Tom's Tech Talk (#13): Coverage Solutions when Narrowbanding
With the current mandate by the FCC to narrowband radio frequencies below 512MHz, we have observed coverage issues. Many engineering documents speak of loss of coverage anywhere from 3db to 6db in coverage. We have noticed a 3db loss in systems, especially if those systems have spotty coverage to begin with. We always use the rule if you have a full quieting RF signal now, you most likely won’t notice the change. If you have poor coverage in some locations, you will notice a difference. With that said, there are a few options available to choose from.
First you need to determine what the issue is. For repeater systems you have talk out to the unit in the field and talk in from the unit in the field. An attempt must be made to determine if it is a talk-out issue, talk-in issue or both. The intent of a basic repeater is to re-broadcast a signal at a higher power and usually at a higher antenna height. The repeater is usually a high power radio with an antenna that may be on a tall building, tower or water tower. When the repeaters receiver opens up and hears a unit in the field, it sends this audio to the transmitter to be rebroadcast out. This higher power and antenna height lead to greater coverage, thus extending the units ability to talk.
If the problem appears to be that units in the field cannot talk back to dispatch, you have a talk in problem. There are options to extend the ability of a repeater to hear by the use of Voting systems. Voting systems place receivers in strategic locations to enhance the ability of the repeater to hear the unit in the field.
If the issue is a talk out problem, or dispatch talking to the unit in the field. You may have to file for a modification to the license to either increase the antenna height or power of the transmitter. As this may be an almost impossible task to modify the license, the alternate solutions are:
1) Simulcast: This involves putting a second transmitter in an alternate location to allow for the ability of the system to transmit from two separate locations. The issue with this will be the timing, frequency error and deviation of the system to accurately allow for the two transmitters to come up at EXACTLY at the same time, frequency and deviation. A good solution yet expensive. This will require an additional location added to your license.
2) Multicast: This solution is similar to simulcast because it requires the addition of a second transmitter. The difference being another transmit frequency is needed. The concept behind this allows for both transmitters to key up at the same time without interfering with each other. Simple, inexpensive, yet this will require an additional frequency which may be a drawback. For the Chicagoland area, frequencies are hard to come by. This will require an additional frequency being needed.
3) Transmitter Steering: This solution mixes Voting and Simulcasting. As receivers listen to specific areas, transmitters get associated with those receivers. As a unit drives through Town A, the receiver from that town may hear that units transmissions. If it gets voted with the best signal, the transmitter associated with that site will actually rebroadcast. The alternate transmitter is associated with other receivers and is used in a different location. The advantage with this is you don’t need the expensive equipment to do the simulcasting as the units never come up at the same time. The challenge lies in setting up the system correctly to insure that the voting receivers hear the proper area. Again this will require a license modification.
As a final option, look into digital. Although this may require a complete replacement of your system, you will gain your coverage back with the ability and functions to do more than you can with analog radio systems.
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