>Hello,
>
>I am in the planning stages for a new tower and antennas and would like any
>comments pro/con for the following installation.
>
> HDX-555 US Tower (55' self supporting crankup)
> CM-20 Cromolly (20 foot mast)
> Yaesu G-2800 SDX rotator
> Force 12 EF-420/240 @ 1' above tower
> Cushcraft ASL2010 or Force 12 4BA @ 11'
> Small (10 element) 144 and 432 MHz yagis near the top
> Cushcraft AR270B 144/432 dual band vertical on the top
>
>Total wind load @ approx 22.4 square feet and weight about 268 lbs (including
>mast).
>
>I'VE BEEN WAITING ON THIS FOR 38 YEARS!!!
>
>I would appreciate your comments. I would like to hear specific comments on
>the Force 12 and Cushcraft log periodic antennas. Please respond direct to:
>
> k8mbh@aol.com
>
>Thanks in advance.
>
>73's es DX,
>Dave in West Virginia
>Atlantic & Pacific Telecom, Inc.
You asked for comments so I will give you mine even though you will
definitely NOT like them.
First, I will admit that I hate crankup towers. I have never owned one and
never will. I will move before I ever put one up or get out of ham radio
entirely. They are dangerous and seriously injure people all the time.
There are dozens and dozens of stories about it. Rohn used to make and sell
crankups but because they are inherently unsafe, they got out of the crankup
business, and Rohn likes to make money just like any other business does!!
Enough of that. Now for some suggestions that you might actually take:
1. I looked up the specs on the HDX-555 and found it is rated for 30 square
feet at 50 MPH winds. You said your antenna load was only 22.4 square feet
which sounds, on the surface, like you have plenty of safety factor.
2. Does your 22.4 square feet include the windload of the mast, rotator,
and feed lines? Your mast, alone, will add another 2 square feet, at least.
3. At what point on the tower does US Tower say the 30 square feet can be
located? I suspect, right at the top of the tower. You are planning to put
a lot of it considerably ABOVE the top of the tower on your 20 foot mast
which is a very different story and will stress the tower much more than if
it were at the top only. I know you can't put all of your antennas at the
same place, but maybe you need to consider fewer antennas or a shorter mast,
or both.
4. What peak wind gusts do you really expect to see in your area? I will
bet they are a lot higher than 50 MPH. K7LXC has some data about all areas
of the US and he can tell you what to expect in your area. Your tower
should be designed to take the maximum expected gust when fully cranked up.
5. Others have told me they expect to keep the tower fully nested until
they want to use it. Only then will they crank it up. This works for about
three ups and downs of the tower after which the owner decides it is too
much work and leaves it up. One answer is a motorized winch but that adds a
lot to the cost and requires some more maintenance. Even then, some hams
leave their towers up since they don't want to wait the 5 minutes or so for
the tower to extend.
It is foolhardy to think you can listen to weather forecasts and race home
in time to crank the tower down in time to beat a wind gust.
6. Wind loading goes up by the SQUARE of wind speed. In other words, a 100
MPH wind puts FOUR times the force on a tower that a 50 MPH wind does. On
the surface, you might think you could compensate for this simply by
reducing the antenna load to one fourth of the 50 MPH spec and have a tower
good for 100 MPH. Not so!! The tower itself has wind load and if you want
to reduce the load on a crankup rated at 50 MPH so that it will stand up at
100 MPH, you have to reduce the entire load (including the tower itself) to
one fourth. You may find that a completely stripped (no antennas at all)
fully extended crankup will collapse (fold over) at 100 MPH even though it
has what appears to be very good ratings with lots of square feet of
antennas at 50 MPH.
Bottom line: I would ask US Towers some very specific questions before
making an investment in their tower.
First, I would get the wind information from K7LXC. Next, I would ask US
Tower what sort of square footage their tower, fully extended, will support
at that wind speed, not at 50 MPH. Next, I would ask US Towers what the
impact will be if you put a significant part of that wind load several feet
ABOVE the top of the tower. They will probably want a diagram of what you
are planning to do. Don't forget to put your feedlines on the diagram as
they add windload, too. Last, I would ask them to send you a letter
(signed) telling you they believe your planned installation is safe.
I will be very surprised if you can get such a letter out of them, but if
you do, keep it in a safe place for future reference, just in case.
Do not take my word for any of this. Consult a qualified structural
engineer and then heed his advice. Get his advice in writing and keep it on
file as you may need these documents later. If he won't put his advice on
paper and sign it, it is worthless. Find another structural engineer who
WILL put his advice in writing.
Good luck with this project, Dave, and I sincerely hope you can get a safe
installation in the air. Please let us all know what US Towers has to say
and what your structural engineer advises.
Stan w7ni@teleport.com
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