EFHW Antenna: A Detailed Analysis (NEC4.2)

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9XMT, Mar 29, 2018.

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  1. K4DJM

    K4DJM Ham Member QRZ Page


    I'm somewhat surprised by your response. I thought we were somewhat in agreement. I was challenging the utility of modeling an EFHW with a coaxial low-impedance feed. This is not how anybody serious about these antennas deploys them, and it is not how the (controversial) commercial versions are deployed either.

    The sims and real life experiments illustrate the sensitivity of the ground or counterpoise connection when using a 50 ohm coaxial feed, or the feedline radiation when such a connection is omitted. Both your experiments and mine show no such sensitivity when the 49:1 unun is included.

    To be clear. Although I don't see the original simulation as sufficiently relevant to the unun deployment scenario, (and to directly address your query) I don't pretend to know how to fix it.

    My theory -- and I do not know how to prove it -- is that the "counterpoise" afforded by (lets say) the coax shield is generally of such a low impedance at the feedpoint that anything you attach at the transformer ground lug (in parallel with that Lo-Z) would have minimal effect. Worst case is the feedline is a quarter wavelength long -- but keep in mind that the feedline is not hung like an antenna. It is capacitively coupled to all kinds of things as it runs over the roof or on the ground or around other objects. I doubt that the counterpoise Z would ever be anywhere near as high as the antenna side. 2.5K of reactance is afforded by only 18pF at 3.5MHz. I wonder how much capacitance to ground is generally seen by the shield of a coax feedline?

    WB2UAQ likes this.
  2. KD6RF

    KD6RF Premium Subscriber QRZ Page

    Homework assignment ===>
    a) Determine the capacitance of your coax and analyzer to fence and ground.
    b) Determine the capacitance from the transformer to fence and ground
    c) Add the two values
    d) Calculate Xc
    e) Then realize that it only takes 10 or 20 pF from your coax and analyzer to the fence and to lossy ground for it to be the "counterpoise".....

    Because as always, there are no 1-wire counterpoise-less antennas...... as basic circuit principles, every model ever run, every measurement ever made (including yours), and the rest of this thread proves :cool::eek::cool:
    N0TZU likes this.
  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

  4. K4DJM

    K4DJM Ham Member QRZ Page


    See my reply to Mike for details, but my thoughts are that it's not the transformer per se, but the Hi-Z that changes things. Consider the likely impedance (ground referenced) of the coax shield at the feedpoint looking toward the shack. The very worse case would be when the length of same is 1/4 wavelength, but the coax is not hung and isolated like an antenna so it sees a number of opportunities along the way to see significant capacitance to ground. If the antenna had a Lo-z drive impedance (like 50 ohms), the impedance of the coax shield could be significant, but what if the drive point Z is 2.5Kilohms? My working theory is that the coax braid will nearly always be very low-Z compared to that and will look like a short to ground instead of a practical radiator in that environment.

  5. K4DJM

    K4DJM Ham Member QRZ Page

    Bravo! I just made this point in a couple post replies.

    This is one of the reasons modeling the counterpoise currents with a 50 ohm feedpoint impedance is not relevant to the 49:1 unun case. The coax shield could be an effective radiator in a 50 ohm environment, but be more like a short to ground in a 2.5K environment.

    73, Don
    KD6RF likes this.
  6. W5DXP

    W5DXP Ham Member QRZ Page

    Might some people be thinking electrical lumped circuit model rather than electromagnetic wave model?

    A short to ground for RF is not like a short to ground for DC. Both an open-circuit and a short-circuit at RF reflect the EM wave. Common mode RF usually forms standing waves.
    Last edited: Mar 31, 2018
    SWL37632 and N0TZU like this.
  7. K4DJM

    K4DJM Ham Member QRZ Page

    But if you are a Hi-Z source, and your load (right there, at X=0) is very Lo-Z, how much energy actually gets coupled into the attached distributed structure (going towards the shack)? If the bulk of the reflection is at X=0, then that is pretty consistent with my thesis.
  8. SM0XHJ

    SM0XHJ Ham Member QRZ Page

    That makes more sense. Whether the transmitter is hi-z from the beginning (and feed line to) or 50 ohm and then transformed to hi-z near the feed point can have no effect on the function of the antenna itself.
    When the current in the transmission line reaches the feed point it will split over all available paths. If the feed point impedance is low compared to other paths, almost all current will go into the intended radiators.
    If the feed point impedance is high, as in this case, more current will likely go as common mode current down outside the feed line. How much is extremely difficult to predict. It will, among other things, depend on the lengths of the feed line, what it goes near on the way down, and what is connected to in the shack. One thing is quite certain through, it will not be grounded by a perfect ground there. Any CM current that reaches the rig will continue out through any path availabe. If many paths are available, each individual path will receive only a small part of the current.
    All this explains why some people are successful with these antennas while others are not. It also explains why they are pretty much impossible to simulate. One can try to simulate best or worst case scenarios, but no real realistic case.
    SWL37632 and WB2UAQ like this.
  9. K4DJM

    K4DJM Ham Member QRZ Page

    I suspect that most of the people that are unsuccessful are not using the proper transformer. Many people use 9:1 xfrms because they are so available or no transformer -- with ALL the matching done back at the shed.

    I think that thinking in terms of currents can be misleading. Instead, consider power flow. The antenna side of the transformer secondary sees a fairly good match into the antenna. The antenna is 1500 to 3000 ohms at resonance and the source is 2500 ohms -- a pretty good match and much of the available power is delivered to the antenna.

    Now consider the other side of the transformer secondary (same as one side of the primary, right?). As I've pointed out, the Z there is almost certainly very low compared to 2500 ohms. There is a huge mismatch loss there, so very little POWER gets coupled into the sleeve. A small bit propagates down, but this is easily managed by a 1:1 unun isolator at the shack entrance.

    But...without using the transformer (source now 50 ohms instead of 2500), the sleeve (counterpoise) side may be a BETTER match to the feed than the antenna itself. This is asking for RF in the shack. It is a bad situation.

    W4KJG likes this.
  10. W5DXP

    W5DXP Ham Member QRZ Page

    Does your thesis agree with EZNEC?

    KD6RF likes this.

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