[VHFcontesting] Yagis on masts

[nosigma at aol.com]

Patrick,


I am facing a similar problem with my mobile "fixed site" set up (https://www.qrz.com/db/KM4KMU).  Scroll to bottom for set up (less 6M).   Three 12 foot boom Yagi's (2M, 1.25M, 70cm) and a 6 foot boom 3 element 6m yagi on a 15 foot fold over mast.  I have done a bunch of loads and stress analysis (BS Aerospace) and came to reliable solution for winds up to 100mph.  I hope to have the rig (Jeep) ready to go by the January contest.


The guy set up is he most critical part. I think what Rohn is most concerned about is the long column buckling caused by the VERTICAL load of the guys on the mast coupled with the BENDING load (torque) induced by the wind that induces a moment into the mast.  The two in combination are deadly to a long thin mast structures.  The higher you can put the guy lines (close to the cross arm or antenna) the lower the bending load.  Small diameter tubing is very strong in compression but it will buckle quickly with a torque applied (bending not twisting). 


I suggest one or two intermediate guy lines to restrain the buckling and keeping the top guy set as close to the cross arm or antenna as possible to minimize the bending torque applied.  Being able to lower the mast quickly if winds get scary is very very helpful.  Most weather fronts pass quickly (30 min to a couple hours). Getting the mast down fast when winds pick up and then back up fast to maximize operating time is worth the effort.


John
KM4KMU



-----Original Message-----
From: Patrick Thomas <p-thomas at mindspring.com>
To: vhfcontesting <vhfcontesting at contesting.com>
Sent: Tue, Dec 27, 2016 11:34 pm
Subject: [VHFcontesting] Yagis on masts

Hey all,

I've been considering augmenting the park-and-go rover setup with a push-up mast.  However, I have noticed that the Rohn H50, etc., explicitly state that they are "not recommended" for use with yagis.  And sure, physics would suggest that if you have a 15' antenna boom, you are likely to have more off-axis "arm" acting on the top of the mast, versus a vertical or even a dish.  On the other hand, I don't know if they only say that to protect themselves against people who use H50s as semi-permanent rooftop installations in 80MPH wind zones with heavy ice.

So what's the verdict?  I'm sure it's "possible" but is it safe to use a large 2m yagi on an H50 when properly guyed, on a day with gentle breezes?  Or is it indeed "not recommended" under any conditions due to instability, etc?  For that matter, what about the weight of a rotor that high up?

(And yes, I'm still working on the halo stack, so worst case I could always hike that up the line... but would love to have the extra gain and directionality from the yagi.)

Thanks,

Patrick
KB8DGC
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