The power behind the station's voice

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When buying a new transmitter, you will most likely be replacing an old unit or building a new site. If you are planning on removing your old transmitter and replacing it with a new one, your task is clear and simple. Determine the amount of RF output power needed, the amount of space available in the room and verify the available ac power source.

This Entercom Kansas City installation houses four stations.

Other factors to consider include the location of the transmitter's RF 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 interface will work with the new transmitter. Most modern transmitter remote controls offer the flexibility to accommodate most circumstances, but knowing that the old transmitter used maintained closures or logic-high connections, while the new transmitter uses logic-low connections will save time and frustration during installation. Be sure to answer all these questions before taking the old rig off the air and beginning installation of the new one.

From main to backup

Many transmitter sites already have two transmitters, but of different generations. Most likely, the new transmitter represents the most current generation, and the oldest transmitter (once someone's pride and joy so many years earlier) is slated to go out the door. The middle generation becomes relegated to backup status.

Many of the critical questions are answered; the amount of space available and the capability of the ac source power are known. Because the job won't have to be completed overnight, installation details aren't quite as important. There will be time to replumb the coax or to install new conduits for ac power.

Another possibility is that you are adding a new transmitter and keeping the old one as a backup for a station that has never had a backup transmitter before. If this is the case, more planning and perhaps 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 multiple transmitters.

For starters, the additional transmitter has to fit into the available space. Make a drawing of that space; not only the footprint, but critical details of the surrounding walls and ceiling. There may be some aspects of the transmitter room that can be changed depending on the budget. Don't get caught discovering incompatibilities when the new rig is set into place.

The transmitter facility must have sufficient ac power resources to handle the additional load. There will be times when both transmitters will be operating, so the service input and the main disconnect need to have the current handling capability to run both transmitters simultaneously. Refer to the transmitter literature to find out the amount of ac power the new transmitter will require, then hire an electrician to help determine if the service needs to be upgraded. Double check the air-handling capability in a similar fashion; consult with an HVAC company and make sure to consider the ac power requirements for any upgrades in the HVAC system.

There is little point in having a backup transmitter if it can't be put on the air quickly. The ability to switch transmitters via remote control is critically important. For FM transmitters, there are several different ways to do this.

The most rudimentary way to accomplish this is to add another antenna, and to connect it directly to the second transmitter. The advantage is simplicity, but with simplicity come two disadvantages. Usually a station's second antenna is inferior in some way to the main antenna, so when one transmitter fails the station be forced to use the inferior antenna. Also, you won't be able to run both transmitters without one interfering with the other.

A better way is to use a single, four-port coaxial relay. This allows you to switch either transmitter to the single antenna. The transmitter not selected for the antenna is connected to a dummy load.

The ability to remotely switch transmitters between the main and auxiliary antennas and a dummy load will minimize off-air downtime.

The ideal arrangement is to use two coaxial relays, or one relay and one patch bay, so that either transmitter can be switched to either antenna 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 second antenna can serve this purpose, there are interference issues to consider if two transmitters are running at once. I know of one AM station that actually has a backup antenna ready for use. The second transmitter will be as reliable as regular testing and maintenance will allow. If it can't be tested into a load and it can't can't be put on the air by means of a remote control, that transmitter will simply become a dirty hulk of a museum piece.

Keep in mind that dummy loads are purely resistive, while antennas have 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 far need to be considered when building a new transmitter facility. However, in most cases the entire site design is driven by the size of the transmitter itself. The standard approach is to first determine the required total power output (TPO) and then work backwards from there.

There are lots of choices in transmitter manufacturers. Regardless of the different brands, the ac power requirements are going to be similar with respect to the RF power level. With this information you will determine the size and type of the ac power service required, such as single- or three-phase, 240Vac or 480Vac. The transmitter specification sheets should list the ac power to RF power efficiency, and from that the amount of waste heat the new transmitter will generate can be determined. In consideration of that waste heat and other environmental issues, such as the elevation of the site and the typical ambient temperatures, the size of the transmitter room, along with other heat generating equipment, the HVAC requirements can be specified.

The natural rotation of transferring a transmitter from main to backup status is common. Staying on the air is the critical element even though the backup may not have all the features or capabilities of the new main, such as stereo or SCA capability.

Once this has been done, pick the transmitter brand. Once the brand is decided, it may be necessary to reiterate some of the design aspects to be sure that they will work with the specific transmitter.

Considering IBOC

While IBOC is not yet an approved and mandatory standard, it makes sense to consider IBOC operation for an analog transmitter purchased today. 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 for analog and the other for digital. This requires two transmitters, two transmission lines and two antennas. This requires no change to the existing analog transmission facilities, but requires twice as much equipment and tower space. This method is still under consideration by the FCC. An analog transmitter bought today would still be useful for analog operation later with a separate antenna system.

High-level combining is fairly straightforward. The output of an analog and digital transmitter are combined through a high-power combiner. For the analog signal, the insertion loss in the combiner (analog input to analog output) is about 10 percent or -0.45dB. If, for example, the station's TPO requirement is 10kW, the analog transmitter will need to produce about 11.1kW into the combiner to provide 10kW of analog going up to the antenna. If this method is in your facility's plan, have at least 10 percent power headroom in the analog transmitter.

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 level and then feeds a single power amplifier for both signals. To pass the digital signal, the transmitter must be linear. Some newer designs plan for hybrid operations, and they are identified as such.

For MW IBOC, there are at least two manufacturers that have IBOC-ready transmitters available for sale and delivery.

Choosing the brand

If you are charged with making the decision about which brand of transmitter to buy, remember that the most important characteristic about a transmitter is its reliability. Every transmitter will eventually break, so the next most important thing to consider is the ease with which it can be repaired when the inevitable happens. When it breaks, you assume the company is still in business.

Before making the decision on the brand, ask the salesperson some questions. 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 the manufacturer well known and well established? What is the availability of parts and technical service from this manufacturer on a 24-hour, seven-days-a-week basis? Can I interview other users of the same model I'm considering?

While these are questions that can be answered objectively, the reality is that your decision will be subjective. You should plan on being able to use the new transmitter for at least 10 years, so choosing wisely now will make your life (or perhaps your successor's life) that much easier in the foreseeable future.

Irwin is director of engineering services, Clear Channel San Francisco.

Resource Guide

Transmitter manufacturers and their websites




Armstrong Transmitter


Broadcast Electronics Inc

Broadcast Technology

Continental Electronics

Crown Broadcast

DB Elettronica

Ecreso - RFTS Broadcast


Goodrich Enterprises


Itelco USA Inc

Kenneke Communications


Marti Electronics


Nexus Broadcast


OMB America



Ramsey Electronics


Silicon Valley Power Amplifiers

Superior Broadcast



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