Spanish Version (Español)
and Tricks From the Engineer
How to Keep Your Transmitters and Amplifiers in Good Working Order. By Franco Piagentini,Senior Engineer
The purpose of this overview is to put my 45 years of experience in the Broadcast field at the service of anyone who is not completely familiar with this complicated industry. Over these past years, I have experienced a lot of problems related to site maintenance because I was the one running to the sites at whatever time to fix those issues as they occurred. I know what it means to do everything possible to avoid these unwanted trips. I am aware that the following suggestions might sound expensive to many of you, but in the long run they will save you money. A breakdown is always possible because the equipment works day and night without rest. However, if these units work in the most ideal and safe situation possible, then they will last much longer. (In this brief outline, I will be using the metric system as reference because this is commonly used in the greatest part of the world.)
ROOM/UNIT VENTILATION
Usually the transmitting equipment is located in remote areas that are not easily accessible and for that reason they must be installed with the proper care to avoid frequent trips to the sites and wasted time and money. Let’s begin with the place in which the equipment will be working. The environment in which the unit has to be installed is very important. The room has to be clean and large enough to allow for proper air circulation. It should also be equipped with a proper system of fans, if not air conditioning to extract the warm air and allow the fresh air to enter. Because warm air is lighter than the fresh air, you need to let the cold air enter the room through an opening (usually about 65 cm in diameter) equipped with the right air filters and located as close as possible to the floor. On the contrary, the warm air should be extracted with a big exhaust fan through a large hole (same diameter) located as close as possible to the ceiling. The fan can be activated by a thermostat set at the proper temperature range.
When possible, an air conditioning system is always recommended.
When we test our transmitters, they are able to run at temperatures up to 50 degrees Celsius, but this does not mean that they can run for a long period of time in these conditions without having the internal devices of these units suffer.
The internal heatsink that dissipates the heat of the mos fets is usually 30 degrees Celsius higher than the ambient temperature in the room; so the cooler the ambient temperature of the room, the better the unit works.
As an example we usually compare a transmitter to a regular car. If the temperature of the coolant rises too much, the engine will suffer. Moreover, you must consider that a car is running only for some hours while a transmitter is working 24/7. It is also very important to properly install the transmitter. Usually the easiest and most reliable thing to do is to place or mount the unit in a rack together with the other equipment such as a stl receiver, stereo generator etc.In this case, always place the transmitter at the bottom (remember the fresh air stays closer to the ground or floor) and leave about the same space, 15 to 25cm,of open space above the transmitter when installing another device in the rack as it allows for circulation around the unit. The sides of the rack, if removable, can also be removed to allow maximum air circulation. Keep the rack or cabinet 15 to 25 cm from the back wall to allow for maximum air flow around the unit.
KEEP THE ROOM CLEAN (Remember that dust and/or small bugs are the greatest enemies of RF units)
ELECTRICAL SYSTEM
Once the room is in good condition, it is important to figure out the best way to supply AC power to the units. Depending on the different sites in the world, the electricity is subject to more or less fluctuation.
Fluctuations in voltage are not the only issue you can have with Electricity. At times, you can have spikes and peaks of power that last a very short time, but are enough to damage your equipment. There are many devices on the market that protect the AC line and these products are all very helpful (surge suppressors, stabilizers etc.). However, only one is really mandatory to avoid disasters caused by the AC power to the devices. It is called an ISOLATION TRANSFORMER.The isolation transformer creates a physical separation from the input of your AC line to the transmitter. Anything that comes through the AC stops in the transformer and cannot reach the transmitter.
The reason why this device is mandatory is due to the fact that in the last 10 years the Broadcast Industry has developed new smaller power supplies that are very light and allow the units to be smaller in size and lighter for easy transportation. This has had a very positive impact on the Broadcast market. Unfortunately, on the other hand, the big transformers that were located inside the units and that were offering more protection have been removed and the new AC-DC power supplies do not give protection. The Isolation transformer is then necessary to give that protection back to the unit. The good news is a big isolation transformer installed where the AC enters the site can protect your unit as well as any other equipment in the same building.
If you want to protect your specific unit and you want to know what size of transformer you need; you can multiply the output watt power of your device by 2.5 and this will give you the watts required for your isolation transformer.(Example: for a 1000 W transmitter you need at least a 2.5 KW isolation transformer). If you would like to protect more devices you should add the wattage of every unit and multiply by 2.5.
Once you have your transformer installed, then you should verify how good your ground system is. Almost every good electrician should have the proper device to measure the Ground. Note that a good ground should be less than 5 Ohms. Money invested in measuring ground and fixing any problems found,can avoid costlier future expenses. Every rack and every unit must be connected to ground.
ANTENNA SYSTEM AND FEEDING LINE
Now that the electrical system has been analyzed and possibly set up, it is time to think of the radiating system (antenna) and the appropriate feeding line. There are many kinds of antennas on the market but generally I will divide them between NARROW BAND antennas and BROADBAND antennas. The choice of a narrow band system or a broadband system is very important and I’ll try to explain why in few words. Until 15-20 years ago,antennas were mostly narrow band and tuned on the assigned frequency. A tuned antenna provides a little more gain than a broad band antenna. At that time, transmitters were almost all TUBE type and the disadvantages of a narrow band antenna were not noticeable because the tube could handle big mismatches without problems. Now days, however the use of tube transmitters is almost at an end and the MOS FET technology has created smaller units that continue to be more powerful but so delicate!!!
Per my experience, I will always recommend a BROADBAND antenna when using mos fet amplifiers or transmitters and there are multiple reasons why. First of all, a broadband antenna gives a better guarantee to consistently keep the SWR at an acceptable level (most narrow band antennas change their impedance with rain, wind, snow etc.). Second, a broadband antenna will work on any frequency in the FM band. Therefore, if, at some point in time, you are re-assigned a frequency, you will still be able to use your antenna. Whereas, with a narrow band antenna system, you would need to have the system re-tuned in case of a frequency change. Last, but not least, a broadband systems allows you to place various frequencies into the same system by way of a combiner.
Any antenna must be ELECTRICALLY IN SHORT CIRCUIT. This means that you must measure continuity between the inner conductor of the connector and ground. This is an indispensable feature of every antenna because it reduces the risk of damage to the transmitter due to lightning and other electric charges present in the atmosphere.
Another very important precaution is to use a grounding kit on the feed line. The cable will be grounded to the pole or tower every some feet to insure a better ground system. In addition, our transmitters and amplifiers are equipped with a ground path inside the final amplifier module for further protection.
As I mentioned before, a breakdown is always a risk due to external causes (storms, lightning,etc.) or internal (defective components, ambient temperature etc.). These recommendations are a general guideline to give the best possible environment for your installation.
I sincerely believe that these guidelines will give you the most reliable set up possible.
The purpose of this overview is to put my 45 years of experience in the Broadcast field at the service of anyone who is not completely familiar with this complicated industry. Over these past years, I have experienced a lot of problems related to site maintenance because I was the one running to the sites at whatever time to fix those issues as they occurred. I know what it means to do everything possible to avoid these unwanted trips. I am aware that the following suggestions might sound expensive to many of you, but in the long run they will save you money. A breakdown is always possible because the equipment works day and night without rest. However, if these units work in the most ideal and safe situation possible, then they will last much longer. (In this brief outline, I will be using the metric system as reference because this is commonly used in the greatest part of the world.)
ROOM/UNIT VENTILATION
Usually the transmitting equipment is located in remote areas that are not easily accessible and for that reason they must be installed with the proper care to avoid frequent trips to the sites and wasted time and money. Let’s begin with the place in which the equipment will be working. The environment in which the unit has to be installed is very important. The room has to be clean and large enough to allow for proper air circulation. It should also be equipped with a proper system of fans, if not air conditioning to extract the warm air and allow the fresh air to enter. Because warm air is lighter than the fresh air, you need to let the cold air enter the room through an opening (usually about 65 cm in diameter) equipped with the right air filters and located as close as possible to the floor. On the contrary, the warm air should be extracted with a big exhaust fan through a large hole (same diameter) located as close as possible to the ceiling. The fan can be activated by a thermostat set at the proper temperature range.
When possible, an air conditioning system is always recommended.
When we test our transmitters, they are able to run at temperatures up to 50 degrees Celsius, but this does not mean that they can run for a long period of time in these conditions without having the internal devices of these units suffer.
The internal heatsink that dissipates the heat of the mos fets is usually 30 degrees Celsius higher than the ambient temperature in the room; so the cooler the ambient temperature of the room, the better the unit works.
As an example we usually compare a transmitter to a regular car. If the temperature of the coolant rises too much, the engine will suffer. Moreover, you must consider that a car is running only for some hours while a transmitter is working 24/7. It is also very important to properly install the transmitter. Usually the easiest and most reliable thing to do is to place or mount the unit in a rack together with the other equipment such as a stl receiver, stereo generator etc.In this case, always place the transmitter at the bottom (remember the fresh air stays closer to the ground or floor) and leave about the same space, 15 to 25cm,of open space above the transmitter when installing another device in the rack as it allows for circulation around the unit. The sides of the rack, if removable, can also be removed to allow maximum air circulation. Keep the rack or cabinet 15 to 25 cm from the back wall to allow for maximum air flow around the unit.
KEEP THE ROOM CLEAN (Remember that dust and/or small bugs are the greatest enemies of RF units)
ELECTRICAL SYSTEM
Once the room is in good condition, it is important to figure out the best way to supply AC power to the units. Depending on the different sites in the world, the electricity is subject to more or less fluctuation.
Fluctuations in voltage are not the only issue you can have with Electricity. At times, you can have spikes and peaks of power that last a very short time, but are enough to damage your equipment. There are many devices on the market that protect the AC line and these products are all very helpful (surge suppressors, stabilizers etc.). However, only one is really mandatory to avoid disasters caused by the AC power to the devices. It is called an ISOLATION TRANSFORMER.The isolation transformer creates a physical separation from the input of your AC line to the transmitter. Anything that comes through the AC stops in the transformer and cannot reach the transmitter.
The reason why this device is mandatory is due to the fact that in the last 10 years the Broadcast Industry has developed new smaller power supplies that are very light and allow the units to be smaller in size and lighter for easy transportation. This has had a very positive impact on the Broadcast market. Unfortunately, on the other hand, the big transformers that were located inside the units and that were offering more protection have been removed and the new AC-DC power supplies do not give protection. The Isolation transformer is then necessary to give that protection back to the unit. The good news is a big isolation transformer installed where the AC enters the site can protect your unit as well as any other equipment in the same building.
If you want to protect your specific unit and you want to know what size of transformer you need; you can multiply the output watt power of your device by 2.5 and this will give you the watts required for your isolation transformer.(Example: for a 1000 W transmitter you need at least a 2.5 KW isolation transformer). If you would like to protect more devices you should add the wattage of every unit and multiply by 2.5.
Once you have your transformer installed, then you should verify how good your ground system is. Almost every good electrician should have the proper device to measure the Ground. Note that a good ground should be less than 5 Ohms. Money invested in measuring ground and fixing any problems found,can avoid costlier future expenses. Every rack and every unit must be connected to ground.
ANTENNA SYSTEM AND FEEDING LINE
Now that the electrical system has been analyzed and possibly set up, it is time to think of the radiating system (antenna) and the appropriate feeding line. There are many kinds of antennas on the market but generally I will divide them between NARROW BAND antennas and BROADBAND antennas. The choice of a narrow band system or a broadband system is very important and I’ll try to explain why in few words. Until 15-20 years ago,antennas were mostly narrow band and tuned on the assigned frequency. A tuned antenna provides a little more gain than a broad band antenna. At that time, transmitters were almost all TUBE type and the disadvantages of a narrow band antenna were not noticeable because the tube could handle big mismatches without problems. Now days, however the use of tube transmitters is almost at an end and the MOS FET technology has created smaller units that continue to be more powerful but so delicate!!!
Per my experience, I will always recommend a BROADBAND antenna when using mos fet amplifiers or transmitters and there are multiple reasons why. First of all, a broadband antenna gives a better guarantee to consistently keep the SWR at an acceptable level (most narrow band antennas change their impedance with rain, wind, snow etc.). Second, a broadband antenna will work on any frequency in the FM band. Therefore, if, at some point in time, you are re-assigned a frequency, you will still be able to use your antenna. Whereas, with a narrow band antenna system, you would need to have the system re-tuned in case of a frequency change. Last, but not least, a broadband systems allows you to place various frequencies into the same system by way of a combiner.
Any antenna must be ELECTRICALLY IN SHORT CIRCUIT. This means that you must measure continuity between the inner conductor of the connector and ground. This is an indispensable feature of every antenna because it reduces the risk of damage to the transmitter due to lightning and other electric charges present in the atmosphere.
Another very important precaution is to use a grounding kit on the feed line. The cable will be grounded to the pole or tower every some feet to insure a better ground system. In addition, our transmitters and amplifiers are equipped with a ground path inside the final amplifier module for further protection.
As I mentioned before, a breakdown is always a risk due to external causes (storms, lightning,etc.) or internal (defective components, ambient temperature etc.). These recommendations are a general guideline to give the best possible environment for your installation.
I sincerely believe that these guidelines will give you the most reliable set up possible.
