Rosemount

Keeping a transmitter site clean and organized has many benefits.

It is usually beneficial to employ a redundant closed-loop HVAC system to keep dirt, pollen and humidity out of your shelter.

Transmitters that operate with high voltages especially will benefit from the clean environment because high voltage tends to attract dirt and can lead to unwanted arcing. Proper air flow in transmitters, audio processing gear and computers is also critical; and dirt and pollen can quickly clog air filters leading to overheating of components and eventual failure of critical systems.

In addition, having redundant HVAC systems helps to assure that unattended transmitter sites will continue to stay cool if one of the air conditioners were to fail, until repairs can be facilitated. It’s now common to install a tertiary ventilation system that will simply bring in filtered outside air and draw it across the room to cool the area in the unlikely event that both HVAC units were to fail.

An ideal transmitter facility will have redundancy for most of the equipment. In many cases the costs associated with buying “two of everything” are impractical.

If you are able to duplicate equipment, one strategy that has proven to increase overall reliability is to have a main and an alternate “chain.”

A facility in which each transmitter is fed by its own set of processing and STL path helps to ensure that in the event of a problem (i.e., RF off the air, audio off the air or audio problems like distortion etc.) you have the best chance of bypassing the problem as quickly as possible until the problem can be addressed by simply switching to the alternate transmitter chain, including the RF amplifier.

Depending on your individual circumstances the times will vary; a popular method is to alternate systems on a quarterly basis. This helps prove to yourself that both systems are working properly; in the event of a failure on any one system, you will be confident that you can switch reliably to the alternate system and your business will not be compromised.

If you have a primary/backup system, your backup transmitter may not be as modern and therefore may not offer you the efficiency and reliability of your main transmitter. It is extremely important to test it regularly and maintain it as well as your primary.

Often, preventive or required maintenance is performed on transmission gear that has lethal voltages inside. While all manufacturers of modern broadcast equipment take precautions to interlock their equipment when a door or access panel is removed, occasionally it is necessary to bypass these safety measures in order to troubleshoot the problem.

When this happens, the engineer opens himself or herself up to added risks. The consequences of making a careless mistake can be deadly. Compounding the problem is the fact that many of these maintenance sessions are performed late at night at a time when the engineer may be fatigued, and could carelessly make a mistake.

The issue is a serious one, and requires one carefully to manage the risks associated with transmitter maintenance. Some of the tools available to the engineer to lower this risk include using checklists, making sure the shorting stick is used before contacting any potential high-voltage points and never working alone.

The equipment itself needs to also be protected. If you employ an air-cooled dummy load, make sure that it is interlocked to remove the RF source from whatever transmitter is connected to it, should the cooling fan in the load fail to operate.

The same is even more relevant with a water-cooled dummy load. If sufficient water flow is not maintained, the load element will be destroyed in a matter of seconds. A properly adjusted, high-quality external water flow switch is usually required to protect a water-cooled dummy load.

Just as the load needs to be protected, those who use a manual or motor-driven RF transfer switch need to make sure that these switches are interlocked to any transmitters to which they are connected. Usually moving an RF contactor with RF present can cause serious damage to the switch.

Probably the most important — and unfortunately one of the most widely overlooked — interlocks needed at transmitter site are the VSWR protection interlocks.

Even though modern transmitters provide internal VSWR fold-back protection, it is absolutely necessary to have an external VSWR meter capable of interlocking the transmitter(s) installed at an appropriate place in the transmission line system, and tested on a regular basis.

The cost of installing and maintaining such a device is a small insurance premium to pay for a whole lot of protection afforded to your expensive transmission line and antenna system.

Another vitally important consideration for those using transmission lines with an air dielectric is proper pressurization of the transmission line system with dry air or an inert gas such as nitrogen. The object is to keep the voltages between the inner and outer conductors of the transmission lines from arcing over and destroying the line.

Using a dehydrator to dry the air to a very low dew point guarantees that the moisture content in the line will be low enough to avoid arcing.

The other popular method that is effective is to infuse the line with nitrogen gas from a bottle or a nitrogen generator. This effectively removes any moisture in the line and helps to prevent flash over.
Having an electric pressure switch to monitor the transmission line’s gas pressure and relay the status to the operator on duty is imperative so that if an air leak develops, it does not go unnoticed.

Finally, for air-cooled tube type transmitters a routine testing program for the air flow safety systems is important. Even though all tube-type air-cooled transmitters employ a safety air flow switch, unless they are tested on a regular basis you can’t know whether they are actually working to protect your equipment.

One popular method is to slowly restrict the air flow to the intake filter with a piece of cardboard on the transmitter under test until the air interlock is no longer satisfied, and verify that the filament gets turned off.

Most tube manufacturers furnish a data sheet that along with specifications for operating parameters depicts recommended filament voltage settings. Typically it is advantageous for a brand-new tube to be run at the rated filament voltage for the first several hundred hours and then at a reduced voltage for the life of the tube.

Eventually when the emission drops off the voltage can be increased to a certain point to extend the useful life of the tube further.