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Antenna Feed Line Length

Discussion in 'CB and Export Equipment and Accessories' started by Paris, Dec 10, 2006.

  1. Paris

    Paris Member

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    I’m interested in anyone’s practical experience regarding the proper length for antenna feed lines in a mobile CB installation. The goal is, of course, to achieve the lowest SWR and highest antenna efficiency. I’ve read quite a bit of contradictory advice on the subject which seems to fall into several different schools of thought: (1) the line should be as short as possible, (2) the line should be 1/2 wavelength, (3) the line should equal the effective length of the antenna to which it is matched, either 1/2, 1/4, or 5/8 of a wavelength.

    The advice I’ve seen most often is that the lead in wire should be 17 feet long, but this does not seem to make sense if the object is to have the wire be some fraction of a wavelength. Follow my math –- and I’m happy for anyone to point out where I’ve gone wrong here. Assume we want to use channel 19 or 27.185 MHz as our starting point: the wavelength (in feet) = 984 over 27.185 or 36.196, the velocity factor of the coax is .82 (assume Belden 9258 RG-8X), thus the wavelength in the coax for that frequency would be 29.68 feet (36.196 X .82), and 1/2 of that would be 14.84 feet, 1/4 = 7.42, 5/8 = 18.55, and 3/4 = 22.20.



    Obviously none of these numbers is close to the common wisdom that the feed in line should be 17 feet, which is why I am asking for practical advice. My thought, FWIW, is that the line should be as short as possible without any sharp bends.

    [edited to correct math]
     
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  2. Hamin' X

    Hamin' X Active Member

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    Ah Grasshopper, you have discovered one of the great mysteries and your BS filter is working well. These people that tell you to use a specific length of coax, don't have a clue. I was reading the back of a packaged Francis 5.5 Hotrod antenna, the other day. It was saying to use different lengths of RG-58, RG-8X, or RG-59 coax, depending upon your installation. While it is true that when you co-phase two 50 ohm antennas, that they be fed with 75 ohm coax, of odd 1/4 wave electrical multiples, you have to know the velocity factor of the coax used. Just because it is RG-59, tells you nothing. Depending upon the construction of the coax, you could have a velocity factor of .66 to .95. A big difference when calculating electrical lengths. It is the same with all coax.

    The long and short of it is: With a single antenna, the correct length of coax is the amount it takes to reach the antenna. If you are having to use a specific length, then the coax has become part of the antenna and you don't want any part of it in a mobile installation. Anybody that tries to tell you different is just perpetuating a myth.

    Rich
     
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  3. KingCobra_CDX882

    KingCobra_CDX882 W9WDX Amateur Radio Club Member

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    Very true..

    this BS been going on forever(and many buy into it unfortunatly)

    single antenna......shortest needed coax feedline
     
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  4. King Mudduck

    King Mudduck FEAR THE DUCK!

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    http://www.ocarc.ca/coax.htm

    Check this out, it may help you and others further understand the mystery of antenna feed lines a little better.
     
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  5. Paris

    Paris Member

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    That's a very nifty dipole calculator. Thanks!
     
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  6. trash

    trash Member

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    We have a very good tutorial on our ATV club web page too that deals with testing transmission lines. Everything you might ever want to know from the basics up. There is even a section on why you notice a difference with your SWR meter when you cut the cable to different lengths.

    http://www.satvg.org/smf/index.php?topic=9.0
     
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  7. KingCobra_CDX882

    KingCobra_CDX882 W9WDX Amateur Radio Club Member

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    A good tutorial
     
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  8. C W Morse

    C W Morse Active Member

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    Coax length is a case of getting tangled up in the details! :D Yes, you can see a "difference" in the readings. "Yes", there may be some cases where doing so is necessary. But! It isn't as important as some of us make it out to be! :D Except for some specific cases, I have never even given the slightest thought to "trimming" coax, indeed, had never even HEARD of doing that until I ran into the CB guys that just swore that you had to do that. Gee! Don't tell my antenna, or it might quit working! :p If you want to fool with coax trimming, that's fine. You won't get an argument from me! But, for the most part, you should trim the antenna elements/whip to resonate your antenna. The coax-cutting formula is:

    "A length directly proportional to the distance from the radio to the antenna!" ;)

    73

    CWM
     
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  9. Hamin' X

    Hamin' X Active Member

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    Just as an example of what I have been saying about coax length, here is what happened to me several years ago.

    I installed a screwdriver antenna on my pickup bed rail and used it with my 706m2g. Everything was fine 80-10 except for 7250-7300kHz on 40 meters. Now I check into a net on 7268.5 every day, so this situation was not acceptable. The internal SWR meter on the 706 and the FS meter that I tune the screwdriver by both agreed, things went south above 7250. Back to the shack to grab the Bird 43 and a 12" jumper for a closer look at the antenna feedpoint. Hooked up the 43 and jumper cable and WTF, it tunes perfectly. Back and forth a few times, clean the grounds, check connections, no change. With jumper and meter in-line , the antenna worked perfectly. I replaced the feedline with one about 16 inches longer and it still works perfectly.

    Moral of the story: If it works, use it. If you want it to work every time, figure out why it worked. In this case, there was obviously something wrong with the installation, but the coax change made the radio happy and the signal reports are great, so it is not worth my time to delve any deeper into the problem. As far as I am concerned, problem solved.

    Rich
     
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  10. freecell

    freecell BANNED

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    SWR has virtually nothing to do with antenna radiation efficiency. once again this is demonstrated best by terminating the load end of the feedline with a value of resistance that is identical to the Zo of the line, as in the case of an rf "dummy" load. it's further demonstrated by the fact that free space has an intrinsic impedance value of approximately 377 ohms. it's funny to see you all pontificating over your myths when the real problem presented by SWR in any system is the reduction in the amount of power delivered to the load, a problem that is easily addressed and remedied once it is realized that whenever the load does not equal the Zo (characteristic impedance) of the line that the line impedance is determined by a widely varying range of values of E/I all along the line which can be used to restore full power output from the (transmitter) source to the load. that's the real problem.

    antenna efficiency is determined by BIRP. Balance, Impedance, Resonance and Pattern, only one of which has anything remotely to do with SWR. what has escaped most of you is the fact that once the impedance of the line becomes anything but Zo and if the values of E/I along the line are known that those points can be used to terminate the line and provide the necessary conjugate interference to match the output of the transmitter to a line input impedance that is anything but 50 ohms. this solves the problem created by the load mismatch presented at the other end of the line.

    an SWR meter doesn't measure antenna efficiency any more than a flat SWR is a guarantee of optimum antenna performance. if you're feeding a balanced antenna with coaxial feedline (which destroys the pattern) and the antenna is not resonant at the frequency of choice then achieving a flat swr is meaningless. by the same token, antenna systems are floating around in space with swr present that would make any cb'er (and some hams) squeamish while the transmitters feeding them are delivering over 90+% of their available power to those same mismatched antennas.

    the principles at work here are the same ones that require the francis line of antennas (under 8') to be used with specific lengths of differing types of feedline for best performance (and lowest swr) when used singly and the same principles involved in a situation that arose recently when one or two customers called me complaining that their PowerStikII antennas (which matched well in their previous tractor rigs) were not seeing the usual low swr in their new Freightliner XL tractors with the original 18' lines. quite the contrary, the swr has risen to 3:1 when relocated to the new tractors. one of the drivers with which i have spent time on several occasions explaining to him how antennas and feedlines operate, particularly under mismatch conditions, decided to take my advice and replace the 18' RG8M lines with 9' RG58 lines and was happy to see that the swr (as measured from the line input) dropped from over 3:1 to just over 1:1 when the 9' lines of RG58 were installed.

    in a line with a Zo of 50 ohms and a load that represents anything but 50 ohms the differing readings that are seen using the swr meter when placed in various spots in the line are only fooling you if you don't know what they're trying to tell you and specifically what that is is that since VSWR=EMAX/EMIN what it's trying to tell you is that the impedance along the line varies with the placement location of the bridge in the line, which makes absolute sense since in an unmatched line impedance (Z=E/I) changes from one spot in the line to the next.

    the only time a feedline with a Zo of 50 ohms is exactly that occurs only under two sets of conditions: either the transmitter, (source) the antenna (load) and the feedline are all presenting an impedance of 50 ohms or the feedline is unterminated at either end in which case the characteristic impedance of the feedline is solely dependent on its physical characteristics only, length not being one of them. whenever a source and a load are attached where either deviates from the Zo of the line, then and only then does the range of dynamic impedance values appear on the line.

    Z (Impedance) = E (Voltage) / (Divided By) I (Current)
    for example, given that E and I measured at a particular spot in the line are both equal to 50 volts and 1 amp respectively, the impedance at that point in the line is 50 ohms. i can assure you this is only the case where both the source and the load are both equal to the Zo (characteristic impedance) of the line.

    where it is otherwise the line reverts to a rather wide ranging set of values that are easily visible at *multiple spots along the line, especially if that line is *longer than 180 degrees at the operating frequency in question. real resistance values are known to be present minimally anywhere from 33 - 78 ohms wholly dependent on the length of the line for absolute terminating values.

    as to the question of the tuned 1/2 wave line. do you want to use it? if you do then know this. when you do the exact values of resistance and reactance that are present at the feedpoint of the antenna will be mirrored almost exactly at the transmitter input to the line. that's not a bad idea if you can measure and know that the resistance at the feedpoint is + or - 50 ohms and the reactance is near a zero net value. if they're say 33 ohms and j-15 ohms capacitively reactive you have to ask yourself a question. do you really want those exact conditions to exist at the transmitter? only if the transmitter is equipped with its own adjustable PA matching circuit. with a fixed 50 ohm transmitter output and 33 j-15 at the antenna the last thing you want to use is a tuned 1/2 wave line, unless you want reduced transmitter power output or maybe you just want to be able to see the conditions at the antenna feedpoint as it may be inconvenient to make measurements at the antenna for one reason or another. if the transmitter is fixed 50 ohm output then you may want to consider the fact that what's going on with the match between the transmitter and the line might be of equal if not somewhat more concern since it can be managed from either end of the line anyway. that's what the AT in the nomenclature in TS850SAT does. it's not really an antenna tuner per se since what it really does is makes sure that the transmitter is always working into as close to a 50 ohm line input as possible at all times, regardless of the mismatch at the load
    up to a certain point. ranges of matching values achieved by these "antenna tuners" are anywhere from 16 - 150 ohms and wider. the same thing to a lesser extent (33 - 78) can easily be done by knowing where these values of impedance can be found along an unmatched line under similar conditions.

    every length of feedline is a matching transformer....Cebik.
     
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  11. bob85

    bob85 Supporting Member

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    thanks freecell, now how much pain am i in?? lol.
     
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  12. Paris

    Paris Member

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    Death, Taxes, and Coax Woes

    In an ideal system, the length of the coax antenna feedline should not make any difference to either the antenna’s SWR or its efficiency, see ARRL Handbook, 2006 Edition, Sec 21.1. However, as others have pointed, out its length is often critical real-world mobile installations. I suggest (as Hamin' X did) that this can only happen if the coax shield is radiating RF energy. In order for the coax antenna lead to work as an ideal feedline, the current carried on the outer braid must be equal and opposite from the transmitted signal traveling along its core; otherwise, the signal attempting to radiate from the core cannot be cancelled out. The ideal feedline does not radiate any of the transmitted signal, and only the antenna forms the resistive load. However, if the feedline does leak, it must then be considered part of the antenna, and it’s length will certainly effect both efficiency and SWR.

    When you think about it, it seems inevitable that you will not be able to achieve an ideal ground in your car or truck. Almost certainly the chassis to which the coax shield is grounded will carry stray RF from whatever electronic control systems are found in modern cars, including the ignition, the CPU/ECU, the fuel injectors, the fuel pump, among others; not only this, but chassis itself is probably segmented, and unable to act as an efficient ground plane. The result will be common-mode currents in the coax shield not be present in the core, which will, in turn, allow the transmitted RF to escape from the core. In short, your feedline in a mobile installation will almost certainly become part of your antenna.

    Since the nature of the stray current introduced onto the outer braid of the coax feedline cannot be predicted, it would follow that it is impossible to predict any specific length of feedline necessary to achieve a low SWR. Thus, the proper length of your feedline will have to be determined by experimentation on a case by case basis, but it’s important to understand that it will make a difference, nevertheless.

    Perhaps one solution is to coil the feedline so that it acts as an RF choke, as I’ve seen suggested in some articles, but then other writers advise strongly against doing that. Has anyone tried this?
     
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  13. Toll_Free

    Toll_Free Active Member

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    And the long of it is, if you use a piece of coax that is not a multiple of a quarter wave, electrically, and you don't measure your VSWR at the FEEDPOINT of the antenna, you don't have a friggin clue as to what your real feedpoint match is.

    Don't buy into this. The proper way, is to use an electrical quarter wave to match your system. Then, use as long a piece as necessary, if you want to. I stick with the electrical quarter waves. Works for me, and I KNOW my measurements are accurate, not reading the capacitance values in the coaxial cable itself, and not reading a skewed reading from the feedpoint reactance.

    --Toll_Free
     
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  14. Toll_Free

    Toll_Free Active Member

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    Well, that's one way to look at it.

    I guess the other way is that coaxial cable length can play into account when your trying to get a flat match, and don't know what the complex impedance is on the other side, huh?

    Let me get this straight. When you measured as close to the feedpoint as possible, it worked OK, and you got a smaller amount of reflected power.

    But, when you put a piece on that was "convenient", your radio saw a worse match.

    And you think that your coaxial cable isn't being used as a radiator? As part of the counterpoise of your antenna system?

    Please, I beg you. Explain this? How a good match at your feedpoint can be repeated as a bad match at the 'OTHER' end of the coaxial cable, but it really doesn't mean anything, other than the foldback circuit in the radio will cut your power back, you won't see max power transfer, and you can see hotspots on the coax, the radio, etc.

    Not to mention, this causes more stray RF around.

    --Toll_Free



    --Toll_Free
     
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  15. Toll_Free

    Toll_Free Active Member

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    Wow. Someone that actually reads, comprehends, and gets it.

    And you don't post with an amateur radio callsign? Howd you get so smart :)

    --Toll_Free
     
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