Commercial equipment for carrier current broadcasting. Carrier current means that instead of sending your radio signal to an antenna, you place it on already existing wires, most commonly power lines that enter a building. From there the signal might feed up the power lines and into the power grid, to reach radios near the lines.
Carrier current was common with student radio stations on college and high school campuses, but it can also be used for community broadcasting in a neighborhood; you don't have to be a school to broadcast by carrier current. No license is required or available as it's considered a Part-15 radio service, in the same league with cordless phones and garage door openers.
Carrier current broadcasting is mostly done on the AM broadcast band and was used in the early days of radio as a kind of cable audio service. During World War II, radio transmissions by the public were forbidden, so amateur radio operators used carrier current to communicate with each other over power lines. Colleges experimented with their own stations in dorms and other areas, where they proved to be successful at reaching the close-in populations on a campus, so the practice spread to many schools in the USA after the 1940s.
In 1960, Low Power Broadcasting, or LPB was formed to serve the college radio market, building transmitters, power line couplers and other equipment for school broadcasting programs. They had a reputation for solid, high quality equipment for about 40 years, and virtually locked up the college market.
This is a review of their TCU-30 Transmitter Coupling Unit (click on any photo to view it larger)
Here's the mean looking 3 phase power cord. Note that this is for coupling the radio signal into the power grid, not to power the unit as it might imply! Typically the TCU-30 coupler unit, along with the transmitter, is mounted on a wall or in an equipment rack close to the power distribution panel for the building. This cord comes out of the bottom of the TCU, where it's plugged into a convenient 3 phase power receptacle, feeding all three phases, so that different parts of the building can be reached by the broadcast signal.
SEMO is Southeast Missouri State University. So far I haven't been able to find any history on what AM broadcasting program they might have had, but the school's low power FM signal had been struggling to stay on the air. Rage FM, KDMC-LP went off air in 2014, SEMO surrendering the license to the FCC after 12 years of operation.
The RF input connector, SO-239, receives the radio signal from the transmitter by coaxial cable. Will accept a signal from any AM band transmitter from 1 to 30 watts, with a 50 ohm output. Below 1 watt you might have trouble calibrating on the meter for best output, but it will still couple even a tenth of a watt signal to the line.
Operating instructions sheet attached to the inside of the coupler's lid so it isn't lost, though some of these sheets will come unglued and flop down inside of the case.
Cover off, view inside. There's not much in the way of open wiring; a double sided glass fiber circuit board handles all connections between the input cable and signal output to the lines.
Closer view of the main board, pretty clean, no smoke or dust, so the unit was probably not operated in a boiler room like some of them were. Knobs are clean, even in the milled edges, so the coupler probably hasn't been used a whole lot, or it was adjusted and left that way.
Multi-meter for combination of output wattage and VSWR. VSWR is Voltage Standing Wave Ratio, a measure of how much signal power is being reflected back from the power line it's connected to, and hence how much is getting out. Going by the chart, you adjust for a dip to the lowest VSWR, hopefully getting the meter into the green area. With careful tuning it might even be possible to reach the bottom of the scale, indicating that most of the power is getting into the line, and very little is being reflected. Guitarists might be drooling over those orange capacitors to the left of the meter! Those are the coupling caps to allow the RF to pass to the three phases of the line, while blocking most of the 60 hz AC voltage from feeding back into the tuner. The caps are rated at .1 microfarad at 1000 volts.
Impedance selection toroid, a ferrite donut core wrapped with coils of wire having many taps, which are selected by the dial switch above to match the RF impedance of the power line, which varies widely, and is often pretty low, hence the selections for 72 ohms down to 1.1 ohms.
Capacitance decade, in series with the signal to the line and used to tune out the reactance, or resistance of the line to the radio signal. Dipped mica capacitors are in a matrix that can be adjusted in fine steps for more or less capacitance.
5 wire multifilar coupling toroid
TCU-30 coupler's specification sheet. It can handle up to 30 watts!
LPB equipment like this coupler is usually found on the used/recycler market after schools close their broadcasting programs. Since LPB equipment was engineered well and carefully built, many of these units are still in good condition or can be repaired easily to help you to get on the air. There are guides available to help you understand how to hook it up, rules to go by and how to stay safe while using it, since after all you're connecting to AC lines.