have a perfect SWR match you will have voltage and current peaks and valleys that don't move on the feed line. If the system had a perfect SWR match, you would be correct, but in this case it specifically doesn't have said perfect SWR match. Here is a video that will demonstrate this, among other things, using a Shive Wave Generator. Essentially, two AC signals of the same frequency traveling on the same medium in opposite directions will always have that type of effect.

    Truth

    There are devices that actually measure SWR this way, I have three of them, however I think it is important to note that the standard SWR meters used in CB and Ham radio do not actually measure SWR in this way. They are measuring something else entirely and using the forward/reflect switch as a means to create the ratio that is SWR, essentially, as seen from a different point of view. There are several ways of getting SWR, your described method and the SWR meters in actual use by the people who frequent this forum are but two of them. In the end, different methods, same result. The commonly used meters are not, and are in fact unable to, measure impedance directly. If they did, they would cost as much if not more than the antennas themselves...

    As most SWR meters used by said radio operators would have little to no effect on the impedance of the feed line, and don't actually compare the impedance of a signal to the impedance of the given meter, your next few statements will not be correct when using said meters. If you have a meter that does measure impedance directly, they would be correct.

    It would be correct to say that any transmission line that has losses has more loss with higher SWR. I see what you are saying, however, you don't need to limit it to longer transmission lines. The fact that there is more signal that has been reflected back on to a lossy medium means that there will be more losses in play. If you want to say it will be more noticeable with wires of a given electrical length or longer, that is fine, but even if you don't notice it, the additional losses are present either way.

    And here you touch on one of the things that SWR can actually be used to determine. Is is safe to plug this feed line into the radio? You don't need the perfection that a lot of people insist on getting, you just need to be low enough to be safe.

    The other thing SWR tells you is if something has changed in the system. If you had an SWR of 1.5, and now suddenly you have a SWR match, or perhaps your reading are now way off, this tells you something changed, and is a sign you should check out said antenna system.

    A third thing that SWR can be used for is checking to make sure the impedance is near what it should be. Maxwell's writings (the Another Look at Reflections .pdf and his books) have a chapter called "Low SWR For the Wrong Reasons". That being said, this won't apply to most people here as most wouldn't know what SWR they should have to begin with, or they primarily use antennas that are designed to present a low SWR where the antenna is intended to be mounted. This is more for when designing and building antennas.


    The DB
     
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  6. Captain Kilowatt

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    Maybe I am wrong but I am thinking he means voltage and current minima and maxima as opposed to peaks and valleys as in an AC waveform.
     
  7. CTR

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    Traditional practice when discussing sinusoidal AC or RF voltage or current levels is to assume RMS voltage and current unless otherwise stated. What you are saying only complicates a discussion by bringing time and distance into things needlessly.

    This is not a discussion of time varying levels, but rather what a meter sees..
     
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  8. undertaker

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    Ok so am MFJ analyzer is not accurate? Just wondering...
     
  9. CTR

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    Properly used, calibrated, and in good shape they are suitable for Ham and CB use. They have a very stable SWR bridge, but the bridge is normalized to 50 ohms. Like all bridges, it does not actually read standing waves. It reads deviation from the normalized impedance of 50 ohms at the point where it is connected.

    It actually measures or "knows" four things:

    1.) (Voltage) imbalance in a 50 ohm Wheatstone-style resistor bridge (Vr)

    2.) Voltage across the series resistor from the source in the bridge (Vs)

    3.) Voltage across the load (Vz)

    4.) RF generator source voltage (Vgen)

    The most accurate readings are SWR, but we have to remember like ALL single point bridges it really measures deviation from an impedance, in this case 50 j0. So. like all single point bridges, it is not actually reading the ratio of standing waves.

    The least accurate reading is reactance when reactance is low. This is because, to calculate reactance, it has to look at Vz and Vs. If the sum of Vz and Vs is more than Vgen it knows the load is reactive. A complex formula is use to calculate reactance, but even a 1 bit error out of 256 bits can be several ohms error. This doesn't matter, because we know if SWR is low the reactance also has to be very low. It is impossible to have high reactance and low SWR. As a matter of fact, the software in the 259 is supposed to (I don't write code, but I suggest how it should be written) look at "SWR" and prevent reactance from being high if SWR is low, because that would indicate a calibration or "bit" error.

    This is partly why I get a hoot out of reading tuning for low reactance is better than tuning for low SWR. The other reason is any changes in reactance around 1:1 SWR are meaningless to system operation. If the reactance becomes meaningful, the SWR cannot possible be 1:1. This is why I am pretty uncomfortable with any claim tuning for minimum reactance magically produces maximum field strength.

    An explanation of how the 259 works is here: http://www.w8ji.com/mfj-259b_calibration.htm
     
  10. HARDROCK

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    Thanks for posting CTR . good to see you back.
    Since you have educated us on swr and meters,
    My post about reactance being wrong. I think i should have posted impedance matching.
    Maybe you can enlighten us on the benifits of matching,

    Tony 73
     
  11. 543_Dallas

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    I think he did.
     
  12. HARDROCK

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    I must have missed it. Thanks
     
  13. The DB

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    Here is a question. I am assuming that said device is using an ADC, and by the sound of it it is an 8 bit ADC, which has a 256 bit range. Would it really be so difficult to modify the device to be use-able with a 12 bit ADC? That would give it 4096 points of measurement, that is 16 times the measurable points in the same voltage range. Such a change would potentially have a significant effect on how accurate the measurement is would it not? And they really don't cost that much these days, I know some micro controllers that cost $20 or less that have 20 plus 12 bit ADC's built in...

    I wasn't going to bring this back up as I really didn't want to get in to this again, so I will ask a question in place of what I would normally write.

    Before I do I want to make sure one thing is clear, I never once said that resonance was where maximum field strength was, in fact, I agreed that resonance has nothing to do with field strength, and I believe I said I can demonstrate that. By extension, the problem with your argument is that SWR, when it is in an acceptably low range, will have exactly the same effect as resonance, you won't notice the difference when tuning for minimum SWR. You said as much above and I agreed with you.

    Why do you so strongly espouse one when choosing from either option will make no noticeable difference? You have stated a fact out of one side of your mouth that you won't notice the difference, while at the same time you are saying that you should use this one and only method, period, end of story out of the other side of your mouth. That is the problem I have with what you are saying, you are trying to have your cake and eat it to. I should mention at this point that I don't personally use either of these points to tune an antenna, if you want to know what I tune for you will have to find it above in one of the previous posts I mentioned it.

    If you look above, this discussion between us is really pointless as on a technical level we agree on pretty much if not everything. How many times did you say something and I respond with "I agree" or "yes thats true" or "I know"? Keep to the technical side of things and we are golden. Its when you put forth opinion as fact when the facts you also present disagree with your own opinion that we bump heads.

    Now this is just me, I can't speak for anyone else that had issues with what you said.


    The DB
     
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  14. HARDROCK

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    Here it is 543

     
  15. 543_Dallas

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    You beat me to it but the statement about how SWR meters work caught my attention too.

    I feel this is misleading. I believe my antenna analyzer works this way but every swr meter I own measures reflected power and is scaled to give an swr reading in relation to forward power. The same thing I do by turning the slug in a Bird 43.
     

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