For a PDF version of this article, which includes diagrams, click
Interoperability among various public safety agencies has been a goal of most U.S. states for some time. Many states are still working toward that goal. Many different approaches can be taken to achieve this end. The intended purpose is to allow personnel of various agencies to communicate directly to coordinate efforts on a particular incident. Many agencies and states have spent millions in pursuit of this goal but still find themselves short of total interoperability.
In South Carolina, a Motorola SmartZone 800MHz system is being built to achieve statewide coverage, supposedly. The intended goal is to have every agency (at least those involved with public safety) on this 800MHz system. However, one size does not fit all. The 800MHz system is not the great panacea that it is thought to be by many. Current users of conventional VHF highband radio systems (as well as conventional UHF) are generally satisfied with the coverage these systems provide.
At least one agency that has a VHF highband system also has some radios on the 800MHz trunking system. Most of the comments from those users are negative in regard to the 800MHz system. Instead, they rely on their old VHF system when the message must get through. Running a dual system requires two radios in each installation — an expensive proposition.
Maybe the day will come when all public safety agencies (maybe even non-public safety) will be forced to move to the 800MHz system. And, maybe the completed 800MHz system infrastructure will provide fair radio coverage statewide. Even if that does happen, it will take a long time and lots of money from the agencies involved. So, what to do in the meantime? Let’s examine a few options for the short term and maybe even for the long term.
Certain 800MHz frequencies have been designated for mutual aid on a national basis. These are used in conventional 800MHz operation. A common CTCSS (EIA’s continuous tone coded squelch system) tone of 156.7Hz is designated for all these frequencies. These standard frequencies are listed in Table 1. The ICALL frequency is used for calling to establish initial contact. Once contact is established, another agreed-upon channel is used for communications.
A few years ago, I built a VHF/VHF repeater or, more aptly, translator by configuring and linking two Motorola 16-channel Maxtrac 300 radios. It seemed logical to do the same with a VHF/800MHz conventional setup. Using radios with the 16-pin accessory jack on the rear should be a cinch — I thought. Figure 1 shows how I configured the two VHF Maxtracs to form a repeater/translator. Figure 2 shows how the overall arrangement was configured.
Next, pin 14 on the accessory connector on the rear of each radio was programmed to go low (near ground potential) with the reception of a signal with the proper CTCSS tone. This is done in the radio programming software.
With this setup in mind, I ordered a used 800MHz conventional Maxtrac radio to link with a VHF Maxtrac radio. However, when I received the radio I discovered that I couldn’t program the accessory pins because of a software issue. Furthermore, I discovered that my manual didn’t cover the logic board in this radio. So now I was flying by the seat of my pants.
At this point, it came down to trying to locate a point in the audio/squelch circuitry that would go low with the application of an input signal with the proper CTCSS tone. How hard could it be? Just get out the voltmeter and probe around until I found the right place. So, to begin, I fed the receiver input with a signal at the programmed frequency and with the proper CTCSS tone. While probing with the voltmeter all around the audio/squelch circuitry, I found a point that would go high (near 15V) with the application of the proper signal input and back to low with the removal of the signal — just the opposite of what I wanted. Further probing yielded no better results.
After thinking about it for a while, I decided that this might be a good place to use an optoisolator. Yes, I know I could have used a simple transistor switch by using the 15V through a resistor to forward-bias the transistor. Still, I preferred the optoisolator arrangement.
I recently built a PSK31 interface between my computer and ham radio by using an optoisolator for keying the radio from the computer serial port. The optoisolator prevents ground loops from causing problems and it worked fine in that case.
The optoisolator I used in this project was the ECG3044. The switched transistors are a Darlington pair. (See Figure 3 for the pin connection diagram of the optoisolator.) If you want to see a pdf file on the ECG3044, go to www.nteinc.com/specs/3000to3099 and click on the pdf beside the 3044. The NTE3044 is the equivalent of the ECG3044. Figure 4 shows the details of the wiring of the interface between the two radios. Notice that pin 5 on the 800MHz radio’s 16-pin accessory connector is shown in red. Pin 5 on the radio was disconnected from the normal location in the radio and connected directly to the point on the board that produced the 15V after applying the proper receiver input signal.
When the 800MHz radio receives a signal on frequency and with the proper CTCSS tone (156.7Hz), pin 5 will go high (near 15V). This is applied through a diode to the optoisolator input. The diode inside the optoisolator turns on and produces light. This light turns on the Darlington pair transistor, which places the output (pin 5) at near ground potential. This is applied to pin 3 of the 16-pin accessory connector on the VHF radio, thereby keying the transmitter of the VHF radio.
Receive audio from the 800MHz radio (pin 11) is then fed to the VHF transmitter audio input (pin 2) through the 2.2k resistor. Thus, an 800MHz transmission is heard in VHF receivers tuned to the frequency of the VHF radio in the translator configuration. The setup works bi-directionally. That is why we call it a translator although it is (technically speaking) a repeater.
The photo shows the wiring of the interface board on a simple perforated project board from Radio-Shack. It is housed in a small plastic box. Figure 5 shows the block diagram of the overall setup. Each radio requires its own antenna — one for 800MHz and one for VHF.
The SmartBridge model SB-100 is designed to provide a gateway to an 800MHz SmartZone or Smart-Net trunking system from a conventional VHF/UHF radio system. The basic operation is similar in nature to the setup previously described but far more sophisticated. A radio base station in the conventional VHF/UHF system is interfaced with an 800MHz trunking radio. For example, for the SmartZone link a Motorola MCS2000 might be used to serve as the gateway into the 800MHz trunking system.
The SmartBridge SB-100 is from Radio Systems Technologies of Australia. For more information on this product check out the company’s Web site at www.rstradio.com/sb100/sb100datasheet.html.
The ACU-1000 interconnect from JPS Communications of Raleigh, NC, is yet another way to facilitate interoperability. For complete details on this device visit the Web site at www.jps.com/products/prodinfo/acu1000.html. For related information on this and other interoperability information visit these Web sites:
Another recommended site is www.pswn.gov. Lots of good information on this topic can be found on the Web.
As you can see, many methods can achieve interoperability. These range from the simple translator using mobile radios to fairly sophisticated systems such as those from Radio Systems Technologies and JPS Communications. It is wise to investigate all the options before taking the ultimate plunge. As the old saying goes: “Look before you leap.”
Until next time — stay tuned!
Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. His email address is email@example.com.