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Thread: High gain dipole design

  1. #1

    Default High gain dipole design

    I am turning to the ham community as I have not been able to find answers anywhere else.

    I am interested in making a dipole antenna with a gain higher than what the nominal 1/2 wave dipole achieves. I hope to achieve around 9db gain omnidirectional in the horizontal plane (maybe more). This is for the 2.4 GHz band used on wifi equipment. I have tried to do a lot of research on the web, but am still finding I have specific questions which are not being answered.

    I am familiar with the fact that 6db and 9db direct screw-on antennas are sold for 2.4GHz wifi equipment. The ones I have seen have been called dipoles.

    The closest thing I have come to which addresses somewhat in detail is a Youtube video (https://www.youtube.com/watch?v=bs8hvXGJdhM&t=94s) where the presenter dissects one such antenna. I have included screen shots which show the dimensions of element. Note that in the author's notation regarding the length of the coil, the 70mm refers to the length of the wire section before coiling.

    The high gain antenna referred to appears to have evolved from a 1/2 wave dipole (shown in one of the screen shots,) in which the grounded element is a 1/4λ sleeve which runs over the outside of the transmission line. The high gain design seems to be an extension of this, where the feed portion of the element is extended, and includes a loading coil at some point along its length, but the grounded portion is still the 1/4λ sleeve.

    As the author of the video does not give details as the the reason for the physical dimensions, I have some question about this. As I have said I have done much searching and while gaining quite a bit of valuable knowledge on basic antenna design in general, I haven't been able to find specific answers regarding these more advanced versions of the dipole. All questions are referring to the high gain version of the antenna shown in the video except where otherwise noted:

    1) While the feed portion of the element has been extended, the ground portion still remains 1/4λ (using the sleeve.) Does that mean that it a dipole does not necessarily have to be fed in the center? (assuming however it would have to be at some multiple related to λ.) For example, could a 5/4λ dipole (in theory) have a 1/2λ conductor for the ground portion, and 1λ conductor for the feed conductor?

    2) The coil in the example is constructed from a 70mm section which has been coiled. What is the total effective length of this element? Does it somehow work out to be a 5/4λ element?

    3) I am assuming the coil in the element is for impedance matching. Let's ignore impedance matching for a moment and just focus on resonance. Would an antenna using a 1λ length of straight wire connected to the feed, and a 1/4λ sleeve connected to the ground like shown in the photos, result in what is called a 5/4λ antenna, and have the same directivity?

    4) Is the following statement correct: While a 1/2 wave dipole has about 73 ohms feed resistance, and thus no coil is needed, a coil is needed for impedance matching if a > 1/2 wave antenna is made.

    5) When adding a coil for impedance matching, how does one determine the length of wire for the coil, the length and diameter of the coil, and the length of the segments of conductor of the antenna?

    6) Referring to the video, the presenter was asked what is required to make a longer antenna. The presenter replied saying that additional segments of (1/4λ conductor plus a coil section) could be inserted before the end, implying that adding more would increase directivity and thus theoretical gain. Does this seem reasonable? And if it is correct, why several coils distributed along the element? Why not just one coil of proper values at one end?

    7) How would one continue to increase the length of the antenna to give higher gain?

    Thank you very kindly,
    Kevin Jones

  2. #2

  3. #3

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    First, this is a monopole. As a result it must be installed on a ground plane, aka a sheet of metal as might accurately remember 1/4 wavelength or maybe it is a 1/2. Without which, performance sucks. It has been a while since I did any of this and I am too lazy to look anything to help move this forward. However, there is a diminishing returns issue that rapidly works against higher gain. For instance, every 3dB gain requires twice as much wire over the previous.

    I bought a range extender for my purposes sometime ago. Look up reviews on Amazon and Newegg and buy the one with the most positive reviews that you can afford..
    -Jeff NE1U

  4. #4

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    What he is referencing is a dipole that uses a hollow sleeve as one half of the driven element that has been modified in a collinear manner using phasing elements allowing the high impedance at the end of the dipole to drive the low impedance at the end of a 3/4 wave wire. This modification should actually be done to both sides of the dipole instead of just one to make it balanced. For this reason, it is not considered a monopole or even an off center dipole. Its a dipole that has a phasing coil and collinear element on just one side ...unfortunately making it unbalanced.

  5. #5

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    I think that is a bazooka balun sans the other half of the dipole
    -Jeff NE1U

  6. #6

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    Quote Originally Posted by brandon lind View Post
    What he is referencing is a dipole that uses a hollow sleeve as one half of the driven element that has been modified in a collinear manner using phasing elements allowing the high impedance at the end of the dipole to drive the low impedance at the end of a 3/4 wave wire. This modification should actually be done to both sides of the dipole instead of just one to make it balanced. For this reason, it is not considered a monopole or even an off center dipole. Its a dipole that has a phasing coil and collinear element on just one side ...unfortunately making it unbalanced.
    Quote Originally Posted by Psi* View Post
    I think that is a bazooka balun sans the other half of the dipole
    Sorry for the late response, I have been considering your answers and trying to research as much as I can before replying. Thank you very much for all your responses.

    I see I have probably jumped into the deep end of the pool with trying to understand antenna theory through these examples. I am a retired electronics engineer, but my work never involved RF. However the design that has been dissected in the video I believe was a fairly common approach for these "rubber ducky" style mounts on consumer wifi equipment (before blade antennas started becoming more popular, which print the elements on a PC card.) I am assuming the industry used this design as a way to achieve the desired directivity/gain in the shortest length possible. To simply create a balanced dipole with identical, thus inherently balanced elements, and use > 1/2λ designs to achieve higher directivity/gain, would quickly become impractical even at 2.4GHz.

    Jeff, regarding your suggestion that the sleeve may be a balun, which off-hand seems reasonable, when the presenter of the video was duplicating the antenna in his example, he connected the shield of the transmission cable to the top of the sleeve. From what I have researched about bazooka baluns, wouldn't the shield need to be connected at the bottom? So I am not sure what to think about that suggestion; although there may be principles regarding the bazooka balun of which I am unfamiliar.

    Is it possilble the sleeve is just acting as a ground plane for a monopole? Or is the radial diameter just too small for that?

    Though Brandon, you seem convinced that this is neither a monopole nor an off-center dipole, although you do say it is unbalanced. (Hence it would require a balun which seems to support the bazooka balun suggestion?)

    Brandon, you refer to this as collinear, which is throwing me a bit, as there is only a single element. Are you viewing the sections above and below the phasing coil as two separate elements in this regard, and thus collinear?

    Also, why is it not "even an off center dipole?" Are you saying that even with an off-center dipole, both elements would have a phasing coil? Nonetheless, isn't an off-center dipole inherently unbalanced (thus making me wonder what is the distinction?)

    I also am not able to get my head around this statement, "allowing the high impedance at the end of the dipole to drive the low impedance at the end of a 3/4 wave wire."

    Thank you kindly.

  7. #7

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    (NOTE: I believe I submitted this post last Thursday. After waiting a day and not seeing it appear, I contacted the forum via [email protected] about the matter but I never received a reply of any kind. I checked again today and it still has not been approved. So I am posting it again assuming there has been some technical glitch somewhere along the way. I apologize if this is not according to your policy.)

    Quote Originally Posted by brandon lind View Post
    What he is referencing is a dipole that uses a hollow sleeve as one half of the driven element that has been modified in a collinear manner using phasing elements allowing the high impedance at the end of the dipole to drive the low impedance at the end of a 3/4 wave wire. This modification should actually be done to both sides of the dipole instead of just one to make it balanced. For this reason, it is not considered a monopole or even an off center dipole. Its a dipole that has a phasing coil and collinear element on just one side ...unfortunately making it unbalanced.
    Quote Originally Posted by Psi* View Post
    I think that is a bazooka balun sans the other half of the dipole
    Sorry for the late response, I have been considering your answers and trying to research as much as I can before replying. Thank you very much for all your responses.

    I see I have probably jumped into the deep end of the pool with trying to understand antenna theory through these examples. I am a retired electronics engineer, but my work never involved RF. However the design that has been dissected in the video I believe was a fairly common approach for these "rubber ducky" style mounts on consumer wifi equipment (before blade antennas started becoming more popular, which print the elements on a PC card.) I am assuming the industry used this design as a way to achieve the desired directivity/gain in the shortest length possible. To simply create a balanced dipole with identical, thus inherently balanced elements, and use > 1/2λ designs to achieve higher directivity/gain, would quickly become impractical even at 2.4GHz.

    Jeff, after researching, I think your suggestion that the sleeve may be a bazooka balun may be correct. At first I questioned it as I was under the impression that a bazooka (or sleeve) balun had to be connected at the bottom of the sleeve, and the examples in the video connect it to the top. However after reading this your suggestion seems more plausible:

    https://www.w8ji.com/sleeve_baluns.htm - "Depending on which layout is mechanically easier to build and if you want the sleeve to radiate, the sleeve can be connected like this or reversed. In the layout below the sleeve is non-radiating. In a collinear antenna we might want to reverse the sleeve so it forms half of a dipole element."

    Brandon, you seem convinced that this is neither a monopole nor an off-center dipole, although you do say it is unbalanced. (Hence it would require a balun which seems to support the bazooka balun suggestion?)

    I am not able to get my head around this statement, "allowing the high impedance at the end of the dipole to drive the low impedance at the end of a 3/4 wave wire."

  8. #8

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    The bazooka balun theory... If the sleeve is connected to the shield at the end of the coax, it is acting like one side of a dipole that simply has its coax ran through the sleeve to keep it out of the way. If it was a balun, it would be connected to the outer surface of the coax shield 1/4 wave from the end.

    The statement "allowing the high impedance at the end of the dipole to drive the low impedance at the end of a 3/4 wave wire." I will try to explain the best I can while keeping it simple:

    At the end of an antenna (or the open end of a transmission line) carrying a signal, the voltage is highest at the open end. The reason for this is because, well, it’s an open circuit and there is nowhere for current to go. If the current is minimum, the voltage must be at a maximum. This happens because of waves reflecting back in such a manner to make this infinite voltage (or current for a shorted end) possible. As we get farther from the end, the ratio of voltage to current (impedance) changes.

    An open-ended coax stub wave long will appear to be a short circuit at its input end. A short-circuited stub will appear to be an open circuit at its input end. The only way for what’s at the end of the coax to not affect the impedance at the input end is for the load to be a perfect match to the coax which prevents any reflection and thus prevents impedance changes seen along the length of the coax. Between these 2 extremes, mismatches of any magnitude will result in some level of reflection and that translates into a standing wave. W2AEW has a great video on understanding standing waves and visualizes them using a scope and a strip of transmission line. https://www.youtube.com/watch?v=M1PgCOTDjvI

    This is also how bazooka baluns work... For currents that try to flow back along the outer surface of the shield, the bazooka makes the outer surface look like a 1/4 wave shorted stub, and being a 1/4 shorted stub has an extremely high input impedance, the currents no longer can flow in that direction preventing the coax from becoming part of the antenna.

  9. #9

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    Quote Originally Posted by brandon lind View Post
    The bazooka balun theory... If the sleeve is connected to the shield at the end of the coax, it is acting like one side of a dipole that simply has its coax ran through the sleeve to keep it out of the way. If it was a balun, it would be connected to the outer surface of the coax shield 1/4 wave from the end.
    The article at the link I posted in comment #6 seems to indicate that the balun sleeve can be connected at one end or the other, in one case making the sleeve also act as one side of the dipole, yet still perform the function of a balun. Are you saying then that this isn't true, that it can only either act as a balun OR a dipole element?

  10. #10

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    I think you are right, it would make sense that it would still function as a balun. I never thought of it that way.

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