No-Bull Tips for Transmitter Sites

by John Bisset

Ever have equipment installation interface problems? Here's one solution.

Mark Ward is chief at WTSN(AM)/WBYY(FM) in Dover, N.H. Last summer, his stations received a windfall in the form of a Comrex BlueBox and Matrix. These units provided the stations with a quantum leap in the audio quality of remote broadcasts, compared to conventional analog POTS or cellular phone service.

When used for commercial remotes on the FM, installation was straightforward, with a mix-minus feed from the audition bus of the console straight into the Blue Box.

The AM presented a challenge, however. One of their goals was to use the Comrex for live call-in talk shows while the host was at a remote location. This would require three busses on the board: program to feed the transmitter, audition to feed one mix-minus to the Blue Box, and a telephone bus to feed a different mix-minus to the telephone hybrid. Alas, the AM station's mid-1960s board has but two busses; and any source can only be assigned to one bus at a time.

Mark created a list of "who needed to hear whom":

1) the remote host needs to hear the callers, as well as cues from the board operator;

2) the callers need to hear the remote host; and

3) the audience needs to hear the remote host, the callers and all the other elements of the show.

The first requirement was met by wiring a Shure mixer into the BlueBox line input. This mixer was fed by the caller output of the hybrid on the line input, and a local mike for cues. Mark already had installed another Shure mixer for in-studio guests and wired its line output into the send input of the hybrid to provide a mix-minus for the callers when the show was at home.

Fig. 1: Mark Ward uses this simple mod to customize a Comrex application.

A double-pole, double-throw switch was inserted to select either the in-studio mix-minus mixer or the BlueBox to feed the hybrid, thus satisfying the second requirement. Finally, the BlueBox line output was wired into the board to complete the installation. Fig. 1 shows the wiring interconnect.

Operationally, some remote hosts prefer to have the local mike open all the time, while others find hearing the (delayed) return of their own voices, from the studio speaker, distracting. Care in setting audio levels both ways provides excellent results. Mark operates exclusively in "voice" mode, which provides 7 kHz audio bandwidth at any data rate from 14.4 kbps on up.

Typically, the initial connection will be established at 19.2 to 24 kbps, but they drop the rate down to 16.8 kbps before air, and "retrain" the Comrex hourly during long remotes, to allow some margin for error correction of the telephone line.

Note that if errors aggregate beyond a certain threshold, the Comrex will perform a "retraining" by itself, with an accompanying loss of audio for 10 seconds, which can seem like an eternity.

Consensus among those concerned is that this is the way remotes should sound - "like you're in the room."

Asked for comment on this tech note, Comrex called it a clever way to solve the problem. The company notes that its Mix Minus Bridge is designed to scratch this itch; it costs $1,500, so it's a more pricey solution, but it may save time.

Mark Ward can be reached at mark@am1270wtsn.com.

. . .

Here's a site to keep bookmarked for finding obsolete RF connectors.

Aviel Electronics, a division of RF Industries, can fabricate functional equivalents for most RF coaxial connectors and adapters that are obsolete, discontinued or no longer available.

The connectors come in all common miniature interfaces, including SMA, SMB, SMC and MCX, as well as larger interfaces such as BNC, TNC, N, C, SC, HN, LT and LC. Typical body materials include brass, stainless steel and aluminum. The connectors can be plated with gold, silver or nickel; and can be anodized or olive drab. Body configurations include straight, right angle, bulkhead, flange mount, blind-mate, tri-axial and others.

The site is www.avielelectronics.com, and you can contact the factory with your requirements at (702) 739-8155.

. . .

Fig. 2 shows another transmitter site road - or in this case, "road."

The engineer submitting the picture asked to remain anonymous, for obvious reasons.

Fig. 2: Tower for Sale - rowboat included.

It seems the previous owners wouldn't spend any money on the site, other than to keep it on the air. A real road to the tower has since been put in.

Wouldn't you love to see the FCC check the base ammeter calibration at a site like this?

. . .

For Bill Agresta, chief of Crawford's KBRT(AM) in Los Angeles, as for many engineers in California, landslides have become an everyday occurrence. This makes driving old mountain roads dangerous. Fig. 3 is an example. This was taken after rocks were cleared from the road.

Fig. 3: Rain and loose rocks don't mix

Wet weather brought other problems, too, like keeping a ground system in place. It's a battle to keep rain and wind from washing radials away. Bill also has had to be creative in keeping ATU components dry at the base of the tower.

Then there's the problem of local wildlife. Bill saw an unusual sight out his home's back window, and shared it in his group's engineering newsletter. See Fig. 4. He's considering this "no bull" approach to keeping vandals away from the transmitter site.

Fig. 4: Thwart site vandals. Order your guard buffalo today.

Thanks to Bill and Crawford DOE Cris Alexander for sharing photos.

John Bisset has worked as a chief engineer and contract engineer for more than 30 years. He is the northeast regional sales manager for Broadcast Electronics. Reach him at (571) 217-9386, or jbisset@bdcast.com.

Submissions for this column are encouraged, and qualify for SBE recertification credit.