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Resonance of antennas (rods, spheres etc)

  1. Jun 20, 2012 #1
    How can you find the resonant frequency of an antenna whether it is a rod shape or an irregular shape or a sphere? I understand they have a capacitive reactance and you may need an inductive load but can someone explain it all in detail and include which formulas to use. I have a radio antenna but I curious to know what frequencies it resonates at.

  2. jcsd
  3. Jun 20, 2012 #2
    Unless you are talking about the simplest of antennas, like a dipole, there will not be formulas. You will either have to measure or model. A popular tool for modeling antennas is NEC2. It is free and fairly easy to learn. I'm assuming you are talking about a receiving antenna?
  4. Jun 20, 2012 #3
    Okay I downloaded the program and went through the tutorial. I got some results but I'm not sure how to interpret the graph that is generated.
  5. Jun 20, 2012 #4
    Good for you!
    Which graph are you studying? Maybe you could attach the NEC2 source file?
  6. Jun 20, 2012 #5
    Ok long story short, I couldn't start a thread so I made a second account and it just got banned so I'm back to my original account. I attached a screen shot of my results. To be honest I'm not sure how to interpret any of it. I chose 900Mhz because that was in the tutorial. I had to specify frequency and height and radius of the antenna wire/rod but I was under the impression that I could pick a height radius and receive information regarding what the resonant frequency of the rod would be.

    Attached Files:

  7. Jun 20, 2012 #6
    First, your antenna only has one wire segment. This will not be very useful.
    Your best bet for getting up to speed on NEC2 is to open one of the examples under the \4nec2\models subdirectory. Look at their source code and play around with the simulator to get a feel for it.

    All of this assumes that you have some basic understanding of certain antenna concepts.

    Backing up a little, what exactly is your goal? Learning about antenna? Building an antenna for a specific purpose?
  8. Jun 20, 2012 #7
    My goal is to learn about antennas and its physical relationship to its resonant frequency. If you're going to suggest a reading I'm all ears but I'm hoping it will be an easy read.
  9. Jun 20, 2012 #8


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    The main point of using a resonant antenna is the efficiency with which it will radiate its power and the fact that it is easier to 'match' to a transmitter or receiver. the current flowing in a very short dipole has a similar distribution to that of a half wave dipole and their directivity patterns are pretty similar, too. The (half wave) dipole is better for transmitting because, when you try to feed it at the centre, there is no reactive component to its impedance. A short dipole is fine for receiving, though.

    You want to associate dimensions with resonance - well, resonant structures have a large standing wave on (or in) them, and half wavelength structures tend to support a good standing wave.
    Last edited: Jun 20, 2012
  10. Jun 20, 2012 #9
    Google search for "A Beginner’s Guide to Modeling with NEC". It is a 4 part article that provides a gentle introduction to modeling antennas with NEC2. Part 3 discusses matching and resonance.
  11. Jun 20, 2012 #10


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    You can tell a bit about that antenna from the information given.

    That diagram shows a slice of the antenna radiation pattern and since the antenna is symmetrical about the vertical axis, you can mentally rotate it to get something like the top half of a donut.
    This tells you the antenna works by radiating most of its power horizontally or up at low angles, but it radiates very little straight up.

    The input impedance is 179 - j341. For resonance, the j number should be zero.
    In this case, the antenna is being operated below its resonant frequency. You can tell this by the negative sign.
    So, either the antenna should be longer or the frequency should be higher.

    You can calculate the resonant frequency of this type of antenna with the formula:
    Length (in feet) = 234 / Frequency.
    So a 900 MHz quarter wave antenna should have a length of 234 / 900 or 0.26 feet or 3.12 inches.

    A dipole is about a half wavelength long. A wavelength at 900 MHz is 300 / 900 or 0.333 meters long so a half wavelength is half of this, or 0.166 meters or 6.56 inches.
    The "300" in the above formula comes from the speed of light.

    A formula for half wave dipole is Length (in feet) = 492 / frequency in MHz
    So a dipole at 900 MHz would be 492 /900 = 0.546 feet long or 6.56 inches long.

    I find the program EZNEC free Demo version 5 easier to use since all the information is in one program.
    You can get this program from www.eznec.com.
  12. Jun 20, 2012 #11
    Wow, thanks a lot. Everything makes a lot more sense now. I just have a few questions. How did you know to look for the j value of the impedance and not the current or the parallel form? And also, will voltage j value always be 0?

    Also, the formulas for half wave dipole and quarter wave dipole, are the values "492" and "234" respectively constants?

    I will check out eznec right now.

    Thanks everyone for your help I really appreciate it.
  13. Jun 20, 2012 #12


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    The impedance is normally always given in the series form and Eznec does not even give the parallel form.

    You adjust the frequency of the input signal until the impedance is as close to being unreactive as you can get. So the J factor becomes close to zero. It changes from negative to positive as you go through resonance if you are increasing the frequency.
    Note that whatever is left is a pure resistance and this has to be matched to a feedline and feedlines only come in certain fixed values. 75 ohm and 50 ohm feedlines are common.

    The voltage is whatever you make it, but unless you are feeding two antennas at once and want to introduce a phase shift on one of the feeds, then you would always select the unshifted version. It is only a reference point anyway and the internal voltages and currents in the antenna will be given a phase angle relative to the applied voltage.

    Yes, those numbers are constants. They would be different if you wanted an answer in metric units like meters, etc.
    A dipole should actually be slightly less than a half wavelength in air and how much less depends on the diameter of the wire you use to make it.

    Eznec is good. It may look a bit crude when you first look at it, but it has a great interface to actually produce usable results. You can make a change and then see the result immediately, without saving it to a text file.

    You enter "wire" data by changing an existing antenna's wires. Then you can do an SWR sweep to find out where the antenna is resonant. "wires" can be 1 inch pipes or even bigger metallic objects.
    Then you can plot the radiation pattern of the antenna.

    The program works on "segments" which are considered as discrete radiating points. So, a wire may be split up into 5 segments, for example.

    You can have up to 20 segments on the free version of the program. For a dipole you might use 19 of these and the odd number will mean that the dipole can actually be fed at the center.
    That is probably confusing. If you have 9 segments, then the one that is fed, then another 9 segments, then the feed is in the middle.
    If you had 20 segments, then the feed would have to be offset from the center, on segment 10 or 11.

    Use a much lower frequency, though. Try 10 MHz. This will give you a feel for real life antennas.
  14. Jun 21, 2012 #13


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    It may help to do some actual reading about this. (Hundreds of sites and even BOOKS???.) Learning -by-doing can be very effective but you risk wasting a lot of time up blind alleys. It's a huge topic with so many varieties of radiator.
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