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My Crystal Radio Page As a youngster I made several crystal radios1 and one-tube radios, and various amplifiers and many transmitters (mostly illegal!). I even made a special five-tube radio designed to mount in the steering column of a 60 Series scooter. It worked swell, but the filaments all fell out of the tubes after about 5 miles on that rough scooter. Since then, both in business and as a radio amateur, I have designed and built a multitude of receivers, transmitters, and other electronic equipment, but I never forgot my early fascination with the crystal set. More information about crystal sets appears at the bottom of this page. The HRO Dial Hobbydyne I finished this latest Hobbydyne™ in April, 2007. It features a HRO dial and gearbox that I picked up on eBay. It also uses an antique Pilot Centragrad 500 pF straight line frequency variable capacitor in the detector circuit. The detector coil is 192 uH wound with 660 strand #46 Litz. The combination provides a band spread of five divisions between each broadcast station, evenly spaced all the way across the broadcast band. Two Styrene couplings were staked together with a short piece of pipe and cement to provide enough length for the detector coil on one end of the form and the smaller 2.3 inch antenna coil inside the other end. The smaller coil rotates with a knob on the front panel to adjust the coupling. A small air variable is used to trim the high end of the band to exactly 1.7 kHz with the capacitor plates fully open. It tunes to down to 530 kHz with the capacitor plates fully meshed. This set uses my Hobbydyne™ circuit with a tiny differential capacitor to enhance the selectivity. In this particular set there is also a small piston trimmer between second set of stator plates and ground to null out any frequency change when the differential capacitor is rotated.
The front panel spacing is not as nice as some of my previous sets, but it was necessary for this particular dial/coil combination. This set equals my previous large coil sets in both sensitivity and selectivity. There is no measurable degradation from the use of the smaller size antenna coil. The proximity of the smaller coil to the detector coil causes only a very slight reduction of the larger coil's Q. The Q with the small coil in place tops out my HP Q meter at 1000. The antenna tuning capacitor is one of the tiny two gang Russian military surplus units with both sections in parallel. It requires a turn and a half to go from minimum to maximum capacity and I find this just right for quick, yet accurate trimming of the antenna coil. This set is an absolute dream to tune with all the band spread that it offers.
The Hobbydyne™ Pro This Crystal Radio, made in June, 2006, I call the Hobbydyne™ Pro. It has two high Q ferrite coils wound with 100 strand #46 Litz wire that track using a dual 365pF tuning capacitor, plus a third independently tuned ferrite Litz wound coil that is variably coupled to the other two. It has better selectivity than any other set I have built. In place of my usual Hobbydyne variable capacitor for adding additional selectivity a two position knife switch, upper right on the panel, is used to select normal or high selectivity. A very small air trimmer is used as compensation to insure that a frequency change does not occur when the selectivity is changed. This allows very precise dial calibration. The large National Vernier dial is used to tune these circuits. Future plans call for a frequency calibration chart in the lid.
The two tracking tuned circuits use a modified version of the Negative Mutual Inductive Coupling that J. W. Miller used in their 1950's Hi-Fi Crystal Radio Tuner. In this set a ferrite cored mutual inductor is used to allow the coupling between the stages to be adjusted to the desired value. Individual trimmer capacitors are used on each tuned circuit, plus a padder capacitor on one of them to obtain near perfect tracking across the broadcast band. More information on the Miller tuner can be found at the bottom of this page. The third independently tuned circuit used for the antenna input provides impedance matching and additional selectivity. The amount of coupling between this tuned circuit and the following circuits is fully adjustable by rotating the coil with the upper left knob on the front panel. This circuit is tuned by the dial at the lower left. Initially this was a vernier dial. After operating the radio for a while I decided that a vernier dial was not necessary for this stage because faster tuning was desirable, so it was modified to provide a direct drive. For casual reception the knife switch at the upper left can be thrown to bypass the antenna tuned circuit and connect the antenna directly to a link on the first mutually coupled coil. Both the sensitivity and selectivity are reduced some, but tuning is simplified since only the large National vernier dial is used for tuning.
This is a crystal radio made in July, 2004.. It has a pair of NOS National Velvet Vernier dials made of bakelite that are over 70 years old. This radio was designed to have a "military" appearance and will be installed on my ham radio table. It has a straight line frequency silver plated ceramic tuning capacitor and the same Hobbydyne™ circuit as my other sets. The detector coil turns are space wound with a strand of silk thread between each turn and it has a Q of over 800 at 1 kHz as measured on a Hewlett Packard Q meter. The rotary rod that turns the antenna coil is controlled from the center knob with a dial cord drive. The rear of the drive spool can be seen between the two coils in the second picture. It is very sensitive and selective. This set is featured on pages 79-80 of CQ Magazine, November, 2005.
The "Son of Hobbydyne" pictured below inspired this easy to build, easy to duplicate set with the same performance. I guess I can call it "Hobbydyne Jr". It uses the same circuit and the same parts as its Dad but it is built on a simple open chassis. Actually, by moving the audio board over it could be put in an antique looking case like the set below. This crystal radio uses a European Jackson Brothers detector tuning capacitor and a Xtal Set Society dual gang antenna tuning capacitor. This radio uses the Jackson capacitor for the detector because it has ceramic insulation. The less expensive and easier to obtain single gang capacitor sold by the Xtal Set Society will work fine. The Like its Dad, this set is super sensitive and super selective. I consider it a state of the art crystal radio. The ferrite coils used in this set and the one below give excellent performance. Substituting the very expensive 6.5 inch 660/46 Litz wound coils for the ferrite rods gives only a tiny improvement in selectivity. So little difference that it takes a calibrated signal generator to tell if there really is a difference. This set also uses a Schottky diode and the small variable capacitor for detector coupling. It is designed to work best with an earphone load of at least 100K ohms, although it will work pretty well with plain old 2000 ohm dc headphones. The operational adjustments are the same as the Hobbydyne™ and the Son of Hobbydyne pictured below.
Pictured here is my "Son of Hobbydyne". This set was constructed using only parts that are readily available commercially, except the two National dials. It uses quality miniature tuning capacitors with ceramic insulation and 1/2 inch Ferrite rod coils wound with 100/46 Litz wire. The detector coil is wound on a 3-inch rod, the antenna coil on a 2-inch rod. The Q of the 3 inch rod coil is only about 10 percent less than the 6.5 inch Litz coils in the original Hobbydynes. The 2-inch rod is rotated with a knob on the front panel to vary the coupling between the two coils, and the shorter form was used to hold down the cabinet size. All other components are far enough away from the coils that they have little or no affect on their Q. The schematic of this set is essentially the same as the original Hobbydyne. It uses a Schottky diode as its detector. The detector diode, the bypass capacitor, the DC equalization resistor and capacitor, and the RF chokes are mounted on the small board at the lower left in the rear view picture. I use a pair of WW2 sound power phones through a 150K ohm matching transformer, but it also works well with a pair of quality 2000 ohm (DC) headphones. The extra hole is for a switch to add capacity to the antenna circuit which will be add later. The set is very sensitive and very selective. As the coil coupling is reduced the tuning of both coils becomes very sharp, and the signal will increase when the correct coupling is found. Then the differential detector coupling capacitor can be used for an additional increase in selectivity with very little loss of sensitivity. Using the coil coupling and detector coupling controls, while carefully tuning both the antenna and detector coils can really bring a weak station up out of the noise. My next project will be a breadboard version of this set that is easier to construct. I will have an actual size CAD layout available for those who wish to build one. If there is a demand I will also provide a parts kit.
Here are two pictures of my Hobbydyne II crystal radio This set is very similar to the Hobbydyne, but uses a second Litz wound coil in the antenna circuit in place of the ferrite rod. This set is a little more sensitive and a lot more selective than its predecessor due to the higher Q antenna circuit, and the ability to vary the coupling between the coils. Both capacitors are silver plated and ceramic insulated, both coils are space-wound on threaded forms using 660 strand #46 Litz wire. The detector coil is 6.5 inches in diameter and has a Q well in excess of 1000. The antenna coil is 4.5 inches in diameter with a Q of about 900. When the two coils are rotated to loosen the coupling the selectivity is really sharp. The presently unused control location is for a switch to add capacity to the antenna coil, but this is only required with a very short antenna. The differential capacitor circuit has been changed slightly, and has a second balancing capacitor. With the balancing capacitor there is no change at all in frequency as the detector coupling is adjusted. It uses four Aligent 5082-2835 Schottky diodes in parallel for the detector. A 200K to 1K transformer feeds a pair of WW2 sound powered earphones. Slightly more sensitive and selective than the Hobbydyne, it receives about 30 stations clearly in the daytime and many more at night.
This one-tube radio uses a 1Q5 beam power pentode with a screen potentiometer for regeneration control, just like the one I built in 1950. It follows the classic one-tube radio design from years ago, but has a modern feature or two. If you look in the middle of the coil you will see a 1/2 inch ferrite rod. The coil of Litz wire on this rod measures 250uH and it is used as an auxiliary antenna input coil. The set also has a six turn conventional antenna winding with a tap at one turn. A ground binding post plus three antenna binding posts on the front panel allow the selection of any of the three inputs. The ferrite rod input is superior to the conventional antenna input coils on most antennas, especially at the lower end of the broadcast band. The main tuning coil is wound on a 3 inch ABS sewer coupling from Genoa. It is wound with 100/46 Litz wire at 37 turns per inch. The form was threaded to allow a little spacing between the turns. The other windings are wound with #22 solid which is also 37 turns per inch when close wound. The feedback winding was optimum with only five turns. The recess to the right of the tuning capacitor is for a custom made 36 volt battery. The plate circuit is choke coupled to avoid DC voltage across the earphones. It works very well and the regeneration is very smooth.
This is the underside picture of the radio. Notice one more modern feature, the printed circuit board that is used as a common ground plane. It does not extend up under the coil to avoid lowering the coil's Q. Feed thru capacitors are staked into the board and used both as capacitors and as connection points. There are also several Teflon insulated posts staked in the board for connection points..
Pictured here is my newest crystal set, the Hobbydyne™. It uses 700 strands of #46 Litz wire on a 4.375 inch diameter detector coil. Seven individual strands of 100/46 were paralleled and lightly twisted in the correct direction. The coil form was made from two Styrene pipe couplings. They were joined and lathe turned to 1/2 their former wall thickness. They are grooved lightly at 19 TPI to give the Litz wire a little space between adjacent turns. The unloaded Q of this coil is over 1000 at 1000 KHz, higher at the low end of the band, and a little lower at the high end. It has absolute single-channel selectivity across the band except directly adjacent to my two local "barn burners" where it has acceptable alternate channel selectivity. Here in Central Florida this set receives 27 stations clearly in the daytime, and almost every assigned 10kHz channel at night. The input circuit has individual tuning capacitors for series or parallel tuning, or any combination of the two. It is similar to the Benodyne. It has two selectivity adjustments. The first is the ferrite antenna tuning coil that slides in or out of the detector coil controlled by a knob on the front panel. The second is a miniature differential capacitor used to couple the detector coil to the diode, also controlled from the front panel. For general listening both of these selectivity adjustments can be pre-adjusted and left in position. For weak DX reception they can be tweaked as necessary. When using series antenna tuning with the antenna capacitor left fully meshed the detector tuning works just like a "real" radio. All 27 daytime stations can be heard clearly. Any chosen station can be further enhanced by tuning the antenna capacitor to resonance at the station's frequency. The diagram below shows a 1N34 detector; it has been upgraded to an Aligent Schotty Barrier diode which provided a nice increase in both selectivity and sensitivity. Using a capacitor to provide variable coupling to the detector is not a new idea. In fact, it was used in the 1950's in the Miller Tuner circuit, Set #5 below. For some reason it is seldom used by experimenters, even though it is far easier and works much better than tapping the coil. It does require the use of a good quality RF Choke to provide a DC return for the diode. Although it cannot be seen in the picture, (some parts are hidden in a milled recess inside the white Teflon capacitor base) it has a variable resistor to equalize the AC and DC load for the detector. I retrofitted capacitor diode coupling into my set #3 shown below to replace the taps and the tap switch, and it enhanced its performance. Thanks guys, I borrowed
ideas for this set from many of you.
#3 Crystal Radio adds a few refinements such as ceramic insulated variable capacitors, twin National Velvet dials and ferrite core inductors wound with Litz wire. The unloaded Q of these coils measure well over 400 and it is the most selective set I have built. In this set the detector coil has two selectable taps. Note the front panel knob provides variable coupling between the two ferrite coils. By turning the antenna coil 90 degrees to the detector coil the coupling can be reduced to a very low value. Less coupling means increased selectivity and it helps separate the weak stations from the strong ones.
#2 Crystal Radio was made for nostalgia purposes and it is based upon the circuit of the Mystery Crystal Set which was made famous in Australia in 1932. It is the only single tuned set I have built, and because of this it does not have the selectivity of the other two models, although it does work well. The unloaded Q of its coil is quite high because it is wound on a 4.25 inch Styrene form with Litz wire. The majority of the wiring is concealed in wire passages routed underneath the wood chassis. As an experiment I put an antenna tuner ahead of the Mystery Crystal Radio. The selectivity and sensitivity were both greatly improved, and I decided to built a dedicated tuner for it. The tuner features both series and parallel tuning at the flip of a switch . It has #19 wire space wound at 18 turn per inch on a threaded styrene coil form. Its Q is also extremely high. No connections are necessary between the Mystery set and the Tuner because the tuner was designed to provide correct coupling when the two chassis are sitting side-by-side. With the tuner the Mystery Crystal Radio is a real DX go-getter and has excellent selectivity. The tuner has it's wiring concealed in routed passages under the chassis the same as the Mystery radio. Extra concealed passages are in place so it can be converted to a stand-alone crystal set very easily.
#1 Crystal Radio is my experimental model. It has had several types of coils including a 2 inch cylinder, a rook which is a variation on the cylinder, a honeycomb and the present radial arm spider coils. I tried coils wound with solid wire and wire with 100 strands of #47 Litz wire. The Litz is far superior, providing a Q (quality) factor almost twice as high on some of the coils. The empty hole in the center of the front panel was the location for a switch that was used to select ten different detector coil taps. One particular tap was almost always gave the best performance. To eliminate any possible loss caused by the large number of coil taps, I rewound the coil with only that one tap. The present coils in this set are radial arm spiders wound using Litz on Teflon hubs with Polystyrene radials. These coils have an unloaded Q of about 285. Note that spacing between the coils can be adjusted. Best overall operation is with the two spider coils spaced three inches apart. When I want to try something new this is the set that gets picked on.
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It is ironic that the basic circuit I am using is just about the same that my Grandfather, Edwin E. Frederick used in the crystal radios he manufactured and sold in 1914 and 1915. This was long before any broadcast stations existed. His radios were used to receive the Arlington, VA time signal that was operated by the Government. The Arlington time station ran a power of several hundred thousand watts2 on a very low frequency, and it could be heard for hundreds of miles. Jewelers were his customers, and they used the sets to receive the time signal and set their clocks. Jewelers often had crystal radio displays in their show windows and they drew many visitors. Some jewelers even had a ball in their show window that they dropped at noon. Radio was very new and people were fascinated with it. People would gather each day and watch the ball drop. We still have one of his sets, and it is pictured above. The little hinge-out door on the right side is for access to the crystal. I also have several newspaper articles, some customer testimonials, and about 20 hand-drawn diagrams for his crystal radios. Each radio is slightly different from the preceding one. I also have the name of each person who purchased one of his radios, the date of purchase, and the price paid. To see two 1915 newspaper articles and a typical diagram of one of his radios click here.
Here is a nice single-tuned Crystal Radio built by Eddie, the great grandson of Edwin Frederick who made the set directly above. Eddie is off to a running start with a 3 inch coil using Litz wire.
#4 Crystal Radio is a homemade copy of the Heathkit CR-1 Crystal Radio from the 1950's that is built in a 6 x 3 x 2 inch deep plastic box. It has a double tuned circuit using ferrite coils and is similar to set #3. The spacing between the coils is adjusted once for best performance and is not adjustable during operation. The Q of the coils is much lower than the coils in set #3 because of their close proximity to the front panel and the tuning capacitors. For best Q nothing must interfere in the near field of the coils. It does perform quite well though, and receives about 80 percent of what set #3 does. The original CR-1 Heathkit goes for over $200.00 at eBay. That is ironic, because most Heathkits of that area, even the scopes, generators and the like, go for much less. This Heathkit reproduction is featured on page 80 of CQ Magazine for November, 2005.
#5 Crystal Radio is a J.W. Mller commercial High Fidelity Band Pass AM Tuner from the 1950's. It uses two ferrite coils isolated from each other and coupled by a special "mutual coupling" coil. The result is a set that offers superior band pass selectivity. The two tuned circuits track precisely and it tunes just like any other AM The trade-off is reduced sensitivity, but it still receives stations from all over the southeastern US in the evening from my Florida location using only earphones. When connected to a high fidelity amplifier like Miller intended the amplifier more than makes up for the slight lack of sensitivity. So, what can I receive with my crystal
radios? How this
foolishness all started What did I learn? Here are a few Crystal Radio links. I will list more soon. Scott's Crystal Radios Radios, earphones for sale and more Dave's Crystal Set Page You have to see all of Dave's sets!
Ben Tongue's Site Crystal
Radio Set System: Design, Measurement & Improvement Peebles Original Radios Parts, radios, plans and more Footnotes: (1) I assumed that ever one knows what a Crystal Radio is. Perhaps I should explain just in case. A crystal radio uses only tuned circuits to select the stations, and a crystal detector to rectify the radio station's signal to enable it to be heard with a set of headphones. It is entirely passive, it has no active components like tubes or transistors and uses no electricity. When you listen to a broadcast station on crystal radio the sound in your earphones comes directly from the station your are listening to. Since there is no amplification of any kind, the circuits, especially the coils, must be very efficient so that sufficient power can be recovered (stolen) from the station to operate the earphones. (2) Many early pioneer radio stations ran powers of many hundred thousand watts. In some areas merchants could "steal" enough power from nearby stations to light their signs. Today the maximum power permitted in the United States for commercial broadcast is 50,000 watts. In those early days the frequencies above our current AM broadcast band were consider useless and were used mostly by amateurs. Go to Jim's Cushman Scooter Web Site Hobbytech.com rev 06/23/07 Copyright 2002 - 2003 - 2007 Hobbytech.
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