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Thread: How does flower pot antenna work?

  1. #1

    Default How does flower pot antenna work?



    I read some sites about flower pot antennas and I cannot understand how it works. For example, how the current flow to form a radiation.

    I know it's constructed by two 1/4 wave sections. The upper part is easy for me to understand as the current goes up to the top. What about the lower part when the coax shield is not peeled off? When pressing a TX button, the upper part (at first quarter of wavelength period) current is + then the lower part is - so the - goes down to the choke? So, how and when the + and - signals reflect up and down to radiate to the air?

    How does the signal lopes of the radiation pattern would look like?

    Here's the text explaining how it works but I don't understand. This is not a flower pot but what the builder got the idea from.






  2. #2

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    The FPA is just a half wave dipole, with a clever coaxial feed system.

    If you understand how a half wave dipole works, then you know how the FPA works. The 1/4 wave outer braid of the coax is just the other half of the dipole.

    Look at the ARRL article picture - ignore for now the "choke" and coax to the left. All that remains is a simple 1/2 wave dipole.

    The "choke" in the FPA design would more properly be called a "trap" - just a self resonant parallel tuned circuit that doesn't allow the 1/2 wave dipole to "see" anything to the left of the trap. As far as the antenna is concerned, everything to the left of the trap isn't even there. (In the real world, there is a bit of interaction, but the effects are pretty small, and the radiation pattern is distorted very little from that of an ideal 1/2 dipole in free space).

    If you look at VK2AOI's write up, near the bottom of the page, you find his data for the self resonant frequencies for the trap or choke.

    Hope this helps.

  3. #3

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    Quote Originally Posted by KD6RF View Post
    The FPA is just a half wave dipole, with a clever coaxial feed system.

    If you understand how a half wave dipole works, then you know how the FPA works. The 1/4 wave outer braid of the coax is just the other half of the dipole.

    Look at the ARRL article picture - ignore for now the "choke" and coax to the left. All that remains is a simple 1/2 wave dipole.

    The "choke" in the FPA design would more properly be called a "trap" - just a self resonant parallel tuned circuit that doesn't allow the 1/2 wave dipole to "see" anything to the left of the trap. As far as the antenna is concerned, everything to the left of the trap isn't even there. (In the real world, there is a bit of interaction, but the effects are pretty small, and the radiation pattern is distorted very little from that of an ideal 1/2 dipole in free space).

    If you look at VK2AOI's write up, near the bottom of the page, you find his data for the self resonant frequencies for the trap or choke.

    Hope this helps.
    Thanks. I think I almost understand the half wave dipole but comparing to dipole, the antenna looks like the other half is cut off. So, how the current flows in the FPA according to this video?


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    hmmm, can't hear the audio on the clip you posted.

    But anyway, not sure what else to say.

    The flower pot antenna *IS* a half wave dipole, and is center fed, just as the 1/2 wave dipole shown in the video. Half of the FPA dipole is the 1/4 wave wire (center conductor of the coax), the other half of the dipole is the 1/4 wave of coax shield.

    Remember, we use coaxial transmission line because (among other reasons), it confines what is inside the shield from anything outside the shield and visa-versa. So, it doesn't matter if we have a wire (the center conductor), many wires, or a bevy of dancing girls *inside* the shield of the coax. The antenna itself sees only the coax shield which is 1/4 wave long, and is the other half of the dipole.

  5. #5

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    Yes, I can imagine the center conductor is the radiation part but I am curious about the shield. If the shield is another 1/4 part of the radiation part, how does the current flow? The current would flow in opposite to the center conductor. In a half wave dipole, the current flows in the same direction. Then how would the radiation pattern be? This is what different to the half wave dipole.

    One of the idea I could think is the upper(center conductor) functions as a 1/4 wave antenna and the lower(the shield) functions as a ground plane(like the 1/4 wave antenna we put on the car roof. Not sure if I am correct.

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    Quote Originally Posted by yoham View Post
    Yes, I can imagine the center conductor is the radiation part but I am curious about the shield. If the shield is another 1/4 part of the radiation part, how does the current flow?
    The same as a dipole.

    Quote Originally Posted by yoham View Post
    The current would flow in opposite to the center conductor.
    No. Remember the description of coaxial cable - there is inside-the-cable, where current flow on the *inner* surface of the coax is just as in the transmission line in the video. And then there is outside-the-cable, where current flow on the *outside* surface of the coax (the other half of the dipole in the FPA) is just as in the dipole in the video.

    Kinda like this:
    FPA.jpeg

    See how the red arrows (antenna current) are the same for all the cases? The tricky part is realizing that the coax shield of the FPA serves a dual function - the outside surface is half of the dipole, and the inner surface is half of the transmission line. They are separable currents - the antenna current on the outside surface of the shield flows in the opposite direction as that of the transmission line current on the inner surface of the shield.


    I think where you are having trouble is visualizing how the current flow on the inside surface of the shield of a coaxial cable is different from that on the outside. Once you see this, the FPA will make sense!



    (Note the asterisk in the drawing - this is a coax fed dipole which is not a good idea due to imbalance - but don't worry about that for now.....)

  7. #7

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    OK, I understand now.
    Is this the same as the "skin effect" I heard before?

    So, in half wave dipole, we connect the shield to one part of a dipole. There will be no current flow on the skin if the shield is connected to the antenna?

    More info: I read somewhere that "higher freq., less depth of surface that current flows". So at high freq. the current flows only on the skin of the conductor. How does this apply to your info?
    Last edited by yoham; Sun 16th Sep 2012 at 20:34.

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    Quote Originally Posted by yoham View Post
    OK, I understand now.
    Is this the same as the "skin effect" I heard before?
    It is actually a bit more complicated than that, but it is a handy visualization tool to think of currents flowing only on the surfaces.

    Quote Originally Posted by yoham View Post
    So, in half wave dipole, we connect the shield to one part of a dipole. There will be no current flow on the skin if the shield is connected to the antenna
    Well, it's not really a good idea to connect a dipole to coax as shown in the middle sketch in my picture (where the coax runs away from the dipole at a 90 degree angle). Pattern skew, RF on the shield, RF in the shack, and VSWR problems crop up unless we use a balun or some other sort of device that prevents current flow on the outside of the coax (which, is the purpose of the trap in the FPA). There is lots written about this subject on the web. A lot of it is even true


    Quote Originally Posted by yoham View Post
    More info: I read somewhere that "higher freq., less depth of surface that current flows". So at high freq. the current flows only on the skin of the conductor. How does this apply to your info?
    True, but worrying about skin depth as a function of frequency doesn't really help one understand how the FPA works. If you can see that differing currents can flow on the inside and outside of the coax shield, then you know about 98% of how the FPA works.

    The other 2% involves Maxwel's eqs, linear superposition, and a host of other details that don't add a lot to the understanding of the FPA's basic functionality.

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