The power behind the station's voice
May 1, 2003 12:00 PM, By Doug Irwin
When buying a new transmitter, you will most likely be replacing anold unit or building a new site. If you are planning on removing yourold transmitter and replacing it with a new one, your task is clear andsimple. Determine the amount of RF output power needed, the amount ofspace available in the room and verify the available ac powersource.
This Entercom Kansas City installation houses four stations.
Other factors to consider include the location of the transmitter'sRF output connector, electrical input and remote control connections.While these do not have to be in the same place as the old transmitter,accounting for their location will speed the installation process.
Speaking of the remote control, also check that the old interfacewill work with the new transmitter. Most modern transmitter remotecontrols offer the flexibility to accommodate most circumstances, butknowing that the old transmitter used maintained closures or logic-highconnections, while the new transmitter uses logic-low connections willsave time and frustration during installation. Be sure to answer allthese questions before taking the old rig off the air and beginninginstallation of the new one.
From main to backup
Many transmitter sites already have two transmitters, but ofdifferent generations. Most likely, the new transmitter represents themost current generation, and the oldest transmitter (once someone'spride and joy so many years earlier) is slated to go out the door. Themiddle generation becomes relegated to backup status.
Many of the critical questions are answered; the amount of spaceavailable and the capability of the ac source power are known. Becausethe job won't have to be completed overnight, installation detailsaren't quite as important. There will be time to replumb the coax or toinstall new conduits for ac power.
Another possibility is that you are adding a new transmitter andkeeping the old one as a backup for a station that has never had abackup transmitter before. If this is the case, more planning andperhaps a bit of research is required.
Some kind of resistive load is critical to maintaining a transmitter.With appropriate switching, a single load can be used for multipletransmitters.
For starters, the additional transmitter has to fit into theavailable space. Make a drawing of that space; not only the footprint,but critical details of the surrounding walls and ceiling. There may besome aspects of the transmitter room that can be changed depending onthe budget. Don't get caught discovering incompatibilities when the newrig is set into place.
The transmitter facility must have sufficient ac power resources tohandle the additional load. There will be times when both transmitterswill be operating, so the service input and the main disconnect need tohave the current handling capability to run both transmitterssimultaneously. Refer to the transmitter literature to find out theamount of ac power the new transmitter will require, then hire anelectrician to help determine if the service needs to be upgraded.Double check the air-handling capability in a similar fashion; consultwith an HVAC company and make sure to consider the ac powerrequirements for any upgrades in the HVAC system.
There is little point in having a backup transmitter if it can't beput on the air quickly. The ability to switch transmitters via remotecontrol is critically important. For FM transmitters, there are severaldifferent ways to do this.
The most rudimentary way to accomplish this is to add anotherantenna, and to connect it directly to the second transmitter. Theadvantage is simplicity, but with simplicity come two disadvantages.Usually a station's second antenna is inferior in some way to the mainantenna, so when one transmitter fails the station be forced to use theinferior antenna. Also, you won't be able to run both transmitterswithout one interfering with the other.
A better way is to use a single, four-port coaxial relay. Thisallows you to switch either transmitter to the single antenna. Thetransmitter not selected for the antenna is connected to a dummyload.
The ability to remotely switch transmitters between the main andauxiliary antennas and a dummy load will minimize off-airdowntime.
The ideal arrangement is to use two coaxial relays, or one relay andone patch bay, so that either transmitter can be switched to eitherantenna or dummy load.
Every station should have a dummy load, whether it is an AM or FM.There is no better way to test a transmitter offline. While a secondantenna can serve this purpose, there are interference issues toconsider if two transmitters are running at once. I know of one AMstation that actually has a backup antenna ready for use. The secondtransmitter will be as reliable as regular testing and maintenance willallow. If it can't be tested into a load and it can't can't be put onthe air by means of a remote control, that transmitter will simplybecome a dirty hulk of a museum piece.
Keep in mind that dummy loads are purely resistive, while antennashave some inductive properties. Test a transmitter into a dummy load,but always tune it into the antenna so it is ready for standby use.
Building a new transmitter site
All of the transmitter site characteristics I've discussed so farneed to be considered when building a new transmitter facility.However, in most cases the entire site design is driven by the size ofthe transmitter itself. The standard approach is to first determine therequired total power output (TPO) and then work backwards fromthere.
There are lots of choices in transmitter manufacturers. Regardlessof the different brands, the ac power requirements are going to besimilar with respect to the RF power level. With this information youwill determine the size and type of the ac power service required, suchas single- or three-phase, 240Vac or 480Vac. The transmitterspecification sheets should list the ac power to RF power efficiency,and from that the amount of waste heat the new transmitter willgenerate can be determined. In consideration of that waste heat andother environmental issues, such as the elevation of the site and thetypical ambient temperatures, the size of the transmitter room, alongwith other heat generating equipment, the HVAC requirements can bespecified.
The natural rotation of transferring a transmitter from main to backupstatus is common. Staying on the air is the critical element eventhough the backup may not have all the features or capabilities of thenew main, such as stereo or SCA capability.
Once this has been done, pick the transmitter brand. Once the brandis decided, it may be necessary to reiterate some of the design aspectsto be sure that they will work with the specific transmitter.
While IBOC is not yet an approved and mandatory standard, it makessense to consider IBOC operation for an analog transmitter purchasedtoday. There are three proposed methods for transmitting a hybrid(analog and digital) FM IBOC signal: use of separate antennas,high-level combining and low-level combining.
The easiest method to plan is to use separate antennas; one foranalog and the other for digital. This requires two transmitters, twotransmission lines and two antennas. This requires no change to theexisting analog transmission facilities, but requires twice as muchequipment and tower space. This method is still under consideration bythe FCC. An analog transmitter bought today would still be useful foranalog operation later with a separate antenna system.
High-level combining is fairly straightforward. The output of ananalog and digital transmitter are combined through a high-powercombiner. For the analog signal, the insertion loss in the combiner(analog input to analog output) is about 10 percent or -0.45dB. If, forexample, the station's TPO requirement is 10kW, the analog transmitterwill need to produce about 11.1kW into the combiner to provide 10kW ofanalog going up to the antenna. If this method is in your facility'splan, have at least 10 percent power headroom in the analogtransmitter.
Even if your station is not ready to install an IBOC system today,consider a transmitter that is IBOC-capable for the future.
Low-level combining creates the hybrid signal at the exciter leveland then feeds a single power amplifier for both signals. To pass thedigital signal, the transmitter must be linear. Some newer designs planfor hybrid operations, and they are identified as such.
For MW IBOC, there are at least two manufacturers that haveIBOC-ready transmitters available for sale and delivery.
Choosing the brand
If you are charged with making the decision about which brand oftransmitter to buy, remember that the most important characteristicabout a transmitter is its reliability. Every transmitter willeventually break, so the next most important thing to consider is theease with which it can be repaired when the inevitable happens. When itbreaks, you assume the company is still in business.
Before making the decision on the brand, ask the salesperson somequestions. Has the model I'm looking at been available for some time?Is it a proven design or are there many in the field? Is themanufacturer well known and well established? What is the availabilityof parts and technical service from this manufacturer on a 24-hour,seven-days-a-week basis? Can I interview other users of the same modelI'm considering?
While these are questions that can be answered objectively, thereality is that your decision will be subjective. You should plan onbeing able to use the new transmitter for at least 10 years, sochoosing wisely now will make your life (or perhaps your successor'slife) that much easier in the foreseeable future.
Irwin is director of engineering services, Clear Channel SanFrancisco.
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