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Ham Radio - An end fed folded dipole experiment. Part 1, the idea and design.

Discussion in 'Amateur Radio News' started by KB7TBT, Jul 7, 2019.

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  1. KB7TBT

    KB7TBT Ham Member QRZ Page

    Ham Radio - An end fed folded dipole experiment. Part 1, the idea and design.

     
    WA4EMU, AB3ZI, KD0XL and 8 others like this.
  2. WN1MB

    WN1MB Ham Member QRZ Page

    I see either a flattened/stretched out full wave loop or a shorted 1/2 wave balanced line.
     
    K2CAJ, K4AGO, K2XT and 1 other person like this.
  3. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    I agree, but if the feed currents are out of phase then the field will cancel just like a parallel transmission line. If he feeds it with a Balun, and the current in both sides is in phase, then is should behave like any other end fed half wave antenna.
     
  4. WD8JQN

    WD8JQN Premium Subscriber QRZ Page

    I is for current Z is for impedance
     
    KA1JY, KB1GKN, KA1VT and 1 other person like this.
  5. W6QWN

    W6QWN Ham Member QRZ Page

    Hmm... Seems to me that the theoretical impedance of a shorted half wave is zero..
     
    KA1JY, AB7E, W4AEE and 3 others like this.
  6. W6QWN

    W6QWN Ham Member QRZ Page

    You mean short both ends and feed it with a balun ??
     
  7. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    I was thinking that he should leave the far end open. If the current on both wires is in phase, then it should radiate just like any other half wave antenna.
     
  8. K6HPX

    K6HPX Premium Subscriber QRZ Page

    Go look up a c-pole from years ago.....

    Ken K6HPX
     
    K6PB and KB1GKN like this.
  9. AC2NZ

    AC2NZ Ham Member QRZ Page

    Is it possible to use the correct terminology? By definition, a DIPOLE is a BALANCED, CENTER-FED antenna; it has an equal length of wire on both sides of the center insulator/feedpoint.

    Thus, an "off-center fed", while being an antenna, is NOT a dipole. In fact, it's a WINDOM antenna.

    And an end-fed is a random wire or longwire antenna, NOT a dipole.
     
    AK5B, K7YB, KU4X and 12 others like this.
  10. AC2NZ

    AC2NZ Ham Member QRZ Page

    Exactly - all it would achieve is to increase the *wire size* a smidgen.
     
    K4AGO, KR3DX, KA4DPO and 1 other person like this.
  11. K8AI

    K8AI Ham Member QRZ Page

    Not much different than the old flat-top "hammock" aerial idea used by AM broadcasters in the 20's and early 30's except it only has two wires and "fed" at the end.

    As was mentioned above, adding extra wires effectively increases conductor cross section, increasing capacitive end-effect therefore increasing bandwidth, if I'm not mistaken.

    Also, the larger the conductor diameter, the lower the feedpoint Z is going to be at the ends, so that 1000-2500 Ohm figure will vary depending on the size of the conductor.

    The whole idea of the folded dipole is that the center can be series fed with typical balanced line that has a higher Z0 (300-600 Ohm) - hence a more appropriate feed point Z for balanced line.

    See "cage dipole" in Orr's Radio Handbook or ARRL Antenna Manual.
     
    K9UR and KA0HCP like this.
  12. K8JHR

    K8JHR Ham Member QRZ Page

    Even a fool is thought wise ... and discerning if he holds his tongue. Proverbs 17:28
     
    K4AGO and KR3DX like this.
  13. KQ6XA

    KQ6XA Ham Member QRZ Page

    No comment.
     
    Last edited: Jul 8, 2019
  14. K8AI

    K8AI Ham Member QRZ Page

    Ouch! I wonder who that was directed toward.
     
  15. W4XA

    W4XA Ham Member QRZ Page

    Chuck,

    You're right.

    A (half-wave) folded "dipole" with one end open, (I.E. "shorted" center feedpoint) and the opposite end shorted is simply a 1/2 wave length of transmission line.
    Also known as a 1/2 wave shorted "stub" (which isn't used)

    Since it is 1/2 wave length of transmission line (not a dipole antenna) if you open the opposite end, the the input impedance will be infinite. (open circuit impedance)

    For the OP, you can prove this using a Smithchart, you can calculate it or you can measure it, but it's going to be the same at the frequency for the 1/2 wave length.

    This is basic transmission line theory, proved in every Electrical Engineering lab using balanced lines, coax or wave-guides. Every EE student does it in the lab to design tuning stubs for different types of antenna systems.

    Using a slotted line, you can take a transmission line and measure voltage and currents along the line to calculate impedances at a particular freq for different line lengths (measured in fractions of a wavelength) with either shorted or open "stubs" to create a matching network.

    There's more here about this stuff.
    https://www.microwaves101.com/encyclopedias/slotted-line-measurements

    Now if you're talking about shorting the feed feed "end" of the transmission line, I.E. like using a piece of parallel transmission line as an end fed antenna, using a single wire feed to both wires of the "twin-lead"

    That might be different but probably not much different than a single end fed wire.

    Regards,

    Rick
     
    Last edited: Jul 8, 2019
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