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I designed a Yagi antenna using 4nec2, assuming that I would use a non conductive boom.

If I use an aluminum boom but I insulate the elements from the boom, can I keep the same design? How does it affect performance, in particular SWR?

It is a 2m (144-148mhz) designed to be a portable VHF / UHF (not mounted on a mast).

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  • $\begingroup$ I suspect that the answer you're looking for would depend on specifics of your antenna design. If you have a 4nec2 model of your Yagi design with non-conductive boom, why not just make a copy of the model and modify the copy to have an aluminum boom? Then you'll know for sure. $\endgroup$ – rclocher3 Apr 4 '16 at 15:47
  • $\begingroup$ I haven't found a model of a Yagi on 4nec2 that has a boom, so I don't know how to properly design it, I tried adding another element representing the boom crossing all other element but the driven one. I saw no difference at all, no mather how I place other element (crossing the boom or not) $\endgroup$ – GmodCake Apr 4 '16 at 16:04
  • $\begingroup$ You may have answered your own question. Most Yagis have metal booms, so if the models don't include booms, then my guess would be that it's common knowledge among Yagi designers that an insulated boom doesn't affect the antenna's impedance (and the SWR) or the pattern. But I'm just guessing. $\endgroup$ – rclocher3 Apr 4 '16 at 16:16
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    $\begingroup$ There is a thing called boom correction (BC) when using a conductive boom, according to my reading from now on, the BC for my antenna considering a boom of 20mm of diameter is to make reflector and director 2.5mm longer ... that is quite a bit of precision to have when cutting it. But I want to be double sure. Here's a link it's a chart for BC depending on the mounting type. $\endgroup$ – GmodCake Apr 4 '16 at 16:39
  • $\begingroup$ Interesting. So there must be an effect, but a fairly slight one. By the way, if you're planning on actually building an antenna according to a 4nec2 model then you'll want to initially make the elements a few percent longer than the model specifies, and then trim to get the impedance you're looking for, usually resonance or low SWR. The dimensions of the finished antenna are often slightly different than the model says, because of variations in the velocity factor of the real-world materials. Have fun! $\endgroup$ – rclocher3 Apr 4 '16 at 17:26
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So, by design, the elements of any Yagi have a zero current going through the center point. That's pretty obvious be symmetry: assuming you excite the "left and right" halves of the driving dipole with exactly opposing voltages, everything should be symmetrical across the plane through the middle of that dipole.

Hence, if you approach that plane from left and right, you should see the same voltage, at any time. If two points have exactly the same potential, no current will flow. That's why Yagis work even if their elements are non-perfectly isolated from a metal boom.

The boom correction factor is really just the effect of having a metal object of non-zero left/right dimension in the way of your EM field – that will basically "swallow" a bit of E-field, but that effect should be relatively small for reasonably sized booms.

SWR will practically not be affected; the couple of millimeters you'd have to add to the driving dipole to compensate for a conductive boom are, from a matching perspective equivalent to using the antenna with a couple 0.001 mismatched cables – you'll be hardly pressed to find a matching circuit that is good enough to make the effect even measurable. Also, remember, you're not going to use that antenna for a CW of exactly the maximum efficiency frequency of that Yagi, but a couple of MHz around; now, "±1.5MHz around the optimal frequency" means your wavelength-to-antenna element ratio is off by around an easy percent, anyway – it's really no use to over-optimize your antenna here.

Things like non-perfect impedance, dielectric and ohmic losses, geometry imperfection, non-ideal free space impedance (funny fact: air moisture does change the $\epsilon_r$ of air) will probably outshadow this.

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Without all the technical banter take a look at G0KSC site on Yagi design. Justin explains it quite well... The bottom line is when using a non conductive boom or simply isolating the element's from a conductive boom the Yagi becomes much much quieter as far as the signal to noise floor goes. A matter of hearing the someone or not?? My conclusion from building many different Yagi's for vhf/uhf/hf.... is I agree..

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  • $\begingroup$ I like this answer because it goes beyond the intended behavior of the antenna, and considers that the boom is an element at some frequency, and can couple noise into the other elements. Remember, too, that metal-on-metal joints, especially when secondary to the antenna's intended operation, may be constructed so that the junction corrodes, adding oxide layers between the boom and the elements. The oxide layer is non-linear, adding noise in the intended passband by mixing other noise sources -- an intermodulation effect. $\endgroup$ – cmm Aug 3 '18 at 13:26

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