EZNEC+ feedpoint impedance display question

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WB5AGF, Aug 11, 2020.

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  1. WB5AGF

    WB5AGF Ham Member QRZ Page

    For EZNEC+ v.6.0.17

    Is there a way to cause the software to display an antenna's feedpoint impedance in complex form (showing the resistive and reactive components) ?

    I am aware that when the software is displaying VSWR the mouse can move the cursor along the VSWR curve and, as it does so, the resistive & reactive components are shown in text form at the bottom of the screen.

    What I am looking for is a way to display (graphically) the resistive and reactive components of an antenna's feedpoint impedance over a range of frequencies.


    Paul Nix, WB5AGF
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Paul, with Eznec running, just click on the "Src Data" button. Is that what you are looking for?
    That works for a single frequency at a time.

    If you want to print out tabular data during a frequency sweep, then use the SWR dialog box to set up a list of frequencies to try and what you want to output, and then use the "Setups" button to switch to "frequency sweep". Finally, click the "Freq Sweep" button. That gets you the numbers, but not a plot.

    Since I have the companion to EzNec called AutoEz (see AC6LA's web site), it is really easy for me: Here is the R and jX of a 10m long dipole in free space as a function of frequency near the fundamental resonance as plotted by AutoEz:


    Suppose we wanted to see what happens over a really wide frequency sweep: upload_2020-8-11_13-55-12.png
    fyi: AutoEz called EzNec as a subroutine 155 times to make that plot...
    Last edited: Aug 11, 2020
  3. WB5AGF

    WB5AGF Ham Member QRZ Page

    (This is very curious .... When I first saw your response I did not see anything but your first comment. I composed a response but then noticed that there was more to what you said. Most of that 'additional' information negated what I was going to say.)

    I 'found' AutoEz awhile back but became confused when trying to understand what/how it functioned (and so 'put-it-aside').

    I'm trying to look at what happens when a wire approaches a half-wavelength .... But what I seem to be observing is that over a sufficiently wide frequency sweep some very interesting things happen if one is not fixated on any one impedance.

    I have also become interested in linearly-loaded dipoles with just a single 'fold-back' (i.e. NOT the three-wire 'COBRA' antenna).

    I'm attaching the VSWR curve of a 30 Meter long linearly-loaded dipole (it looks like a folded-dipole except that there is a 'break' with a gap of a Meter).

    I constantly wonder if anything I'm doing with EZNEC has value as I get all manner of error messages.

    Thanks for your assistance.

    Paul, WB5AGF

    Attached Files:

  4. HAMHOCK75

    HAMHOCK75 QRZ Member

    Or try 4NEC2 which is free for plots like this,

  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Paul, the learning curve with NEC is pretty steep. I learned by reading almost everything that L. B. Cebik (sk) ever wrote on antenna modeling. There is a great archive of Cebik's work available on the web at http://on5au.be/Cebik documents.html

    I can likely help you get over some humps. Since you are folding back an antenna wire and running it parallel to another, I can guess where you are having trouble.

    Would you be willing to post your .EZ file? To do so, you have to play a trick on the QRZ forum's file attach method. It has a very short list of acceptable attachments, and .ez is not on the list.

    Here is what I do: I rename the file in Win10 from (e.g.) mike.ez to mike.ez.txt. That gets around QRZ's filter, and the file can be uploaded to your post.
  6. WB5AGF

    WB5AGF Ham Member QRZ Page

    I used to work in a product development group at Texas Instruments and our IT people started blocking files with a zipped extension ... so we did the same thing ... i.e. changed the extension to .txt and the files got through.

    I've renamed the file from having a '.ez' to a '.txt' extension.


    Paul, WB5AGF

    Attached Files:

  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    EZNEC allows you to view SWR sweeps in Smith Chart form which of course gives you the real and imaginary components normalized to the system impedance. As you drag the cursor over the smith chart plot the specific real and imaginary impedance components are displayed for each test frequency.

    If you want the complex impedance plotted on rectangular coordinates then AutoEZ is a great way to go as Mike pointed out but personally I prefer seeing impedance sweeps on a Smith Chart and that's a default EZNEC feature under the View menu on a displayed SWR plot.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Got it, Paul.

    I will just tabulate the things that it takes to make a model that obeys the NEC rules and improves the "Average Gain Test" of the model (look that up in the help file).

    1. In order for the source to be at the geometric center of the model, Wire #1 (W1) has to have an odd number of segments, so I changed 100>101 segments. 101segments for W1 is not a bad guess at what it will take to make a reasonable model, as I will show later.

    2. Whenever you model two closely spaced (0.3m in your file) parallel wires, you have to align the segment dots. This means that the segment dots on W4 have to align with those on the left half of W1, and the segment dots on W5 have to align with those on the right half of W1.

    I do that by dividing the length of W1 = 30m by 101 (per my segmentation) to get a segment length of 0.29703m/seg. To make the gap between W4 and W5, I can only make the gap either one seg long, or three seg long, or five seg long (and still keep the seg dots aligned.) I choose to make the gap 3 seg long to make the actual gap close to your original gap, meaning that W4 has 49 segs, and so does W5. That puts the right end of W4 at 15 - (49*-0.29703) = -0.44553 and the left end of W5 at +0.44553.

    3. the short end wires (W2 and W3) should be segmented so that they have about the same seg length as the other wires, at least within a factor of three either way. I changed them to only 1 seg each...

    4. I found a load that I didn't understand, so I deleted it. Now, when I do an AGT (lossless wire, free space, no dissipative elements in the antenna model), I have a model that has an AGT=1.000, which is a necessary, but not sufficient condition for a "good" model...

    With lossless wire, free space, the fundamental resonance of the "fixed" model is at 3.00MHz, where the Z=21.4Ohms. It resonates again at 6.90MHz, where Z= 290 Ohms.

    Here is the fixed model. If you need to optimize it, that is AutoEz's forte, so tell me what you would like this antenna to do?

    Attached Files:

  9. WB5AGF

    WB5AGF Ham Member QRZ Page

    I have to go fix something to eat but wanted to comment ...

    I had-so-hoped that I'd discovered something that
    would get my name into antenna texts for generations
    ... sort-of a modern-day W8JK ....

    (Oh Well ... )

    - Paul, WB5AGF
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    I have deceived myself a few times...:(

    EzNec hasn't made me invent a "new" antenna, but it sure has made me understand the "standard" ones. It has also debunked a few "mystery" antennas...

    A data point about the one you posted (and I "fixed"): It could be used as a "shortened" 80m antenna. I found a length that puts the Swr minimum at 3.75MHz: If L= 23.58m (77.2ft), then Z=21 Ohms, which is much shorter than a normal dipole for that frequency (125ft)

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