Glenn O'Neil, a volunteer firefighter in Charles County, used to hear Loudoun County school bus drivers chatting about their routes and sharing their recipes.

That was annoying, yes, but it would not have been a serious problem if the chatter hadn't come over the fire department's emergency radio systems.

Messages about how someone's child had missed the bus sometimes drowned out emergency communications, and O'Neil said it was especially bad in the western part of the county.

"We've had rescue boats out in the Potomac on water rescue, and they were not able to communicate with the dispatcher," O'Neil said. "We've had several instances of that" -- as recently as this year.

Loudoun and Charles counties had been broadcasting on the same frequency. This miscommunication, like many other problems with Charles County's radio system, is supposed to become history now that the county is spending $18 million to switch emergency workers to a new 800-megahertz system.

The county's old radio network, a VHF system, had been used since the mid-1980s, but the big growth in the county's population and in its public safety infrastructure meant that the technology was quickly growing obsolete.

"The system was 15 years old, and you couldn't buy parts," said Tony Rose, Charles County's communications chief. "There were no VHF frequencies available to expand our system." He said buying a new radio system was the "only viable option."

The radio system had technological limitations beyond just being antiquated.

Anyone who has used a car radio knows that reception can be finicky. Under the old system there were dead spots throughout the county where signals couldn't be received -- even in places such as the sheriff's station in Waldorf.

"In the District 3 station in Waldorf on VHF sometimes you can't use your radio," said Lt. R.J. Williams, commander of the communication section of the Charles County Sheriff's Office. He said the VHF radios failed in schools and shopping malls, too, because of the heavy walls and weak signals.

"Down the bottom of the hill in Bensville, you couldn't talk," Rose said.

The improvements to the radio system over the past three years included five new radio towers to send a stronger signal.

Large buildings and thick walls obstruct radio signals, but as part of its deal with Motorola, the county has designated 55 densely populated buildings -- hospitals and schools, for example -- in which the contract requires the communications company to guarantee radio service.

The new 800-megahertz system, which St. Mary's, Fairfax, Montgomery, the District of Columbia and many other jurisdictions use, can fit many more users onto the same number of channels, thanks to a computer that assigns incoming transmissions to whatever frequency is open at the moment.

The three-year project to get the radio system up and running is effectively complete; all that remains is a grueling test of the system in which communications workers will go to every part of the county to gauge the signal's strength and the quality of the voice it carries.

Officials divided the county into a grid of 2,000 squares from Waldorf to the waters of the Potomac. Motorola has guaranteed adequate reception in at least 94.5 percent of the grid, Rose said, and in mid-July communications workers began their county-wide odyssey to test the signal in each sector.

This final test will take about 4,000 man hours, Rose predicted. But he expects the results will be much better than those with the VHF system.

"If you measured the old system by the same standards as we're about to test," Rose said, "about 50 percent of the county wouldn't pass."

A device attached to a radio tests Charles County's new 800 megahertz radio system. Chris Campbell of the Charles sheriff's office uses a portable device to test the radio signal. A grid on a computer screen shows areas in which communications workers test the signal's strength and the quality of the voice it carries.