Technical ยป DX Jack on Tower Base Installation
 
DX Jack on Tower Base Installation PDF Print E-mail


Editor's note:  The following comments originated as an email from Jack Sproat W4JS, written May 20, 2008, to Bruce Brown WA8IQF in response to Bruce's inquiry on tower base installation.  Bruce was in the process of putting up a homebrewed hexbeam.  The comments are reproduced here, unchanged, with DX Jack's permission.



Regarding the base for your pending tower, let me offer some comments.

 

1)  You are going to bracket your tower off your house, but I've no idea just where.  Is it going to be bolted into the concrete block wall, bolted through the fascia, or what?  (Note that using lag screws into a fascia board is just asking for trouble.  If you go through a fascia board, you have to bolt into a framing member that is structurally sound.  If you bolt into the wall, you will have to drill six bolt holes into the concrete block wall and install expansion bolts (I do not know of what diameter.)  The location at the house will determine the size house bracket needed.  The Rohn HB25AG sets the tower a maximum of 15" out from the wall; the HB25BG sets it a maximum of 24" out from the wall.  The former costs about US$100, while the latter costs about US$140, all plus freight (prices from February 2008 Texas Towers catalog).  How far out from the wall you place the tower will determine where the foundation will be located, so the house bracket should be installed first.

 

2)  For our 60' Aluma Tower, we had to put one cubic yard of concrete into the foundation.  I was fortunate to hire a young fellow to dig a 3' x 3' x 3' hole in the ground.  It took him all of 45 minutes and I gave him US$20.  Based on the Rohn catalog info for the hinged baseplate, the foundation should be 2' x 2' square.  As the tower will be bracketed off the house and you don't have much antenna wind load, probably a 2' deep foundation would be adequate.  2' x 2' x 2' = 8 cubic feet, or 0.3 cubic yard of concrete.  You can buy bags of concrete mix at Lowes or Home Depot, but I don't recall what portion of a cubic foot each bag will yield.  (They are labeled with the yield, which I think is about 1/2 cubic foot per bag.)  Yuli and I mixed the concrete for our foundation in a wheelbarrow, bag by bag.

 

You form up the foundation with 1/2" x 4" wood.  For a 2' square foundation, you make a wood frame (having kind of an open "H" shape) that has a 2' square opening in the center.  One pair of the side boards are at least 3' long, such that these long sides support the frame over the excavation.

 

3)  The tilt plate requires four 1/2" x 12" long anchor bolts.  The easiest way to tackle this is to buy four 1/2" threaded rods, with four 1/2" nuts and one "fender washer" for each rod.  At the bottom of each rod, you install the fender washer between two nuts.  At the top of each rod, you install one nut under the base plate and one nut on top of the plate.

 

The easiest way to get the tower plumb is to install the bottom 10' tower section into the base plate, support the base plate above the wooden foundation form (frame), and attach the tower section to the house bracket.  Plumb the tower in both directions with a level.  Place the concrete into the excavation, making sure the tower section remains plumb.  Bring the concrete up to the top of the form, but with the nuts under the base plate just above the concrete.

 

3)  The form work can be stripped off the next day.  The first section of tower is unbolted from the house bracket and tilted over to ground level, such that you can complete assembly of the tower sections.  As the used Rohn 25 and 20 tower sections do not have any bolts, you will need three 1/4" x 1.5" and three 5/16" x 1.5" galvanized bolts and nuts at each junction of the tower sections and at the base plate.

 

4)  This all assumes that the tower will be mounted against the house such that it hinges out perpendicular to the house.  If that isn't the case, let me know, as such a problem can be overcome.  However, when the installation is complete, the tower must be set such that one leg faces the house and the two outer legs are bolted in the house bracket.

 

I hope this gives you enough guidance.