Your tower’s wind load is a critical component. If you have a crank-up, it’s
ability to handle wind load is typically measured at 1 foot above the top of
the tower. When you stack antennas and move wind loading higher, the overall
wind load needs to be recalculated as the load increases exponentially the
higher it goes. Guyed towers have their own considerations, but I have never
had to address those as all my towers have been free standing crank-ups, so I’m
only addressing crank-ups.
The mast also makes a big difference, as it’s bending and torque capabilities
are important. DOM1026 or chromolly masts are much stronger than a simple
galvanized mast made of, for example, irrigation tubing. I’m not an engineer,
but I’ve had to address tower wind limits when planning for a tower to meet
load requirements imposed by municipal codes for a building permit.
Two considerations on stacking can conflict with one another: Wind load and
antenna interaction. Putting max wind load lower is better for total wind load
on the tower. All things being equal, and putting your SteppIR in the middle
may be best as you can home 5e elements and make it “invisible”. Also
installing your 40M at 90 degrees to the other 2 beams will help potential
interaction issues. Ditching the 40M monobander and using a single boom SteppIR
with 40M elements will help both overall wind load and interaction issues by
putting your HF elements in the same plane lower on the tower.
73
Lou, W0FK
Lou Laderman
Sent from my iPad
>
>
> Message: 6
> Date: Tue, 17 Mar 2020 10:52:45 -0400 (EDT)
> From: Art Greenberg <art@artg.tv>
> To: TowerTalk@contesting.com
> Subject: [TowerTalk] Yagi statcking order?
> Message-ID: <alpine.DEB.2.21.2003171028540.6340@mint19p3-vm.artg.tv>
> Content-Type: text/plain; format=flowed; charset=US-ASCII
>
> I have a SteppIR 3 element yagi. I want to add 40m and will probably do it
> with a 2 element yagi like the Shorty-40 or XM-240. And I'd like to have
> an 8 or 10 element yagi for 2m as well. I have about 17 feet of 2 inch
> 0.25 wall galvanized mast to work with.
>
> Just guessing that normally yagis are stacked in order of decreasing wind
> cross section. But all three yagis are pretty close in cross section at
> around 6 square feet and in weight at around 50 pounds. The Shorty-40 has
> a 14 foot boom, the SteppIR has a 16 foot boom, the XM-240 has a 22 foot
> boom. I'm inclined to put the 2m yagi at the top of the mast, but should
> it go between the two HF yagis?
>
> What stacking order makes the most sense?
>
> I do realize that I won't have optimal spacing. Adding the 40/30 driven
> element to the SteppIR is a possibility but I'm interested in the gain a 2
> element yagi will provide on 40m.
>
> --
> Art Greenberg
> WA2LLN
> art@artg.tv
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