Fox Hole Radio Pt. 2

I've written about foxhole radio before. But back them MAKE hadn't made a nifty video on how to make one yet. The instructions are simple, and even cover some basic problems that may come up in the process. The Tools and materials are very simple. If you are even reading this post, you will be able to do this.

You'll note he had a little trouble with the razor blade. The rectifier is a little unclear for me still though. The pencil lead is a mix of clay and graphite. Knowing that the clay isn't a part of it we're making a connection between graphite and steel. So in that model the razor blade is the whisker and the pencil is the crystal. It's sort of backwards to the traditional idea of a crystal detector.

Graphite electrodes were used in Tungar rectifiers, and mercury arc rectifiers in early car batteries. This was a diode of sorts, used to convert AC to DC. This was around the early 1920s. More recently graphite has been used in Schottky Diodes. More here.But I'm not sure why the razor has to be blue. You can steel blue by heating it (to 530 - 600 F) as in the video but what does that do to the steel? I have also read that you can blue steel with a solution of sodium thiosulphate and lead acetate. There is no way to know what method was used on that specific razor blade.  But any non-conductive coating could act as a dielectric. My thinking is that the heating here actually just burns off a lubricant (or other coating) that interferes with the connection.

Perikon detector

Strange as it may seem, the crystal in crystal radios wore out. The rectifier was a contact metal–semiconductor point-contact junction, aka a Schottky diode. I mentioned this once before so I wont get too much into the diode. But suffice it to say that not every part of the crystal would rectify the circuit. Rectification in this case means "act like a diode." More here.
Because the whisker was made of wire or a needle, and the crystal was softer, the surface was worn by use. OK hypothetically not every "crystal" was softer, but they were almost inherently of different harnesses. The very soft Graphite was used as a whisker sometimes. and at the other end, carborundum was considered durable. So either the whisker or the crystal was experiencing wear and usually it was the crystal. So hobbyist chose a crystal and whisker based both on it's sensitivity as a detector but also it's longevity. The hunt was on from the start for a "permanent detector."

Lets look at the hardness of some known whiskers and crystals:
WHISKERS
Graphite: 1-2 Mohs
Bornite: 3 Mohs
Iron: 4 Mohs
Steel: 7-8 Mohs
Bronze: 3 Mohs

CRYSTALS
Galena: 2.5 Mohs
Zincyte: 4 - 4.5 Mohs
Molybdenite: 1 - 1.5 Mohs
Iron Pyrite: 6-7 Mohs
Carborundum: 9 Mohs
Silicium: 7-8 Mohs
Chalcopyrite: 2.5 Mohs

As narrow as that variation is, remember that the Mohs scale is only a 10 point same topping out with diamond. not every whisker and crystal worked well together and even when they did, their mismatched harnesses could be problematic. Some combinations were proprietary and many of those were discovered by Mr. Greenleaf Whittier Pickard. He found both the Zincite /Bornite rectifier and the Zincite / rectifier. He called it the Perikon Detector.

He also tested versions with iron pyrite and silicon crystals. Silicon was novel enough he patented it's use in 1906. His Zinc Oxide detector was patented in 1909. He developed one with Molybdenite the same year. More here.

Walter Schottky invented everything

I actually started writing this post in 2006. But Walter Schottky invented so many things, and improved so many others, it's difficult to narrow the field to a few hundred words. Let me cover three that now bear his name: Schottky Effect, Schottky Barrier, and the Schottky Diode.

Schottky Effect: In 1935 he noticed that a vacancy in a crystal lattice results when an ion from that site is displaced to the crystal's surface, a type of lattice vacancy now known as the Schottky defect.

Schottky Barrier: A potential barrier formed at a metal-semiconductor junction which has rectifying characteristics, suitable for use as a diode. The largest differences between a Schottky barrier and a p-n junction are its typically lower junction voltage, and decreased (almost nonexistent) depletion width in the metal.

Schottky Diode: a special type of diode with a very low forward-voltage drop. When current flows through a diode, it has some internal resistance to that current flow, which causes a small voltage drop across the diode terminals. A normal diode has between 0.7-1.7 volt drops, while a Schottky diode voltage drop is between approximately 0.15-0.45 – this lower voltage drop translates into higher system efficiency.

Now the obligatory history: Schottky was born in 1886 in Switzerland, but grew up in Germany. His dad was a university mathematician in Berlin. He did an amazing ammount of work in physics and electronic theory. In college Schottky was taught by Max Planck. Yes that's the same Planck who championed Einstein. In short Schottky = smart mofo.

Schottky moved back and forth from commercial work to academics. Schottky's achievements can be divided into two phases: the first being research into vacuum electronics and the second, starting in 1929, covering semiconductor electronics. He invented the tetrode, ribbon loud speaker, ribbon microphone (with Erwin Gerlach) and the superheterodyne .

*Edwin Armstrong is typically credited with the invention of the superhet solo. But Schottky discovered the same principle on his own and developed his own in 1918. Even though it went unused and unnoticed.. he beat Armstrong by decades.

I've covered ribbon mics before and the superhet under Armstrong. But let's get into that tetrode. He invented the tetrode in 1919 when he was working at Siemens. It was the first multi-grid vacuum tube. A tetrode contains four electrodes, two grids -that's the primary grid and a second grid called a screen. The screen prevents the tube from generating ancillary oscillations. Tetrodes are what made VHF transmission practical. Plate-to-grid capacitance in a tetrode is lower, thereby reducing output stage instability. That includes all FM radio, and television channels 7 - 13.

He retired in 1958 and died in 1976 at the age of 90.

The power behind crystal radio sets

In the 1920s the U.S. Government actually distributed literature on making crystal radios. Soon crystal radio sets will no longer work. In February of next year analog TV will cease in favor of HD TV. It is expected by most parties that despite severe problems, radio will go the same way, both AM and FM if you side with Clearchannel's Jeff Littlejohn. Whenever that happens, if it happens, these simplest of tuners will die.

A crystal radio requires no direct power source. It is entirely powered by the radio waves it receives. This is the simplest kind of radio receiver. It's very difficult to name a single or most important inventor. But that didn't stop me from writing about many of them in the past. [Before you nit-pick I'll state for the record that this is the simplest of all radios.] I'll list the parts below but only one of them really needs additional explaining, the diode. I'll get into that in steps, but first lets' cover the radios components:

1. Antenna
*I've covered this here.

2. Coil
Loops of wire, usually copper. Tuning different frequencies is manages with fewer or additional turns of wire in the coil. Iit needs to be grounded. Coil + capacitor = resonant circuit.

3. Diode
This is where the crystal comes in. It's not a lump of quartz. The octahedral crystal is a lump of galena a.k.a. lead sulfide, an ionic compound of lead and sulfur. A wire contact is used to form a junction with the crystal until it forms a diode. The wire is called a "cat's whisker." I hate the name but that's the nomenclature. The wire is usually phosphor bronze. To tune, one literally had to probe the surface of the crystal until it generated sound confirming the complete circuit.

Note: There are also iron pyrite and silicon diodes but they are less sensitive. Modern crystal fans use germanium. The Schottky diode is a silicon diode. Walter Schottky's design was sound, and compensated for it's lack of sensitivity with greater efficiency. But suffered from thermal instability. In other words, it gets hot enough to break shit. See below.

More complicated was connecting the diode to both the coil and the earpiece. The normal heat of a soldering iron could damage the already irascible wad of galena. This was often instead soldered with wood's metal. Wood's metal is an alloy of bismuth, lead, tin, and cadmium. It has a melting point of under 160 °F. Still, it releases toxic fumes at that temp. This was really truly such a pain in the ass many people just improvised clamps.

4. Ground
You need to ground the circuit, this is usually between the antenna and coil.

5. Headphone
For a crystal set, these need to have high impedance.

The basic way it functions is that the antenna receives the signal. The coil stores the energy in it's magnetic field. Through it the wave energy moves to the diode. The diode rejects half of the alternating current converting it to pulsating DC current. This allows the headphone to work. If you fed AC current into an earpiece the alternating sides of the wave cancel each other out and you get an amplitude of zero. More here and here.

The Microphone Part 7

Despite improvements, in both carbon and condensner microphone design audiotechnicians in radio and recording wanted something better. With the popularity of the radio there was even a genuine cash market for the product. There was a need as they say...

In 1924 the ribbon microphone was invented by Walter Schottky and Erwin Gerlach (of Siemens.) This first ribbon mic consisted of an extremely thin (0.002 mm!) concertina ribbon of aluminum placed between the poles of a permanent magnet. The directional pick up pattern was a very useful quality since it makes it possible to eliminate acoustic feedback. http://www.coutant.org/ribbons.html

Many articles claim that Harry F. Olson of RCA invented the Ribbon Mic. this is not so. He did pioneer many of the early popular Ribbon Mic designs. But ac much credit as he and RCA deserve for the success of Ribbon mics in the market place, even these early commercially successful models were bulky and eventually fell out of favor.

The moving element of the ribbon microphone is a thin corrugated aluminum ribbon suspended between the poles of a strong magnet. In this arrangement sound moves air particles vibrating the ribbon in the magnetic field. This motion causes an alternating voltage to be generated in the ribbon, the amplitude of which is proportional to the velocity of the air particles. The output voltage and the electrical impedance of the ribbon are raised to a value suitable for transmission of the signal to an amplifier, by a transformer built into the microphone case. The transformer is well shielded against stray magnetic fields by multiple shields of mu-metal and copper. Article here.

While it was (and is) widely considered to be the most natural sounding microphone ever made, it had a down side. The first ribbon mics were very heavy, about 8 lb. they were very susceptible to moisture damage and also could easily be damaged by shock or blowing into it. Simply blowing into a Ribbon mic ruined them. Great article on modern Ribbon Mics here.

In 1932 RCA introduced the RCA Type 44-BX, A bi-directional velocity mic. This is a classic robbon mic. more information here: http://www.coutant.org/2.html Ribbon mic technology improved slowly moving toward smaller chassis and less delicate hardware. In 1958 Eugen Beyer changed all that with his introduction of the world's first robust, 'short diaphragm' ribbon mic. Its capsule shared dimensions similar to the moving coil transducers of the time and his original designs are still manufactured today.

The 44BX pic to right is from the illustrious K-BAY Microphone Collectors Website. Mr. James U. Steele over there knows more about mics than the rest of us radio geeks combined. http://www.k-bay106.com/photos.htm