Intermod Squad

Co-location is the de facto law of the land. With several transmitters cheek-to-jowl, how can you tell who's causing the intermod?

When Ralph Henes heard about an application for a wireless tower near his Madison, WI, home, he hardly slept a wink, and he was a half-hour early for the zoning hearing. No, he wasn't there to argue against it. He just wanted to invite co-location on his 48-foot-tall ham-radio tower.

"I have joked for years about having one of those nice, big towers for my antennas, so when this possibility came to light in the newscast, I again jokingly told my wife, 'Here is my chance to get my tower,'" Henes said. "When I realized this was a united effort to stop the company from putting it up about eight blocks south of me in a grade-school yard, I thought, why not my tower?"

Today, Henes has a brand-new tower, courtesy of Airadigm, his tenant for the next 25 years. He isn't alone, either. At least six other hams also are leasing space to providers, and one in Connecticut has four different providers on his tower.

It isn't to the stage of being a cottage industry, but ham towers have one thing going for them: They tend to be less of an RF thicket than those that are home to other wireless providers or broadcasters. A relatively quiet site is a real find as more zoning boards require co-location. Even Henes' tower had to be built to accommodate other tenants.

Choosing a co-location site starts with an intermod study to assess the RF environment and determine what effect the new signals might have. As with RF-compliance studies, if the tower or rooftop is owned by a site-management company, an accurate database of the other tenants' powers, frequencies, air interfaces and equipment helps both with the initial study and with tracking down any problems that arise later. How assiduously that database is updated and whether that information is disseminated to other tenants depends on the lease's terms.

"If we add other antennas or coax, we are to inform the tower owner that we're making those changes," said Marc Leclair, vice president of engineering for Sol Communications, which is 100% co-located with several companies. "As far as adding channels or increasing power, it says it in the lease, but we haven't seen anything that's said, '_____ is increasing power by 50W.'"

That information is handy when choosing a site because intermods are predictable mathematically.

"If you know all the frequencies transmitted at a location, you can calculate all the intermods so you'll know all the threats to a particular receive channel," said Les Polisky, Comsearch director of field services.

Suppose that provider A's intermod study doesn't show any problems for itself. Does that mean that its arrival won't affect other tenants?

"A's transmit signals may combine with B's transmit signal to contaminate C's uplink," said Brad Deats, Summitek Instruments director of engineering. "This is where a tower manager can be of great use. When considering allowing A onto the tower, the potential impact on all existing tower residents' uplink spectrum should be considered."

A thorough intermod study helps avoid costly fixes after you've moved onto the site.

"The rule of thumb in the industry is if (there's) interference, it's assumed that the last guy up is probably causing it," said Bill Mayberry, BellSouth Wireless director of technical operations. "In an extreme case, that guy could have to turn off their equipment until they prove that they're not causing it."

Bye, Products!
Most providers avoid co-locating on sites with FM or TV antennas mainly because broadcasters can create severe, even-order intermod products not seen on sites with just other providers.

"When you have a lower frequency emitter that can mix with your normal frequencies, that creates another order of intermods," Polisky said. "Intermods aren't necessarily the mixing of two frequencies. It could be the mixing of three. So having AM, TV, FM or police repeaters makes the possibility of harmonic interference worse."

Other variables to consider:

• Run-down towers and tenant equipment. Wireless is a young industry, so many of its towers are fairly new, but rusty bolts or water-logged antennas aren't uncommon.

"Anything that becomes non-linear can be a source of regenerating problems even though they may not exist in the transmitters or receivers themselves," said Lou Meyer, Decibel Products vice president, applications engineering. "It can act like a mixer."

• Intermittents. Some neighboring transmitters, such as paging, might be off during the intermod study. That's one more reason why an accurate tenant database is helpful in avoiding surprises.

"We had a pager transmitter that was mixing with an AMPS forward control channel into the CDMA reverse band," said Jim McDaniel, director, product management at Grayson Wireless, whose CellWatcher tool surveys the RF environment for other signals.

• False readings. Even a high-quality spectrum analyzer can provide an incomplete picture.

"If you know you're having an intermod hit on a particular channel, you can put some additional (bandpass) filtering in front of the spectrum analyzer so that the spectrum analyzer itself doesn't become a source of the problem," Meyer said. "If you overdrive the spectrum analyzer with too many signals, it will generate intermod itself."

Elbow Room
Vertical separation helps avoid problems such as a neighbor's transmitter desensitizing your receiver. But between the trend toward co-location and site managers' desire to squeeze as many tenants as possible onto each site, ad- equate spacing might be increasingly difficult to come by. Most providers aim for a minimum of 15 feet between antennas. An intermod study can help determine whether that's enough.

"Transmitters operating in different frequency bands have differing sensitivities to co-location," said Summitek's Deats. "For example, using the site-planning mode of (our) BaseStAR Interference Analyzer, you can determine the sensitivity of one receiver co-located with two different transmitters. If the receive band of interest cannot be affected by any possible combination of transmit signals from the two transmitters being considered, spacing between the transmitters is less of a concern."

Getting enough separation on a rooftop site often isn't easy, especially if the building is in a location that other providers also consider ideal.

"Horizontal isolation is a lot less than vertical isolation, at least for vertically polarized antennas, which most of our industry uses," said Decibel's Meyer. "A vertically polarized antenna has a 'cone' of silence below that vertically polarized area, and that's what helps get good isolation when you're stacked on each other. When you're on a building top, you don't get the benefit of a cone of silence."

One solution is to hang the antennas off the roof's edge, but if the antennas are angled to cover, say, an adjacent road, other problems can arise.

"Too much of an angle can skew the pattern, particularly depending on what the building is made of," Meyer said. "The more metallic content, the more it's going to tend to skew that beam."

With that many variables, it's easy to forget that intermod doesn't always come from your neighbors.

"It could be from our own equipment," said Sol's Leclair. "That's something that we need to realize in the industry: It may be us."