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

    Answering the challenge
    Here is the first test setup partially answering Rege's (AI3V) challenge of placing a small transmitting source at the end of an EFHW to see what happens. Rege wanted 80m and a far field measurement. This will be at the more manageable 30m and use a commercial magnetic field probe relaying measurements via WiFi to a distant computer. Likewise the ~150 mW transmitter is remotely controlled via WiFi. Both are battery powered and have no additional wiring to spoil the antenna isolation.

    I've confirmed all the gear works as designed, but not without some pain of bricking Raspberry Pi Zeros, etc.

    Here is the test setup...


    Important points include:
    • I made two dipoles rather than morphing one to both purposes due to the different feed mechanisms.
    • The center-fed 30m dipole is split with a strain relief and then attached to a Banana-BNC adapter for attachment to the R-Pi 30m transmitter module.
    • The end-fed 30m dipole is one continuous wire with the same broadband LNR autotransformer from my dissection years ago. See picture here from this article about the LNR 40-20-10. A UHF to BNC adapter allows connection of the same R-Pi 30m transmitter module.
    • The dipoles are suspended with non-conductive rope (or equiv.) atop non-conductive masts.
    • The mag sensor will move in small increments measuring the magnetic field about one inch from the dipole wire.
    • This distance between wire and mag loop sensor is fixed by the non-conductive hanging system I made. It's quite a sight. Pics soon.
    • The ~150 mW of applied power, the probe spacing from the wire, and the sensor electronics produce useful numbers in the range I can digitize. With effort I could make this an absolute measurement, but for this A-B comparison, relative will have to do.
    • Magnetic field being proportional to current, we will effectively map the current magnitude along the wire for each case.
    • The center-fed dipole is made from copper wire, while the end-fed is nichrome wire. Just kidding... seeing if you are awake. Both are, of course, made from precisely the same copper wire.
    • Both wires required trimming to obtain good SWR using a good ole MFJ meter hanging as free as I could get it in my initial tests. Both wound up the same length... no surprise.
    • Both dipoles have no conductors anywhere near... except earth of course. The height and tautness will be kept as consistent as we can, but many data sets will hopefully average out any variations.
    This test is a long time coming. I've been careful about its planning, but welcome suggestions for improvement. Keep in mind I'm testing only one thing... how that ~150 mW turns into current magnitude differing only in source placement at center or end.

    Why not use a wraparound rf current meter?
    Weight mostly. The wireless sensor system can ride on an antenna element with ease and not droop it too much. Remote measurement also means I can test with the sensor well away with dipole elevated to great heights. Nothing wrong with rf current meters. I have the MFJ product. This is just less intrusive to the system and easier to read remotely.

    Predictions please
    It's time. If you believe the above approach can work, it's time to place your bets on the outcome of this test. Precision not required. Possible broad answers include:
    1. Both antennas have mag fields (and hence current, hence performance) within a couple dB of each other given the LNR transformer introduces at least a little bit of understandable loss to the system.
    2. The end-fed antenna fails to operate much like a 1/4 wave whip on an HT ruins performance.
    3. Something in between.
    I've already seen enough preliminary data to suggest #1, but will let the final data represent my opinion. A hint is buried in the setup bullet points above.

    This is just data folks. Let's please refrain from this thread devolving into jackassery as oft times happens here on the Zed. I'd really appreciate your help keeping a broad smile on the moderator's face.
    K9AXN, KD6RF, WN1MB and 3 others like this.
  2. AK5B

    AK5B Ham Member QRZ Page

    The Rege Challenge will finally come to fruition. Cool.
  3. SM0GLD

    SM0GLD Ham Member QRZ Page

    I will follow this test.
  4. 4X1WD

    4X1WD Ham Member QRZ Page

    Me too,

  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    I'll bite.
    Since amateur radio operators come from all walks of life - here is a my challenge to all Sheldon's Cooper's among us.
    From long forgotten theory - mathematically demonstrate how the "energy pulse" AKA charge applied to a transmission line ( yes , antenna can be analyzed as a transmission line ) travels along the TL and CONVERTS the electrical to electromagnetic energy while doing so.

    Again I am no Sheldon Copper , but I believe there is something called "unity pulse" to help to analyze such events.

    As far as this "Rege" experiment goes - I hope it will concentrate on facts and not get bogged down with what color the wire used for the experiment is, did it rain yesterday etc.

    There are numerous mathematical articles analyzing the TL and most of them will point out that "lossless TL is assumed".
    Not addidng losses to the TL equations makes the math much simpler.
    After all - the losses are also mathematically defined.

    I wonder if utilizing "Reverse Beacon Network" would be helpful to "demonstrate" how the setup performs in practice.
    But it may be on borderline of "subjective" opinions which should be avoided.
    Best of luck
  6. KX4O

    KX4O Ham Member QRZ Page

    Way too windy or rainy this weekend gents. Will keep trying.
  7. KX4O

    KX4O Ham Member QRZ Page

    I did get a couple WSPR beacons into the network using my transmitter in the end-fed configuration during pre-testing, but the power is so low at 150 mW and 30m so popular for this, it was hit or miss. As well RBN reports S/N (not very useful for relative measurements if N varies) even though each value is available independently in WSPR. Perhaps using a local receiver running the actual WSPR software might work, but then if we are that close, I'll probably just use a good spectrum analyzer for amplitude measurements. It does make a lot of sense to have someone nearby measuring signal strength well away from the test antennas during this. Kill two birds with one stone... but I won't let it delay the primary tests.

    I may be confusing the WSPR network with the Skimmer network though.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I thought that Rege and everybody else believes that if you suspend a 1/2 wave length horizontal wire, get a current flowing in it, it radiates the same, regardless if you feed it 50% from the end, 33% from the end, 20% from the end or 5% from the end.

    I thought that Rege's beef is that if feeding the horiz wire down near the end, and if you leave off the 5% of the original wire on one side of the transformer, leaving only the 95% wire, and substitute 50 or so feet of coax shield with a bit of common mode current on it for the missing 5% wire, then the antenna stops radiating, and your shack will blow up because of the common-mode current that flows toward your shack grounding system?

    If it is the former, then why will not Nec modeling suffice? If it is the latter, then why not just measure the magnitude of the common-mode current on the feedline of an EFHW antenna fed through a 1:49 transformer?
    Last edited: Apr 29, 2019
  9. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    Neither, I'd say. I think the purpose was to prove/disprove the claim (made by several manufacturers and many on this forum) that the EFHW does not need any counterpoise or ground at all.
    Simply measuring common-mode current on the feed line may sound easy, but it isn't. How the common-mode current is distributed along the feed line and any cabling inside the house etc will vary greatly from site to site (it will likely NOT be strongest near the feed point).
    Testing the antenna as I suggested in another thread, with a small battery operated transmitter and suspended far away from conducting objects should answer how it works with (almost) no counterpoise or ground.

    As to my prediction, with the size of the transmitter used in this test, I'd guess the maximum current measured along the antenna wire will be in the order of 1/10 of that on the centre fed dipole (or 1 % of radiated power compared to the dipole).
  10. W9XMT

    W9XMT Ham Member QRZ Page

    An additional source of CM current flowing on the outer diameter of the outer conductor of a coaxial feedline to an EFHW is its coupling to the fields radiated from the nearby single conductor considered to be "the antenna."

    The NEC4.2-generated graphic below includes the effects of such coupling.

    Last edited: Apr 29, 2019

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