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When modelling a Yagi in NEC the radiation pattern is essentially identical whether a conductive boom is included or not, see pictures below. An explanation says this is a limitation of the method of moments method in NEC-2; "only currents in the axial direction on a segment are considered, and there is no allowance for variation of the current around the wire circumference".

Radiation pattern (boom) Radiation pattern (no boom)

However when I sweep VSWR over frequency I get quite different plots for the boom vs. no-boom case. Why does NEC give different impedance results, and is this a true effect or an artefact of my modelling?

VSWR (no boom) VSWR (boom)

Edit: Here are some more details about my NEC setup;

  • Auto segmentation with 20 segments/wavelength.
  • Boom radius 12.5 mm, element radius 5 mm.
  • (Boom wire) connects (reflector left and right) connects (boom wire) connects (driven element left and right) connects (boom wire) connects ... (directors and boom wires)
  • Source is attached to the left driven element, at the connection to the right boom element and boom (see picture).
  • Diagram of driven element
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    $\begingroup$ Hello and welcome! You are correct, but NEC is tricky and sensitive. Please add some information about how you are feeding the yagi - which segment(s) are excited and how, and also the segmentation in general - how many segments in each element, in the boom, and the wire thickness. $\endgroup$
    – tomnexus
    Aug 13, 2023 at 6:06
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    $\begingroup$ Hi thanks! I added some more NEC parameters and a diagram - let me know if there's anything else that would be useful. $\endgroup$ Aug 13, 2023 at 11:37
  • $\begingroup$ A little bit of both? It doesn't look like you're modeling a realistic feed arrangement, so you shouldn't expect a realistic impedance/SWR. But also, requiring at least half of the DE to be insulated from the boom is common in real life. $\endgroup$ Aug 13, 2023 at 23:18
  • $\begingroup$ Would you be able to provide any tips for a more realistic feed arrangement? Should I have 2 sources (one on each arm of the dipole) that are equal in magnitude and opposite in phase? When I tried this it removed the SWR peak > 100 in the 4th image. Having separate sources also would allow me to model an air gap between each dipole arm. $\endgroup$ Aug 14, 2023 at 3:24

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In a Yagi there shouldn't be any longitudinal current on the boom, if the structure is perfectly symmetrical. But this is not the case in your simulation.

If you look carefully at the structure currents in your output viewer, I'm sure you will see some current flowing on the boom, near the feed.
Because you only have a single feed, the current spreads out into all three wires available to it - the other half of the driven element, and both directions of the boom.

To feed a dipole with an even number of segments like this, you need two sources. Each will show half of the total impedance (half voltage, same current). Pay careful attention to the direction of the sources - they must add in phase, so look at what your NEC has done to place the wire segments. The simplest is probably just to try changing one of the sources to -1 volts if it doesn't look right.

I also think you may have far too many segments. In the last picture it looks like you have 49 segments on a 0.35 m element, making them 7 mm long, and they're 5 mm radius... I think you can go as short as "cubical" segments, but this is really pushing your luck. Elsewhere you said 20 segments per wavelengh, which should be fine.
For resonant wire antennas like a Yagi, NEC only needs 10 segments per wavelength, so 5 or 6 is quite OK for a yagi. The only minor issue with 6 segments and a boom is that the twin-feed is quite spread out now, and doesn't match the single-point feed.

I suggest the following experiments:

  1. Simulate the Yagi alone, with 9 segments per element, one source in the centre of the driven element
  2. Change to 10 segments per element, change to two sources. Check that the impedance is almost exactly half.
  3. Then add the boom, carefully, and see what changes.

Some more quick thoughts:

  • You don't model the gap between ends of the dipole, as a) the gap is small compared to wavelength, b) it is bridged by the wires to the coax, which is even smaller, and c) there is no gap because current is flowing all the way through the dipole, right? There's no part of the dipole that doesn't have the current going through it.
  • be super-careful of wire junctions - make sure they are numerically identical, no rounding errors or anything. NEC won't connect them until they're very close together, and overlapping unconnected segments will cause trouble.
  • also be careful of the length-to-diameter rules near junctions. You can get away with quite fat segments in NEC, but not at junctions, where the observation point of one segment can be inside another segment. If in doubt, make the wires thinner - you will get almost the same answer, but with fewer numerical errors. If you see any large change going from 5 mm radius to 2 mm radius, then something is not OK with one or both of these.

To briefly quote the manual:

The current expansion used in NEC enforces conditions on the current and charge density along wires, at junctions, and at wire ends. For these conditions to be applied properly, segments that are electrically connected must have coincident end points. If segments intersect other than at their ends, the NEC code will not allow current to flow from one segment to the other. Segments will be treated as connected if the separation of their ends is less than about 10-3 times the length of the shortest segment. When possible, however, identical coordinates should be used for connected segment ends.

The angle of the intersection of wire segments in NEC is not restricted in any manner. In fact, the acute angle may be so small as to place the observation point on one wire segment within the volume of another wire segment. Numerical studies have shown that such overlapping leads to meaningless results; thus, as a minimum, one must ensure that the angle is large enough to prevent overlaps.

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  • $\begingroup$ Thanks for this. It appears the main improvement with the boom has come my adding 2 sources, one on each half of the dipole. I didn't think to look at currents before and they were uneven on the dipole, with significant current coming off some boom segments as alluded to. Once I put 2 sources this current disappeared and the SWR peak > 100 is removed. $\endgroup$ Aug 16, 2023 at 6:43
  • $\begingroup$ Once I was using 2 sources, the impedance plots did not change much with small changes in segmentation. $\endgroup$ Aug 16, 2023 at 6:44

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