Categorie: Hackaday

When I started working in a video production house in the early 1980s, it quickly became apparent that there was a lot of snobbery in terms of equipment. These were the days when the home video market was taking off; the Format War had been fought and won by VHS, and consumer-grade VCRs were flying off the shelves and into living rooms. Most of that gear was cheap stuff, built to a price point and destined to fail sooner rather than later, like most consumer gear. In our shop, surrounded by our Ikegami cameras and Sony 3/4″ tape decks, we derided this equipment as “ReggieVision” gear. We were young.

For me, one thing that set pro gear apart from the consumer stuff was the type of connectors it had on the back panel. If a VCR had only the bog-standard F-connectors like those found on cable TV boxes along with RCA jacks for video in and out, I knew it was junk. To impress me, it had to have BNC connectors; that was the hallmark of pro-grade gear.

I may have been snooty, but I wasn’t really wrong. A look at coaxial connectors in general and the design decisions that went into the now-familiar BNC connector offers some insight into why my snobbery was at least partially justified.

Keeping the Impedance

The connector that would eventually become known as the BNC connector when it was invented in the 1950s has its roots in two separate connectors developed in the 1930s and 1940s for the burgeoning radio and telephone industries. When it comes to wires and connectors for DC and low-frequency AC circuits, pretty much anything that will carry the current and provide a firm mechanical connection will do. But once a circuit is into the radio frequency range it’s a different story. At such frequencies coaxial cable is preferred for transmission line, and any connectors inserted into the line need to be engineered to minimize changes in impedance, which could cause reflection of the signal and generate standing waves that can cause damage.

Paul Neil, an electrical engineer who had been with the Bell Company since 1916, was well versed in RF systems. In the 1940s he identified a need for a coaxial connector capable of working well at microwave frequencies and designed the Type N connector. Like all coaxial connectors, it was designed to present as little change in the characteristic impedance of the feedline as possible by keeping the spacing between the center conductor connection and the outer shell as close to the feedline dimensions as possible. Neil’s connector had a female threaded outer shell on the plug that mated with male threads on the matching socket, and was designed to be weatherproof. The N connector took its name from Neil’s last initial and is still in use to this day.

Meanwhile, an engineer at Amphenol Corporation was working on his own design. Carl Concelman’s connector, similarly dubbed the Type C connector, used the same approach to reduce impedance changes in coaxial connections. However, he chose to make his connector quick-disconnect; rather than tediously screwing and unscrewing the outer shell, the C connector had a bayonet connection. The outer shell of the socket had lugs diametrically opposed on its outer surface. These lugs would mate with the long arm of L-shaped grooves machined into the inner surface of the outer shell on the plug. The shell would be rotated to move the lugs into the short arm of the groove, locking the two connectors together mechanically and electrically.385

Best of Both Worlds

Both the N and the C connectors enjoyed success in the marketplace, but neither was ideal. The N connector was slimmer in profile than the C but had all that pesky threading and unthreading to deal with; the C connector has that nice quick-disconnect but was bulky. In addition, neither connector was particularly easy to manufacture as each required some fairly fancy machining. With those shortcomings in mind, an engineer at the Hazeltine Electronics Corporation named Octavio Salati came up with his own design. It would have the bayonet locking feature of the C connector and the slimmer profile of the N connector. It used the same techniques as both connectors to minimize reflections due to inline impedance changes.

Salati’s connector was patented in 1951 with the unexciting name “Electrical Connector.” Unlike its predecessors, it would not be dubbed the “S-Connector” but, in a gentlemanly gesture, it was called the BNC, for “Bayonet Neil-Conselman.” To support the RF work for which it was originally designed, the connector had a 50-ohm characteristic impedance; later, a 75-ohm version was made for the television industry. The connector is usable up to around 11 GHz, although it’s not ideal past 4 GHz or so owing to the slots cut into the conductor for the outer shield, which start to radiate signals.

The BNC connector has seen widespread acceptance as a coaxial connector in industries far beyond its original target markets. From public service communications to scope probes to computer networking, Salati’s design, and by extension both Neil’s and Conselman’s, has delivered solid performance for the past sixty years.

Ham radio operators bouncing signals off the moon have become old hat. But a ham radio transmitter on the Chinese Longjiang-2 satellite is orbiting the moon and has sent back pictures of the Earth and the dark side of the moon. The transceiver’s main purpose is to allow hams to downlink telemetry and relay messages via lunar orbit.

While the photo was received by the Dwingeloo radio telescope, reports are that other hams also picked up the signal. The entire affair has drawn in hams around the world. Some of the communications use a modulation scheme devised by [Joe Taylor, K1JT] who also happens to be a recipient of a Nobel prize for his work with pulsars. The Dwingeloo telescope has several ham radio operators including [PA3FXB] and [PE1CHQ].

You can find technical particulars about the satellite on its web page. There are also GNU Radio receivers and information about tracking. If you want to listen in, you’ll need some gear, but it looks very doable. The same page details several successful ham radio stations including those from [PY2SDR], [CD3NDC], [PY4ZBZ], [N6RFM], and many others. While the Dwingeloo telescope is a 25-meter dish, most of the stations have more conventional looking Yagi or helical antennas.

If your Mandarin is up to it, there is live telemetry on that page, too. You might have more luck with the pictures.

For working conventional satellites, you often need an agile antenna. We suspect the lunar orbiting satellite appears to move less, but you’ll have other problems with more noise and weak signals. Although hams have been bouncing signals off the moon for decades, they’ve only recently started bouncing them off airplanes.

AI is currently popular, so [Chirs Lam] figured he’d stimulate some interest in amateur radio by using it to pull call signs from radio signals processed using SDR. As you’ll see, the AI did just okay so [Chris] augmented it with an algorithm invented for gene sequencing.

His experiment was simple enough. He picked up a Baofeng handheld radio transceiver to transmit messages containing a call sign and some speech. He then used a 0.5 meter antenna to receive it and a little connecting hardware and a NooElec SDR dongle to get it into his laptop. There he used SDRSharp to process the messages and output a WAV file. He then passed that on to the AI, Google’s Cloud Speech-to-Text service, to convert it to text.

Despite speaking his words one at a time and making an effort to pronounce them clearly, the result wasn’t great. In his example, only the first two words of the call sign and actual message were correct. Perhaps if the AI had been trained on actual off-air conversations with background noise, it would have been done better. It’s not quite the same issue, but we’re reminded of those MIT researchers who fooled Google’s Inception image recognizer into thinking that a turtle was a gun.

Rather than train his own AI, [Chris’s] clever solution was to turn to the Smith-Waterman algorithm. This is the same algorithm used for finding similar nucleic acid sequences when analyzing genes. It allowed him to use a list of correct call signs to find the best match for what the AI did come up with. As you can see in the video below, it got the call signs right.

With any hobby, it’s easy for things to get out of hand. Equipment can get scattered around the house, chargers lost in the car while cables languish in the shed… but it doesn’t have to be this way. With a go-bag or go-box, everything required is kept together in a ready-to-go condition. Heading out for a day of filming? Grab the go-bag and you’re all set. [oliverkrystal] wanted to apply this to a ham radio setup, and built this ham shack-in-a-box.

Wanting to use proven components and keep things rugged and usable, the build starts with a 6U-sized plastic rack mount case. This saves weight over plywood versions and is nice and tough. A combination of off-the-shelf rack mount parts and 3D printed pieces are brought together to make it all happen. [oliverkrystal]’s printed cable organisers are a particular treat, and something we think could help a lot of builds out there.

It all comes together as an impressive self-contained unit with two radios, an antenna tuner, in-built illumination and other useful features. No longer does one have to scramble around preparing gear for the weekend’s hamventures – grab the box and you’re ready to go!

Perhaps you don’t have a lot of ham gear, though? Try this setup to get going for less than $100.

Chances are you have at least one radio that can receive FM stations. Even though FM is becoming less used now with Internet and satellite options, it still is more popular than the older AM radio bands. FM was the brainchild of an inventor you may have heard of — Edwin Armstrong — but you probably don’t know the whole story. It could make a sort of radio-themed soap opera. It is a story of innovation, but also a story of personal vanity, corporate greed, stubbornness, marital problems, and even suicide. The only thing missing is a long-lost identical twin sibling to turn it into a full telenovela.

Early Days

Armstrong grew up in New York and because of an illness that gave him a tic and caused him to be homeschooled, he was somewhat of a loner. He threw himself into his interest in electric and mechanical devices. By 1909 he was enrolled in Columbia University where professors noted he was very focused on what interested him but indifferent to other studies. He was also known as someone more interested in practical results than theory. He received an electrical engineering degree in 1913.

Unlike a lot of college graduates, Armstrong didn’t go work for a big firm. Instead, he set up a self-financed independent lab at Columbia. This sounded good because it meant that he would own the patents on anything invented there. But it would turn out to be a two-edged sword.

Tubes and Villians

Every good story needs a villain or two, and this one is no exception. The first one is Lee de Forest, the man who invented the triode. History hasn’t painted de Forest kindly, and some of the reasons for that are because of his interactions with Armstrong. However, there’s more than that.

Technically, Thomas Edison invented the vacuum tube as an offshoot of experimenting with light bulbs. He knew electrons were streaming away from the filament and put an electrode in — what we would call a plate — to collect them. He didn’t have any real idea what to do with the device, though.

In 1904 John Fleming realized that the device operated like a check valve allowing current to flow in one direction but not the other and demonstrated using it as a rectifier. This is why people in some parts of the world call tubes Fleming valves.

What de Forest added to the mix was to put a grid between the filament and the plate. He was actually trying to build a radio detector that used ionized gasses and filed a patent on a two-terminal device in 1907. The grid was initially on the outside of the glass tube, which didn’t work well. Once it was moved inside the tube, it allowed a small signal on the grid to be amplified at the plate. De Forest called this tube the audion. There were a few reasons it didn’t work very well, not the least of which is that de Forest erroneously thought that having a little gas left in the tube was essential for its operation. We know now, you want less gas, not more.

This all fits in with the historical accounts that de Forest didn’t fully understand the tube. He was just trying different things to see what would work — not always a bad thing, especially in those days where others worked with a similar methodology. He even reportedly said:

I have arrived, as yet, at no completely satisfactory theory as to the exact means by which the high-frequency oscillations affect so markedly the behavior of an ionized gas.

If he was just a practical inventor, that wouldn’t make him a villain, though. However, when Marconi, who held the Fleming patent, sued that the audion infringed, de Forest took the position that the two devices were completely different. Of course, they were not. A court sided with Marconi although since the grid was a patentable improvement, so the two sides agreed to exchange rights.

We think of protracted court battles over intellectual property as a modern problem. Perhaps it is true that the more things change, the more they stay the same.

Back to Armstrong

Armstrong grew up experimenting with gassy low-quality audions. He was determined to understand how the device worked in a scientific way. While at Columbia he did comprehensive studies and found that using positive feedback could create much higher amplification — enough to drive speakers instead of headphones. This is the basis behind a regenerative receiver. The signal is amplified many times over getting stronger each time. In addition, Armstrong learned that if you increase the feedback, you get sustained oscillations. This would be a huge breakthrough for radio to have a reliable way to generate radio waves electronically.

Armstrong filed for a patent in 1913. Lee de Forest predictably discounted Armstrong’s work for a few years. Then in a surprise move in 1915 he filed patents for the same inventions claiming he had priority because of a lab notebook he had dated in 1912. World War I intervened, however, so things moved slowly.

Regenerative receivers were sold until another Armstrong invention would replace them. Regens are still popular with hackers because they generally have a very low parts count. If you want to learn more about how they work, check out Stan’s video analysis of one based on a FET which isn’t very different from a tube.

War Time

During the war, Armstrong also developed the superheterodyne receiver: a common architecture even today where a frequency of interest is converted to a single intermediate frequency for amplification and filtering prior to detection.

By 1919 Armstrong was in court on two fronts on the de Forest patents. To finance his legal fees, he had licensed several companies to make regenerative receivers for amateurs and experimentation. He was also shopping for a big corporation to buy the rights. Westinghouse wound up with both the regenerative and superheterodyne patents. By 1928, the courts would actually decide a Frenchmen named Lévy invented the superheterodyne first.

The Regenerative Patent

The legal front on regeneration was quite different. Both the court and the patent office decided that de Forest’s patents were not valid. However, Armstrong didn’t want to settle for the compensation offered by de Forest. This allowed de Forest to appeal the case, which he eventually won through two further appeals up to the Supreme Court.

This move shocked most people in the radio business at the time. Armstrong attempted to return an award he received from the IRE (the Institute of Radio Engineers) and the institute refused to accept the return, publically stating they rejected the court’s findings.

Although Armstrong didn’t do well in court, he did have a little luck. While dealing with the legal end of things, he stumbled on an improvement to regeneration called super regeneration. That patent netted him $200,000 and 80,000 shares of RCA stock which made him the largest shareholder. Keep in mind, too, that $200,000 in 1922 was a fortune. RCA wound up never actually producing radios using this technology, as the superheterodyne turned out to be far superior.

Which Brings us to FM…

In case you forgot, all this was leading into the invention of FM radio. AM radio is very prone to noise and fading because these show up as changes in amplitude — the A in AM. During the 1920s, Armstrong was trying to think of ways to improve AM radio. FM — modulating frequency instead of amplitude — had been largely dismissed because of an incomplete analysis of FM done by John Carson showing that FM would not improve on the quality of AM.

By 1928, Armstrong started working with FM despite its detractors, and the key was using a wider bandwidth. Armstrong filed for patents in 1933. RCA had the right of first refusal on his patents by this time, but they were unimpressed with a system that was complex and was not compatible with existing equipment.

Armstrong went to smaller radio companies like General Electric and Zenith. He also got the FCC to allocate a band for this new kind of radio with 40 channels in the 42 to 50 MHz range. You might notice that this isn’t where the FM band is today. That will play a part in the story to come. There’s a lot of pictures of old FM radios, for this band online. Oddly enough, this band displaced another attempt to do “better” radio called Apex radio — a topic we will cover in the near future.

The Million Dollar Question

At first, RCA saw FM as a threat to their existing businesses and did everything they could to prevent Armstrong from demonstrating the system to the public. Despite this, Armstrong did get the FCC interested in FM and even built his own FM station W2XMN to help get things moving.

The first broadcast was in 1939. There were only 25 FM receivers in the world at that time, so the audience wasn’t very large.

RCA finally wanted to get into the FM game, but they didn’t want to pay Armstrong royalties. In 1940, they offered him a cool million dollars for a non-exclusive but royalty-free license. Armstrong didn’t feel like it was fair to other companies that were paying 2% on their sales. He refused and this would become a fateful and ultimately pointless decision.

To the right, you can see a magazine cover from 1940. The picture shows a million volt arc that totally ruins AM reception but didn’t interfere at all with the FM radio.

Band Adjustment

Because of World War II, there were comparatively few FM receivers and stations in service on the new frequency band. I say comparatively because ultimately there would be nearly 400,000 receivers in service compared with millions of AM radios.

Signals around 50 MHz are subject to propagation effects that can cause interference. RCA lobbied fiercely to move the FM band and Armstrong vigorously countered it. In his opinion, RCA only wanted to disrupt the existing base of FM stations and receivers, perhaps because he wasn’t willing to take their million dollar offer.

Since you know the current FM band is from 88 to 108 MHz, you can probably guess which side won in 1945. Still, Armstrong was convinced that FM was the future and even hired a public relations firm to spread the word about FM’s superiority.

RCA would eventually develop what they claimed were non-infringing FM patents and even encouraged other companies to stop paying royalties to Armstrong. He sued, but RCA was able to tie the case up for years.

The Bitter End

The two obvious villains in this story were de Forest and David Sarnoff of RCA. However, there’s a third villain: the courts. Being constantly embroiled in legal battles with a giant company takes its toll on your pocketbook and on your mental health.

Facing bankruptcy, Armstrong approached his wife Marion (who had been, by the way, David Sarnoff’s secretary) about returning money he had given her to put aside for their retirement. She refused and in 1954 he took a swing at her with a fire poker. Unsurprisingly, she left him.

Armstrong lived in an apartment on the 13th floor of the New York River House. With his wife gone and three servants done for the day, Armstrong removed an air conditioning unit, put on a nice suit, a hat, overcoat, and gloves. Then after writing a two-page note, he walked out the window, plummeting to his death on a third-floor balcony. The New York Times reported that he was heartbroken over the loss of his wife and regretted hurting her.

It is ironic that Armstrong turned down the million dollars. After his death, Marion settled with RCA for — what else — a million dollars. She also pursued other court cases, defending his patents and receiving infringement awards from other manufacturers. FM would really take off after General Electric added stereo to FM in the late 1950s.

A sad end to a prolific inventor that created a lot of technology we still use today. It is hard to say for sure if the villains in a story like this were really as bad as they appear or just unable to present their side of the story. On the other hand, history is written by the victors and Armstrong certainly wasn’t the victor. That’s got to mean something.

As I was writing this, though, one thing did strike me. Most of the world — including the United States — has gone to a patent system where “first to file” gets priority. I’ve always thought that is bad for us hackers because we are less likely to quickly file patents and, thus, more likely to get knocked out by a big company spewing out dozens of patent disclosures a day. But this is a case where first to file might have totally changed Armstrong’s life for the better. It also reminds me that even though most of us don’t file patents often, maybe we should think about it. Maybe big companies are going to control all the upcoming innovations because — unlike Armstrong — we are letting them.

A gimmick capacitor is made by taking two strands of insulated wire and twisting them together; the length and the tightness of the twist determine the capacitance. Tightening or loosening the twist, or trimming some of the wire off, makes it tunable.

These are most commonly found in RF equipment or high-speed logic because of the small capacitance involved — usually about 1 to 2 pF per inch of twist or so. The thicker the insulation, the less capacitance you’ll get, so it is common to use magnet wire or something else with a thin insulating layer. You can take this one step further and decrease the spacing by stripping down one wire as long as it isn’t going to touch anything else.

Obviously, the insulation needs to be good enough for the voltage on them, an important consideration in tube circuits, for instance. But other than that, a gimmick capacitor is a straightforward tool to have in your box of design tricks. Can we take this further?

PC Board Gimmicks

You might wonder if the technique can be applied to PC boards. The answer is yes — sort of. Unless you use very thin boards, or thin layers in multilayer boards, it takes a lot of board real estate to get even a small capacitance. Also, typical PCB material can change over time with moisture or other effects. Practically, unless you use special board material and thicknesses, it isn’t very useful. There has been work on laying out linear capacitors on IC substrates using fractals, but we aren’t sure how that would translate into a PCB layout. We’ve seen lots of other PC trace components like antennas, shunt resistors, inductors, and transmission lines.

You can see I made a gimmick just bigger than two inches. I then went looking for something around the lab that had the ability to measure such a small capacitor. The component tester couldn’t. I have a nice digital multimeter that has a special plug-in for measuring capacitors and thermocouples, but it wouldn’t reliably read anything under 25 pF. I was thinking about building up a circuit to test when I realized I should search Hackaday first.

Hackaday Saves the Day

[Jonathan’s] capacitance meter is just what I needed and I even threw it out to an Arduino that was already hooked up using the Arduino Create web interface, so that was easy. I actually used the newer “Mark II” code but it works the same for the low values I was measuring. I calibrated with a garden variety 10 pF ceramic. It probably isn’t that accurate, but I really only wanted to see the change more than the actual value, so I thought this was sufficient.

The two inch (call it 6 cm) gimmick reads about 5.5 pF. That might not be totally accurate, but I was expecting about 4.5 pF and the magnet wire insulation is quite thin, so it’s in the right ballpark. Let’s take it as a baseline to measure change. I then cut about 1.5 cm of the capacitor away — about 25% — and the reading became 3.7 pF. Another centimeter brought it down to 2.6 pF.

Of course, hand-wound pitch isn’t very accurate, nor were my cuts or measurements, but that works out to just around 1 pF per centimeter. Obviously, your results are going to depend on your winding and the kind of wire you use. [Harry Lythall] suggests folding a single piece of wire, holding it with pliers, and twisting. Then you cut the loop when you are done.

That’s a Wrap

It is easy to forget that any two conductors near each other will have capacitance. Another common makeshift capacitor is a length of coax with connections at one end and open at the other. RG-8, for example, is about 30 pF per foot of cable. There’s even an online calculator that will tell you how much coax you need for any given value. This varies by coax type, of course, so remember to cut a little long and trim!

The next time you need a small adjustable capacitor — especially in a lab setting — don’t forget about the gimmick. Be sure to experiment with different kinds of wire if you are trying for larger values. We’ve seen this trick used in RF filters. In the case of the gimmick, you may be thinking small, but when you are really looking for high voltage capacitors, you can make those, too.