Question about a dual end fed antenna - possible?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4LLZ, Aug 5, 2020.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: abrind-2
ad: L-MFJ
ad: Subscribe
ad: Left-2
  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Actually, Jim, the feed point impedance of a 20m long center-fed wire at 10m agl over average dirt is not 10,000 Ohms at its resonant frequency of 14.463MHz; it is 5245 Ohms and varies with frequency as shown below:


    If I were going to build this antenna, I would make the wires a bit longer (1.20wl) and have an EDZ, which has about 2dB more gain in the favored directions and a much easier to feed impedance. (390 - j1461 Ohms)!

    Let us see you show this using "transmission line and antenna logic and calculations as understood in the 50's and 60's before the computer tools emerged."
  2. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Now go back and center feed it using two wires then post the results.

    Then check to see if there any limitations when feeding an anti-resonant --- truly end fed ---- 1/2 wave wire.

    Thanks for the reply.

    Regards Jim
  3. W9XMT

    W9XMT Ham Member QRZ Page

    To K9AXN - I might be inclined to follow your commands except for your view quoted below, taken from another thread here on QRZ:

    "It would be more productive to revisit transmission line and antenna logic and calculations as understood in the 50's and 60's before the computer tools emerged, and displaced logic."

    What would be the point of my providing you with information by the use of post-1960, logic-displacing methods/software you think not to be very productive?
    Last edited: Aug 9, 2020
  4. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Wasn't a command at all. The current Electronic CAD tool set is a superb tool and if I was actively pursuing design work, I would spend whatever time it takes to master them.

    Problem is, in the hands of folks that are unwilling to take the time to review the limitations and learn the rules of the product e.g. the limitations in dealing with the end fed 1/2wl wire or any anti-resonant feed point.

    Case and point, the theory that a 1/2wl wire cannot be fed at the end because it presents a near or infinite impedance is absurdly incorrect.

    If I sounded a bit aggressive, it might be that you, and a handful of others respond with "You are uninformed" etc, etc, to anyone that disagrees with you. You then refer to charts derived from the very tools you claim I think to be useless and or complex math that you don't understand.

    If you are to be authoritative learn the tool set or the logical understanding of the theories at hand.

    If I sound brisk I apologize. I ask you as a gentleman to rerun the tool using a configuration that consists of a Center fed 1 wave length wire. The configuration in post #10 may be a 1wl wire fed with a single wire connected in the center. That is not a center fed 1wl configuration --- it is a T.

    What will be revealing is the center fed 1wl wire will have the same radiation pattern as a center fed 1/2wl wire but narrower and added gain.

    A good day to you Regards JimK9AXN
  5. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Please go back and post the radiation pattern --- thanks Jim K9AXN
  6. W9XMT

    W9XMT Ham Member QRZ Page

    ANSWER: Obviously K9AXN didn't comprehend the graphic in Reply 10. It shows a single conductor, only.

    What he apparently took as "may" be a 2nd conductor connected to the center of the antenna is the vertical (Z) axis of the 3-D volume of space above the origin that includes the NEC wire model.

    My graphic in Reply 10 shows the net operating parameters and 3D radiation pattern envelope of a single, 1WL, center-excited, horizontal conductor installed 10m above a ground plane of "average" dirt.

    Then, K9AXN offered this advice to me:
    It is K9AXN that needs to learn the logic and use of a modern tool set such as M-o-M (NEC) software.

    Also it would benefit him to "look before he leaps."
    Last edited: Aug 9, 2020
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is the comparison of three cases. Center-fed horizontal dipoles with a wire length L of 10m, 20m, and 24m. Dipoles are 10m above Average (Cond=5mS,Dielec=13) Earth. The third case is the EDZ antenna, which by definition is non-resonant.

    I find the 20m resonant frequencies of the 10m and 20m dipoles, show the feedpoint impedances, the peak gains and efficiencies:

    First, the azimuth patterns at 30degrees above the horizon (dipole wires lay along X axis), so peak radiation is broadside to the wire:

    Second, the elevation pattern at 90degrees azimuth (broadside):
    Last edited: Aug 9, 2020
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    ps to post #17:

    The reason nobody would ever use a 1wl long dipole is because it takes 3300 Vp-p to drive it at 1kW!
  9. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Good morning Mike,

    I disagree. Your attempting to justify the 5500 Ohm quote from eznec by using vp-p not RMS. I believe you will find in the limitations set out in their documentation regarding anti-resonant (end fed 1/2wl wire) that they support only the OCFD 2 wire design; not end fed. As a result they found the best feed point on the wire would be at the 2500 ohm point which makes a good configuration for a multi-band antenna.

    Here is a 20,000 ft overview of the design process from the paleontological era. It will be drilled into later.

    Only two things need to be known in order to design a dipole driven from either the center or end: The Z0 of the wire and Q.

    The Z0 of the wire is 600 Ohms and Q of a 1/2wl dipole driven from either end, which is 8.3333.

    We will provide a teaching experiment developed in 1964 to measure the capacitance, inductance, radiation ratio, and velocity fraction of the wire. In the next iteration we cover a comprehensive cycle by cycle explanation of both the resonant and anti-resonant wires.

    This is for a .5λ wire with 500 Watts applied.

    First we use the Z0 of the wire, 600 Ohms to calculate a base. (500W÷ 600 = .83333) (√ .83333 = .91287A) (.91287 x 600 = 547V) (.91287 x 547 = 500W)

    Now we take the Q of a .5λ wire = 8.3333 (This will be proved when we drill down to the next level). Now render the √ of 8.3333) (√8.3333 = 2.8867)

    We now have everything needed for final calculations.


    First calculate the voltage, current and feed point impedance of the resonant (Center fed wire). Voltage (547 ÷ 2.8867 = 189V) Current (.91287 x 2.8867 = 2.635A)

    (189 ÷ 2.635 = 72 Ohms) (189V x 2.635 = 500 watts).

    END FED:

    (547 x 2.8867 = 1579V) (.91287 ÷ 2.8867 = .31623A) (1579 x .31623 = 500 Watts) (1579V ÷ .31623 = 5000 Ohms)

    Mike, is there anything about Vp-p in these calculations or was that a Trojan horse?

    CONCLUSION: A 1λ center fed wire will have a 10,000 Ohm feed point not 5000.

    More to come:

    Regards Jim
    Last edited: Aug 10, 2020
  10. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    YES/ Just put a relay in it.

    a simple single pole double throw/ like a 10 amp relay will handle plenty of power. run a 12-volt line down there or suck 12 volts off of your coax with an injector
    Then switch one or the other in or out

Share This Page