1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quarter wave V shaped antenna

  1. Sep 29, 2014 #1
    Does anyone know what the radiation patter is suppose to look like for a quarter wave V shaped antenna with the angles pi/4 and 3pi/4. The matlab looks like 2 lobes which is not what I was expecting. I was expecting 2 dounuts in the respective angles.
     
  2. jcsd
  3. Sep 29, 2014 #2

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    How is this antenna fed? With a small antenna of any shape, you'd expect more or less a single, circular 'donut' shape with nulls, top and bottom. But a diagram would help, so as we're all talking about the same thing. Does it have a ground plane? (etc. etc.)
     
  4. Sep 29, 2014 #3

    berkeman

    User Avatar

    Staff: Mentor

    Google Images is our friend! Try a Google Images search on this: radiation pattern inverted v
     
  5. Sep 29, 2014 #4

    davenn

    User Avatar
    Science Advisor
    Gold Member

    An inverted V antenna is usually a 1/2 wave tho ....which are what are described in that google search

    The OP said a 1/4 wave V shaped antenna
    to the OP ... please clarify ... also comment on your freq of interest and if this is a wire(tube) antenna or a pcb etched antenna
    how its being fed and any other relevant info

    else we are all just going to play the annoying 20 questions game

    Dave
     
  6. Sep 29, 2014 #5
    I did not see an option to post my matlab image. When I put in 0.5 for L matlab returns a complex variable error (probably has to do with the current function. I also just saw the attach file option, I must have been tired and missed it. Also when I change the angles to 0 and pi I get a perfect dounut which is what you would expect because that would be a non V hertzian dipole.

    I guess I just did not expect the attached shape from a V of theta pi/4 and theta 3pi/4.

    Here is the code:
    theta1=0.25*pi;
    theta2=0.75*pi;
    theta=[0:pi/100:pi];%theta vector
    phi=[0:pi/100:2.*pi];%phi vector
    L1=.25;
    L2=.25;
    B=2*pi;
    r=1;
    w=1;
    u=1;
    I=1;
    %Radiation pattern for E1
    [PHI, THETA]=meshgrid(phi,theta);
    C1=sin(THETA).*cos(PHI).*sin(theta1)+cos(THETA).*cos(theta1);
    E1=((exp(1i.*B.*L1))./(1i.*1i.*B.*(C1-1))).*((exp(1i.*B.*L1.*(C1-1)))-1);
    E2=((exp(-1i.*B.*L1))./(1i.*1i.*B.*(C1+1))).*((exp(1i.*B.*L1.*(C1+1)))-1);
    E=(1i.*w.*u.*I.*(E1-E2))/(4.*pi.*r);
    Eth=-(cos(THETA).*cos(PHI).*sin(theta1)-cos(theta1).*sin(THETA)).*E;
    Eph=(sin(PHI)).*(sin(theta1)).*E;
    EthC=conj(Eth);
    EphC=conj(Eph);
    %Radiation pattern for E2
    C2=-sin(THETA).*cos(PHI).*sin(theta2)+cos(THETA).*cos(theta2);
    E1p=((exp(1i.*B.*L2))./(1i.*1i.*B.*(C2-1))).*((exp(1i.*B.*L2.*(C2-1)))-1);
    E2p=((exp(-1i.*B.*L2))./(1i.*1i.*B.*(C2+1))).*((exp(1i.*B.*L2.*(C2+1)))-1);
    Ep=-(1i.*w.*u.*I.*(E1p-E2p))/(4.*pi.*r);
    Ethp=(cos(THETA).*cos(PHI).*sin(theta2)-cos(theta2).*sin(THETA)).*Ep;
    Ephp=-(sin(PHI)).*(sin(theta2)).*Ep;
    EthCp=conj(Ethp);
    EphCp=conj(Ephp);
    %Combined Radiation pattern (Etotal = E1 + E2)
    E2=((Eth.*EthC)+(Eph.*EphC))+((Ethp.*EthCp)+(Ephp.*EphCp));
    % converting to spherical coordinates
    [x,y,z]=sph2cart(PHI,pi/2-THETA,E2);
    %plotting routine
    mesh(y,x,z,E2);
    colormap(winter)
    axis equal
    title('Radition Pattern for Dipole Antenna')
    xlabel('x-axis--->')
    ylabel('y-axis--->')
    zlabel('z-axis--->')
     

    Attached Files:

  7. Sep 29, 2014 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member

    rppearso

    the matlab stuff doesn't help me, maybe others can model it

    can you please answer the questions I asked in my previous post :)

    cheers
    Dave
     
  8. Sep 29, 2014 #7
    Hello Dave,

    The work I am doing with Antennas is purely theoretical at this point. I am building up the derivations and matlab tools to be able to model radiation patters of various antennas. The next one we are doing is a loop antenna. The frequency would be proportional to the wave length and in my case it is a quarter wave length in length so the frequency would just be an input, but maybe im wrong and that may be where my error is. My issue is that when I put in a half wave length I get a complex number error which would lead me to believe my code is wrong but when I put in a 1.25 wave length I get a very legitimate looking pattern. For my generic purposes I don't know if it matters if its a wire antenna or a small antenna etched on a chip or if its fed from a coaxial cable or a wave guide or a transmission line.
     
  9. Sep 29, 2014 #8

    davenn

    User Avatar
    Science Advisor
    Gold Member

    normally it would matter because how you feed an antenna and how it is mounted is going to at least affect its feedpoint impedance and its radiation pattern
    Those, I would have to assume, would need to be taken into account in matlab ?

    eg a 1/4 wave whip is going to have a very different radiation pattern in these 3 different scenarios ....

    1) free standing ... no ground plane
    2) free standing ... with ground plane
    3) etched on PCB again with or without some form of ground plane

    others may be able to elaborate further ... as said, matlab is not something I know :)

    and I and still curious as to why you want to kink the radiator ?

    Dave
     
  10. Sep 30, 2014 #9
    I am just learning antennas and I imagine we are going to get to ground effects and we have talked briefly about scattering. I am pretty sure we are going to revisit impedance matching for the feed lines to the antenna as well, though I hate smith charts lol. Right now we are just doing radiation modeling assuming no ground effects or scattering. Also I would assume this is a wire antenna as microstrip antennas are not for another 9 chapters.
     
  11. Sep 30, 2014 #10
    Is that a quarter wave per leg or total? A 1/2 wave vee has a pattern nearly identical to a dipole. On the third harmonic, you'll have additional major lobes.
     
  12. Sep 30, 2014 #11
    p.s. Assuming free space.

    You can play around with a great freeware program called 4NEC2, which makes it really easy to model any antenna over any kind of ground (or none)
    Eric
     
  13. Sep 30, 2014 #12

    davenn

    User Avatar
    Science Advisor
    Gold Member

    this is one of the things I have been trying to ascertain from the OP and still haven't got a definitive answer
    nor an answer as to why there's a V shape in the antenna
     
  14. Sep 30, 2014 #13
    Ah yes, I am terribly sorry, each leg of the V is a quarter wave monopole. I did the math for a quarter wave mono pole and then summed the energy terms. I will have to check out 4NEC2.

    Dave I am terribly sorry I have not answered your questions adequately, I am just learning this material, we have not got to ground effects or anything fancy yet. We derived the EM wave equation, solved it then started applying that to simple antennas in free space.

    So yes this would be a V shaped antenna in free space with a total of a half wave (quarter wave for each leg), I just want to make sure my radiation patter looks correct in my post #5 attachment.

    We will be going over the code tomorrow but I have been working on the loop antenna but did not want to move forward if my V antenna pattern is messed up.

    Once we have a small library of matlab code for various antennas im guessing we will go back and start applying ground effects and other real life modifications.
     
  15. Sep 30, 2014 #14

    davenn

    User Avatar
    Science Advisor
    Gold Member

    OK so now that we have established that this is a 1/2 wave dipole
    Confirm that the feedpoint is in the centre
    and if so, then the comment Berkeman made in post # 3 is applicable
    do some googling with his key words

    Us amateurs ( ham radio guys) Berkeman, myself and prob. a few others on here use inverted V antennas mainly on the HF bands 1.8 - 30MHz

    dipoles.GIF


    just for a little practical antenna use

    top diagram a horizontal dipole ... radiation directions straight up, down, in and out of page .... nothing off the ends
    this is not ideal, as it makes the dipole very directional, also a lot of the signal is radiated straight up bounces of the ionosphere
    and comes straight back down ... waste of energy ;)

    using an inverted V lowers the overall radiation pattern by lowering the angle and making better use of the ionospheric reflection
    it also improves the antennas performance by making it less directional

    I appreciate this doesn't directly answer your matlab queries. Just giving you some practical use

    cheers
    Dave
     
    Last edited: Sep 30, 2014
  16. Sep 30, 2014 #15
    Thank you dave, we got it sorted out in class today. I made some really dumb errors when I summed the 2 mono poles. I should have summed the theta hat and phi hat of each mono pole taken the conjugate and then added them but I did it backwards. Also I had to take the absolute value because there were residual imaginary components. Now on to the loop antenna.

    I believe my aircraft radio antenna is a V dipole and that VOR stations are hertzian dipoles. I am excited to get to fractal antennas and scattering. Fractal antennas were invented by a ham radio operator whos condo association would not let him have big antennas on his back deck area, pretty clever. He made it out of coat hangar. Have you tried spiral, fractal or loop antennas?

    I would imagine for a ham radio you would not want it highly directional, the radiation pattern I got in class was really bloby for a V antenna and yes the feed was from the middle but I could modify each length and model the pattern if I wanted too.

    Do you talk to people across the world? Do you speak another language?
     
  17. Oct 1, 2014 #16

    berkeman

    User Avatar

    Staff: Mentor

    For local HAM comm, we usually use symmetric antenna patterns, but for more directed comm, we use multi-element Yagis or other directional antennas.

    The English language is usually used for international HAM communication, but that may not be true for in-country comms in other countries (I don't know). :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Quarter wave V shaped antenna
  1. V(m) and V(peak) (Replies: 2)

  2. Purpose of Antenna (Replies: 3)

Loading...