COMPUTERIZED DISPATCH

HOW WELL DESIGNED IS YOUR TRANSMISSION INFRASTRUCTURE?

by Don McCurdy

Recently, I was contacted by a driver at a company at which I formerly managed. He was upset that the radio infrastructure had been altered and, of course, a channel failed leaving significant portions of the city with reception problems.

Initially, the company had two data channels at different antenna sites that covered the entire city. In anticipation of an increased volume of business in a rapidly growing suburb they moved one of the sites north of the city to get better coverage in that area. Unfortunately they lost their citywide coverage and left themselves with no backup in the event the primary channel failed. The good news is that they're planning a third channel. So what could have been done to prevent this?

Coverage

Initially, the company had one data channel.  Their business volume increased to a point that it could no longer be successfully dispatched by voice. Various radio issues pointed up the need for an additional radio channel. It was total chaos when the data channel failed. It was decided that both channels should be able to cover the entire city in the event a channel failed.

I highly recommend that a company have redundant coverage, especially in the highest business volume areas.  Every potential antenna site should be able to provide you with an antenna propagation chart that should give you a good idea of what coverage you can expect at your frequency and output power.  Make sure that your high volume areas are covered before you select a new antenna site.

Baud Rate

Basically, baud rate is how fast computers talk to each other. Different mobile data terminals "talk" at different speeds.  The slower the baud rate the greater distance a terminal will be able to receive.  

While it's fairly common for a base station to transmit at 100 watts, mobiles only range from 25 to 40 watts. Often a vehicle can receive a message, but not be able to respond to the message due to the disparity of output power.

It would make sense, therefore, that the base station could talk at a faster baud rate with no ill effects due to its greater output.

Why this scenario is not more common is a mystery to me.  This issue is important if you are considering upgrading your mobile data terminals. A company I worked at "upgraded" their terminals from a 4800 baud version to a 9600 baud version and suddenly had "coverage" problems.  No one warned them that their coverage would be reduced, so consider yourself warned.

Backup Power

The majority of taxicab fleets in the US rent tower space for their transmitters and antennas. These towers may or may not have generator power in the event of a power failure. There is usually a tower site available that will have backup power to prevent your system from failing due to a power loss.  It generally costs more to have backup power on a transmitter site, but the extra price it worth it if you end up being the only company transmitting in your area.

A less expensive alternative is a battery backup. Transmitters can be fitted with a battery that powers the system and is charged by the transmitter power supply when there is active power. This will keep the system up through short power outages and allow time for contingencies if the outage appears to be going to last for a longer period.

Mobile Systems Preventive Maintenance

Some mobile data terminals require "tuning" to balance them with the mobile radio. This allows optimum modulation for the radio attached to the MDT.

An important system check that is often overlooked is the power out of the radio and the standing wave ratio (SWR).  If you are not using "loaded" antennas the length of the coax and the length of the antenna will affect the ability of the mobile transmitter to transmit cleanly, that is, with a low SWR.

If there are corroded connections much of the output power of the transmitter may be reflected back rather than transmitted out.  This is a common problem and should be checked every time the vehicle comes in for onboard computer system issues.

The vast majority of RF (radio frequency) power meters will have an SWR setting to check the standing wave ratio.  If SWR is high the onboard computer system will not function correctly. Early antenna mounts had a rubber center with a chrome outer ring. As the rubber center aged it would begin to allow the antenna connection to flex and disrupt communications when the vehicle was moving. If your company uses this type of antenna connection I recommend replacement with the plastic dome style that the antenna mounts into with a set screw. It will save you a lot of headaches.

Antenna Site Maintenance

Like the mobile systems, the transmitter sites may also develop problems. Usually, a short maintenance visit will be able to determine if your system is having issues. Checking power out and SWR can point up issues that have not come to light as a result of a complete system failure. A high SWR at an antenna site may be the result of your antenna taking a lightening strike that has damaged the antenna but not killed it completely. I recommend a maintenance visit by a qualified technician at least once a year.

Vehicle Maintenance

Like any computer system, unstable power can cause issues with the onboard computer systems utilized in computerized dispatch.  A failing alternator can fluctuate  power in and out or have excessive ripple voltage which may result in unstable computer operation.  A weak vehicle battery may also cause similar issues. It is wise to load test these systems when a vehicle comes in for a computer related issue.

Single Point of Failure

Whatever type of dispatch you use you should attempt to eliminate any single point of failure. That is any single component that can render your system useless if it fails. Some of these items are expensive, but what is the cost if it fails?  Some parts may take over a week to replace, can you be down for a week? It's best to have backup equipment and written procedures on how to implement contingency plans. Waiting until you are under the gun can be a very frustrating experience.

To sum it all up, proper setup and care of your radio infrastructure is critical to your operation. Do your best to fully understand the ramifications of any changes you may want to make and maintain your equipment. Help is available at most competent radio shops.

 —dmc

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