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I am looking at using NEC to model a $0.5\lambda$ rectangular folded dipole, for closely spaced larger sides (choosing a separation of $0.01\lambda$).

In Antenna Theory, Analysis and Design by Balanis, the folded dipole is touched upon, and an equivalent antenna mode circuit is presented:

balanis-figure-9.14

I created a NEC model for this to show the geometry and pattern. Auto-segmentation is used with 20 segments/$0.5\lambda$. The larger sides of the model point in the same direction, so the voltage source matches the antenna mode circuit. The magnitude of the impedance is $213\Omega$.

folded-dipole-model

Some questions I have:

  • How can I verify this model? The pattern and peak gain is very similar to the standard dipole. Are there any limitations due to NEC?
  • Is the purpose of using 1 voltage source on each of the larger sides is to ensure to current is identical in each?
  • The gain of a standard 0.5$\lambda$ dipole can be theoretically derived as $2.15$ $dBi$. Does a similar derivation exist for a folded dipole, taking into account the separation length?

Edit: As suggested, I have made the following updates:

  • Updated segmentation to 10 segments/0.5$\lambda$.
  • Wire radius at $0.001\lambda$, which satisfies $r < L/10$ before/now.
  • Using only a single source, in the $+\hat{x}$ direction.
  • Simulate for separation of $0.01\lambda$ (left) and $0.1\lambda$ (right).

folded-dipole-model-seperation

The single source makes the elevation pattern non-constant, peaking at $\pm 15\deg$ and $\pm 55\deg$ respectively to broadside. The magnitude of the impedance is $417\Omega$ and $647\Omega$ respective.

The standard dipole impedance was $95\Omega$. In Balanis it was stated the impedance of a short-separation folded dipole is 4 times the impedance of the standard dipole, which is approximately true in this model.

Some follow up questions:

  • How can I know if the pattern and gain of my folded dipole is correct? I was under the impression a folded dipole pattern/gain is very similar to the standard dipole.
  • If I feed from the $-\hat{x}$ direction as shown in Balanis, my front lobe is smaller than my back lobe. Why is this the case?
pymekrolimus
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  • Your model looks OK - quite a lot of segments, you should probably drop to 20/lambda. Make sure you respect the 2:1 thickness rule, better stick to 5:1 (so with short segments like this, the wire radius must be quite small).
    • – tomnexus Dec 11 '19 at 07:29
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  • But in NEC you don't really need two sources, unless you want to fully investigate the two superimposed modes of the folded dipole. A folded dipole is fed from one side. Effectively the two superimposed sources on the other side add up to zero - see how they have opposite polarity. So remove one source and see how it looks - edit your question to include impedance too.
    • – tomnexus Dec 11 '19 at 07:32
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  • When you've removed one source, you should start to see a slight variation in the "azimuth" pattern - it won't be completely flat. More separation makes it more variable. Perhaps increase the separation to lambda/10 to make it more obvious. You might need to adjust the graph scale to see it.
    • – tomnexus Dec 11 '19 at 07:36