Monthly Archives: August 2020

1970 Carrier-Current Remote Speaker

Fifty years ago this month, the August 1970 issue of Popular Mechanics showed how to put together this somewhat ambitious project, a remote carrier-current speaker for your hi fi. A one-transistor transmitter was hooked to the output of your amplifier, and the receiver could be plugged in to any outlet in the house to listen remotely. The project also gave you an easy method of having an outdoor speaker, as long as you had an outdoor outlet. If desired, you could construct additional receivers for use in other rooms, all tuned in to the same transmitter. The speakers were equipped with individual volume controls.

To simplify the project, the receiver used a pre-wired audio amplifier available from Lafayette Electronics. The electronics all mounted in a small box that could be affixed to the back of the speaker.

Both transmitter and receiver used a standard 455 kHz IF transformer for their tuned circuit. These were tweaked to get them down to about 400 kHz, the frequency used by the system. That signal was transmitted through the power wiring. This “antenna” was coupled to the circuit on both ends through capacitors.

Transmitter schematic.

Receiver schematic.

Emergency Hospital Antennas, 1945

I was a little unclear on the concept when I first saw this illustration in the August 1945 issue of Popular Mechanics, and I suspect that whoever wrote the caption might have been similarly confused.

The caption is “emergency antenna for individual hospital patients,” and is then followed by some details for each of the illustrations. I believe that what they really mean to say was that these were four different ideas for emergency antennas.

I guess I was confused because of the picture on the left. It appears that these wires are to be attached to the antenna terminal of the radio. I was confused because a similar idea, wrapping the radio with wire, can be used to connect an antenna to an external antenna, even though the set doesn’t have an antenna connection. The loop of wire couples inductively to the internal antenna coil. But in this case, the coil of wire apparently is the antenna.

Figure 2 is simply a loop of wire wound around a piece of cardboard and, again, hooked to the radio’s antenna connection. Figure 1 is the same antenna, but cleverly concealed behind a photograph. Figure 4 looks like a lot of work, since it consists of a spider web coil hidden inside the speaker.

It seems to me that a more suitable solution would be to toss a length of wire out the window when the nurse wasn’t looking, although she seems to be very watchful.

19th Amendment

Bemidji (MN) Daily Pioneer, Aug. 18, 1920.

One hundred years ago today, August 18, 1920, American women got the right to vote, with the ratification of the 19th Amendment to the U.S. Constitution. Tennessee was the 36th of the 48 states to ratify the amendment, and as the headline here notes, it happened in time for the 1920 presidential election.

Popular Mechanics Finds Its Way to Communist China

Our readers enjoy perusing old issues of Popular Mechanics and other magazines, so we understand the excitement of U.S. Marine M/Sgt. Elmer C. Bender in finding the magazine’s August 1947 issue. The flyer, stationed at Tsingtao, China, took off the morning of October 1948 to get in some hours in a training flight.

Due to a faulty fuel gauge, he and Navy Electrician’s Mate William C. Smith were forced to land on a beach only 15 miles from their base, where they were quickly surrounded by Chinese Communists who surmised that they were spies. They were held for over a year until their eventual release.

The two men reported that they weren’t mistreated, but were taken from village to village. Their main problem was boredom, but the Lieutenant charged with their custody would occasionally bring English reading material. The highlight of their stay was when he showed up with the magazine, which both men devoured. Bender reports, in the magazine’s August 1950 issue, that he was especially interested in the issue’s feature on how to solder. When the men were eventually released, the Communist Lieutenant, noting their enthusiasm for the magazine, asked them to send him some more copies upon their return to the U.S.

While there’s no evidence that either man became a ham, it’s likely that they passed many hours reading about the 5-band AM-CW transmitter shown here. When their families got an FM radio, they probably both instinctively knew the trick of placing a folded dipole antenna under a carpet, as shown in the magazine.

Drive-In Educational Programs and Meetings for COVID-19

I’ve previously written about why I believe outdoor spaces should be utilized for learning and other group activities during the COVID-19 emergency. Since I’m in the business of providing continuing education programs for attorneys, a few weeks ago, I presented an outdoor program at a city park.

The weather for that program was excellent, and everyone enjoyed the opportunity to be outside. However, the weather can be unpredictable, and it won’t always work to be outside. For that reason, today, I presented a live drive-in program in a parking lot. The attendees watched from their car, and listened on their car radio. If there had been questions or feedback, they could have phoned me at any time.

Why a Drive-In Program?

It was a bit more work, and it would actually have been more pleasant to just get out of our cars and do it outside, as with the previous program. But a drive-in program is a very viable option for any group that needs to be meet in person, since the weather won’t always cooperate completely.

I didn’t bring it today, because the weather was nice, but if there had been a chance of rain, I would have brought a canopy such as the one shown here. For an outdoor presentation of any kind, the speaker obviously needs to dress for the weather. But the attendees can take advantage of their car, or even run the car for heat or air conditioning if needed.

My transmitter was the InfOspot Talking House transmitter, which I previously reviewed. As expected, my transmitter easily covered the parking lot. I parked near a tree, and tied the end of the 10 foot antenna to a convenient branch. I should have tested everything, since the audio quality wasn’t quite as good as I had expected. In most of my tests at home, I fed the transmitter with an MP3 player, and the audio quality was absolutely astounding. Today’s audio quality was quite good–certainly good enough for a lecture. But if the transmitter was used for any kind of musical presentation, it wouldn’t have been quite good enough. I’ll need to experiment with a different mike, or possibly run the existing mike through a preamp and feed the auxiliary input rather than the mike input. (The audio quality through the line-in jack is excellent, so if you’re broadcasting from a location where another PA system is in use, that’s the best option, but you will need an inexpensive isolation transformer to prevent 60-cycle hum.)

Caveat Emptor: The Importance of a Legal Transmitter

When buying a transmitter, it’s important to buy a transmitter that is legal. Unfortunately, most of the transmitters in use for drive-in events these days don’t appear to be legal. In particular, it would be very difficult to use a legal FM transmitter for an event such as this, because FCC rules (section 15.239) limit the transmitted signal to an extremely low level. Amazon and eBay are full of FM transmitters that claim to be legal for unlicensed use, but even a cursory examination of their specifications reveal that their power level is many times what is allowed. On the other hand, the rules governing AM transmitters (section 15.219) are much less restrictive. The Talking House transmitter is certified as complying with this section of the rules, and has a very good range for this type of use. It is, therefore, the transmitter that I recommend for this application. Chances are, nobody will complain to the FCC. But if they do, and especially if there is interference with aircraft communications, which are near the FM band, the fines are typically in the range of $10,000. It’s best to use a transmitter that you know is legal, and the Talking House is.

Here’s another writer who agrees with me that most FM transmitters used for this purpose are illegal. As he puts it, they are all lying when they say on eBay or Amazon that they are legal. And in my experience, as both a lawyer and a licensed radio engineer, he’s right. Some of the listings claim that the transmitters are FCC certified. In some cases, the certification appears to be for use by a licensed radio station. In one case, the transmitter appeared to be certified for unlicensed use. I checked the FCC website, and sure enough, there was a report from a Chinese lab certifying that the transmitter was compliant. The report even contained test results showing compliance with some of the requirements. But noticeably absent was any report showing that the transmitted field strength was compliant. It couldn’t have been, because the transmitter was using hundreds or thousands more times power than allowed. It appears than an overworked FCC simply failed to notice that this critical measurement was missing from the report. Let the buyer beware. Just because an eBay listing says that a product is legal doesn’t mean it’s legal.

A Good Way to Conduct In Person Meetings

As with my previous program, we could have just as easily done it over the phone, or via Zoom. But some activities are best done in person, and after a long quarantine where everything has been done remotely, it’s refreshing to interact with others in person, even if they are six feet away and/or on the other side of a windshield. Creatively making use of outdoor locations, whether it’s for business, education, or worship will serve an important role until things get back to normal. And history tells us that yes, eventually, things will get back to normal.

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V-J Day

Today is the 75th anniversary of the end or World War II.

After the atomic bombings of Hiroshima and Nagasaki on August 6 and 9, 1945, surrender quickly followed. The Japanese communicated their intention to surrender on August 15 Japan time, which was the evening of August 14 in the United States. While the official U.S. date for V-J day was the date of the surrender in Tokyo Harbor, September 2, V-J Day was celebrated in the United States the night of August 14 and August 15. The newspaper shown here is the Milwaukee Journal, August 15, 1940.

US Navy photo, via Wikipedia.

1940 Lafayette One Tube Kit

Eighty years ago, the beginner couldn’t go wrong with this one-tube radio kit from the 1940 Lafayette catalog. For $3.95 (plus tube, batteries, and headphones) he or she could put together this set and pull in broadcast stations, both local and distant. And for only 98 cents more, they could buy coils to cover four shortwave bands from 16 to 200 meters.

A Preview of Postwar Television

When this issue of National Radio News, August-September 1945, went to press, the war was still raging in the Pacific. But the end was in sight, and despite the cynicism of skeptics, the editors of the magazine knew that television’s role in the postwar electronic era was a foregone conclusion, “witness the rapt attention of these comely lasses as they view the screen of the General Electric receiver.”

1940 Toy Submarine

If Junior is looking for a spectacular science fair project that can be built at little expense, but still uses a slightly dangerous chemical, he can’t go wrong building this real submarine, according to plans contained in the August 1940 issue of Popular Science.

When released in the local pond, the submarine repeatedly submerges and surfaces, all the time moving forward in the water. The secret of all of this is a small amount of calcium carbide, the same chemical used in old-fashioned miner’s lamps. When the chemical is exposed to water, it generates a high pressure gas, which is used to propel the vessel and provide buoyancy. The chemical is readily available at Amazon, and probably at your friendly local hardware store.

School will eventually reopen, so your young submariners can take advantage of their summer vacation to build and test the craft, which can be made out of scraps of sheet metal and tin cans. Soldering is required, but kids naturally enjoy working with molten lead. When school reopens and the science fair is on, the submarine can be used for a variety of scientific experiments involving buoyancy, propulsion, or chemical reactions. The teacher has probably seen old movies with miners wearing headlamps, but this will probably be their first exposure to the actual chemical used. A blue ribbon is almost guaranteed.

1950 Five Tube Superhet

These lucky youngsters have probably been drawing Social Security for about a decade, but in 1950, they were tuning in a favorite program on this attractive five-tube radio put together by their dad and big brother. The internal circuitry was a typical “All American Five” superhet, with a tube lineup of 12SA7, 12SK7, 12SQ7, 50L6, and 35Z5 rectifier. Since this was an AC-DC set, the article cautioned that the screws holding the chassis to the wooden case should be covered up for safety.

The cabinet is designed to resemble a circus wagon. The front is carefully painted with red and white enamel paint, with wooden dowels making the cage. Then, after the paint has dried, animal decals are applied.

The plans appeared in the August 1950 issue of Popular Mechanics.

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