[SCCC] Tower Mast Question

[Marty Woll]

Hi, Juan.

You have received some good advice so far.  Stay away from pipe (not the 
same as structural tubing), use high-strength alloys, drop the mast lower 
into the tower, and use thrust bearings and plates to help manage the side 
loads and facilitate rotator servicing.

Back in the mid-70's I wrote a BASIC computer program to determine mast 
stresses for a stack of up to ten antennas.  One thing you find playing with 
the numbers is that, for a given alloy, the outer diameter of the mast is 
the biggest determinant of how much load it can handle at a given height and 
wind speed.  Almost all my installations used oversized masts of 4130 or 
4140 alloy (yield stress in the 90,000- to 120,000-psi range), which meant 
customizing the top section.  My biggest stack went 16 feet up (with 8' 
down) from the top of the tower, which was fixed at 100'.  This included 3 
el on 40m, 5 el each on 20 & 15m. 8 el on 6m and 16 el on 2m.  I had done 
the calcs, consulted a RPE, and I slept fine on windy nights.  If you give 
me the info below, I will hunt up the program and run the numbers, but - as 
Leigh pointed out - your tower is probably rated for a load not more than 
one or two feet above its top section, so derating of the tower will be 
needed the higher up you go.

Here's the data that you'll need:

- Mast outer diameter, wall thickness, height above the tower, and yield 
strength in kpsi (I can look the latter up for some common alloys)

- For each antenna, flat-plate area (for booms & elememts, that's diameter x 
length) facing north/south, flat-plate area facing east and west, and height 
above top of tower.  Thus, for an antenna mounted at right angles to a 
north-pointing main stack, the boom area would be considered 
north-south-facing, like the elements of normally oriented antennas. Don't 
forget to add the area of the boom-to-mast plate(s)

- Maximum design wind speed  (50 mph is too low for Southern California; the 
minimum you should consider is 70 mph with a healthy safety factor to boot)

- Desired safety factor (A factor of 1 means yield stress equals max design 
stress; i.e., no margin for error.  A factor of three means the yield stress 
is three times the maximun design stress.)

Warning: I am NOT an engineer; I'm just a retired professional 
number-cruncher (CPA).  If the preliminary analysis gives you results  too 
close to "the edge", or if you have any unusual conditions at your location, 
you would be well advised to consult with a registered professional 
engineer.  I have done so with many of my installations.  Even if they bless 
my calculations, I'm more comfortable (and have something to show the 
insurer or the courts if worst comes to worst).

73,

Marty N6VI

----- Original Message ----- 
From: "Juan" <w6now at hushmail.com>
To: <sccc at contesting.com>
Sent: Sunday, January 28, 2007 1:57 AM
Subject: [SCCC] Tower Mast Question


> Hello contesters, I'm looking for advice on the size/type of mast  that I 
> should be looking for. Some decissions have not been made
> yet about the antennas that would go up at my new place.
>
> My Tower option would include either 55' or 72' (not sure yet) (Rated at: 
> 50 MPH Winds -- 37 sq feet; 300 lbs) 55'
> (Rated at: 50 MPH Winds -- 35 sq feet; 350 lbs) 72'
>
> I've seen some folks go after a chromoly mast, and others after a 
> stainless stell and others using a reinforced mast. Ideally I would
> like a 25' mast (4' for rotator inside tower, and 21' for antenna 
> installation)