[Top] [All Lists]

Long mast / crankup tower

To: <>
Subject: Long mast / crankup tower
From: (Wendell - W5FL)
Date: Thu, 19 Dec 1996 18:39:05 -0600

I don't know why any tower company can justify rating their towers at =
50mph wind loading.  I just finished looking at towers and decided to =
buy a Rohn 45 Foldover (64 ft).  I couldn't afford a crankup that would =
carry the load needed in Texas Thunderstorms.  Enclosed is a table I =
developed for different windspeeds for a self supporting tower.  I live =
in a 70 mph windspeed area, but have to design for 100 mph wind gusts:  =
(If these tables don't come out ok, let me know.  They are in Microsoft =
Word.) =20

This table reduces the wind load based on the tower load being low with =
respect to antenna load (which is not true in all cases).  These are the =
very best case (and probably less than 20% high) but do not include =
effects of feedlines (which are sometimes nearly as large in windloading =
as the tower itself).

mph     wind press      Ant Load
50      10      30.0
60      14      20.8
70.7    20      15.0
86.6    30      10.0
100     40      7.5
111.8   50      6.0

As you can see the crankover is rated for 30 sq ft at 50mph, and less =
than 15 sq ft at 70mph,  and less than 7.5 sq ft at 100 mph.  All =
building codes will require that you design to at least 70 mph depending =
on where you live.  The matter is made MUCH worse when you start putting =
antennas above the top of the crank up tower as their area must be =
compensated by their distance above the top.

The KT34XA   is      9 sq ft
The 40-2CE    is      7.6 sq ft (compensated for being 10 ft above =
2 in mast        is      2.3 sq ft (12 ft long and compensated for being =
5 ft above tower)
Rotor              is      0.5 sq ft

TOTAL ANT LOAD is 19.4 sq ft and your tower is rated for less than 15 at =
70 mph.  Many people do it, but taking down a bent crank up is quite =
expensive and virtually impossible without a crane. =20

The safest bet is to get a PE to look at your whole setup since you are =
more than doubling the tower load with the 40 mtr antenna and mast.


* The tower is heavy duty model, rated @ 30 sf / 50 mph  (I know, I =
* the wind blows harder than that!!!)
* For antennas, I will be putting a 40-2CD above a KT34XA.  Mast is fine
* for this.  I keep tower down 10' when not contesting, which adds

FAQ on WWW:     
Administrative requests:
Sponsored by Akorn Access, Inc & KM9P

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