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@mindspring.com>
To: vhfcontesting <vhfcontesting@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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