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Comparing currents along end-fed and center-fed dipoles: Predictions please

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX4O, Apr 27, 2019.

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  1. KX4O

    KX4O Ham Member QRZ Page

    I'll add it to my list and see what I can do.
  2. KX4O

    KX4O Ham Member QRZ Page

    I just remembered the primary and secondary have a connection at one end...

    Not sure how well I can measure the capacitance between primary and secondary in this particular situation without the value being spoiled by the short. Thoughts welcome.
    WB5WPA likes this.
  3. WB5WPA

    WB5WPA Ham Member QRZ Page

  4. KX4O

    KX4O Ham Member QRZ Page

    Yeah the simple meter based measurement didn't work out. Sorry.
  5. KX4O

    KX4O Ham Member QRZ Page

    May I ask... where were you going with this train of thought?
  6. WB5WPA

    WB5WPA Ham Member QRZ Page

    Well, I see that you have the bottom of the secondary tied to the primary 'ground' so my point is moot now.
  7. AI3V

    AI3V Ham Member QRZ Page

    Why is there a so239 connector? I thought the whole experiment was to use a tiny "altoids tin" transmitter at the end of a single 1/2 wavelength wire?

  8. KX4O

    KX4O Ham Member QRZ Page

    It's built into the two different LNR transformers I am using to energize the Hi-Z end as mentioned in the OP. I have one of Danny's transformers too that I may try. The transmitter, be it Altoids or otherwise, connects to this. The transmitter I came up with is pretty small, not as small as you would like, but small nonetheless. Here it is in the center-fed test...
    ...and again in an earlier end-fed test with the LNR transformer...
    ...except now I have one less adapter between the transmitter and the LNR transformer... no more rubber bands either.

    My next test will connect the fabled 0.05 wire to the other side of the transformer and again measure the energy in the dipole and this new wire... just because I can at this point.
    WB2UAQ and AI3V like this.
  9. KX4O

    KX4O Ham Member QRZ Page


    Here is the transmitter module as tested with the shorter adapter to the smaller LNR transformer at the end of the 30m dipole.
    That white blade to the left is the H-field sensor that hangs on and moves along the dipole for measurements. The battery is AA size for a reference.

    Here is a view of the entire system differing from the previous image only in the position of the sensor (dipole midpoint)...
    As small as an Altoids approach? No, but pretty close and certainly much much smaller than the antenna under test.

    Here is the end-fed feed end with the much larger transformer from the LNR QUAD product...

    Here are measured magnetic fields of both LNR transformers feeding the 30m dipole at its end compared to the same transmitter feeding the 30m dipole in the center...

    Large or small both LNR transformers have about the same 1.6 dB or so power loss with respect to the center-fed case. Dialog with Owen and others suggest this loss is what one should expect from 1:49 (impedance ratio) step-up transformers give or take tolerances, materials, etc. The larger point is the dipole radiates pretty well whether fed in the center or from the end without the aid of a 0.05 or any other wire past the end. Yeah the end-fed transformers along with the transmitter R-Pi stack do provide conductive area, but it's certainly quite small, but not zero certainly. I will be testing this again with the 0.05 wire just to see what happens.

    All this feedlineless testing is impractical for radio operations in Realville, but sets a solid foundation of understanding how these EFHW antennas function. The fact they work with no extra conductors is a good thing to build upon.
    Last edited: Jun 14, 2019
    WB2UAQ likes this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    One other thing to consider is what is the output impedance of your test transmitter? Does it deliver max power into 50 + j0, or some other impedance. What is to prevent it from putting more power into, for example, a center-fed that has a Z of 58 Ohms vs an endfed that presents a Z of 35 Ohms?

    When I used to run a mobile antenna efficiency contest, I used a signal source that had a 20db 50 Ohm attenuator between the source and the antenna under test. This normalized the source impedance to be very close to 50 Ohms. The contestants could observe the field strength, and could adjust their antenna match to get maximum field strength. Highest absolute field strength won...

    With respect to the 0.05 wl short wire at the transformer, I find that the long wire primarily effects the resonant frequency, while the short wire primarily effects the match. I am able to feed an EFHW by using a Common-Mode-Choke placed on the coax at a few feet from the transformer, such that the total length of the antenna is from the choke to the far end, and the coax shield "tail" is just the right length to make the Swr50 at the transformer very low, as shown here.
    WB2UAQ likes this.

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