Modern Mechanix, June 1939.

In 1928, WBBM in Chicago and KFAB, then in Lincoln, Nebraska, both operated on 770 kHz with 5000 watts.  And they both carried the CBS network at night.  They generally coexisted well, but there was a problem for listeners, mostly in Iowa, who were equidistant from the two stations.  Both stations would come in equally strong, but interfere with one another.  These listeners complained to CBS, and the two stations worked at solving the problem.

Eliminating any heterodyne (the squeal caused by two signals on very close frequencies) was an easy enough problem to solve.  The two stations simply needed to make sure that the transmitters were exactly on the same frequency.  But there was another problem.  The signals from the network came by telephone lines, and those signals travel at approximately the speed of light.   Since Lincoln was 500 miles further away from New York than Chicago was, the program reached Lincoln about 23 milliseconds later than it reached Chicago.  Therefore, the two stations weren’t transmitting the exact same program.  KFAB was sending out the program with an additional 23 millisecond delay.  (When a different phone line was used later, the delay grew to 35 milliseconds.)

This caused a problem for listeners in Iowa.  The signals from Lincoln and Chicago traversed the airwaves to Iowa in the same amount of time.  But since the Lincoln signals started with a built-in delay, the effect in Iowa was that the there was an echo effect when listening to CBS on 770.

The stations solved the problem in a number of ways.  First of all, WBBM paid KFAB to sign off at 10:00 PM, after the end of network programs.   After 10:00, WBBM had a clear channel as far west as its signal would go.  The two stations also coordinated their station ID’s so that one announcer was not talking over the other.  But the biggest problem to solve was the delay.  To solve the problem, WBBM had to delay the network feed.  With digital processing today, this would be a trivial problem to solve.  But in 1928, it was a major engineering challenge.

The WBBM engineers eventually came up with an electronic solution involving 19 stages of filtering, equalization, and amplification.  A series of filters, consisting of a capacitor and inductor, were carefully chosen.  Each filter attenuated one frequency range, but also introduced a delay.  Since they didn’t want the attenuation, the equalization was needed to restore the audio to its final form, and the amplification was needed to make up for the loss in the filters and equalizers.

But until that system was designed, WBBM engineers came up with a Rube Goldberg solution that worked amazingly well.  The speed of light is about 300 million meters per second.  But the speed of sound is about 1080 feet per second.  To generate the necessary 23 millisecond delay, sound would need to travel about 23 feet.  So the WBBM engineers procured a 23-foot section of lead sewer pipe, mounted a speaker at one end and a microphone at the other end.  The sound was simply fed through the pipe before going on the air.

The system wasn’t perfect, since echos from the microphone reflected back, adding a new echo effect, what they were trying to get rid of in the first place.  But this echo was solved by wadding fabric into the pipe.  Close to the microphone, this consisted of gauze.  Closer to the speaker, thicker material was needed.  Fabric from a pair of overalls belonging to one of the staff turned out to fit the bill, and they were stuffed into the pipe.

The result was a very high quality audio signal, with a dynamic range of 100-5000 cycles.  Eventually, broadcast standards called for slightly better audio, and the electronic system using filters was used.  But for a time, WBBM’s programming passed through 23 feet of sewer pipe before hitting the airwaves.

References

• C.B.S. Robot Timer, Radio News, June 1938.
• Modern Mechanix, June 1939.
• WBBM Radio Yesterday and Today.

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Eighty years ago this month, March 1935, Radio Craft magazine featured this one-tube broadcast radio that could be built for a dollar. The only manufactured radio part was the type 30 tube, which ate up 75 cents of the budget. Everything else was scrounged from household goods. The author reported receiving a station in Dallas, 1500 miles away from his location, the first night.

The two fixed condensers were made of tinfoil and waxed paper. The filament condenser consisted of 36 feet of 36 gauge wire wound on a spool. The grid leak condenser, which would probably be about 1 megohm, consisted of a pencil mark on a piece of wood. The tuning condenser consisted of two metal plates separated by celophane. The tuning coil was home wound on a cardboard form, and the tube socket was four paper clips.

The diagram of the completed receiver is shown below.

Fifty years ago this month, the cover of the March 1965 issue of Electronics Illustrated showed this integrated circuit, the Motorola MC556G. The case of this IC measured 5/16 inch in diameter, and the chip itself measured about 1/10 inch square. It contained six transistors and eight resistors. The accompanying article noted that it was now on the market at a price that hobbyists could afford to use for experimental projects,  $3.35.

To put the new device in perspective, the article compared it to the still ubiquitous 5-tube radio, which consisted of about six basic circuits using about 20-30 components. The article noted that the day would soon arrive when one or two IC’s would constitute a “complete radio that is equivalent in performance to that five-tube AC/DC job.”  That prediction came true only seven years later, in 1972 with the ZN414 AM radio IC from GEC-Plessey. The modern functional equivalent of that IC is the MK484/TA7642 am-Radio IC, which is a complete radio in a chip, requiring as its only external components a battery, coil, tuning capacitor, and earphone.

While the eight transistors in a 1/10 inch package was revolutionary at the time, transistors in current IC’s are in the range of tens of nanometers in size, allowing several billion transistors per chip. But building something with an IC was revolutionary fifty years ago, and Electronics Illustrated featured two projects making use of the MC356G. The first was a square-wave signal generator, and the second was the AM radio shown below. In this diagram, the portion shown in black is internal to the IC, and the components shown in red are external. As you can see, the circuit makes use of four of the chip’s eight transistors, and four of its resistors.

The IC was designed for use as a logic gate, but transistors are transistors, and they could be used for their amplification function. For the radio in particular, getting the circuit to work took several experimental designs, but the author finally “hit upon one that has a decent amount of sensitivity, selectivity, and audio output.” The author noted that most of the headaches in designing the radio were caused by the close proximity of the components on the chip. Having only 1/10 of an inch to work with presented leakage paths between the circuits that would be out of the ordinary for a radio designer.  The finished project is shown here.

In the design, two of the transistors are used as RF amplifiers, with the signal being fed back through a regeneration control. The second of those transistors also amplifies the audio, and there are two more audio amplifier stages. The actual detector consists of two external diodes.

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An interesting ionospheric effect was first noticed about 80 years ago, and reported 80 years ago this month in Radio Craft magazine, February 1935.  Radio Luxembourg operated on 252 kHz, with a powerful 150 kw signal designed to provide coverage in England.

The phenomenon was discovered in 1933 by B.D.H. Tellegen, in Eindhoven, Netherlands, who was listening to a station in Beromunster, Switzerland, on 652 kHz. In the background of the Swiss signal, he could hear the audio of Radio Luxembourg. This same phenomenon was reported by other listeners. Due to the distance between the three points involved, it could not be explained by the receiver being overloaded. The Luxembourg signal could be heard only when the Swiss station was transmitting.

Tellegen noted that the three points were in a straight line: When the signal from the Swiss station made its way to the Netherlands, it passed directly over Luxembourg. He correctly theorized that the carrier of the Swiss station’s signal was being modulated in the ionosphere as it passed through the strong signal of Radio Luxembourg in the ionosphere.

The ionosphere had only recently been discovered, and was not totally understood. It was previously supposed that the ionosphere was a linear medium, through which radio waves passively reflected. But the existence of the Luxembourg Effect showed that the ionosphere could be artificially “heated,” to produce non-linear effects.

Interestingly, the carrier frequency of the signal didn’t seem to be critical.  The modulation of the interfering signal was superimposed on the other signal without regard to the carrier frequency.  Subsequent research showed that most of the effect took place in the lower range of the audio frequencies.

Much to the dismay of conspiracy theorists, this is the phenomenon that the High Frequency Active Auroral Research Program (HAARP) was working with. It’s relatively easy to generate a strong radio signal in the High Frequency (HF) region. HAARP had transmitters that could generate 3.6 MW signals from 2.8-10 MHz and radiate them toward the ionosphere. This strong signal was able to generate the same kind of “heating” effects caused by Radio Luxembourg.

It’s more difficult to generate signals in the Extremely Low Frequency (ELF) region. Among other things, ELF signals are used to communicate with submarines. The main idea of HAARP was to generate these signals not in a transmitter, but in the ionosphere itself, by mixing two strong HF signals. For example, it would be practically impossible to generate a radio wave of 0.1 Hz with a transmitter. But by beaming two signals into the ionosphere, one at 4.000000 MHz, and one at 4.0000001 MHz, the result would be a radio wave, generated in the ionosphere, with a frequency of the difference, 0.0000001 MHz, or 0.1 Hz.

The phenomenon is sometimes called the Luxembourg-Gorky effect, since the powerful longwave transmitter at Gorky, USSR, produced similar effects.

References

• Probing the Northern Lights, Popular Mechanics, July 1999.
• PHD Dissertation by Juan Valentin Rodriguez, 1994, at page 5.
• Radio Science for the Radio Amateur by Eric Nichols, page 11-7

Radio parts were in short supply during the War, and radio enthusiasts had to make do with what they had. “H.T.,” a resident of Bothell, Washington, apparently had in his junk box a 1D8GT tube, and a low-impedance earphone, and wanted to know what he could do with them. So he wrote to the editors of Radio Craft magazine asking for a diagram of a receiver covering the broadcast band making use of the parts he had. He wanted to mount the earphone in the cabinet for use as a small speaker.

The editors indulged him and provided this diagram in the February 1945 issue. It was reprinted from the July 1940 issue, and showed how the combination diode-triode-pentode tube could be used in this circuit. The triode section of the tube was an RF amplifier, followed by the diode detector, with the pentode serving as an audio amplifier. Unfortunately for H.T., the low impedance earphone would need to be used in conjunction with an audio transformer. This set would drive a pair of high-impedance headphones, but to use it with his low-impedance earphone, it would need to be wired as shown for the speaker. So H.T. had to find himself either a set of hi-z headphones, or the output transformer, in addition to what he already owned.

The other hard-to-obtain part would be the variable capacitor. The circuit here shows a ganged condenser, but the response pointed out that two separate condensers would provide better results.

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Most hams who have been around a while have encountered the “Benton Harbor Lunchbox.”  This was a series of transceivers from Heathkit, and the most common were the HW-30 “Twoer,” which covered two meters, and the HW-29 “Sixer” for six meters.  Less common was the HW-19 “Tener” for, you guessed it, ten meters.

These were very popular in their day.  They were a single-band transceiver.  The transmitter put out about 5 watts of AM, and the receiver was superregenerative.  The tuning was very broad, but once they locked on to a signal, they were surprisingly sensitive.

By the time I became a ham in the 1970’s, VHF AM was virtually gone.  There was one six-meter AM net in the Twin Cities that hung on, and I was a regular check-in with my Sixer and later a Gonset Communicator.  But FM had taken over two meters by then, and Twoers were basically given away for practically nothing, even though they were often in pristine condition.  I owned many of these little rigs, and at one time I owned a complete collection.

My collection included the lesser-known cousin, the Model CB-1 CB transceiver shown here.  The CB model came out in about 1960, and is shown here in this ad in the February 1960 issue of Popular Electronics.

It sold in kit form for $42.95, and was also available wired for $60.95.  It featured one crystal-controlled channel (the crystal was included).  The receiver was the same superregenerative receiver used in the other Lunch Boxes, and was calibrated for channels 1-23.  It had a built-in power supply for 120 volts.  For mobile use, it used an external power supply, which consisted of a vibrator and transformer.  The power was supplied to an octal plug on the back (the same as the bottom of a tube).  The 120 volt power cord and the DC power supply had  octal sockets on them, along with appropriate jumpers.

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