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. 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.
     
    K7JOE and KA0HCP like this.
  2. 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.
  3. KQ6XA

    KQ6XA Ham Member QRZ Page

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

    K8AI Ham Member QRZ Page

    Ouch! I wonder who that was directed toward.
     
  5. K7MYR

    K7MYR 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
    KE0RFZ likes this.
  6. N1BCG

    N1BCG Ham Member QRZ Page

    What's missing with the initial assumption of Z at the end of a folded dipole is that it relies on the existence of a certain impedance at the feedpoint in the center. If that's shorted then the whole thing becomes a (useless) 1/2 wave stub as mentioned.

    A ~300 Ohm resistor could be placed at the original center feedpoint and then the endpoint impedance could probably be matched, but you just end up with a radiating dummy load.

    Since useful power would be dissipated by the resistor, it could be replaced with the feedpoint of another antenna with matching Z. Now things are coming off the rails and phase relationships have run amok.
     
  7. KB0R

    KB0R Premium Subscriber QRZ Page

    So what did you get for a final impedance?
     
  8. KY5U

    KY5U Subscriber QRZ Page

    Guy is well spoken but boring because he yammers on. He took 13 minutes to cover 5 minutes of material. What he's doing is good, but he should have an outline off camera to follow and "keep moving" in big letters on the sheet. All that said, he's Walter Cronkite compared to most other ham videos.

    I learned this by watching some of my own boring videos.
     
    G8FYK likes this.
  9. PA3BUL

    PA3BUL Ham Member QRZ Page

    If you use a 1:9 unun at the end of a 1/2 lamda, you get an antenne that behaves like a dipole.
    So if it is as long as a dipole, behaves as a dipole it might as well be a dipole.. an end-fed dipole.
    In many cases it is easier to feed the dipole from the end. It may save coax. Also a vertical dipole is easier to feed from the lower-end. GD DX to all, Math, PA3BUL.
     
    AG5SU likes this.
  10. PA3BUL

    PA3BUL Ham Member QRZ Page

     

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