[SCCC] Tower Mast Question

[Leigh S. Jones, KR6X]

Although pipe is manufactured in standard 21 foot lengths, pipe is not made 
with the strength required of a mast for use with antennas in the size range 
you are considering and should not be considered as a mast.  Some of the 
decisions regarding a mast have already been made for you by the 
manufacturer of the tower -- especially the outside diameter of the mast. 
Your tower will probably not accept any size except 2" OD, although it is 
not entirely out of question that another mast size might be required in the 
case of a tower rated for 35-37 square feet of antenna load.

Actually, 35-37 square feet is an immense load.  But a 50 MPH wind rating 
for the tower suggests that the tower will be at least partially retracted 
most of the time in order to increase the safe wind survival range somewhat. 
You might go to bed with a mild breeze of 10 mph, but it's not unusual for 
the wind to increase in a few hours to around 70 once in a while.   To 
overcome this, a typical crank-up tower working around its maximum wind load 
should be retracted to the point where the section overlap is approximately 
doubled for most of its lifetime.

The problem is that your mast is not retracted when the tower is cranked 
down.  If you know that a storm is coming, you can crank your tower down to 
protect it, but the mast remains at full extension.  So the mast needs to be 
engineered for higher winds than 50 miles per hour.

And, its a safe bet that you won't be using a 25 foot 2" OD mast rated for 
35-37 square feet of antenna right at the top.  In fact, if you look into it 
you will probably discover that your tower is rated for those kinds of loads 
on a mast only a foot or so above the top of the tower.  If some of the 
antenna area will be mounted up the mast a way, then you'll need to derate 
your tower accordingly.  This is especially true because the tower's ratings 
are probably limited by its ability to withstand forces at the bottom of the 
top section.  If the top section extends 17 feet out of the second section, 
then mounting your antenna surface area 17 feet above the tower doubles the 
leverage that it has against the weak point at the bottom of the top 
section.  Add to that the fact that a long mast adds wind loading all by 
itself.

Stainless steel is not structural steel.  It's pretty and it doesn't rust, 
but it's expensive and relatively weak by comparison to chromoly.  So the 
suggestion has to be that you should go with chromoly because it's the 
strongest material easily available.  You will probably find a nice 20 foot 
length of galvenized quarter inch wall chromoly at Texas Towers.  In the 
70's when I was most active in antenna building, chromoly tubing wasn't 
available in galvenized form, so I cleaned the oil off of the chromoly with 
paint thinner and spray painted the masts with gray rustoleum. Texas Towers 
might have 21 foot lengths, but in the 70's 2" masts of structural steel 
tubing and chromoly were available from tower manufacurers like Tri-ex in 20 
foot lengths.  Tri-ex also made larger masts for their Sky Needle series of 
tubular towers.  I think I remember them being 2 7/8" OD quarter inch wall 
and around 25 feet long.  I don't really suggest anything longer, unless you 
are focused on stacking arrays of microwave or UHF antennas with tiny wind 
loads.

Of course, steel is made and sold in various sizes, and alloys, but buying 
piecework in custom sizes is potentially very expensive.  Someone with 
contacts in industry could probably find appropriate material for masts, but 
that's what we depend upon tower manufacurers to do for us.

For HF antennas like "Christmas Trees" of 20-15-10M yagis, you'd be doing 
yourself a favor to get the Texas Towers reinforced chromoly galvenized mast 
with a quarter inch wall at the top and reinforcing added at the bottom.  I 
don't know of any other source of galvenized chromoly masts (assuming you 
are not going to take the mast to a plating house and have it galvenized as 
custom piecework), and I don't know of any tubing source in the new high 
strength structural steel alloys such as is used in modern automobile frame 
construction.  I sure would like a tower and mast to be built out of that 
kind of steel -- but unfortunately it isn't happening yet.

Leigh Jones, KR6X
I used to put up towers for hams professionally with Ted Gillett, W6HX

----- 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)
>
> thanks in advance.