End fed dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AA7EJ, Aug 10, 2011.

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  1. AA7EJ

    AA7EJ Ham Member QRZ Page

    I read an article stating that one cannot feed the dipole at the end with only one wire of the feed line.
    For example when using "ladder line" both wires need to be terminated.
    The argument was that there is a need for physical path.
    Now my question is - how can this explanation be applied to simple center fed dipole.
    Is is not that the "gap" of the center insulator "circuit" is completed via EMF?
    So why same does not apply to end fed dipole?
    In theory , the antenna circuit should also complete via EMF.
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Nah. A center fed dipole is a complete circuit; current is applied to each half by direct connection and diminishes to nothing at each end as power is "lost" in the wire by radiation due to its radiation resistance (which if matched to the transmission line should result in nearly 100% efficiency). If you pull the two ends around in a loop until they meet and make the wire twice as long, now it "looks" more like a complete circuit (no open ends) but it works the same way with a different radiation pattern and somewhat higher radiation resistance.

    With an end-fed wire, you've lost a connection for return current. If you don't provide one, the earth will, eventually, but that's an uncontrolled and typically lossy path. If you can pump current into the wire, it will still radiate but probably won't be as effective as the 2-post connection center-fed of the same length unless you get lucky.

    The "1/2-wave end fed" commercial antennas on the market make an attempt to match the transmission line to the much higher feedpoint resistance of that configuration but still use the outer conductor of the coax to form the "return current path," resulting in feedline radiation. In an optimal configuration, that probably works great. In a less optimal configuration, it won't.
  3. K9STH

    K9STH Ham Member QRZ Page

    End fed 1/2 wave antennas fed with 1/4th wave open wire feed line with only 1 side connected to the antenna have been in use for basically a century. These are called "zepp" antennas because they were first used on Zeppelin aircraft. Those antennas work on powers of 2 times the lowest frequency of the antenna. That is 2^0 = 1, 2^1 = 2, 2^2 = 4, 2^3 = 8, and so forth. As such, an antenna "cut" for 160-meters works on 80, 40, 20, and 10-meters but does not function well on 15 meters.

    Glen, K9STH
  4. W0BTU

    W0BTU Ham Member QRZ Page

    Last edited: Aug 10, 2011
  5. K8JD

    K8JD Ham Member QRZ Page

    End fed Disaster.

    My first experiment with a "Zepp" antenna was quite discouraging. I strung up a halfwave of wire for 40M and END fed it with balanced line and used my new MFJ balanced line tuner to match it to the rig.
    It turned into a RFI disaster in the shack..A radio man's nightmare. I could tune up with 5 watts but turning up the power locked up my keyer and also stuck the radio on transmit even if I turned off the keyer. Reset my digital clocks and shut down my computer.
    I reconfigured the antenna for a center feed point and lengthened to a double Zepp for 30M.
    That also happens to be an extended double zepp length for 20M and a 5/8 wave dipole on 40. no more RFI ! Everything works fine now.
    If they would have had solid state eqipment in the Zeppelin fleet, that antenna design would have been thrown away quickly !

    Last edited: Aug 10, 2011
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Maybe a spark from the Zepp antenna is what made the Hindenburg go ka-pow.;)

    Remember the Zepp isn't just fed with open wire from one end, it's fed via a critical length of open wire that acts as a transformer, and the bottom end of the "unconnected" wire is grounded. If you use just "whatever length reaches," that isn't good.
  7. K9STH

    K9STH Ham Member QRZ Page

    When configured correctly, the Zepp works fine. However, like any end-fed antenna you do need a good ground/counterpoise.

    A random length of feedline generally will not work and, by definition, using a random length of feedline makes the antenna not a Zeppelin antenna.

    The best working wire antenna that I ever had was a half-wave on 160-meters fed with a quarter-wave of 300 ohm transmitting twin lead. This type of feedline was available back in the 1950s and 1960s. It had like 14 gauge wire and a VERY heavy insulator between the wires. The antenna worked very well on 160, 80, 40, 20, and 10-meters. The antenna did not work well on 15-meters but that didn't matter because I had a 2-element yagi for that band.

    Since my transmitters, at the time, had wide-range pi-networks no antenna tuner was needed (WRL Globe Chief 90A that could match up to at least 1200 ohm and a Heath DX-100 that could match up to at least 300 ohms).

    Glen, K9STH
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Reminds me of the time I went to the CA DMV and told the lady that I had taken my amateur radio license plates off of my RV and put them on my new pickup. She said, "You can't do that!" I said, "Too late. I already did that."

    The antennas used by the Zepp airships proves, "that one indeed can feed the 1/2WL dipole at the end with only one wire of the feed line". Here's what EZNEC says about the current on that 1/4WL of feedline. The Zepp airship antennas were probably link-coupled which discourages common-mode problems because the common-mode fields cancel in the link-coupled coil.

  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The band Led Zeppelin proved categorically that if you turn up the volume loud enough you can see God.

    And He's holding one end of an antenna.
  10. AA7EJ

    AA7EJ Ham Member QRZ Page

    Thanks for replies.
    Call me stubborn, but I am still not convinced.
    It seems that anytime this or similar subject comes up
    the discussion turns into matching the Zeppelin antenna with 1/4 wave transformer.
    I have no quarrel with matching the antenna to the rig.

    So if the feed line radiates that means the currents in the conductors , either coax or open wire,
    are not balanced, hence the "cold" (return) conductor needs to be "terminated" at either end?
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