I'll take a crack at some of these, since this is what I do for a living...
Ive been doing some research online and can't seem to find the specific answers to some of my questions.
I'm trying to research and figure out exactly how the public safety systems, radios, repeaters all work together in harmony. I ask a lot of questions so I can have a better understanding of things. Im curious and would love someone to help me out. Ive been listening to scanner sense I was 10 and now I'm 25 and want to know the little details. Im not trying to be annoying, If someone has a link to explain this questions Id appreciate it a lot.
1. Whats the advantage over digital conventional system verses a analog conventional system? Encryption I'm guessing?
Others have pretty well covered this. One thing they didn't mention is occupied bandwidth. Since the amount of radio spectrum is limited, using it efficiently is encouraged. One way of fitting more radio systems into a limited amount of spectrum is to use as little amount of "space" as possible. A specific radio channel uses a certain amount of bandwidth. 25KHz is considered standard for FM. It's really easy to fit a human voice into 25KHz of bandwidth. As technology advanced, it became possible to fit voice onto 12.5KHz of bandwidth. Both 25KHz and 12.5KHz FM channels were done via analog audio. Easy and cheap to do. A few years back the FCC mandated that almost all radio users in the VHF High and UHF bands needed to stop using 25KHz channels and switch to 12.5KHz channels. This becomes a requirement on January 01, 2013. You will often see this referred to as "Narrow banding" (not to be confused with Rebanding). So, what does this have to do with the Analog vs. Digital question? Well, a couple of things. First is that while it's easy to do analog voice on a 12.5 KHz channel, people are anticipating that eventually the FCC will require users to stop using 12.5KHz channels and go to something narrower (6.25 is what the FCC says they will do next, but has not set a date). To go any narrower than 12.5KHz, you realistically need to use digital. You can easily fit digital voice traffic on to 6.25KHz of bandwidth, and two voice paths into 12.5KHz. So, it's an efficiency thing.
The other reason is that once you go to digital, it becomes easier to do much more with a radio. You can encrypt voices in digital easier than you can in analog. You can do text messages, send GPS coordinates along with voice, radio ID information, basically lots of different options.
So, since Digital is the way things will be going in the future, a lot of agencies and companies are choosing to make a bigger step now, rather than just taking the step to analog 12.5 KHz bandwidth.
Also, the Federal Governement has mandated that any Federal Grant money that is used to buy public safety radio equipment must be spent on P25 digital capable radios.
2. If the police use lets say six or eight repeater towers, do the repeater towers all repeat to each other to cover the whole radius of the area that's supposed to be covered?
Repeaters don't, They pick up a signal on one frequency, and transmit it back out on a second frequency with more power. Say they receive on frequency "A" and transmit on frequency "B". Since they receive and transmit on different frequencies, they would not "hear" each other transmitting.
Often what is done is that many tower sites around a specific area will all tie back via special phone lines, network, T-1, Fiber, or radio links to a central point. A device called a "voter" will take all the received signals from the receivers and select the best signal. That signal will then be sent back out to the transmitter (or transmitters). You can transmit the same signal from multiple locations, that is called Simulcast. It gets a bit tricky since each of the transmitters have to transmit the exact same signal at the exact same time or the receiving radios will hear and echo.
3. How many watts does a police officers portable radio give off verses the one in the car? And does the portable transmit to his/her car or the nearest squad car in the same department and get re-amplified using the mobile radio inside the car and than send to the closest transmitter.
Others have pretty much covered this. I'll add that it varies by frequency, usually. Most VHF Low portable radios transmit with about 6 watts of power on average. VHF High portables tend to be around 5 watts. UHF tend to be around 4, and 700 and 800MHz are usually 3 watts. Has to do with the efficiency of the RF amplifiers in the radios a different frequencies, etc.
An officers radio can work 3 different ways (usually):
1. They can transmit "simplex", which is transmitting and receiving on the same frequency. This usually only works over "line of site", not very good, but sometimes used because it doesn't rely on a repeater. Good for covering small areas, or in areas where there is no repeater coverage.
2. They can transmit directly to a repeater on a tower or mountain top.
3. Using a thing called a Vehicular repeater, they can transmit from the portable radio to their car, where a radio picks up the signal and then sends it to the higher powered mobile radio in the car. Same thing operates in reverse, when the mobile radio picks up a signal, it uses the vehicular repeater to transmit the signal to the portable radio. This is really useful to cover big areas as often a hand held portable radio won't have enough power to transmit back to the mountain or tower top repeater. California Highway Patrol does this.
4. Is the input frequency on the portable the same as the one in the squad cars? I know the repeater is different.
Depends on the way they are using it. If the system is simplex (radio to radio directly, no repeater) then everything is on the same frequency.
If they are using a repeater on a mountain or tower top, then the mobile and hand held radios will transmit on one frequency and receive on another.
If they are using a vehicular repeater system, then the portable will transmit on, say, frequency "A" and the vehicular repeater will receive on frequency A. The vehicular repeater will connect to the higher powered mobile radio. The mobile radio will transmit the signal out on frequency "C". When the mobile radio receives a signal, the mobile repeater will transmit that back to the portable. It can get more confusing depending on the rest of the system, but that's a basic overview.
5. What happens if two officers are on a Trunked system and are trying to talk at the same time to dispatch on the same channel, does the computer stop that and let the officer know to hold his/her traffic? Ive never heard two officers trying to talk on the same channel on my scanner, whether its trunked or conventional how does that work?
It can get pretty deep explaining exactly how a trunked radio system works, and there isn't enough space for me to get into it all here, but I'll give it a quick try:
A trunked system has what is called a "central controller". The central controller "directs traffic" for the system. They (usually) won't let two people transmit at the same time on the same "channel". The second person trying to transmit will get a tone letting them know they can't transmit yet, and then they will hear the conversation already in progress. There are ways to change this. Most systems have a system that allows prioritizing certain radios or channels. So, one radio could technically have a higher priority than another, and it would get priority. Conventional analog is a free for all, sometimes. It would be possible for two radios to transmit at the same time, however due to something called "capture effect", usually the receiver will lock onto the strongest signal. It is possible to program most modern radios with a function that won't let a radio transmit if there is already a signal being received, however. I don't usually use that on any of my public safety radio systems since in an life or death situation, you want someone to be able to get through.
6. If there are only 5 system frequencies on a Motorola type 2 smartnet system and more than 5 people in different talk groups including public works, sanitation, etc are all trying to talk is that going to cause a problem being only 5 frequencies to use? If not how come?
Yep, it will. It's actually a bit different than that. A 5 channel Moto system like that will always have one channel working as the "control" channel. This channel is running a slow speed data that handles the function of the trunking. It has to do with assigning radios to the available frequencies. So, a 5 channel system like that will only have 4 voice paths available. So, what happens when all 4 "dispatch" channels are in use? The next users will get a "busy", and it sounds basically like a telephone busy signal. The "5th" users will get put into a queue and when a channel is available, their radio will beep to let them know. Again, there is a priority system that lets certain radios or talk groups have priority over others. On the system I run, our PD and Fire have highest priority, and then the other no public safety talk groups have lower priority. Also, you can assign priory levels to specific radios.
7. In my county of Saint Charles Missouri the sheriff uses Digital Conventional. And theres only one dispatch channel. With the county being so big it seems like one channel is just not enough to help these officers out. How do they do it?
Discipline. Being brief on the radio and getting the messages across as clearly and quickly as possible is one of the skills they have to have. Often, they will also have other resources. Some departments may use cell phones for some stuff, or computer terminals in the cars. But, basically it the discipline that makes the difference.
