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1/4 Wave, 1/2 Wave terminology

Discussion in 'Becoming a Ham - Q&A' started by KB1TJY, Mar 5, 2010.

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  1. KD8KSX

    KD8KSX Ham Member QRZ Page

    If you Google "USMC Antenna Handbook" you'll get a few links to a pdf copy of the same. It's a nice introduction to antenna theory and practical application without getting too deeply into the "make your eyes glaze over" details.

    At least this Old Army Grunt found it easy enough to read.

  2. KD8KSX

    KD8KSX Ham Member QRZ Page

    Deleted double post.
    Last edited: Mar 8, 2010
  3. KF7AJU

    KF7AJU Ham Member QRZ Page

    FYI those formulas given were for feet not meters.
  4. AE5JU

    AE5JU Ham Member QRZ Page

    So, you might ask, why a half wavelength and not a full wavelength to transmit that radio wave?

    In simple terms, that half wavelength of antenna is producing the wave for positive half of the cycle, 180*, and that same half wavelength for the negative half of the cycle, or 180*.

    OK, but what about 1/4 wave? :confused:

    That is really a half of a halfwave dipole. The other half is reflected in the ground plane, earth ground, radials, etc.

  5. K0CMH

    K0CMH Ham Member QRZ Page

    Another important piece of information is that different lengths of antennas, and different designs of anetnnas, have different input impedances.

    Impedance is important because whenever a wave encounters a changed in impedance, only part of the power of the wave passes through, and the other part is reflected back, and often lost.

    Modern radios are made to have the impedance of the output connector 50ohms. We use 50 ohm coax so that the impedance is the same, and the wave traveles from the connector into the coax without any reflected power or lost power.

    We want the connection at the antenna ("feed point") to also be 50 ohms impedance ("input impedance"). When all this happens, almost all the power gets sent into the antenna.

    A center fed dipole that is 1/2 wave length long, when suspended above the ground about 1/2 wavelenght high, has very close to 50 ohms input impedance at the specified frequency. Therefore, almost all the power will go into that antenna. A half wave length end fed wire antenna has a much higher input impedance, and we can suffer a considerable amount of reflection and power loss where the coax connects to it.

    Here is an example. We want to transmit on 14.250 MHz. with a half wave dipole. The metric formula says to divide 142.56 by the frequency in MHz. to determine a full wavelength: 142.65/14.250 = 10.01 meters. A half wavelength is half that: 10.01/2 = 5.005 meters. That would be a good length to start buuilding our antenna.

    But not all antennas have 50 ohms input impedance, no matter how hard we try. Maybe we want a design that will never have 50 ohms input impedance. We may want to feed the antenna with 450 ohm balanced feed line.

    We can make a circuit that electronically converts the input impedance to 50 ohms, or close. There is usually some small loss of power through those circuits, but they much improve the amount of power going into the antenna. One of these "circuits" is called a "tuner". It is adjustable so that a range of input impedances above and below 50 ohms can be adjusted to nearly 50 ohms. Some antennas come from manufacturers with "matching coils" or "matching boxes". These do the same as a tuner but are made for only one frequency or band.

    Hope this helps you understand why antenna length is important.
  6. KB1TJY

    KB1TJY Ham Member QRZ Page


    Many thanks for the detailed explanations, greatly appreciated! :)

  7. NA0AA

    NA0AA Ham Member QRZ Page

    Now that you have the immediate answer to your question, you should take some time to do more detailed reading about antennas - the ARRL Handbook or the much more comprehensive ARRL Antenna Book will give you a bit more background.

    Antenna design ranges from the dead simplicity of a resonant dipole to the sublime absuridty of 100' long rope based VHF beam antennas and a million variants inbetween.

    When you get more into the hobby, a better understanding of how antennas work will help you more effectively use what you have, and better understand how a compromise antenna can work when ideal antennas are impractical or impossible.

    Besides, maybe you will come up with something new.
  8. KB1TJY

    KB1TJY Ham Member QRZ Page

    Thank you, I will do that. I also need to join AARL as well. I passed my General last evening.

    Thanks again,

  9. K7JEM

    K7JEM Ham Member QRZ Page

    I think you need to re-check your formula, this is drastically wrong.
  10. KE5PGE

    KE5PGE Ham Member QRZ Page

    Clarity .......

    I'll try to make my question simple. If I decide use a long wire for an antenna and make it at full wave. Lets say an end fed #10 copper at 40 meters. What I am confused about is by making the wire 40 meters in length, how do I maintain 50-ohm impedance? I know the coax that I use as a feed wire is rated at 50 ohm. Is the impedance rated by the wire size or type? Does it change depending on the length of the wave or the wire? Would I stand a better chance of having less deviation from the 50-ohm if I were trying to make a full wave at 160 meters rather than at 40 meters? Would it be better to make a dipole and split the 40 into 2- 20's using one side from the center conductor of the coax and the other from the braided shield? If I need to maintain a 50 ohm impedance for proper antenna and radio ops, the wave length or the wire length shouldn't play into the mix…...or should it? I am sure that I am making this much harder than needed and I can't begin to express my appreciation for this forum. Also, How does the grounding portion of the feed line play into the equation? Everything that I have is grounded with 8' ground rods. I have been working with electricity/electronics for 35 years and am ashamed to say that I have never understood impedance as a practicality. Until I needed to grasp a better understanding of antenna theory, it never was a priority. I am a relatively new HAM and am a Tech. I am currently studying for my General class but this is one hurdle that I must tackle before that happens. I haven't been as involved as much as when I started because I have had to stop to rebuild my home after Hurricane Ike paid us a visit. But now I am back at it. I would certainly like to be able to get a good grasp of this once and for all! Thank You for your help.
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