Care & Feeding of Antennas

For base-station antennas, the great outdoors are anything but. A site that's subject to weather extremes can suffer if water sneaks into antennas, connectors and feed- lines, where freezing and thawing can be a double whammy. Even the weather at the time of installation can result in problems later on.

"When it's -20*, guys are 200 feet in the air, the electrical tape is brittle, and you tape around these connectors (between the feedline and the antenna), it's not going to be a good seal," said Keith Radousky, BellSouth Cellular director of engineering. "Any time an antenna goes up in bad weather, it's a likely candidate for the water to penetrate the connector, get in the jumper and cause a problem."

As a result, any extra effort put into selecting an antenna with, say, a certain intermod value can wind up being confounded by the damage.

"Company X has tested it for third-order intermodulation products, and they said it's better than -100dBm," said Andy Singer, director of marketing and technical services at RFS (formerly Celwave and Cablewave). "But now you put that antenna in the environment. In a lot of cases, because of the construction of the antenna and what the environment will do to it, it may not look anything like that -100dBm three years later."

Symptoms of such problems aren't always easily identified. An increase in blocking is one potential clue, but it also can be a sign of other problems, such as insufficient capacity. A trip to the site, although helpful in ferreting out the problem, isn't always an option.

One way to reduce legwork and still catch problems is by installing sensors in key places. BellSouth's transmitting infrastructure continually monitors the VSWR and sends an alarm when it hits 2:1. On the receive side, the infrastructure looks for diversity error.

"It measures the power coming in from the two antennas, and if one of the antennas is consistently stronger by 3dB to 6dB, it creates an alarm," Radousky said. "It could be a bad cable or a bad antenna. We typically flop the receive antennas, and if the fault follows, we troubleshoot it."

One low-tech safeguard is periodic visual inspections, but without x-ray vision, they reveal only external problems."The kind of things you could visually inspect wouldn't be a 1:1 correlation," said Linda D'Evelyn, Ball Wireless Communications applications engineering manager. "If there were any corrosion inside the antenna, that would be far more damaging than anything outside the antenna."

Sweeping the antenna periodically is another safeguard, but like visual inspections, sweeps can create a false sense of security.

"They would detect only a catastrophic failure," D'Evelyn said. "Sweeps are primarily to test the cable integrity. We would guess that perhaps 50% of the elements inside the antenna could be disconnected, and you couldn't detect it from a standard sweep from below."

Even so, periodic sweeps still can be helpful if the results are used for trending.

"It's surprising how well you can overlay those and get an idea of what's changed in that antenna system," said Eben Jenkins, Tektronix product-marketing manager, wireless-communications-test group.

One caveat: Test the antenna system before it's installed.

"We've seen brand-new cables, right out of the box, from well-known cable manufacturers that have several dB of discontinuity at a particular frequency," said Cliff Morgan, Tektronix product-marketing manager, spectrum analyzer products and EMI test receivers. "It wasn't until sweeping that cable with a scaler network analyzer -- a spectrum analyzer and a tracking generator hooked together -- that we found that discontinuity. We've also seen cases where the connector wasn't put on correctly, and that caused a big problem. So the initial characterization of the cable can be extremely important for providing a good baseline."

Even at wireless' low power, arcing tests can be helpful. One way to catch arcing is to use high power during the test.

"A lot of times, a problem won't show up under 1W tests, but if you put in the full RF load of maybe 250W, sometimes you'll see a breakdown in a junction or something that you wouldn't see under low-power testing," said Vince Spatafora, BellSouth Cellular system-engineering manager. "If you watch reflected power, you'll see it increase as all the junctions and elements warm up, and it will usually stabilize after a few minutes."

Sweeps aren't the only place to look for trends. If a site's traffic steadily decreases while overall network traffic remains constant, antenna degradation could be the culprit. That's what Radousky learned when one site's coverage went from great to poor. At the site, everything looked good and swept fine, so he changed the antenna and discovered that water had seeped in.

"It changed the pattern, but it was still showing good from all test equipment," Radousky said. "So I think the final test is knowing what your coverage is like. If you have an area that loses coverage for no apparent reason," it could be a sign that the antenna is suffering.

AN OUNCE OF PREVENTION The best way to reduce the amount of legwork in catching and tracking down problems is to reduce the probability of their occurring in the first place.

"The No. 1 thing is defects in manufacture," Radousky said. "About 12 years ago, we had a big batch of antennas from a major antenna supplier. They all went bad within a year because the rivets were loose, and they had created intermodulation products even though the SWR checked out okay.

"We've purchased probably 10,000 antennas from another major vendor, (and) we've had no more than a half-dozen go bad. That's attributed to good manufacturing procedures. I think the No. 1 issue for operators is to go to the factories and really examine from a physical standpoint how the antennas are manufactured. Once they're up, they typically don't go bad if they're manufactured well."

Design is another issue.

"Depending on the internal construction, certain antenna types are more susceptible to environmental degradation than others," D'Evelyn said.

For example, if the installation is prone to tower vibration, select one that doesn't have multiple internal solder joints. "Once you have a lot of internal solder joints, the problem of how to ensure that those would never have stress cracks becomes fairly challenging," D'Evelyn said.

The site's location also is a factor in choosing an antenna that's less likely to degrade. In a coastal location, for example, salt fog can cause problems, while in other areas, pollutants such as acid rain might be the main concern. IEC 68-2 is the standard for environmental tests, but that in itself isn't a guarantee.

"Within those, there's a whole range in the magnitude of tests," D'Evelyn said. "Somebody passing the test isn't really saying the test is a salt chamber for one day or three weeks. Most of those standard tests don't test against, for example, acid-rain-type chemical corrosions."

The quality of a manufacturing process also can be measured in how far each antenna strays from the specs.

"I went to one manufacturer where each antenna is tuned by hand and needs a little TLC in manufacturing," Radousky said. "I didn't think that was a repeatable antenna from the standpoint of quality, so I avoided that manufacturer."