Date: Wed, 12 May 2010 17:30:07 -0400
From: "Fredric T. Serota" <fserota@msn.com>
Subject: Re: [TowerTalk] Best mast location?
Thanks for responding. Actually, I have read and re-read "Up The Tower"
several times. It actually doesn't give specifics about where on the mast to
go. I don't understand why they sell 15' reinforced masts if they want you
to put the beam right on top of the mast.
### A buddy of mine in NEV, bought a US Tower corp ...'reinforced mast'... and
snapped it like a toothpick in a 85 mph wind. [tower was nested]. The UST
reinforced masts consist of a 20' mast, .120" wall thickness, with an inner
liner,
that is also .120" wall thickness. The inner liner is only 10' long. IE:
the bottom
10' of mast is .240" wall..... the top 10' is .120" wall. The kicker is...
the mast +
the inner liner are BOTH made from 36 ksi steel.. which is bottom of the barrel
stuff imo. Inner liner is welded to outer liner at the base.
## If you are going to use long steel masts... you are better off to use a real
one like 1026 DOM [75/87 ksi].... or heat treated 4130 chromolly steel
[104/107/115/120 ksi] . You can get either 1026 DOM [75 ksi] in any
wall thickness you want. The 1026 DOM 87 ksi stuff is usually only 1/4" thick.
The heat treated chromolly comes in either 1/4" or 3/8" thickness.
Only a collection of very light
weight 2 meter and above antennas would qualify. Also not sure why they pick
a tower height of 55' which is an odd height if comparing wavelengths of
amateur bands, being more than a half wavelength at 20 and less than a half
wavelength at 40.
## 492/7 mhz = 70.29' There is no magic height. For a single yagi
[no stacks tribander's / monobanders on the same band] higher is usually
better. Back in 1977, I went from 48' to 67'... and it was like throwing a
light switch. In a typ residential neighborhood, a lot of it has to do with
clearing 29' roof tops from neighboring houses, and clearing 40' utility poles
which all have a 18" extension on top for the 12.5 kv line, trees, and any other
stuff in the way. 48' might sound like a lot, but you are only clearing the
HV
wires by 6.5'.
So I, too, am reading all I can and asking all I can. On this issue,
actually, the ARRL Antenna Handbook makes the most sense going through the
"moment" calculations. But they come to the conclusion that there is little
difference between the top of the tower and 5' above.
## Nonsense. Huge difference. Think of the mast as the mother of all torque
wrench's.
I just ran your 17 sq feet [ projected area of a cylinder per Step IR.. and NOT
effective area like F-12, etc specs] through the ARRL mast software.
I assumed 10' of mast above the top of the tower... with your 17 sq ft yagi,
mounted
7' above top of tower. I also assumed a 2" OD x .240" wall mast. Also
assumed
35 ksi yield strength. YOUR MAST SNAPS AT 62.47 MPH !!!
## a 1.9" OD sched 40 [.145"] 35 ksi mast, snaps at 49.4 mph.
## a 2.0" OD x .25" wall, 87 ksi mast, snaps at 99.75 mph.
## a 2.0" OD x .25" wall, 104 ksi mast, snaps at 109 mph.
## a 2.0" OD x .375" wall, 120 ksi mast, snaps at 130.6 mph
## a 2.0" OD x .375" wall, 107 ksi mast, snaps at 123.2 mph
## Your TX-455 is rated for 37 sq feet @ 50 mph.... and ONLY
12.3 sq feet @ 70 mph !!! And BOTH of those ratings are assume
the yagi is no more than ONE FOOT above the top of the tower.
## With a 70 mph wind, you have the tower OVERLOADED with your
proposed 17 sq ft yagi..sitting just 1' above the top of the tower... and
grossly overloaded with the yagi sitting 7' above the top of the tower.
## 12.3 sq ft @ 70 mph = 185.149 lbs. 185.149 x 56' [55+1=56'] =
10,368.344 ft lbs
at the base of the tower... and that's just from a 12.3 sq ft yagi... and not
the
windload of the tower itself, nor the mast. Let's use that 10,368.344 ft lbs
as our..'bench mark' . What he proposes is 17.0 sq ft.. 7' above top of
tower. 17 sq ft @ 70 mph = 255.898 lbs. 255.898 x 62' [55+7=62'] =
a whopping 15,865.676 ft lbs at the base of the tower ! This is the bending
moment at the base of the tower from the 17 sq ft yagi @ 62'.... and does not
include the windload of the tower itself, nor the mast... which is a bunch more.
## 15,865 / 10,368 = 1.53 You have just exceeded the US tower corp
base bending moment by 53%. ....... in a 70 mph wind... and NO ice or snow !!
## You have a few options. [a] retract the tower if winds exceed 55 mph
[measured at top
of tower and not grnd level] [b] mount 17 sq ft yagi 1 ' above top of tower,
instead of 7',
and retract tower if wind exceeds 60 mph [ measured at top of tower] [c] use
a smaller yagi
not to exceed 12.3 sq ft.. and mount it no more than 1' above top of tower.
[d] use a smaller
yagi, mount it 7' above top of tower, and retract tower when wind exceeds 60
mph.
[e] obtain a bigger tower.. like a HDX-555 [29.5 sq ft @ 70 mph] or a HDX-572
[ 18 sq ft
@ 70 mph].
## IF you decide to mount the proposed 17 sq ft 7' above the top of ANY tower,
even with
tower fully retracted, you will require a stronger mast.
later.......... Jim VE7RF
Fred
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