[antenna] Why use 3L/4 instead of L/2 elements?

Discussion in 'Wireless & RF Design' started by E:V:A, Mar 16, 2013.

  1. E:V:A

    Thread Starter New Member

    Mar 16, 2013

    I'm trying to get a better understanding of a particular kind of the widely popular stacked colinear antennas. In this case we have an antenna tuned for the 1090 MHz (ADS-B reception).

    Here are 3 typical designs I'm looking at:



    The antenna have the following "elements":

    connector => L/2 + S + 3L/4 + S + <3L/4

    where S = 1/4 wave phase stub.

    My first question is:
    Why do they use 3L/4 (3/4-wavelength) elements for the two top parts and not 1/2 wave elements in phase?

    The reason I'm asking, is because I read somewhere (related to a J-pole) that the 1/4 wavelength "leftovers" on the radiator element (in our case the 3/4) would cancel part of the wave in the far field, resulting in worse gain near the horizon, than otherwise. At least that's how I understood it from reading HERE and HERE.

    My second question has to do with the 2 phase stub loops (S).
    In some designs the 2 loop's handedness are opposing each other. What would be the effect of this, if any?

    (It would be cool to see some simulation or model of this, but I don't have that kind of SW (e.g. HFSS etc.) to play with, so if there are any specialists here, I would very much like to know more about this.)

    Thanks in advance,