Results tagged “broadcasting”

LPB TCU-30 carrier current coupler

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.


Who's Who in PA radio 1927

From the estate of Peter Granba's trash can comes this Who's Who In Radio publication, 1927 Pennsylvania Edition, created as an historical document.

In the Preface, W. E. Johnson writes,

"I have tried to make this as complete a history of every prominent radio person and organization in the state of Pennsylvania, as possible."

Who's Who In (Pennsylvania) Radio with 8CRK, 1927

8.5 x 11 sheets folded in half, thicker paper cover, 65 pages, illustrated with line-art drawings.

Sections include:

  • Broadcasting stations
  • Radio Artists and Orchestras
  • Radio Editors
  • Manufacturers and Jobbers
  • Radio Dealers
  • Amateur Operators

There are 34 Pennsylvania AM broadcasting stations listed, most running between 10 and 500 watts. All four of the Pittsburgh stations profiled in this book are still on the air and serving listeners' radios, which seems incredible 89 years later!

KDKA 970, now 1020 with a general talk format. They are a designated 'clear channel' broadcaster with 50 kilowatts.

KQV 1090, now 1410 and an All News format, which it's kept since the 1970s. Before that they broadcast rock and pop music. 500 watts then, 5,000 now.

WJAS 1090, now 1320. In 1926 it was sharing air time with KQV on 1090. Looking at their schedules, one station would broadcast for as little as a half hour, leave the air, then the other would sign on 30 minutes later, and they'd alternate throughout the day. Had a lengthy broadcast schedule during prime-time hours at night, 8-11 pm.

In the 1970s, WJAS jumped on the rock and roll bandwagon and became WKTQ, '13Q', with hot pop, contests and giveaways. By the early 1980s, most pop music and its younger listeners had moved over to the FM band, so the station got the WJAS call letters back, starting a 'Music Of Your Life' adult standards format, and picked up some of the veteran TV and radio personalities from around town, like Bill Cardille and Jack Bogut. In 2014 WJAS picked up a conservative talk format that had been dropped by an FM station. 500 watts then, 7,000 now.

WCAE 650, now 1250. Was WEAE, then WTAE with adult contemporary music and variety. It went to talk, and then Radio Disney as WDDZ, running AM HD radio for a while. 500 watts then, now 5000.

Most stations started with a live, variety format and local news, and weren't run by radio companies, but as a sideline or promotion for another business, such as a local store or hotel. A hotel could conveniently be used as a performance space for musicians and other acts that would then be broadcast.

Here's a complete scan Who's Who In Radio so you can take a journey down memory lane, thanks to Peter Granba's trash can! He's an Amateur Radio operator (ham) in this book, but I like the pride he feels for his own radio station, writing "Radio Station 8CRK, Ambridge Pa." in the front and back of the book, then signing his name.


Note, if the pdf file looks out of focus or pixelated in your browser, please download it and use the viewer on your computer, thanks.


Radio 1946

Radio - Design, Production Operation, a technical magazine from 1946

I've always liked collecting old radio books and magazines, from kid's science project books to Popular Electronics.


I enjoy reading the scans done by other radio freaks trying to preserve history, so here's a contribution from me, scanned several years ago. It's a trade magazine called Radio, with 56 pages, written for engineers and technical people. One could say it's like a 1940s version of Radio World.

I believe that this magazine came from the trash of Peter Granba here in Pittsburgh. In the 1980s there was a big curb side trash pile on a main road nearby, and the trove was so good that I asked my dad to drive me there so that I could pick up some of the radio-related items. Granba was apparently a Ham radio operator and electronics experimenter.

April 1946 contents include:

  • Cover: Western Electric's 'Clover-Leaf' FM broadcasting antenna
  • Inductive Tuned Loop Circuits
  • IF Amplifier Stability Factors
  • Electro-Mechanical Analogy In Acoustic Design
  • AM Transmitter Design
  • D-F For Static Pulses
  • Recent Radio Inventions
  • Bridged T And H Attenuators, Diode Conduction
  • Loss Due To Shunt Or Series Resistance
  • Capacitor Machine, Co-ax And TV, FM With Non-Linear L

In 1946, television and FM seemed to be well on their way, with the FM antenna tower on the cover, and high power FM, VHF, UHF and microwave tubes for airplane instrument landing being detailed. One advertisement is for the Eimac 3X2500A3 external anode tube, capable of 3500 watts output from 88-108 mhz, the modern FM band.

My favorite article is Modern AM Transmitter Design starting on page 30, which is about the latest 250 watt broadcast transmitter, as used in many communities across the country. It's fan-less, so it can be placed right in the studio, uses motorized final amplifier tuning, a high level plate modulator, and has a frequency response from 30 to 10,000 hertz.

250 watt AM band transmitter

Get the full draft quality but readable PDF scan here:



Nice packing comrade!

I'm on the way to building an AM broadcast band synthesized PLL oscillator. What that means is I'll be able to choose a frequency between 530 and 1700 khz, set some little switches, and the oscillator will put out a stable signal at the chosen frequency. This small signal will feed a power amplifier to boost the signal, with a modulator attached to give it sound, making a small broadcast transmitter, an advanced version of the Radio Shack AM kit that I had.

I started to order parts for this project last month, and I was surprised at how these chips came packed. These are CD4060 oscillator/divider ICs, and are CMOS chips, which should be kept away from static electricity, yet these are packed in what appears to be a plain poly bag! They're probably okay, with protection diodes on their inputs, but I'll put the tea kettle on and make sure to open the bags in the presence of steam, to kill any static that might be generated, then put them into anti-static bags or foam. It's the dead of winter here, very dry air and the worst time to be handling static-sensitive parts.



AM Radio Broadcasting Kit

With Radio Shack on the brink of RIP, I thought it would be a good idea to remember them by writing about one of my favorite kits.

I like to build small transmitter circuits for the AM broadcast band, something that started when I was 12 with a Science Fair AM Broadcaster kit, just like the one in this ad.


DJs were hot stuff on the radio; I liked listening to them and wanted to do it too! This was a pretty easy transmitter to build, the most important thing was to get the transistor leads into the correct holes, and wrap them on the springs on the underside of the kit's deck, which in its own way was a double sided circuit board.

The transmitter worked well enough, my 'city grade' coverage radius was the next room and down one floor, but it was fringe outside the house, using my pocket radio as field intensity meter. At this rate, I'd never be able to build a fanbase or any quarter hour listening, the signal didn't even reach to the neighbor's house! Ultimately I was able to reach several houses down the street, but that required higher level engineering, like wrapping the regulation green antenna wire that came with the kit around our rotary telephone, a trick I'd learned from building crystal receivers to get them to receive more stations, with the phone and its line acting as a longer antenna.

That kit was a lot of fun, and a great intro to broadcasting for a younger pup, but it suffered from a problem that all of the simple AM transmitter kits did then, poor frequency stability. The very same tuner knob system that let you dial in any open frequency in your area was its biggest shortcoming too, it would drift over onto other stations causing beat tones, and when sound was applied to the microphone, the circuit would produce both AM AND FM, since the audio was affecting the frequency too. Tuning across the signal with an analog dial radio, it was bright, loud and clear on each side, with a 'hole' right in the middle, where the sound was weak and distorted. I didn't understand that my transmitter was 'FMing', and that I was hearing it through 'slope detection', but with the split signal I thought I'd created my own version of AM stereo!

I built other oscillator transmitters like this, including a couple of tube circuits, by re-wiring the guts of old AM table radios, but all of them had problems with drift and FM, and didn't sound like other stations on the dial. I knew that I wanted something better, but didn't know how to get it. Later I found out about quartz crystals from things like walkie-talkies, but didn't know how to apply that idea to the AM broadcast band to get a transmitter to lock on frequency.

Next I want to get into circuits I'm trying today, for both full tuning across the AM band, and high stability at the same time, stay tuned!