[Top] [All Lists]

[TowerTalk] Wind Loading Question

To: <>
Subject: [TowerTalk] Wind Loading Question
From: (Kurt Andress)
Date: Wed, 13 Oct 1999 07:56:47 -0700
Saad Mahaini wrote:
> Hello,
> I like to install a small (A3) tribander half way (@ 36 +/- ft)
> up my TriEx 72 ft crankup using a rotatable side mount.  Currently,
> there is a TH11, 40M rotatable dipole, and 2M vertical on top.
> My question is the added wind loading.  How does the appx 5 sq ft
> loading of the A3 (4.36 for antenna, and balance for mount, etc)
> @ 36 ft translate at the top?
> Appreciate the help.
> 73s Saad KB6VIR

Hi Saad,

For your tower, placing the antenna at 36' will be the same as having
1/2 of that area at 72'. You need to add the mounting hardware area.

You didn't ask for the rest of this, it is only for the folks that care
about this obscure unpopular stuff. But, it may help you, if you care to
get it right.

You are pursuing the right general approach, by trying to get your
configuration transposed to equivalent areas at the top, as that is most
likely how the tower is rated.

I'd encourage you to dig deeper into the antenna areas you are using.
You need to get the projected antenna areas for each antenna. The TH11
projected areas are (or were) shown properly on the old Hygain website,
I expect the other antenna areas are not right for this exercise. They
are most likely effective areas, and/or some kind of combined
element/boom area, following the old obsolete practices. You might need
to multiply them by 1.5 to get what you need or the values may be

Unfortunately, it really boils down to having to call the mfgrs, and get
a technical person who knows what you are asking to talk to you. 
The nice lady who answers the phone will just read you stuff from the
datasheet in her notebook (not always useful for your purpose). You need
to talk to the fellow who actually did the work to find out what he did.
He should be able to give you the projected areas immediately upon
finding his notebook.
Those are the first areas he determined, before he applied some
reduction to make it an effective area and/or a combined effective area. 
Be sure you ask for the element and boom projected areas separately.
Then look at everything at 0 & 90 degree azimuths to find out which case
is the combined worst case.
This is likely to be frustrating with some Companies. 

As you are going to apply 1/2 the projected area of the A3 @ 36' to the
tower top, you also need to make adjustments in the opposite direction
for any antennas above the top, by calculating a larger area for
application at the 72' level. This is just a straight ratio. If you have
5 SqFt projected at 78', you need to apply 5*(78/72) SqFt at the 72'
level. Do this for all antennas and sum the results for comparison with
your tower
rated area, but only do so when you have the antenna projected areas at
0 & 90 deg. 

This approach will not work for all guyed towers. More obscure annoying

All tower design spec's use projected areas as input. Each spec treats
projected area to derive an effective area for application of loads in
conjunction with the formulae provided. At the end of the day, when the
designer presents an area rating, he means the projected area. 
Rohn has gone one step further, to discriminate between round or
flat/rectangular membered antennas. God help those that use hybrid
hardware, with mixed rectangular and round sections. Not that the
approach is necessarily bad, it's just nearly impossible for the average
person to match up with a tower rating.

I got a big kick out of the "tower designer bashing session" that just
occured on TT. I think everyone should be damn happy that towers are
designed with safety margins. Specially when we are merrily using
incorrect antenna area values for tower design purposes.
Many of these values are far from applicable to our towers. Many folks
have towers still standing by virtue of those margins, others have
thought they were maxed out, when they weren't.

Let's refrain from attacking the tower designers who did their job
according to the spec's they have cited. If you spend the time to do the
work, you will see that is the case! If you have a problem with the
spec, go after the folks that wrote it! Better put all your pencils in
your pocket protector for that one, might try adding a bigger propeller
on your beanie too!

I'll say this one more time. Tower designers are talking apples, many
antenna builders are talking oranges. We don't know who's doing what
(antenna folks), because they don't openly provide that info. 
If you don't dig for the truth, you'll just get mixed fruit, can't tell
if it's an apple or an orange, seems to taste like one or the other, or
is it the other...?
It certainly seems to be an aphrodisiac, creating a false sense of
security for everyone. Someone please send me some of that stuff, so
this will cease to bother me! First guy that sends me some, gets credit
for not having me say this again! The makings of a local TT hero?

Don't give up on trying to figure it out properly Saad!
If you are successful at gathering the right information, the answer you
seek is rather straight forward. 

I'm probably already sorry I said this again, but the situation is
really upside down, but it doesn't seem right to have guys like yourself
out there trying to get it right, when the real situation is working
against you.

If you pursue the path to truth (apples for apples) and have questions
along the way, send them and I'll see if I can help. 

Sorting out this little continuing irritation might just render a whole
lot of anecdotal stuff into the meaningful category for a whole bunch of

73, Kurt, K7NV 

YagiStress - The Ultimate Software for Yagi Mechanical Design

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>