FM Translators

Modern options now give us a lot more flexibility in the system design April 14, 2015

FM translators are very much in the trade news. You may have some amount of experience with the service—it’s been around for years. Unfortunately, the original manufacturers of translators have come and gone; but modern options now give us a lot more flexibility in the system design.

Before we begin looking at our options for equipment and antennas, let’s review some of the FCC’s basic translator rules, since our system design will by necessity have to fit within their 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, commonly owned with the primary station, may deliver the signal to the translator by any means, including satellite delivery.
• The translator must be configured to go off the air if the main station’s signal is lost; and 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.

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

Over-the-air reception. It would seem that receiving the station to be translated over-the-air would be very 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. It seems like a “no-brainer,” but it’s almost inevitable that there will be some sort of problem with reception, though, related to one of three 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.”

The mitigation of any one of those three problems is done in the same way—by purchasing a receiver that works well through these difficult circumstances; by gaining as much signal from the target as possible; and, by filtering adjacent and/or local stations to the extent possible. Let’s take a look at some options.

Aaron 650 (shown above) is Inovonics’ latest FM rebroadcast/translator receiver. It operates in either a “composite pass-through” or “composite-regeneration mode.” Composite pass-through eliminates retransmission delay (latency), while composite-regeneration gives the ability to completely reconstruct the baseband signal, even altering RDS messaging prior to rebroadcast. The back panel provides dual antenna inputs, dual composite outputs, balanced analog and AES-digital audio outputs, and remote IP access. Aaron 650 has self-logging alarms for audio Loss, RF Loss, and RDS Loss (or hijacking). Personnel can be alerted with email or instant text messages when any of the alarms occur. Aaron 650’s Web interface allows complete setup, listening, logging and control from a PC, tablet or smartphone and reliable two-way connectivity is ensured with a built-in Dynamic DNS utility (eliminating the need for a static IP at the translator site). The front panel displays left and right audio metering, local LED alarms and has an LCD screen with jog wheel for advanced control and editing of all operating parameters, such as bandwidth and auto-blending.

Broadcast Warehouse offers the RBRX1 receiver for use in translator applications. Adjustments are provided for over 50 parameters, including audio and IF bandwidths, de-emphasis and blending for stereo, HF and ultrasonic noise. Control of blending levels and attack & release times is also possible. All adjustments, plus factory presets and user stored settings, can be accessed from the front panel or via Ethernet and serial connections. The RBRX1 has a DSP-enhanced composite output that reconstructs a clean peak- level and bandwidth- limited waveform. RDS PI code checking prevents translators and repeaters from being hijacked or jammed. Some of its other features include balanced audio outputs via XLR connectors; configurable email alerts; a headphone jack for listening in; and, SNMP support.

The DB7000 is one of several receivers produced by Deva Broadcast and seems particularly well-suited to translator duty. The DB7000 is highly selective DSP-based FM tuner; challenging problems with adjacent-signals are mitigated by the used high-accuracy, DSP-based IF bandwidth filters. The receiver measures all the important parameters of the FM transmissions, and comes with a built-in monitoring system that generates warning messages in the case of out-of-limit parameters. USB and LAN communication interfaces allow flexibility for local and remote connection and control of the unit. Other salient features include its composite output; left and right audio outputs; an AES-3 version of the output; configurable alarms outputs on the rear panel; and a headphone output for listening in.

If your intention is to put an older receiver with a composite output into service, you might want to consider composite filtering for the purpose. Broadcast Devices is now selling their LPF-100, a linear-phase electronic filter that is designed to reduce base-band noise levels from studio-to-transmitter links, processing equipment or stereo generators. When “translating” off-air reception of composite, the last thing you want is to transmit any excessive noise in the upper part of the base-band that happens to come out of the off-air receiver. It could show itself as “multipath” noise in stereo receivers in the field, or at the very least, rob you of part of the overall modulation level for your translator.

If you are building an FM translator to go along with an AM station, I’m guessing you won’t use off-air reception, at least as a main source; still, having an AM receiver to pick up the station-to-be-translated is not a bad idea. The Fanfare FTA-100P is both an AM (mono or stereo) and FM receiver in one box. It Includes the Carrier Sense option to detect low or no signal at a translator/repeater site. (Keep in mind that FM translators associated with daytime-only AMs aren’t required to go off-air when the AM station goes off overnight.)

One other possibility for an off-air feed for a translator is of course that of an HD radio-derived feed. The Inovonics INOmini 632 is certainly an option in this case. It’s an FM and FM-HD radio that can be used to decode any one of the HD streams (1-8) and importantly, it will ‘stay-put’ in the event that the particular HD stream disappears for a while. (Older HD receivers failed miserably at this.) You configure the basic functions from the front panel jog-wheel and LCD screen. Decoded audio outputs are available as in their analog versions (via XLR) or in an AES-3 format, all on the rear panel. There’s a front-panel headphone jack as well. Alarm outputs for carrier loss, digital loss, and audio loss are available as open-collectors on the rear panel.

Even the best receiver can’t pull-in a weak station without an adequate antenna, and there are many options out there. Scala (Kathrein) is the most well-known manufacturer of antennas in this category. A great example of a receive antenna is the YA-7 FM (and variants). It’s not so much the forward gain figure of an antenna like this, by the way; the real advantage in many cases is the high front-to-side and front-to-back ratios, which allow you to attenuate off-axis, adjacent frequency signals. (It’s all about improving the desired-to-undesired ratio of the signals getting in to your receiver.) Another option is the venerable CL-FM, which is log-periodic type, really meant to be a transmitting antenna; it makes a great receive antenna as well, naturally. Shively makes its own version of a log-periodic known as the 6025. Keep in mind that each of the options I have mentioned are meant for rugged, long-term outdoor service. Don’t try to make use of an inexpensive, off-the-shelf, sale model from Radio Shack for this application.

In some cases, even with the best receiver, and a great antenna, you may still have issues with local, strong FM carriers. After all, if the translator site is good, then there may very well be other transmitters sharing it. Likely they were there first. So—what do you do in that case? It’s likely that the appropriate filter sets will take care of any local issues. Telewave is a good source of cavity filters; their TWPC-1005-x can be purchase in single, dual, or triple cavity configurations. Microwave Filter is another source of band-pass filters, such as the 3634FM. For notch filters, consider the MFC 6367-2 type.

There are many options in terms of RF transmission gear. The commission’s rules describe the basic requirements to be met so that any particular “transmitter” can be used in this service:

• Harmonic content/spurious emissions must comply with 74.1236; basically this says for TPOs less than 10 W, the spurious emissions requirements are slightly less than what we are normally accustomed to, which in turn is described in 73.317.
• Frequency stability must be within +/- .005 percent when the ambient temperature encountered is between -30 and 50 degrees centigrade, and while the “primary supply” voltage varies between 85 and 115 percent of normal;
• Output power must be maintained automatically, with provisions for the monitoring of operating parameters;
• Any transmitter with an output exceeding 1 W shall automatically cease transmission should the “to-be-translated” signal disappears, and;
• For exciters, a means to limit the modulation to +/- 75 KHz must be employed.

Crown Broadcast is a well-known name in lower power stand-alone FM transmitters, and they’re now offering their E Series (built be Ecreso). The E series features a direct-to-channel integral exciter, and an integral stereo generator that accepts L/R analog or AES at up to 192 KHz sample rate and up to 32-bit word length. It also has two BNC inputs for composite or SCAs. Some other features that are available for the E-series include a TCP/IP communications board that allows for the use of a Web-based interface for monitoring and remote control, along with SNMP support; another important one in the present context is FSK ID’ing for translator applications.

Broadcast Warehouse has an extensive line of transmitters, including TX300 V2, a stand-alone transmitter capable of 300 watts. In one box, you get the exciter, power supply, power amplifier, a stereo generator and audio processor (basically lifted right from the design of the BW DSPX miniFM). It comes with analog, digital, and composite inputs; remote control, e-mail alerts, silence detection, logging, and it supports SNMP, Telnet and Serial Remote Control. Another feature is an FSK ID keyer that removes the need for external hardware or hourly audible announcements—a very nice feature for translator sites. RF output is done via a type-N female. The AC input can accept between 85 and 260 VAC, giving you some flexibility with the installation.

Bext is another well-known manufacturer of FM transmission equipment. For a translator application you could certainly look at their LEX series, which tops out at 300 W. The LEX is a single rack unit, frequency-agile transmitter; it has its own internal stereo generator, along with composite and RDS/SCA inputs. The LEX series also includes and FSK ID keyer for use in ID’ing a translator. Output power is continuously adjustable from zero up to the maximum power rating, and it has proportional power fold-back in response to high VSWR. The power supply accepts between 95 and 260 VAC which gives you flexibility in the installation.

When it comes to translators, you’re not precluded from using products from the big transmitter manufacturers; they also make units that will work great in translator applications.

For example, Broadcast Electronics offers the STXe series. Aside from the integral controller, it has an integral low-pass filter, meaning that it can be used as a translator (meeting that 73.317 requirement). It also has an updated GUI, so if your site has IP access, you’ll have complete remote access to the transmitter. SNMP support is also included.   

GatesAir makes the Flexiva series of transmitters, which include a direct-to-carrier modulator, integral stereo generator, and integral low-pass filter. Its stereo generator has analog L/R inputs along with 2 AES inputs, and a built-in RDS generator. AC power input uses between 90 and 277 VAC so that give you some flexibility in the installation. It’s 3 RU in height, and 20.5 inches in depth. The FAX series comes with an embedded Web interface, SNMP support and local GP i/o, as well.

Nautel offers the VS300LP. It has an integral exciter, with direct-to-channel modulation, an integrated stereo generator, and SCA and RDS generators. The VS300LP accepts an AES program input, L/R analog inputs, composite, or IP streams such as Shoutcast, RTP, or Livewire. It also supports MPX over AES for a completely digital connection to many audio processors. It uses 2 RU in height, and about 25 inches in depth with a type-N female connector output. The AC input voltage can operate between 180 and 264 VAC. Remote access is done via the Nautel AUI, which gives the user complete monitoring and control via IP; parallel control and SNMP access are supported, as well.

Inside of a translator’s coverage area, listeners will expect the same kind of performance as they would from any full-service stations that they also happen to receive. If you are competing on this level, then it makes sense to pay as much attention to the system design, and the subsequent maintenance of a translator as it would any other station. If the translator is a success, then you know that there will be just as many demands from programming and management about keeping it on-air and sounding good.

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