Setting up the basic dispatch center
An inexpensive and simple adapter for a Maxtrac radio allows a small agency to use a logging recorder.
Larry Pratt knows everybody. Pratt is the volunteer fire chief in Kearney, MO, the birthplace of Jesse James (forerunner of the telecom attorney). Kearney, a small but rapidly growing community of less than 5,000 people, lies in the northern “exburbs” of Kansas City, MO. Pratt has been the volunteer chief for 25 years and has steadily and progressively updated the department’s training, staffing, equipment-and communications. That’s where I come in-Pratt knows me, too.
The Kearney Fire Department (KFD) was dispatching on-duty and callback responders with conventional two-tone sequential paging and using a desktop Zetron encoder slaved to the base station radio. Paging range was limited by tower height and transmit power. From the days of “separate service” frequency assignments, KFD shared one of the 28 VHF high-band fire frequencies (carrier squelch) with about five or six talkative neighbors, and the system needed improvement. KFD needed to expand paging range, improve wide-area portable radio operation, and minimize co-channel chatter interference.
To improve portable operation, Pratt and I devised a plan to overlay a single-site repeater station and modify all the field equipment by adding a new, additional channel for repeater “talk-in” use. The plan was to curb annoyance chatter by adding CTCSS to user radios. System range would be enhanced by moving the repeater station to a new PCS tower in town where Pratt knew the owners and tenants. As is typical, Kearney’s regularly used simplex channel remained as the repeater talk-out channel.
At KFD’s comm center in the fire station, we replaced the “senior citizen” Regency desktop with two new, inexpensive Motorola Maxtracs-one for a control station and one for backup base operation. The control station, with reduced transmit power and slaved encoder, is dedicated to dispatch operation, and the multichannel base station, with spare encoder, is parked on the statewide fire mutual-aid channel (154.280MHz, simplex). Work on user radios was simplified because most of the equipment was field-programmable by computer.
Because KFD participates in the metropolitan Kansas City E9-1-1 system, it receives 9-1-1 calls as a secondary PSAP. The Clay County, MO, Sheriff’s Department is the primary PSAP that provides initial call answer and then transfers requests for fire or rescue aid. Even for his small department, Pratt could see the need for operations voice logging. (Logging Rule No. 1: The actual time of day is far less important than the time relationship between actions and events of the center.)
During the transition to the new radio upgrade, an opportunity arose for Pratt to acquire a Seltronics logging recorder. After a service trial, KFD bought a model E-500, which is a small, 10-channel desktop unit that operates reliably and fits nicely in the corner of Kearney’s dispatch center (fancy desk) pictured with Chief Pratt in Photo 1 below left.
To provide clean audio for recording from the phones, we picked off the handset receiver audio from the internal network on each telephone set. This simple and reliable method creates a nice audio balance between distant caller and dispatcher voice. It also records “all the lines” at this phone. (Important anti-embarrassment tip: ALL calls to/from the phone are recorded.)
We next addressed the problem of interfacing both sides of each radio conversation with the two-wire inputs on the recorder. We reviewed the literature (dug around in the Maxtrac book), and devised a strategy (scratched out a drawing) to use the radio’s front-panel mic jack, because transmit and receive audio are both present there.
The schematic in Figure 1 above shows the wiring plan. The recorder output is bridged and isolated from separate transmit and receive audio paths and coupled together using two miniature transformers (Radio Shack part No. 273-1374) and a dc blocking capacitor on the microphone circuit. Using this technique, the audio balance is now excellent and the insertion loss is minimal.
For construction, we chose a four-pair AT&T surface jack that we had in stock (found in the back room) that eventually contained all the wiring and signal components. Wiring was either punched down on the type 110 connectors or made as flying connections, wrapped with heat-shrink caps. One four-wire and one eight-wire telco cord from North Supply completed the job, as shown in Photo 2 at the left. In operation, the regular desktop or palm mic plugs into the adapter box, and the eight-conductor line cord plugs into the radio mic jack. The two-wire line cord feeds the recorder interface box.
The vox recording mode works well because transmit and receive audio sources are quiet between calls.
The adapter box is cheap, dependable and assembles quickly. So, because Larry Pratt knows everybody, be sure to thank him for the idea when you see him again.
MRT welcomes David O. Dunford as a regular columnist in 2000. Dunford, who started in police radio as a dispatcher more than 25 years ago, is the police department technical services manager for Lenexa, KS (pop. 40,000), supervising maintenance techs, repair facilities, telecom design, fleet radio operations and mobile installations. He consults with numerous agencies on VHF/UHF/800MHz systems. Dunford is a member of APCO and has served in frequency coordination.
For his new column, we decided the appropriate title would be the 10-code “10-2”-clear communications.