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EZNEC Models of Your Favorite CB Antennas

Discussion in 'CB Antennas' started by CTStallion, Sep 5, 2010.

  1. CTStallion Active Member

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    Well, the XYL didn't have any "Honey-Do's" for me this Labor Day weekend, so I figured I'd take a few moments and model some of our favorite CB Antennas.



    Please note that I am using EZNEC DEMO VERSION, which limits me to only 20 segments, therefore all antennas were modeled at 20' above REAL GROUND (at the feedpoints), using #10 awg insulated wire as the elements (exception: Dipole was modeled using #14 insulated wire).

    FIRST ANTENNA: 1/4 WAVE OMNIDIRECTIONAL VERTICAL WITH FOUR (4) 1/4 WAVE RADIALS (again, this antenna is at 20' high at the base).

    1st attachment shows the antenna and it's currents

    2nd attachment shows TOA (Take Off Angle AKA: Elevation Angle) provides a maximum gain of 2.12 dBi (dB gain over a theoretical isotropic radiator) at a 42.0 deg TOA

    3rd attachment shows only 1.0 dBi gain at a 12 deg TOA (Elevation Angle, Launch Angle... etc...)

    So... how do we make the 1/4 Wave Vertical perform BETTER....

    ENTER THE STARDUSTER (AKA: M-400), next installment
     

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  2. CTStallion Active Member

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    STARDUSTER (M-400 & derivatives) Model

    To make a 1/4 wave vertical perform better (i.e., lower TOA, better match to 50 ohm coax, less likelihood of common mode currents on the outside <shield> of your coax), we slope the radials downward to achieve the following model:

    1 Shows the currents on a Starduster
    2 Shows that by sloping the radials downward, we have lowered the TOA max gain down to 13.0 degrees with a maximum gain of 1.31 dBi.

    But... how do we then INCREASE THE GAIN OF AN OMNIDIRECTIONAL VERTICAL?????..........

    ENTER THE 5/8 WAVE OMNI (next installment)
     

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  3. CTStallion Active Member

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    The 5/8 Wave Omnidirectional Verticals

    The 5/8 wave vertical I am modeling here is 21 feet in length, and incorporates four (4) 1/4 wavelength horizontal radials.

    Picture 1 shows the antenna and it's maximum current flows
    Picture 2 shows that maximum gain (at a 20' feedpoint over real ground) is 2.86 dBi at a TOA launch elevation angle of 32 degrees.
    Picture 3 shows that a a 10 degree TOA, the 5/8 wave beats BOTH the 1/4 wave groundplane AND the Starduster (M-400) by offering 2.35 dBi gain at a nice low 10 degree TOA.

    Please do note however that a 5/8 wavelength radiator is NOT a direct match to 50 ohm coaxial cable; a matching device is necessary. This is WHY antenna manufacturers (i.e.: Maco, I-10K, Sirio, etc.) place a coil at the feedpoint to make it resonate (electrically appear to be an odd multiple of a quarter wave to the coax) in order to obtain a 50 ohm match. But why not just make the radiator length 3/4 wave? Because that would create unwanted high angles of radiation which we do NOT want. More on that later.

    So... is there any way to improve on a 5/8 wave vertical with horizontal radials???

    Enter: THE 5/8 WAVE OMNI WITH SLOPING RADIALS in the next installment.
     

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  4. CTStallion Active Member

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    5/8 Wave Vertical with Sloping Radials

    The model below shows what would happen if you sloped your 1/4 wave radials on a 5/8 wave vertical. The TOA does NOT drop (like it did on the Starduster vs. 1/4 wave Vert), BUT the gain DOES increase somewhat, and the sloping radials will certainly help to decouple the coax.

    So.... ever wonder what would happen if you added one of the radial kits to your IMAX 2000 or your ABS-1600??? You got it.... no drop in TOA, merely MORE OMNIDIRECTIONAL GAIN and less likelihood of TVI. (Both of which are good things <as Martha Stewart would say>.

    1 is the Current profile of the antenna
    2 is the Max gain and TOA profile
    3 is the low angle performance

    (Please note that these models do NOT apply to the Anttron or Solarcon 99 style antennas, Ringo Rangers, or any other kind of END FED HALF WAVES; merely 5/8 wave (21 - 24 ft radiators).

    Since omnidirectional GAIN is a 'good-thing', is there any way to increase the gain of a 5/8 wave antenna? I'm glad you asked............

    Enter the 5/8 WAVE VERTICAL WITH FOUR (4) SLOPING 5/8 WAVE RADIALS in the next installment.
     

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  5. CTStallion Active Member

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    5/8 Vertical with Four (4) 5/8 Wavelength Radials

    Okay, this antenna is LARGE. The vertical radiator will be 21 - 24 feet in length, and each of the four radials will be the same length as the vertical element (21-24 feet each).

    But, does it work?

    I'd have to say that since it only offers an additional 0.03 dBi improvement in gain, and probably would be a bear to match to coax, I'd have to say it ain't worth it. Well, then WHY did I bother modeling it? Well, let's say you want to place your vertical on a 20' pole which you need to guy.... u could use 5/8 wave radial guy wires and take advantage of the extra gain offered in the process.

    Picture 1 shows the current profile
    Picture 2 shows the maximum gain & TOA profile

    So.... having exhausted the 5/8 wave arena.... what say ye CTStallion about the .64 wave antennas (i.e.: IMAX 2000??? and the like).

    .64 Wavelength antennas revealed in the next installment.
     

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  6. Shockwave Sr. Member

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    I'm curious as to the effect the sloping radials on the 5/8 wave has on the impedance and VSWR at the resonant frequency? Thanks for sharing your work in the forum too.
     
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  7. CTStallion Active Member

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    The .64 Wavelength Vertical Radiator

    Okay, so does a .64 wave radiator really outperform a 5/8 wave antenna? If so, then WHY do all the so called antenna 'Gurus' out there claim that the maximum gain figure for a monopole element is at 5/8 wave?

    The answer lies in the fact that there is little difference between 5/8 wave and .64 wave. Simple mathematics reveals that 5/8 is deconstructed as .625 wave. Therefore, how much difference is there between .64 and .625..... hardly any.

    But, for all you IMAX 2000 lovers out there, I offer the following:

    Picture 1 shows a .64 wave (24' long vertical radiator) with four (4) 1/4 wave horizontal radials (I know, I know... the IMAX 2000 doesn't have any radials, but I needed to model the antenna against a counterpoise). Besides, remember what we learned before on how the SLOPING radials will increase the gain, but not the TOA. Picture 1 also shows the rf current profile of the antenna.

    Picture 2 shows the max gain to be 3.37 dBi at a TOA of 28 degrees. Hmmmm.... the 5/8 wave vertical had only a 2.86 dBi max gain at a TOA of 32 degrees, so maybe there IS something to a .64 (24' long) vertical radiator vs. a 5/8 wave (21' long) vertical radiator. WOW!!! (lightbulb in head glows brightly now). And, if I put SLOPING RADIALS ON MY IMAX 2000, there's no other omnidirectional antenna that can touch me (maybe)

    Picture 3 shows that the gain at a low 10 deg TOA of my .64 radiator to be 2.42 dBi. How does that compare to a similar radial'ed 5/8 wave..... a mere 0.07 dBi increase... not even perceptible (which is why I use an Army Stick instead of an IMAX 2000)..... but I digress.

    But again, as with a 5/8 wave radiator, 50 ohm coax doesn't like the .64 wave radiator, so what if we increased the length of the radiator to an odd multiple of 1/4 wave in order to see 50 ohms?

    ENTER THE 3/4 WAVELENGTH MONOPOLE in the next installment.
     

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  8. CTStallion Active Member

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    3/4 Wavelength Monopole

    In the model below, please find the 3/4 Wavelength Monopole (a beast approximately 27-30 feet in length) fed against four (4) 1/4 wave horizontal ground radials. Again, the feedpoint is at 20 feet above real ground.

    Picture 1 shows the antenna RF currents
    Picture 2 shows the awesome gain, BUT, it is all at HIGH TOA OF 55 degrees. This is pretty undesired.

    How can we lower the wave angle of a 3/4 wave monopole to make this large beast useful on 11 meters????

    ENTER THE SIGMA IV (aka Sigma 4, Vector 4000, LW-150, etc) in the next installment.
     

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  9. CTStallion Active Member

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    The 'Mysterious' Sigma IV, Vector 4000, etc. Demystified

    Let me start off by saying: "Don't be a HATER"... I do indeed realize that there are several of you in this forum who absolutely adore this engineering marvel, and I do indeed respect and admire the performance of this antenna. This antenna is BIG.... I'm talking in the order of 27-30 feet or so in length from top to bottom.

    Basically, as the model will show, the Sigma IV is basically a 3/4 wave vertical monopole with four (4) radials which SLOPE UPWARD to provide a 50 ohm match, decouple the coax, and (MOST IMPORTANTLY), take that high elevation angle of a 3/4 vertical monopole, and put it as close as possible to the horizon at a nice 10 degree TOA.

    What's also impressive about the Sigma 4 design is that there is virtually NO LOSS IN A MATCHING COIL OR MATCHING TRANSFORMER which one would need to match 50 ohm coax.

    Figure 1 shows the rf current profile of the Sigma IV

    Figure 2 shows the maximum gain of 2.79 dBi at a nice, low, 10 degree TOA.

    But CT.... that's LESS GAIN THAN MY 5/8 WAVE!!!???!!!.... NOT ON THE HORIZON IT AIN'T... and therein lies the redeeming quality of this 30' BEAST! It concentrates most of your signal on where you want it: ON THE HORIZON. Additionally, one could also conceivably argue that since antennas radiate mostly from the points of HIGHEST CURRENT, the length of the Sigma 4 puts that current HIGHER UP than, say, a Starduster would, so you get more line of sight for your ground wave to mobiles and/or other base stations.

    Well then, what about the Avanti Astroplane... doesn't it radiate from the TOP? The short answer is: NO. The Astroplane, unlike the Sigma IV, is a simple 1/2 wave J-Pole. It radiates from the MIDDLE, as all 1/2 wave antennas do.

    Okay... we've come a looooooooong way here, and I don't wanna burst anybody's bubble or anything, but is there something BETTER than the Sigma 4????

    The answer is so simple it will absolutely shock you:

    <drumroll please>......................................

    Enter: THE SIMPLE 1/2 WAVE HORIZONTAL DIPOLE
     

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  10. CTStallion Active Member

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    The Old Reliable 1/2 Wave HORIZONTAL Dipole

    Up to this point we have been investigating VERTICALLY POLARIZED antenna patterns. Now it's time to share a secret with you..........

    A vertical antenna does NOT obtain "gain" from ground reflection, merely ground LOSSES.

    We run vertical on CB because we want to work other bases and mobiles which are vertically polarized as well, and we want to take advantage of ground wave propagation for local / area work.

    But..... what about DX? What antennas are best for 'ol SKIPLAND? The answer is a FLAT TOP: and you don't even need a 5 element monstrosity on a 60' tower to do so.

    The SECRET is: GROUND REFLECTION GAINS OF HORIZONTALLY POLARIZED ANTENNAS.

    Forget every freakin thing you've ever been taught about antenna (GAIN) from the antenna manufacturers (who only exist to separate YOU from YOUR hard-earned cash) and learn this: HORIZONTALLY POLARIZED ANTENNAS, when mounted at least 1/2 wavelength above ground, will usually outperform vertical antennas at the same height (in the direction of maximum radiation) in DX LAND due to ground reflection GAINS in a horizontal configuration.

    Okay, let me explain further: the antenna manufacturers have always taught you that their antennas exhibit gain relative to a (theoretical... none exist) ISOTROPIC RADIATOR, and express their gain figures as dBi (the i means: relative to a non-existent isotropic radiator... it's all theory). Why don't they compare their gain figures to a simple HORIZONTAL half-wave dipole??? BECAUSE A DIPOLE ALREADY EXHIBITS OVER 2 DBI GAIN OVER AN ISOTROPIC RADIATOR IN FREE SPACE!!! So, they would LOSE AGAINST A DIPOLE....BIG TIME!!! What's even MORE AMAZING is that since none of us live in "FREE SPACE", we have REAL GROUND to contend with... a horizontal dipole will exhibit gain in broadside directions in and can exhibit low angles of radiation when properly spaced over real earth.

    Here's another "shocker".... you've probably always been told that the HIGHER up your antenna, the better.... right? Well, that's true for LOS (line of sight) for verticals when working ground wave..... BUT, for ground reflecting DB GAINS with HORIZONTAL antennas, the height above conductive earth whereby you start to obtain some nice low angles of radiation begins at a modest 5/8 wavelength above ground relative to frequency. For example, on the CB Band, 11 Meters, 5/8 wave is only a modest: 21 feet or so above ground.

    So, (as I said from the beginning), all the antennas I've modeled thus far have been at only 20' above ground, so this simple horizontal dipole too is only 20' above ground. How does THAT work? (Remember, a dipole, bazooka, windom, or any other 1/2 wave HORIZONTAL antenna will maximize it's radiation pattern BROADSIDE, so for omnidirectional coverage you'll need two (2) at 90 deg angles to each other, or you can just run 1 dipole and 1 vertical to fill in the 'holes').

    A simple 1/2 wave HORIZONTAL dipole exhibits a whopping 8.31 dBi gain at a 25 degree TOA!!!, and a respectable 3.94 dBi broadside low angle gain at a low 10 degree TOA. How's that for a gain figure of merit?

    But I wanna work the mobiles and other vertically polarized people without the expense of a fiberglass or aluminum monster on my roof... is THAT even remotely possible with a simple wire antenna 'like' a dipole????

    Enter: The Inverted Quarter Wave Vertical with Only 2 Wire Radials in the next installment.
     

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  11. CTStallion Active Member

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    Inverted Vertical with only 2 Radials

    Take a dipole antenna about 16' long, and add an 8 1/2' vertical radiator to it (just dangling down and floating in the breeze, or you can tie it off to the ground.) Feed it at the TOP, with your center conductor going to the vertical radiator, and your shield going to the two radials. You now have and INVERTED VERTICAL. Remember that the maximum radiation of this antenna will be at the feedpoint (high rf current area), so you will perform almost as good as a 1/4 wave vertical with 4 qtr wave radials, but without all that aluminum. And, even though it only has two radials, it is remarkably omnidirectional. It's unobtrusive, matches 50 ohm coax (although you're gonna wanna decouple the coax to prevent feedline radiation via ferrite beads around the outside dia. of the coax AKA: ferrite choke or a choke balun), and can be easily made from common #14 insulated awg stranded wire from the local Home Depot.

    As the model shows, it offers a clean 17 degree TOA with 1.01 dBi of gain. I'd say that for local work, and a 1/2 horizontal dipole for DXing, and you're in for a world of DX and local contacts on 10/11 meters with the upcoming solar cycle max returning.

    Take care, gud DX'ing................

    and let the flames begin!
     

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  12. Robb Yup

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    Well; everything EXCEPT flaming.


    Thank You CTStallion for that presentation.

    That was a lot of work/effort to start a conversation stimulus package...

    :pop:
     
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  13. Shockwave Sr. Member

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    CT, that was a great presentation. You've offered a ton of information all in one sitting. The simple horizontal dipole does talk excellent skip. For those who only have a groundplane and an interest in talking skip, adding a correctly orientated horizontal dipole would help tremendously. As you probably have realized I am one of those people fond of the Sigma design. I think you'll agree that the Sigma NEC model is not optimized for performance. In any case the work you have done still clearly shows none of the other omni directional examples are capable of providing more gain with a lower TOA.
     
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  14. wavrider W9WDX Amateur Radio Club Member

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    CT,

    Thanks for taking the time to make this presentation, it is very informative and I am sure will stimulate many discussions, debates.

    I especially liked the comparision to the dipole, the old standard halfwave wire.

    Great work.
     
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  15. W5LZ Crotchety Old Bastard

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    Don't put your kevlar thong away yet, it's still early. :)
    - 'Doc
     
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