SO YOU WANT TO BUILD A TOWER?
The planning and erection of an antenna tower today can be a complex project. It is important for the perspective tower owner to realize what lays ahead. As a general rule there are three main specifications needed to choose a tower site; site selection, tower type and tower specification knowledge (or tower code) make up the main criteria before beginning a tower project. Taking on a tower project is serious business and should only be taken on by a person or persons experienced at planning and building projects of this nature. This paper is for those who are interested in building a tower in the future. It will not turn you into an expert overnight; it may, however, prompt you to think about the project placed before you.
Perhaps the first thing to consider is the tower type that best suits your application. There are three tower types from which to choose: guyed, self-support, and monopole (the crank-up tower is really a version of a guyed tower). The self-support and monopole towers are both similar in that neither require guy wires or any other support structure. As a general rule, most choose the guyed type because it is the most inexpensive route. This is where a little common sense comes into play. You want to get that new 20 meter beam above the tree line, but you are not sure as to how to get it there. The property layout dictates how you are going to accomplish this. If you have the land, meaning acres of it, you could choose a guyed tower. But there are pitfalls to a guyed tower, like the guys. Do you and your family want to contend with guy wires strung on your property? The length of the cable run is another important consideration. One option I will discount from the start is to locate a tower on a building. This can be very complex and will require the services of a structural engineer.
One simple and popular option is to secure the tower to a building on your property, like the house or the garage. Although the engineer may not be necessary, this option will take some thought as to where the tower will be attached to. The tower bracket should be located as high as is practical. A critical consideration is to what, the tower bracket is connected to. At least consult a knowledgeable carpenter to determine the best method to get this accomplished. A bracketed tower bears significant torque and pressure on the building. Make sure the tower and your building, once attached, stay that way.
Back to site selection. Don't forget to check your local ordinances on towers. With the build-out of new, wireless phone networks, tower owners are persona non grata to some. As a ham operator, you will probably get less flack than a cellular phone company. But don't bank on it!
Perhaps you have determined that you want to build a guyed tower. Let's look at this. The first consideration is the land. You should have enough land to locate a tower equal to 150 % of the tower height. For example, you should have 250 feet of clearance between your 100 foot tower and an adjacent property line. This allows for a "fall zone" as mandated in many town ordinances. Another consideration is the land necessary to place the guy wires. It is typical to need 70 % of the total tower height to locate the guys. Anything less increases the downward pull on the tower. This can result in higher tower costs, due to the need for stronger and larger tower legs, or this could reduce the safety factor on a non-engineered tower site. Another perspective is the land contour. It can be impractical to locate guy anchors on severe grades. Severe grades can change the engineering of the site by placing too much stress on one or more tower legs.
Let's take a look at foundations. This includes guy anchor points. This too is an important piece of the puzzle. For small towers, simple foundations that use 12 to 18 inch tubes can be adequate. Please note that it is critical to dig below the frost depth to assure no movement of the foundation. Also, don't forget to reinforce your foundation with anchor steel. Now the soil. There can be a great variety of soil conditions out there, from very wet to ledge. The wetter the soil, the larger the foundation needs to be. I recently provided a tower to a client who had wet soil. It took 300 yards of concrete to "float" that foundation in there. Ledge is excellent to place a foundation onto. Make sure to "pin" it with a rock anchor(s). A note on larger towers such as a repeater site. This foundation should be designed by the tower manufacturer. In addition, test borings and a resultant soil report will need to be done by a geotechnical engineer.
Now the tower code. Tower code? Oh, did you know there is a national tower code? Over the years the industry has changed to a great degree. Initially, there wasn't a national code. Instead there were many different codes administered by many different organizations and agencies. Each approached wind, load, and ice in a different manner which resulted in confusion for the tower owner and the manufacturers. A national standard was born in the early 1980s called EIA/TIA-222. Today the latest revision is EIA/TIA-222-F.
One of the first things the code did was to establish a uniform system for determining wind speed. Over time, the industry realized that wind was not uniform both geographically and vertically. Today, wind safety factors are set in place for the location of the tower. For example, average winds are quite different for say Cape Cod and Columbus, Ohio. Therefore, minimum safety wind speeds are set on a county basis. As a general rule the coast has a higher wind speed than inland. For example, Washington County is a 100 MPH zone while Franklin is only 75. Over time, the industry realized that wind speed increased with tower height. This resulted in stronger, more expensive towers.
The ice storm this past winter leveled a few towers. This was actually quite impressive when you consider a similar, yet less severe storm knocked over about 50 towers in Maine alone, about15 years ago. This is directly attributable to the tower code and the fact that it formally addressed ice as a threat to tower survival. Today, ice protection is an option when it comes to engineering a tower. I wouldn't sell a tower without it here in the Northeast. There are two levels of ice protection available to the tower owner. Minimum protection is to employ what is called "1/2 inch ice with 25 % wind reduction." Basically this allows some additional protection yet without a major increase in tower costs. The second, recommended for a known, exposed site (for example Mt. Washington in New Hampshire), is to use concurrent wind and ice. This means that there is a real possibility that the tower will experience both ice and wind at the same time. By choosing the latter option, one can increase the safety margin for a tower in ice conditions, but can pay a dramatic increase in cost.
Finally let's look at the tower load. Antennas have a certain surface area that can determine the strength of the tower necessary to support it. Antennas with small surface areas, like a 2 meter yagi, present only a small load to the tower. Larger antennas, like microwave dishes and HF beams, can present a large load and the tower engineer has to figure it in to come up with a tower large enough to handle it. Other things like support arms also need to be factored in. The cable used to feed the antennas is critical. Cables, as a general rule, have more surface area than antennas. A tower fed with several 7/8 lines has dramatically more surface area than a tower that uses RG8.
I hope this has answered some questions or perhaps has given you pause to consider some or all of the environmental factors that affect a tower project. It takes considerable planning, time, and expense to erect a tower today. Whether a small tower for your beam or a repeater site, you need to consider all the above to do the job right. If you are not sure, check within your club to ascertain who is knowledgeable about tower erection and can give you a hand. A tower site, properly done, is something to be proud of; anything else invites disaster.
Peter Sturdivant, N1PS Email: email@example.com
Peter Sturdivant is President of Wireless Network Systems, a firm that specializes in wireless site development.