But nominally you have 42 sq ft of antenna load! Much more important,
though, Jerry is correct about the 2-el 15 feet up dramatically increasing
the windload. I can't calculate the exact number, but he's probably in the
ballpark with 60 sq ft. Further, the difference between 70 MPH and 90 MPH
isn't linear.
You really need to find out the max wind rating of your friend's location,
and get an engineer to calculate the actual windload at the top of the tower
with the proposed antenna and mast configuration. You're talking about tens
of thousands of dollars worth of tower that could be destroyed, not to
mention other property and possibly someone's life. Paying an engineer
$700-$1000 to do the calculations is cheap insurance.
Another consideration is vertical load. You have hundreds of pounds of
antenna, and that 24'x3" mast will add a lot of weight as well. I believe
someone suggested the load would be on the order of 600 lbs, which sounds
about right to me. It's not an issue of whether the tower steel and welds
will hold up, it's an issue whether the steel raising/lowering cable and
motor can stand up to the combined weight of the heavy-duty tower plus the
antenna/mast load. The whole structure is held up by that steel cable.
Better make sure US Tower specs can handle the weight and windload (adjusted
for the extra mast height.)
73, Dick WC1M
> -----Original Message-----
> From: Edward Sylvester [mailto:navydude1962@yahoo.com]
> Sent: Thursday, December 11, 2008 12:27 PM
> To: towertalk@contesting.com; K4SAV
> Cc: Paul F. Merrill
> Subject: Re: [TowerTalk] Spacing between Antennas
>
> Thank you for all your input thus far. I just want to clarify that
> the heaviest duty materials are being used. The UST 689 is rated for
> a gazillion sq ft and the 3" chromally mast is not for the faint at
> heart. I believe the 689 will handle 35 sq ft at 70 mph or more....
>
> --- On Thu, 12/11/08, K4SAV <RadioIR@charter.net> wrote:
>
> From: K4SAV <RadioIR@charter.net>
> Subject: Re: [TowerTalk] Spacing between Antennas
> To: towertalk@contesting.com
> Date: Thursday, December 11, 2008, 9:19 AM
>
> In addition to the tower loading issue (which seems to be suspect),
> consider the torque issue. Antennas of this size can produce a lot of
> torque on a tower. That may require some professional help to
> resolve,
> because you won't be able to get any torque numbers for the tower out
> of
> US Tower. I tried to get that data once when I was considering
> putting
> a MonstIR on a crank-up. Although the tower, sq-ft wise, was rated
> to
> handle it, I was afraid of the torque issue. Take a look at what
> holds
> a lattice type crank-up together in the torque direction. It is just
> a
> few little ears, and those bang against the main legs of the tower
> when
> torque is applied.
>
> Jerry, K4SAV
>
> John Elsik wrote:
>
> >I just can not help myself.
> >We do not even need to start the SteppIR debate. It will take off in
> a
> direction that will get away from the fundamental issue.
> >That is way way too much antenna for that tower!
> >At least 42 square feet and over 600 pounds!! Not counting the
> rotor.
> >On a crankup no less!
> >You can not just add up the sq ft numbers when stacking antennas, it
> does
> not work that way!
> >You (friend) need to do more research on tower loading.
> >Please stop and rethink the whole plan.
> >John wa5zup
> >
> >
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|