Antenna theory.

Discussion in 'Wireless & RF Design' started by PRS, Apr 28, 2011.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
    I'm curious about the theory that says an antenna is optimized by being 1/4 of the wavelength or a factor thereof. Has anyone in here actually experimented with this concept? I'm wondering if just the plain bulk of metal is a critical factor. Any thoughts? I once used an iron radiator as an antenna and it worked great. The radio on my breadboard would not even work without it.
  2. bertus


    Apr 5, 2008
  3. steveb

    Senior Member

    Jul 3, 2008
    At least you didn't have to hold the radiator over your head while balancing on one foot. :p

    I have childhood memories of using coat-hangers and tin foil trying to watch Tarzan on channel 27. I watched many an episode in a contorted position holding metal contraptions in the perfect position.
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    One hears all sorts of stories about what constitutes an effective antenna for an AM radio. I met one lady who lived in the "bush" and she used to put the metal teapot on her radio receiver to improve reception.

    A quarter wave antenna for the AM radio band wouldn't be practical - at 1MHz a quarter wave antenna would be 75 metres long. Fortunately a short piece of wire is usually sufficient to provide adequate reception in the case of local radio reception at a modest distance from the transmitter. Once we start getting into short wave long distance or TV communications the role of the antenna construction becomes increasingly important.

    Mind you I'm sending this message through a little USB style wireless modem which looks very humble but seems very reliable.
  5. K7GUH


    Jan 28, 2011
    Hams have experimented with antennas for well over 100 years. Most useful designs for wire antennas tend to be multiples of 1/4 wavelength. For multiple element antennas, the dipole is most often set up as the driven element. But there are lots of exceptions. Try the QST archives for articles on antennas. You will find plenty of interesting reading.
  6. Papabravo


    Feb 24, 2006
    The geometry of an antenna "falls out" of the boundary value problem for a transmission line. You see very clearly what happens when the end of the transmission line is open, short, and properly terminated. You can also see what happens when there is an impedance discontinuity. Radiation, absorption and reflection are all there for the curious to ponder.

    I recommend Gonzalez for a description of the solution to the PDE for a transmission line and the explanation of characteristic impedance.
  7. davebee

    Well-Known Member

    Oct 22, 2008
    Antenna discussions so often lead to religious wars about what is "optimal".

    I see two main problems here. First, there are many tradeoffs in an antenna design, and what is optimal for one feature may not be optimal for another.

    Take directionality. You can make an antenna much longer than a quarter wavelength in order to be much more directional. If it is more directional than a quarter wave, and if directionality is what you need, then the quarter wave antenna has just been shown to be less than optimal in that feature. So no, the quarter wave antenna is not necessarily "optimal".

    Second, don't confuse good enough with optimal. Plenty of random hunks of metal, like your iron radiator, may well be good enough. You could even say they're optimal in terms of the effort required to get your radio working!
  8. mjhilger


    Feb 28, 2011
    Search the web for some discussion about a HAM antenna named "steppir". This unique antenna has a stepper motor which spools out a metal strip within a fiberglass sleeve to a length for your specific need (frequency). So these owners can use 1/4 wave or something else if they find it works better. I think you will find that most of them are very near 1/4 wave for the driven element.

    The other factor in play is matching the impedance to the transmitter/receiver. The math tells us that optimal energy transfer happens when the impedance is matched, but if you have no choice on the output of your rig where is the optimal transfer output to field strength? As already stated, there are almost always trade offs.

    Still I think you might find some useful information from the owners and creators of the steppir.
  9. Jack Puddin

    New Member

    May 4, 2011
    One thing you may want to look at are fractal antennas. Not sure if it will work for your application but I do know that they allow you to pick up a multitude of frequencies with a single small antenna. Thats why you don't see cellphones with long antennas sticking out of them anymore, yet able to use more things at once like wifi, bluetooth, cell signal ect.
  10. sim001

    New Member

    Feb 18, 2012
    Hello everybody,

    I am trying to design 2.45GHz patch on ADS using FR4 substarte.But i am unable to find the substarate paramters i.e thinkness,height and dielctric constant.

    I dnt know how should i proceed.
  11. crutschow


    Mar 14, 2008
    When I was a kid, we had a long outside horizontal antenna wire of perhaps 100ft connected to the old tube AM radio, which gave pretty good reception.

    Most modern AM radios use a a coil wound around a ferrite core which makes a fairly good small antenna. The small whip or wire antenna on a radio is for FM reception.