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Author Topic: Building a 2-element 20M Yagi - driven element  (Read 1390 times)
K3VAT
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Posts: 701




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« on: February 13, 2013, 11:10:44 AM »

Our spring project is building a 2-element 20M yagi.  We're using a variety of sizes of available aluminum square tubing.  This is our 2nd posting on this Forum regarding the design and building of this antenna (the first was here: http://www.eham.net/ehamforum/smf/index.php/topic,87910.0.html).

This question concerns the driven element.  We've decided that we can use ~ 1" aluminum tubing for the center portion of the driven element.  We have a ~3' section of fiberglass that will snugly fit inside the aluminum to help keep the elements aligned. 

Looking at various commercial manufactures, photos and tech specs seem to suggest that one could space the two halves anywhere from about 1/2" to over 6".

QUESTION: Is there an optimum distance between the two element halves (perhaps based upon operating frequency, material type or material size) ?

thanks! 73, Rich, K3VAT
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WB6BYU
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Posts: 13043




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« Reply #1 on: February 13, 2013, 11:30:26 AM »

Whatever works mechanically and isn't too close to arc over at your operating
power level.

Overall driven element length is from one tip to the other, regardless of the
spacing between the inner ends.  That's because the jumper wires from the
coax to the element make up the missing length.
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WB3BEL
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Posts: 78




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« Reply #2 on: February 13, 2013, 11:32:13 AM »

Rich,
A huge part of HF Yagi design is driven by mechanical constraints.  If you are making something for field day that will only need to last a few days in benign weather and is located where it's failure will not endanger life or property, then you have one set of constraints.  If you want to build something that needs to withstand 100MPH wind gusts in a residential area with zoning laws and insurance related issues, then you have another set of constraints.

There is a program called YagiStress available on ArraySystems site. I think and some other excel spreadsheet element calculator that you might be able to find on the web which give guidance on element failure due to stress induced by wind forces.

I think that 1inch diameter tubing at the center of full size 20 meter yagi is undersized for all but temporary use in very benign environment.  But this depends very highly on the element taper schedule you intend to use.  If you are talking about some reduced size Yagi maybe you come up with different requirement.

I think that you are asking about how much gap to have in the center of the element where you want to isolate the driven element from the boom and either half as you intend to do some kind of direct or perhaps hairpin feed system.  This gap is driven mostly by mechanical considerations.  What you do here will have some impact on the element lengths.  An Inch or two is more than adequate from an electrical standpoint unless you want to run super QRO.
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K3VAT
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Posts: 701




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« Reply #3 on: February 13, 2013, 01:04:04 PM »

<snip>

There is a program called YagiStress available on ArraySystems site. I think and some other excel spreadsheet element calculator that you might be able to find on the web which give guidance on element failure due to stress induced by wind forces.

<snip>

Thanks for the excellent reply.

Yes, we've have and use YagiMechanical from DXEngineering that we like.  So far, our design specs for the boom and tapered element sizes are OK.  (think it is ArraySolutions)

Quote from:  link=topic=88099.msg653313#msg653313 date=1360783933

<snip>

I think that 1inch diameter tubing at the center of full size 20 meter yagi is undersized for all but temporary use in very benign environment.  But this depends very highly on the element taper schedule you intend to use.  If you are talking about some reduced size Yagi maybe you come up with different requirement.

<snip>

You're correct, but those yagi's are probably using the standard 0.058" wall 6061 extruded aluminum. For our center sections, we're using 0.12" wall.  We may consider going to a center section of 1.25" or even 1.50".

Quote from:  link=topic=88099.msg653313#msg653313 date=1360783933

<snip>

I think that you are asking about how much gap to have in the center of the element where you want to isolate the driven element from the boom and either half as you intend to do some kind of direct or perhaps hairpin feed system.  This gap is driven mostly by mechanical considerations.  What you do here will have some impact on the element lengths.  An Inch or two is more than adequate from an electrical standpoint unless you want to run super QRO.


That's correct.  We will be using a hairpin matching system (again, from DXEngineering).  No, no super QRO.

Thanks again, 73, Rich, K3VAT

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