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Tried to make an 1/8 wave dipole ant. It didn't work

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NQ7T, Dec 3, 2019.

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  1. NQ7T

    NQ7T Ham Member QRZ Page

    I tried to make an 1/8 wave 915mhz dipole antenna. I had a 1/4 wave and figured shortening each leg of the dipole in half would work, but it didn't. The 1/8 wave ant resonates at about 1820mhz. What did I do wrong? The 1/4 wave ant was 78mm for each leg. I shortened each leg to 39mm. Should be good, right?....no so much.

    Thanks
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    Ain't no such thing as an 1/8th wave dipole. It will resonate at 4x the desired frequency
     
    K0UO and NH7RO like this.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    AJ:

    A 1/4th wave dipole is not resonant as well. I really think he means a dipole with 1/4th wavelength on each side. Shortening each side by 1/2 will make the antenna resonant on 2-times the original frequency. One can make the antenna resonant on the original frequency by adding inductance to each side.

    Glen, K9STH
     
    KW6LA, WD0BCT and N7EKU like this.
  4. W7UUU

    W7UUU Super Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    A 1/2 wave dipole at 915 MHz is just a hair over 6" long end to end.... (468/f in MHz)

    And you want to make it 4 times smaller?

    What is the application?

    Dave
    W7UUU
     
    K0UO likes this.
  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    Risking a wrath of Khan....

    DIPLOE is a resonant device , an antenna, 1/2 wavelength long.
    Wavelength is related to frequency.
    Dipole is a mathematical description of TWO (voltage) maxima - hence TWO poles present on such resonant device.

    For definitions used by professionals - see IEEE dipole definition.

    Then you can choose between serious folks and jokers.

    How is DX?
     
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    As others have implied first resonance in a balanced dipole occurs when the end to end length is 1/2 wavelength electrically. Any lengths shorter than that won't be resonant and will have increasing capacitive reactance as you get shorter. By the time you cut a dipole down to 1/4 wavelength end to end it will be very difficult to match and have a very high capacitive reactance, if you did try to cut one down to 1/8 wavelength end to end it would be extremely difficult to match to common system impedances like 50 ohms or 75 ohms.

    In the case of a traditional ground plane vertical the radials or ground plane serves as a reflective surface that mirrors the main vertical element. That means first resonance in a typical ground plane vertical occurs with the vertical element 1/4 wavelength long as the effective antenna including the ground plane reflection is still a half wavelength antenna. There are variations on that theme that can be achieved by changing the relationship of vertical length to radial/ground plane length but the basic idea is the same that the total effective antenna is half wavelength long at the shortest resonant length.

    As you make antennas longer than half wavelength they become resonant again at a full wavelength (resonant but very high feed point impedance and tough to match), one and a half wavelengths and additional multiples of half wavelength. But a 1/8 wavelength wire or even two 1/8 wavelength wires fed as a dipole will not be resonant and practically speaking will be tough to match even with a good matching network at those lengths.
     
    K2XT likes this.
  7. NH7RO

    NH7RO Ham Member QRZ Page

    1) Try making a standard half-wave dipole and see if that "works." Remember the "468 divided by frequency" rule we must have to learn? IF not, proceed to #2:

    2) Get your hands on an ARRL Antenna Book; most libraries stock them.

    3) I really don't mean to sound insulting here; but you're an Extra-class ham asking about 1/8th wave dipoles? No such thing by definition that I'm aware of---where did you get that idea, if I may ask?
     
    KP4SX likes this.
  8. NQ7T

    NQ7T Ham Member QRZ Page

    Thanks guys for the education. One of you asked the application. It is for RC control. I appreciate the quick responses.

    John
     
    NH7RO likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Probably on the internet where everything that is posted is true. :rolleyes:
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    From The IEEE Dictionary: "dipole See: dipole antenna; magnetic dipole; electrically short dipole; folded dipole; half-wave dipole; Hertzian electric dipole; Hertzian magnetic dipole; microstrip dipole."

    "dipole antenna ... Any one of a class of antennas having a radiation pattern approximating that of an elementary electric dipole. ..."

    There is certainly such a thing as a 1/8 (or 1/4) wavelength (electrically short) dipole but it (they) are not resonant. Balanis dedicates 12 pages to the "infinitesimal dipole". However, The IEEE Dictionary says in "common usage", the word "dipole" used alone, implies a 1/2WL conductor. Technically, a 1/2WL dipole on 80m is not a dipole on 20m since it has more that two current nodes on 20m.
     
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