Problem modeling EFHW + autotransformer

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K5DJO, Oct 8, 2014.

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  1. K5DJO

    K5DJO Ham Member QRZ Page

    Hi,

    I'm building an antenna similar to the HyEndFed/LNR commercial offerings, based on the wonderful posts by PD7MAA here and others. I'm wanting to experiment with different inductor designs, as well as wire lengths, so I'm trying to build an EZNEC model. Unfortunately, what I've gotten so far isn't correct based on the feed point impedance EZNEC is calculating. I've tried adjusting wire lengths slightly, to mimic the trimming I had to do with the 'real world' model, but that didn't seem to help. I'm certain I'm doing something incorrectly here (I'm new to EZNEC as well as amateur radio/antenna theory/etc).

    I'm linking to my model, and would very much appreciate it if someone could look at it and tell me what I'm doing wrong. I've been using the help document that comes with the software, but this one has me stumped.

    EZNEC endfed 1/2 wave attempt

    Thank you in advance.
     
  2. KH6AQ

    KH6AQ Ham Member QRZ Page

    I wasn't able to open the file and it's probably something I'm doing wrong. Can you describe the antenna in more detail?

    If you're feeding the wire at one end it won't work. In EZNEC and in the real world there isn't a true end fed antenna. "End-fed" antennas are actually dipoles and in the case of the HyEndFed/LNR the coax shield is the other half. To model such an antenna a 1/4 wavelength wire needs to be placed on one side of the RF source.
     
  3. W6OGC

    W6OGC Ham Member QRZ Page

    Look at AA5TB's website http://www.aa5tb.com/efha.html. Steve has some explanations of what he thinks is going on and some modeling results, and how he did it.

    I believe I understand you have to "fool" EZNEC on this.
     
  4. K5DJO

    K5DJO Ham Member QRZ Page

    The other file was a zip file, this is the raw EZNEC file. Perhaps that will open for you.

    It's hard to describe in a short number of words, it would be best if you could see what I _attempted_ to do. I've got a main wire for the antenna, 10.1m long, a small wire attached at the top of it (all vertical) with a load (the inductor/coil) attached to it, then a short 1.85m piece of wire attached to that. At the bottom of the long wire, I have a transformer leg attached (1:64 transformer), the other transformer leg is attached to virtual wire, which a transmission line is attached to, and then the other end of the transmission line is attached to another virtual wire which the source is attached to. I hope that helps?



     
  5. K5DJO

    K5DJO Ham Member QRZ Page

    Thank you, I'm going to go look at it. That would make sense, since these antennas use the coax as a sort of counterpoise if you don't actually attach one. I suppose I can make the 0.05 WL counterpoise in EZNEC and see if that 'fixes' things.
     
  6. KM3F

    KM3F Ham Member QRZ Page

    This comment is not going to solve the modeling problem but in the last paragraph where the phrase 'counterpoise to (push) against' is a very poor choice of word when talking about antenna theory.
    There is no pushing.
    A counterpoise is an attempt to electrically complete tuned circuit or electrically, a Tank Circuit.
    The term pushing against is confusing where it does not belong in antenna theory.
    For example a dipole is two elements. One side does not push against the other. The RF has to oscillate back and forth between both sides to accept and 'loose power' for communications..
    This may seem nit picking but in a class on antenna theory you should never hear the term pushing and be correct.

    Good luck..
     
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