FM Translator Implementation and Construction Techniques

Many broadcasters have some experience with the FM translator service. It’s been around for years. The original translator manufacturers have come and gone; however, modern options give us a lot more flexibility in the system design and construction. We’ll take a look at that in this article. Before we begin looking at our options for equipment and antennas, let’s review some of the Federal Communications Commission’s basic translator rules, since, by necessity, our system design will have to fit within its guidelines.

• The maximum effective radiated power for any translator station is 250 watts.

 • If the translator is a fill-in translator, the ERP may be further limited by the need to maintain the translator’s service contour within the primary station’s service contour. Composite antennas and antenna arrays may be used. 

• A commercial fill-in translator may receive a primary station’s signal via any terrestrial transmission method, including (but not limited to) microwave, phone, internet and dedicated fiber optic cable. Aural intercity relay frequencies may also be used on a secondary basis, after coordination with local frequency coordinating committees. 

• A noncommercial educational FM translator that is commonly owned with the primary station may deliver the signal to the translator by any means, including satellite delivery. 

• An FM translator station rebroadcasting the signal of an AM or FM primary station shall not be permitted to radiate during extended periods when signals of the primary station are not being retransmitted. Notwithstanding the foregoing, FM translators rebroadcasting Class D AM stations may continue to operate during nighttime hours only if the AM station has operated within the last 24 hours.

• If the translator site cannot be reached at all hours and all seasons, means shall be provided so that the transmitting apparatus can be turned on and off at will from a point that is readily accessible during all hours and all seasons; for that reason, some sort of remote control is needed.

In terms of system design, delivery of the signal to be translated is probably the most challenging part. In many cases, over-the-air reception will work, but it can be fraught with difficulties — not the least of which is inconsistent reception quality. Also, though it is common knowledge that the translator must go off in the event that the “main” station goes off-air, the remote control requirement is not well-known.

Over-the-air reception. It would seem that receiving the station to be translated over-the-air would be simple because most of the time when one travels up to the proposed translator site (in a vehicle) to listen to the targeted station, reception is good. However, it’s almost inevitable that there will be some sort of problem with reception, related to one of four issues: 

• The signal to be received is weak because of distance and/or intervening terrain. 

• Fading occasionally makes the signal even weaker. This could generate an unacceptable amount of noise in the receiver output and thus as heard over the translator. 

• There are strong adjacent channels present. This could lead to noise in the receiver output that is synchronized with the modulation of the adjacent stations, which could also be heard over the translator.

 • There are strong local signals present. This could cause your receiver to be “de-sensed.” 

First, it’s likely the signal to be translated is weak, otherwise there would be no need for the translator in the first place. Intervening terrain and distance are problems that cannot be fixed; the only means to address them is by getting as much signal from the primary signal as possible, usually through receive antenna gain and height. And though it presents a considerable amount of work, you may find some physical locations for the receive antenna are better than others. Resist the temptation to buy anything other than a professional grade antenna for reception. It’s just as important as the transmit antenna (which we’ll cover later). 

Fading of the primary signal can create issues of noise in the translator output at best and outright oscillation at worst, depending upon how close the output frequency is to the input frequency. Refraction, brought on by temperature gradients in the atmosphere, can cause the signal level to vary. Again, the only way to address this issue is by building in as much antenna gain as possible, thus allowing you to develop a fade margin that is acceptable.

The presence of strong adjacent channel signals is likely,especially if your off-air receiver is located on a mountain or hilltop. Any filter inserted in the transmission line going to the receiver must of necessity be of a very high-Q in order to have any beneficial effect. The IF filtering in your off-air receiver will have the greatest effect in mitigating this problem. Presence of strong local signals also comes with the territory if the receiver that is part of your translator system is located on a mountain or hilltop. Receivers can be desensitized by the presence of strong, local signals, even when they are not that close to the frequency at which the receiver is used. Bandpass filters (which pass the channel you want) and notch-filters (which pass everything but the channel you are trying to be rid of) are effective in mitigation of receiver “de-sense.”

Other means of program delivery. From the FCC rules: “A commercial fill-in translator may receive a primary station’s signal via any terrestrial transmission method, including (but not limited to) microwave, phone, internet and dedicated fiber optic cable. Satellite delivery is prohibited. These requirements also apply to noncommercial educational translators in the reserved band (88 to 92 MHz) that are not commonly owned with the primary station. A noncommercial educational FM translator, commonly owned with the primary station, may deliver the signal to the translator by any means, including satellite delivery.” The normal means we use for feeding radio stations can all be used for feeding translators (with the exception of satellite feeds for commercial fill-ins). FM translators can also be fed by HD Radio receivers. FCC rule 74.1231 (g) states: “Originations concerning  financial support (for the translator) are limited to a total of 30 seconds an hour. Within this limitation, the length of any particular announcement will be left to the discretion of the translator station licensee. Solicitations of contributions shall be limited to the defrayal of the costs of installation, operation and maintenance of the translator or acknowledgements of financial support for those purposes.”

This leads to the topic of “store and forward” for translator systems. There are devices available that afford translator licensees a fairly easy way to broadcast sponsorship messages, by sending them out to the translator sites via satellite (or IP). In this way, the messages can be tailored to the specific area covered by the translator. It’s also important to know that aural intercity relay frequencies may be used on a secondary basis, if the prior coordination process shows that its use would neither cause interference to nor preclude use of the frequency by a full-service radio broadcast station; and a relay through another translator station is only acceptable if the intermediate translator provides a signal to a populated area.