4NEC2

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VK2FAK
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4NEC2

Post by VK2FAK » Thu Jan 10, 2013 11:21 am

Hi all.....

I have looked around and in the programs and I can't find one example of modeling a yagi with a boom.

Has anyone come across a Tut. or example of doing it...

John
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John Vk2FAK

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Re: 4NEC2

Post by VK2GOM » Thu Jan 10, 2013 11:29 am

Hi John, you wouldn't normally model the boom... I assume you mean a conductive boom 8) and not a glassfibre one.

At the zero points where the elements are mounted, it can effectively be ignored. And if there is any length correction due to through boom mounting, that can be determined first in a Yagi design program, then the results entered into the modeller, and optimised.

73 - Rob VK2GOM / GW0MOH

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Re: 4NEC2

Post by VK2FAK » Thu Jan 10, 2013 12:11 pm

Hi all.....

Now Rob, thats where my confusion is coming from.....if in the design stage using another program , how do you ever get to see what sort of interaction the boom is having on the antenna ....What design program allows you to move the elements off the boom and specify the distance....or to see what effect close mounting has, but say not touching. Thru the Boom yes I have the software, but nothing you can specify any distances...

John
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John Vk2FAK

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Re: 4NEC2

Post by VK3AUU » Thu Jan 10, 2013 12:38 pm

I expect that you could model the boom using EZNEC+ v.4.0.
Those who model antennae generally ignore the boom because it has such a small effect on the result. The boom correction is added later when it comes to actually constructing the antenna. I believe that there was an article in A.R. some years ago with good results on boom corrections.

David
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Re: 4NEC2

Post by VK2FAK » Thu Jan 10, 2013 1:14 pm

Hi all...

Yes there is a few different ideas to boom correction....brings me to the point, if I change lengths and such to adjust for the boom....I no longer have the same antenna I modeled , did I do it right, have I over/under adjusted ...did I use the correct construction method to match the model....and more questions

I know at the end of the day these things would simply be a trial and error exercise, I was just kind of hoping I could see something on the computer first...that I just did not know how to set up..

John
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John Vk2FAK

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Re: 4NEC2

Post by ZL1RS » Thu Jan 10, 2013 6:06 pm

Metal booms have an effect on elements ... something about eddy currents induced. Corrections to element lengths due to running it close to a metal boom is something else as well. These correction factors are fairly well established from empirical experimentation, as are a few corrections for mounting "insulators", bolts through element centers, tubs or rods, and ??? I depends how pedantic one is and how much time one wants to spend/waste before getting out the hacksaw to produce "something" that can actually radiate a signal !! My experience is model, build with the established correction factors as required, and then check the SWR and radiation pattern to see if you are in the ball park. If the SWR curve more or less follows the computer model and the radiation pattern back lobes are within coo-ee, you have as good as you need.

FWIW one can add a boom in 4nec2, it is just a matter of adding another "wire" with the appropriate geometry ... but as far as I am aware, nec2 based applications (e.g. 4nec2, EZNEC, AO, etc) do not handle "wires" at right angles correctly, so the trouble to do add the boom is futile because the result is inaccurate and thus worthless.

The nec2 engine based applications have even more shortcomings for the pedantic ... that's why we pay $0.00 for them. To get better one needs to spend thousands-$ (tens-thousands-$?) on FEKO, CCS (or is it CSS?) or HFSS and a few weeks in tutorials and training classes to learn how to use them ...

... then they still don't take care of the external influences that will most certainly be encountered in an amateur radio installation which will have more impact on the final result than any finer points that can be modeled (or not) in the actual antenna by the fancy software.

Changes to element lengths to take care of "correction factors" is just one part that deviates the antenna from the software model to real life ... what about that lump of coax along the boom and over/under/beside the elements to the feed point? What about the mast it is mounted on? What about the hardware used to fix the antenna to the mast? What about the matching system? What about the 10/20/30mm of coax ends connecting to the feed point?

Don't get too hung up on the modelling, as much fun as it can be! 4nec2 will get you close enough and modern wideband Yagi designs will take care of minor discrepancies. What ever is built in the real world WILL be different ... the acid tests are: "does it measure similar to what was modeled" and "does it work better than what I had before?"

At this point I should turn off the computer, pick up the hacksaw, and follow my own words! :roll:

73, Bob ZL1RS
Bob, ZL1RS in the Bay of Islands at RF64vs
http://www.qsl.net/zl1rs

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Re: 4NEC2

Post by VK2FAK » Fri Jan 11, 2013 10:08 am

Hi all...

Doing some extra reading last evening....it does seem that the nec2 software is not all that good in regard to modeling a boom, in fact it was mentioned somewhere that it does not do it at all...As mentioned in a previous post, the other software using different methods to do there calculations is much better for booms and other things.....but its costly.

So as everyone has suggested....do the basic model , look at the element corrections. then cut...

I am not shy about doing the cutting...lol

John
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John Vk2FAK

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