Fixed Wireless Technology

Building a Tower Part 2: Tower Design and Site Design

In part 2 of this series, key executives at MapleNet Wireless discuss the fundamentals of tower and site design.

by Alex Goldman ISP-Planet Managing Editor [February 2, 2009]

Anyone can order a tower from a wireless equipment distributor and have it delivered in a few days, notes Gene Crusie, founder and president of Elkhart, Ind.-based MapleNet Wireless. But anyone willing to pay more can get a tower designed to their specifications. An "engineered" tower is far better than a "pre-packaged" one, he says.

"Some companies need to order a 40 foot tower and throw it up in a couple of days. They may hope to fly under the radar, avoiding all the paperwork," Crusie says. "That's not what we do. We play by the rules."

MapleNet sends the tower manufacturer a detailed description of the tower including height, expected load, predicted wind and ice conditions, and the results of soil samples. The company sends a custom tower. "Custom towers are better for you," says Carender, MapleNet's national sales manager. "They will meet your current and future needs, and they're better from a liability standpoint as well."

The site plan takes grounding into account as well. Carender says MapleNet follows the guidelines taught in Motorola's Site Installation Practices Workshop (R56).

Equipment
The radios must be stored. Carender says he prefers enclosures to cabinets, even though cabinets are cheaper to build and require less power to heat or cool. "It's much easier to work inside a shelter than a cabinet," he says. "If you're working in the rain, you have to open the cabinet and expose your equipment to the rain."

People use cabinets because enclosures are expensive, starting at $30,000 used, Carender says. But shelters are more secure, and are hardened against anything from ice falling off the tower to vandals shooting at it.

Enclosures have a full set of alarms: environmental, power, and open door. Most are connected to the grid, although the company has one in a wildlife preserve that runs off solar power, with a generator for backup power.

If you can store the radios at the bottom of the tower instead of placing equipment on the tower, your equipment is easier to manage but your cabling requirements rise.

"When we build towers for other WISPs, we try to mount the equipment at the bottom of the tower," says Crusie, "but we have been forced to go away from that because the manufacturers use Power over Ethernet."

If you can put the equipment at the tower base, you should, they say, even though the coaxial cable such an install would require adds a load to the tower and is more expensive. The difference between Cat 5 3/8 inch coaxial cable and 1 5/8 inch coaxial cable adds up over the length of a tower, especially if the equipment requires several runs of cable.

The tower
MapleNet builds three types of tower: monopole, guyed, and self-supporting.

Monopole towers are the least desirable, says Carender. The company only uses them when required to do so by local laws. Monopole towers are "one big octagon of steel," he says.

It's a single tall pole with a ladder on one side. If, for any reason, you need to work on the other side of the pole, you need to climb up it and rappel down the other side of it (unless it's within reach of your bucket truck). "We had to do this recently, and it took twice as many staff as it would have on a regular tower," Carender says.

The next level of tower is a guyed tower. It's a cheaper tower, and easier to work on, but the guys take up space on the land. "The radius of a guy tower is 80 percent of tower height," says Carender. "So the acreage for a 140 foot guyed tower would be 1 acre and for a 190 foot tower it would be 1.75 acres."

While the guyed tower may the cheapest solution, it's not always practical.

The best tower, in terms of quality but not price, is the self supporting tower. Such towers have three legs (I had assumed they'd have four, but only the largest towers need four legs, Crusie says). With three legs, the tower has a ladder on each leg, and plenty of places to clamp on to. It's the easiest tower to work on, and most likely the safest.

Remember that MapleNet is ordering custom towers based on expected wind, load, and ice conditions. "A structural engineer told me that ice buildup can double the size of a tower from a wind load perspective," says Crusie.

When creating specifications for a tower, MapleNet can use EIA/TIA 222-F or 222-G. The details are beyond the scope of this article, but to learn more, you can read this article from 2004: Changes in tower standards. MapleNet follows EIA/TIA 222-G because that is the more stringent standard and because cellular companies use only 222-G.

For example (brief example), 222-F allows you to assume that your site has "normal soil" whereas 222-G requires a soil sample for every site. "The soil conditions report adds anywhere from $2,000 to $4,000 to the cost of a tower," says Carender. But if the soil is unusual in any way, the survey pays for itself. A guyed tower requires four soil borings, a self-supporting tower requires three borings, and a monopole tower requires one. If the soil is loose (e.g., sand) the foundation has to be larger and heavier

We could talk about other issues, such as seismic load, but in the interest of brevity, we won't. If your area is prone to earthquakes or volcanoes, however, you should consult with an engineer who can help you handle the issue.

Tower manufacturers include Rohn, Nello, and Valmont, among others.

Once you place an order, you're getting a large shipment. A recent pallet delivered to MapleNet weighed 855 lbs., says Crusie.

Standards keep changing and anyone building a tower should consult with a professional who knows what the current standards are. On January 22, 2009, Wireless Estimator reported that 222-G is being revised to take into account the special needs and dangers that can happen while altering a tower.

Standards are complex and they keep changing, but they change for the best of reasons: improved safety.

—End