Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Patch and dipole antenna

  1. May 27, 2015 #1
    Hi

    Ok I have a contradiction in my mind:

    Patch antenna is called a leaky wave resonant cavity because of the magnetic slots which radiates. We all agree on the fact that they are standing waves with clearly identified max and min as a characteristic of a specific mode inside this cavity.

    1) My question is if we have standing waves, in my mind we would have a max reflection (=1) on the magnetic walls so how comes we have leakage if we have full reflection on the wall ??

    2) If we have NOT full reflection on the wall so it means there is no standing wave in the cavity right ? the max and mins are moving ?

    Thanks
     
  2. jcsd
  3. May 27, 2015 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    That's not what I am familiar as a patch antenna

    what you described is a slotted waveguide

    8+8WaveGuideDiag.gif

    these are patch antennas .....

    11310432_f260.jpg

    Antenne_patch_2_4_GHz.jpg


    so what type of antenna are you really talking about ?

    Dave
     
  4. May 27, 2015 #3
    Thanks for your answer. I'm definately talking about the 2 last pics, the patch antenna !!! I try to understand why your thing that I'm describing a slotted waveguide. Maybe because I wrote magnetic slot ? I was talking about the fringing field which can be "remplaced" by 2 magnetic current. The patch is equivalent to 2 magnetic slots....

    when I talk about reflection on walls, I talk about the 4 magnetic walls which are delimiting the patch cavity....
     
  5. May 27, 2015 #4
    What is a magnetic wall?
     
  6. May 27, 2015 #5

    https://books.google.ca/books?id=_e...esc=y#v=onepage&q=patch magnetic wall&f=false

    You see the boundary condition implied by a PEC (pefect electric conductor) ? A PMC (magnetic conductor) is the one which implies the opposite condition. For example, if you have an interface of air and a VERY VERY high dielectric constant matter, this matter will be as good PMC as its epsilon is high...
     
  7. May 27, 2015 #6
    It's my understanding that a patch antenna works by having the electric field fringe between the patch and the ground plane. The magnetic field is caused by currents moving down the edge of the patch and the resulting image current moving in the ground plane. The "end" of the antenna where the standing wave typically forms depends on where the feed point is and the shape of the patch.
     
  8. May 27, 2015 #7
    look what I don't understand is that an incident wave is brought by the transmission line then a reflection occurs inside the cavity (between the metallic patch and the ground plane) at the end of the cavity (limit of the metallic patch). This reflection isn't a full reflection for me since there is a leakage at the end. so it's not a standing wave inside the cavity ? I'm even confused on the phenomena of a partially reflected wave at a load which interfere with itself. It shouldnt be a standing wave if there is NOT a reflection coefficient of 1....to me...

    let me know what you think

    thanks
     
  9. May 27, 2015 #8

    Baluncore

    User Avatar
    Science Advisor

    Any reflected wave will generate standing waves with the incident wave. The position of those nodes and nulls will be determined by the nature of the reflector.

    Once a wave is resonant in a structure, with a Q of say 100, radiation of energy through a slot or fringing field need only be 1% of the circulating energy per cycle for the full power available to be radiated.
     
  10. May 28, 2015 #9
    As a rough rule of thumb, elements less than 1/10th of a wavelength can be lumped together. Since the fringe is less than that, it can be considered a sudden shift to 377Ω; i.e. an open. This will cause a reflection coefficient (∫˜) ~= 1. (Of course nothing is perfect, and this presumption less than most.) As Baluncore pointed out, we don't need ∫˜ to be 1 to get standing waves.

    Remember, EM waves add linearly (at least in most dielectrics). So a ∫˜ of 1/10th will give a standing wave of 1/10th the amplitude (compared with ∫˜= 1). The rest of the wave goes on its merry way, but there's now a new wave with 1/10th the energy just standing there...
     
  11. May 28, 2015 #10
    You anticipated my question. Suppose we are in a high permittivity dielectric cavity which is in contact with air. You said a standing wave appear whatever the reflection coefficient at the interface, fine but it means that the position of the node and nulls will not "move" along the cavity ? Look I'm gonna tell you what disturb me: the amplitude of the standing wave is :

    Vi*(1+alpha) with alpha the coefficient of reflection and Vi the incident wave amplitude. If it's a full reflection, it's the simple case for me, alpha = 1 and the amplitude of the association of the 2 waves is 2 *Vi. Suppose we have no reflection, it should be a progressive wave with alpha = 0 , the position of the max or min is moving at C (speed of light). At any infinitesimal reflection we will have a standing wave ??!!!! How comes we have such a discontinuity in speed ??!! Another question is that what is the min value of a standing wave in a NON full reflection coefficient ?

    thanks

    Ps: Sorry lots of questions but it has to be extremely clear....
     
  12. May 28, 2015 #11
    Interesting ! This means to me that the whole wave obtained after reflection is an addition of the standing wave that you described and a progressive wave. If I had the ability to see the wave (the whole one) with my eyes, the node and the nulls will note be standing, they will be moving at the speed of C right ? Because this speed is introduce by the progressive wave... You see where I wanna go right ? I wanna talk about the discontinuity...
     
  13. May 28, 2015 #12

    Baluncore

    User Avatar
    Science Advisor

    The leakage of signal is not through the open or conductive reflector but from the part of the resonant cavity exposed through a slot or aperture in the cavity wall.
    A resonant dipole has full reflection at both ends and maintains a standing wave. Yet it still transmits a 2.15 dBi signal broadside.
     
  14. May 28, 2015 #13
    That's fine, I'm following you. I thought we have a standing wave only when there is a full reflection. I agree with you we have a standing wave with any reflection and the amplitude of this standing wave is as small as the reflection is. This is solved ok.

    Now what about the whole wave please ? the whole one is NOT a standing wave right ? Their should be a mathematical way to calculate the speed of the full wave. I'm sure it goes at C because of it's progressive part... I'm sure I missed something because of the speed discontinuity...
     
  15. May 28, 2015 #14
    Ok I have an answer for this apparent discontinuity, I will give my opinion later if someone is interested. Actually the partially reflected case if the same as a progressive wave modulated (with addition not multiplication like in an amplitude modulation) by a standing wave. The amplitude of the standing wave and the one of the progressive wave depends on the reflection coefficient. I think we agree on that...

    Now I have only one question. what the value of the minimum of the whole wave ?
     
  16. May 28, 2015 #15

    Baluncore

    User Avatar
    Science Advisor

    If only the world was so simple. What "whole wave" in what "environment" ?
     
  17. May 28, 2015 #16
    Yes but this radiation is not the result of a leakage because since there is a full reflection there couldn't be a leakage, you agree ? Is this radiation is the result of leakage for you ?

    I will talk about whole wave later....
     
  18. May 28, 2015 #17
    A standing wave is not stationary in the sense of propagation. If it were a single wave like a soliton, it would move right along. The standing effect is typically (but not exclusively) caused when two waves moving in opposite directions interfere with each other. Since an incident wave train and its reflected wave wave train (with ∫˜=1) are just such a pair, they form a standing wave. But both wave trains are propagating at full speed.

    At ∫˜<1, there will be a small standing wave equal to the reflected wave plus as much of the incident wave as needed to match that. This will be swamped by the incident wave for ∫˜<<1, but will still exist.

    Other types of standing waves exist, but aren't really what we are talking about.

    What is a "whole wave"? Waves add linearly, so a standing wave will typically be twice the amplitude of the incident wave. But with clever (or perhaps not so clever) design a waveguide resonator could bounce the wave back and forth six ways from Sunday until the voltage breaks down the dielectric and lets the magic smoke out.
     
  19. May 28, 2015 #18
    Since PECs don't exist, nothing has a ∫˜of 1. But it has a nominal value of 1 (Claimed anyway. It sounds reasonable, but it's not my understanding of what goes on.). Since it is a resonator, the voltage keeps building until the small leakage power (out of the antenna) matches the incoming power. At least that's my understanding of the claim. (I'd buy it but haven't done the math myself.)
     
  20. May 28, 2015 #19
    By your last sentence you demonstrate that both waves are propagating and we all agree on that. However how can you say that the addition of these 2 waves which are giving standing wave will not propagate ? your link say the opposite of that no ?
     
  21. May 28, 2015 #20

    Baluncore

    User Avatar
    Science Advisor

    No. You talked about it before. Your term “whole wave” has no obvious meaning.

    If you will not clarify your statements and you will not answer questions then your misuse of the terminology will continue and you will only waste the time of respondents. The more you write, the more nonsensical your thinking on this subject appears to be. That needs to change.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook