Field Report: Marti STL-20C

The original intention was to install a radio communications link between WGUL-AM's studio and transmitter site. But year after year, STL transmitter and receiver products kept coming up short. We couldn't find a composite STL system that could overcome the distance and terrain between WGUL-AM's studio in Palm Harbor, FL, and its transmitter tower in Safety Harbor, FL. The STL systems the station owned lacked the signal strength to make the hop satisfactorily from an 80-foot to-wer behind our studio complex, across 10 miles and over a 60-foot hill to our 280-foot tower in the Florida lowlands.

What we needed was a super STL—an STL able to leap tall buildings in a single bound.

I gave up all hope of finding such an STL until I heard about the Marti STL-20C composite transmitter. With more than double the output power of other STL transmitters, I had every reason to believe this STL's 20W power amplifier would send our programming soaring across the path. Also, the system was more cost effective than the leased equalized phone lines we used to run programming over to the AM site. And, by all indications, WGUL-AM's Music of Your Life format, broadcast in AM stereo, would benefit from the synthesizer used in this STL. Modulation is direct FM, synthesized +/-50kHz deviation for 100 percent modulation. The synthesizer has a flat frequency response up to 200kHz, with less than 0.2 percent distortion. Its average left and right separation is 58dB across the band, and the separation for the 1kHz to 5kHz range affecting stereo fidelity is even more (62dB separation).

Performance at a glance
  • 20W RF output
  • Direct FM synthesized
  • Less than 0.2 percent distortion
  • Flat frequency response up to 200kHz
  • 58dB separation, 50Hz to 15kHz
  • 62dB separation, 1kHz to 5kHz
  • Greater than 76dB S/N
  • Spurious emission better than 60dB below carrier
  • Peak-hold bar graph modulation meter
  • Two BNC inputs
  • 15-pin connector on rear panel provides filtered outputs for remote control and remote power metering
  • Could this clean STL give our station the full sound we were seeking? I decided to find out.

    First, however, I had to overcome other obstacles. STLs and other airborne systems had proliferated in the area over the years. Finding available band space wouldn't be easy. Nor would it be easy to convince our community that we needed to put up more communications equipment. We were already battling the local authorities to keep a pickup dish at the studio complex for the AP network. It was apparent that we needed an extra powerful STL transmitter for just about any contingency. A marginal STL wouldn't do. We needed every millivolt the STL-20C had to offer so the STL receiver would be able to pick up the transmitted signal in a RF congested area.

    By the time we overcame the usual permit hurdles and acquired approval from our local frequency coordinator to transmit a vertically polarized 950MHz signal that wouldn't interfere with another station's horizontal 950MHz STL, my optimism started to wane. Nonetheless, I cleared two rack unit spaces at the studio for the 11-pound STL-20C, hoping against all odds that we would finally be able to make the hop between studio and transmitter using a quality link. When our Marti STL-20C transmitter arrived I was glad to discover that its front panel design had the same feel I'd come to expect in my years of installing Marti STLs for other stations.

    That made setup almost a non-event. I simply brought in composite audio through a BNC connector in the back of the unit, adjusted the unit for maximum power, and monitored everything from a bar graph meter on the front panel. The meter is a peak hold type, providing precision readings on forward power, reverse power, PA current and subcarrier level. LEDs gave me an instant read on whether the unit was transmitting in composite or mono mode. The only control new to me was a switch for synthesizer resolution, selectable in 25kHz steps. Another new and welcomed addition is the STL-20C's automatic changeover feature; all we will need is an ATS-20E for a second transmitter.

    As a testament to the ease of installation, I used the manual as more of a reference than a step-by-step guide. The unit ran consistently as promised, with not so much as a glitch, even though this was the first STL-20C off the production line.

    Test and Ye Shall Receive

    After installing the new STL at the studio tower, I stood back to see if the STL-20C would deliver. To my amazement, its 20W output amplifier sent programming sailing over the hill—as well as over the new housing developments and trees jutting out from atop the hill. The Marti R-15C receiver for the 280-foot receiving tower received the content easily. Incredibly, program content made it from the top of the transmission tower, through a lengthy transmission line and across a ravine to the transmitter building, where it took another beating as it passed through two cavities installed to protect the AM from rogue cellphone signals coming from another site nearby. I added a pre-amplifier in the circuit to give the signal a little extra boost. Overall, the STL overcame a 12.5dB loss in the transmission path.

    When we turned on the radio and listened to the sound, I was in for another pleasant surprise. Not only was the station's reception strong and clean, but there was also a new depth to the music. It was now fuller than ever. Our CEO noticed it right away.

    In the final analysis, I'd have to say that Marti's new STL transmitter has made significant strides in range and quality. Without it, WGUL would still be sacrificing budget and program content to leased phone lines.

    Mueller is chief engineer of WGUL-AM/FM, Palm Harbor, FL.


    P 817-735-8134
    F 817-735-9340

    Editor's note: Field Reports are an exclusive Radio magazine feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.

    These reports are performed by the industry, for the industry. Manufacturer support is limited to providing loan equipment and to aiding the author if requested.

    It is the responsibility of Radio magazine to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by Radio magazine.

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