Trouble with the neighbors?

Try listening (in) a little better. Interoperability: The word is old, but we’ve given it a new meaning in recent years. It’s even taken on a life of its own. The issue is pretty simple: Can messages of useful importance be efficiently radioed to affected public safety workers in a timely fashion?

Street cops, firefighters and medics are attuned to the importance of this charge. Sometimes Radioman and communications management aren’t. So what’s the flap? The answer is simple, but the solution isn’t. It’s all in what we mean by “messages,” “efficiently,” “affected” and “timely” in the above definition.

A common scenario involves a group of agencies serving the same general geographic area (usually including contiguous jurisdictions) whose radio systems use different bands and/or technologies.

(Any dispatcher operating an EDACS system and coordinating a pursuit or joint response with adjoining Smartnet system users can define all four of the problem words above – but won’t have free time to do so until later.)

Technologically, we have various tools at our disposal to “let everyone hear everything.” (The cool new Uniden BC-780XLT scanner can even listen in to SmartNet and EDACS systems simultaneously.) Radioman’s technical solutions get sidetracked, however, when managers step up and state the obvious: “Not everyone needs to hear everything.” The corollary to this postulate is that interoperable communications links, paths or systems (what we called “mutual aid” in the past) often don’t begin operating until an incident has started.

Our city, Lenexa, KS, ran into this problem several years ago when field officers spotted an officer from a neighboring department foot-chasing a “man-with-a-gun” through the parking lot of a shopping mall.

Lenexa’s patrol bosses said, “Enough is enough,” and we set about to devise a simple solution that would allow our field officers and general staff to monitor dispatch activities of the larger city to our east, Overland Park.

That city had just installed a seven-channel, three-site, simulcast EDACS system. The system performed great for them, but none of the neighboring police agencies could monitor their activity.

We devised a simple translator station to receive the dispatch traffic on one EDACS talk group and rebroadcast it in a localized area on a low-powered UHF channel that is readily received by several nearby agencies. By electronically controlling access, the EDACS-using agency is assured that there won’t be “snooping” onto “private” talk groups. To complete the project, we chose on-hand and reliable radio components.

The EDACS device was the receiver of a standard MCX mobile radio that was programmed with the desired talk groups. (We did have to purchase this radio, new, for $969.) The UHF device was a GE Custom MVP operating at 2W transmit power into a 20-foot base antenna. We chose a Mastr Professional card cage with a specially modified repeater card for both the control system and the mounting shelf. The whole package runs on a small 12Vdc power supply.

Operation is straightforward, using the carrier detect/COR output from the MDX that goes “high” on receiver activation. This signal is fed into the card cage backplane and to pin A3 of the specially modified repeater card. We used a GE 19D416675G6 for Channel Guard, but the simpler carrier squelch card would work fine.

`High’ input conversion Two changes are needed to convert the card to “high” input control. Remove R-53 and Q-19, and then jumper base to collector of Q-19, which acts as a signal inverter and allows the “high” from the MDX receiver to activate repeat functions.

PTT for the MVP radio is fed from the D3 “transmit” on the repeater card, and 110Vdc is “borrowed” from the MVP to run the card cage. We used speaker audio from the MDX radio, which was attenuated with a T-pad to feed the MVP microphone circuit. (Be sure to use a coupling capacitor to block the dc on the MVP mic circuit.)

Using the card cage, we have full control of the translator station, including dropout delay and time-out timer functions. As seen in the photo on page 14, it’s also a great shelf on which to stack the hardware.

To help avoid pesky problems with lightning, we coupled the transmitter through a simple bandpass cavity to the RG-213 feedline.

Police from several nearby agencies programmed their in-car scanners to the translator output and were delighted with its performance. The Overland Park Police Department also achieved a step toward “interoperability” – at no cost to its city.