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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?
  11. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Try connecting a light bulb with one wire and see if it lights up.

    Obviously, it won't, since there's no complete path for the current.

    Now, try plugging in your "source" with one wire to the light bulb and one wire to a ground post, and plug in the lamp with one wire directly connected and the other wire to a different ground post, at a different location.

    Does it light up?
  12. W5DXP

    W5DXP Ham Member QRZ Page

    The Zepp antenna was used by many early 20th century airships and it obviously worked. Take a look at the following graph based on EZNEC data to see why and how it worked.

    The current in the unattached wire at the antenna is obviously zero. The current in the attached wire at the antenna is 0.2 amps. The currents are balanced at the feedpoint and are equal to 2.0 amps. Ideally, radiation from the feedline results in the currents being balanced at the feedpoint. The common-mode current is maximum at the antenna feedpoint and is 10% of the differential feedpoint current.
  13. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    And, that's how it did work. It wasn't using "just one wire," but always two.
  14. W5DXP

    W5DXP Ham Member QRZ Page

    Seems I have missed something. Assuming we are talking about a Zepp, one transmission line wire is indeed unterminated (dead ended). The short element on a J-Pole is unterminated (dead ended) in the same fashion.

    There's one wire for the 1/2WL antenna and two wires for the 1/4WL matching section. If that's not the configuration, then by definition, it's not a (conventional) Zepp.
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    When I said "always two wires," I'm referring to the connection at the transmitter end of the line. Only one of the two conductors of the balanced line connects to the radiator. But it wouldn't work unless both conductors are connected at the transmitter end of the line.
  16. AA7EJ

    AA7EJ Ham Member QRZ Page

    Last edited: Aug 11, 2011
  17. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I didn't bother trying to translate it, but there's a "ground reference" right at the area shown as 1 & 2 in the drawing, which shows a link coupling system. Ground doesn't have to be related to "earth" at all. The ground is one of the two terminals of the output link, and it wouldn't matter if it's floating and in no way connected to earth.

    Of course since it was for use on an airship, there wouldn't be any earth up there anyway.

    Very few hams today use link coupling systems, although the Johnson Viking Matchbox does (did).
  18. W5DXP

    W5DXP Ham Member QRZ Page

    Sorry, I never said anything that would result in that inference. With zero current flowing in one wire at the feedpoint and 0.2 amps flowing in the other wire, it is obviously 100% unbalanced at the antenna feedpoint, i.e. it cannot get any more unbalanced than zero vs some value. I have recently moved and don't have any way to test-measure an antenna but I have no reason to doubt the EZNEC results for a link-coupled system. The coil flux generated at one end of the link coupling coil by the common-mode current is equal in magnitude and opposite in phase to the coil flux generated at the other end of the link coupling coil by the common-mode current. Ideally, those two flux components cancel each other. In reality, it is a difficult (but not impossible) task to accomplish. Since the Zepp is a single-band antenna, there is little reason not to use the best anti-common-mode circuit design possible.

    Unfortunately, there are some tuning caps attached to the Matchbox link coupled coil that provide a sneak path to ground for common-mode signals. The original Zepp patent above shows a floating link that would theoretically neutralize common-mode currents better than the Matchbox does.
    Last edited: Aug 11, 2011
  19. K1DNR

    K1DNR XML Subscriber QRZ Page

    A radiating feed line on an air ship might not have been a bad thing. A long outdoor vertical section of radiating feedline might not necessarily be a "bad" thing in some cases. Coax radiating in your house is usually a "bad" thing. RF in the shack is usually a bad thing.

    A radiating feedline on an antenna designed to have a particular pattern, or a system designed to have low common mode current is definitely a "bad" thing...

    Almost anything will radiate. It really comes down to controlling the pattern, which means controlling how the currents flow in the system.

    Otherwise, hook it up, match it well enough that it doesn't blow up your radio, and have a ball...

    * admittedly gross oversimplification ;)
    Last edited: Aug 12, 2011
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