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Omni-directional Antenna Questions

  1. May 28, 2009 #1

    cf8

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    I have been looking everywhere for a simple, easy-to-understand explanation of antenna dimensions/properties. I am referring to omni-directional that are basically just a piece of wire that stands perpendicular to the ground, like Wi-Fi antennas.

    1. I've read that the length of the antenna has something to do with the frequency that the antenna works best at.
    2. I've read stuff mentioning the quarter-wavelength as being an important value.
    3. I've read that a coil can be a part of the antenna to split the antenna into two antennas, each for a different frequency; I've also read that a coil at one end or the middle of an antenna can improve the signal somehow.

    My questions are:
    1. Which of the above things I've read are true, and which are not?
    2. What is the formula used to calculate the length for a given frequency?
    3. What is the formula used to determine the number of turns, diameter, and spacing between turns of a coil described above for a given frequency?
    4. What is so important about the quarter-wavelength? I mean, why would the full-wavelength be less important/not work as well?
     
  2. jcsd
  3. May 28, 2009 #2
    1. All of the items you posted are true.
    2. There are a number of formulas. Simple antennas use either 1/4 wavelength or 1/2 wavelength of the frequency.
    3. When an antenna is other than a multiple of 1/4 wavelength the antenna looks either inductive or capacitive to the transmitter. The coil will make the antenna resonate at the desired frequency, even if the antenna isn't the right length, but it may not be as efficient.
    4. The "standard" antenna is the half wavelength dipole. Half wavelength dipoles are difficult to mount on vehicles and often are too long. Quarter wavelength whips are much easier to mount and simulate half wavelength antennas by using a groundplane as the other half of the dipole.
     
  4. May 28, 2009 #3

    cf8

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    Thanks! So basically, for a simple antenna, the length of the antenna would be 1/4 the wavelength of the frequency it is "tuned" for?

    So the goal is to get the antenna to resonate? What determines whether the antenna will appear to be inductive or appear to be capacitive? Is one better than the other? Do the dimensions of the coil matter, or is the purpose to just move the electromagnetic field in a different direction?

    Ah, so a full wavelength antenna may perform better, but it is just impractical to have one because of the space it would consume? Or is the half-wavelength ideal for some reason?

    I forgot to mention that I have found some "antenna calculators" but none that have an explanation with them. I see that most people seem to agree that 468/MHz = length for half-wave dipole, but where did that formula come from? I mean, how was the 468 calculated?

    I realize that I may come off as being a pain, but I'm really just trying to understand how and why all of this fits together.
     
  5. May 28, 2009 #4

    berkeman

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    Staff: Mentor

    Last edited by a moderator: May 4, 2017
  6. May 28, 2009 #5

    cf8

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    Yeah, I've read almost the entire Wikipedia page a few times now.

    Also, I keep hearing about the ARRL antenna book; I guess I'm just going to have to get it. I didn't know if it was really that good or not, but it must be - everyone recommends it! LOL.

    I didn't know that there were technical libraries; I'll have to look into that.
    Thanks!
     
    Last edited by a moderator: May 4, 2017
  7. May 28, 2009 #6

    berkeman

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    Most colleges and universities will either have a technical section in their main library, or else a separate science/engineering library. You would generally need to be a student to check out their books, but you can generally just go in and look at them there without being a student.
     
  8. May 28, 2009 #7

    vk6kro

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    Just rereading your question.

    There is a reference in there to trap dipoles.
    If you have a piece of wiire that is a half wavelength long at some frequency, it will resonate at that frequency, and be easy to transmit into, at the centre.
    If you put parallel tuned circuits (tuned to the resonant frequency of the dipole) at the ends of the wire, no signal at that frequency can get past the tuned circuits.
    You can now add extra wire at the ends and this will then resonate at some lower frequency. The lower frequency can pass through the tuned circuits, but the higher frequency cannot.

    These antennas let you use the one antenna to operate on different frequencies.
    In practice, they are diabolical to tune up as everything interacts with everything else.

    You asked about the design of traps for dipoles. These are not just coils but have to be tuned circuits, so they have a capacitor across the coil. Maybe leave that one for some other time.

    Coils of wire are used on their own to make an antenna work when it has to be physically smaller than the ideal size, but this is for one frequency only, not an attempt to make the antenna work at different frequencies.

    Although antennas are made that are odd multiples of a quarter wave long, a piece of wire that is a quarter wave long actually makes a very bad antenna. It is only used for convenience and it must be used with an earth or ground connection to make it appear to be a half wavelength long.
    The important length to remember is the half wavelength.

    That formula "468/Freq in MHz = length for half-wave dipole" is derived as follows:
    Speed of light in feet/second is 983 587 000 ft /sec. One wavelength is 983 587 000/ frequency in Hz. Half a wavelength is (492 000 000/ frequency in Hz) or (492 / frequency in MHz).
    The 492 figure then varies with diameter of the dipole and the surroundings, but 468 is a good starting point for normal wire antennas.

    .
     
  9. May 28, 2009 #8
    Yes, but 1/4 wavelength antenna should have a ground plane. Consider the ground plane part of the antenna.

    Yes the antenna should resonate. When the antenna is in resonance it appears resistive instead of inductive or capacitive. When an antenna is too short it appears inductive. Adding an inductor will electrically lengthen the antenna and if it's the right value it will bring it into resonance. Rarely are antennas capacitive because it's usually pretty simple to shorten an antenna. One example where it isn't would be a printed circuit board antenna. Series capacitors electrically shorten an antenna. Being inductive or capacitive are equally bad. Both will cause part of the power to be reflected back to the transmitter. The dimensions of the coil matter only inasmuch as they give the coil the right inductance.

    The halfwave antenna is preferred because at the instant when the radio wave reaches maximum voltage, one end of the dipole is at maximum positive potential and the other end at max negative thus developing the strongest electric field. A fullwave antenna would have the same voltage at both ends.

    The speed of light is 300 million meters per sec., so the wavelength of 300 MHz is
    1 meter and a half wavelength is a half meter. Okay, so the wavelength of 468 MHz is 300/468 meters and a half wavelength would be 150/468 meters or 0.32 meters or about 0.977 feet. Because of some electrical effects at the ends of the antenna, it's actually 1 foot electrically. If you're working in feet use 468, in meters use 147.

    I've made this a lot simpler than it really is but I hope you get the idea.
     
  10. May 29, 2009 #9

    cf8

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    Ah, okay, thanks. I'll check into this. (I just started college.)

    Ah, I see how that works now!

    Ah, this makes sense! I was trying to picture why, and now I get it.

    ...So a coil "electrically lengthens" the antenna?

    Yeah, I figured as much, but I am starting to understand this more.
     
  11. May 29, 2009 #10

    berkeman

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    Yes. The coil has a higher inductance per unit length than a straight wire, by virtue of the mutual inductance of the coil turns.
     
  12. May 29, 2009 #11

    cf8

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    Awesome; thanks!
     
  13. May 29, 2009 #12
    If you have further interest and to see how complex this can become you should check out The Smith Chart. http://en.wikipedia.org/wiki/Smith_chart and http://www.sss-mag.com/smith01.html#bio

    When you understand the Smith Chart it will make the answers to your questions obvious. This chart makes so many functions of matching impedances easy whereas without it they would be very difficult especially without a computer. This chart is pure genius.

    I took a class once from someone who had interviewed Phil Smith. He told us he asked him how he came up with the idea for the chart. Smith answered that there were always so many infinities - inductive reactance would go to positive imaginary infinity, capacitance reactance to negative imaginary infinity and at resonance the impedance would approach real infinity. He just wanted to put all the infinities on one side and zero on the other.

    As you look at the chart, remember this is the shortcut. Imagine what engineers had to go through in 1939 without calculators or computers.
     
    Last edited by a moderator: Apr 24, 2017
  14. Dec 26, 2009 #13

    cf8

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    Okay, I've done some research, and I have one quick question so far.

    If "active antenna(s)" are for receiving, then what is the equivalent device for transmitting?
     
  15. Dec 26, 2009 #14

    berkeman

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    From wikianswers.com:

    So an "active antenna" is just a receiving antenna with the first receive amplifier located with it or very close to it. As the answer says, that avoids having the RX cable loss degrade your receive SNR.

    For transmit, you could position your TX power amp close to the antenna to increase your TX power a bit, I suppose.
     
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