Sep 10, 2019 DXing With The Heathkit CR-1 Crystal Radio If you’re a regular blog reader, you will likely recall my description of “ The Enigmatic Heathkit CR-1 Crystal Radio ” a few weeks ago. Back then I mentioned that I was ‘eager to get my mitts on one’ and that I had arranged to borrow a CR-1 from another VE7 who was fortunate enough to own one. Download HEATHKIT CR-1 CRYSTAL SET SCH service manual & repair info for electronics experts. Service manuals, schematics, eproms for electrical technicians. This site helps you to save the Earth from electronic waste! HEATHKIT SB-650 30MHZ FREQUENCY DISPLAY FREKVENCIA MERO CA.1995 SM.
Heathkit CR-1 Crystal Radio andHeathkit Diagrams, Schematics and Service Manuals - download for free! Including: heathkit 309 c rf probe, heathkit 336 hv probe, heathkit 337 c demodulator probe, heathkit 342 low capacity probe, heathkit 2006 technician, heathkit 2007 general, heathkit 2008 extra, heathkit a 7e amp schematic diagram, heathkit a 9a amp schematic diagram, heathkita9c, heathkit a 9c integrated amp schem. Heathkit CR-1 Crystal Radio N6CC Fun times – I still have the CR-1 and it still works great although the “chicken head” tuning knobs and headset are not original. I’ve built a lot of crystal radios but this one still works the best; due to careful antenna – diode – headset impedance matching.
Miller 595 Tuner InformationThis page gives the circuit and pictoral description of the famous Heathkit CR-1 crystal radio.
It was a double tuned crystal radio from the 1950's with a series tuned primary that had a 365 pf variable capacitor and selectable additional capacitance on the center selector switch. In my opinion, it would have been more logical to have made the center switch for varying the inductance and using a tapped primary coil.
Here is a picture of the inside of the original CR-1.
It was a double tuned crystal radio from the 1950's with a series tuned primary that had a 365 pf variable capacitor and selectable additional capacitance on the center selector switch. In my opinion, it would have been more logical to have made the center switch for varying the inductance and using a tapped primary coil.
Here is a picture of the inside of the original CR-1.
Another photo showing the inside of the original Heathkit CR-1. Note the rectangular pinkish mica capacitors, the configuration of the central antenna switch, the 'cable knit' appearance of the coil windings, and the long screws holding the panel to the threaded stubs that are recessed deep into the cabinet.
Anyway the coils, marked as 'transformer' in the diagram, were both wound on a ferrite rod and the secondary coil was part of a tank with the 365 cap and the germanium diode for the detector circuit. They do not give details of the coil construction but with a little experimentation you can get into the BC band tuning range.
Here is the original circiut:
This shows the pictoral diagram of the inside of the set:
First Prototype
In my first building project replica I used a toroidal core instead of a rod. An Amidon FT 114-61 was my choice for the core, and 45 turns of 24 GA enamel wire for the secondary gets the detector tank in the ballpark. (Amidon's webpage is http://www.amidoncorp.com). I used 30 turns of the same wire would on the same toroid for the primary with taps at 3, 6, 9 15, 24, and 32 turns. I used the center selector switch to vary the inductance of the primary instead of switching in additional capacitors.
The little box is a Radio Shack part #270-1805 that is just about exactly the same size as the original enclosure. The front panel is made of aluminum, so it serves as a common ground, but the antenna and one of the headphone binding posts need to be insulated from it using little fiber washers to prevent those binding posts from making electrical contact with the panel.
The radio was originally sold either with or along side a pair of 4000 ohm Trimm Acme headphones. They were only middle of the road for sensitivity and a good old pair of Brandes Superior or Western Electric 509-W's would be much more sensitive.
Here is a shot of the main parts on the floor. The rotary switch, diode, and box are from Radio Shack. The variable caps were obtained from the Xtal Set Society. The coil is described above and I got the old mica capacitors from my junk box. I ended up using a different coil and no padder capacitors though.
One challenge was to tap a couple of the holes on the front face of the variable caps to allow them to be mounted to the front panel using 6-32 screws. Care must be taken to avoid damaging the capacitor and to keep the metal shavings out of the ball bearing mechanism.
In my first building project replica I used a toroidal core instead of a rod. An Amidon FT 114-61 was my choice for the core, and 45 turns of 24 GA enamel wire for the secondary gets the detector tank in the ballpark. (Amidon's webpage is http://www.amidoncorp.com). I used 30 turns of the same wire would on the same toroid for the primary with taps at 3, 6, 9 15, 24, and 32 turns. I used the center selector switch to vary the inductance of the primary instead of switching in additional capacitors.
The little box is a Radio Shack part #270-1805 that is just about exactly the same size as the original enclosure. The front panel is made of aluminum, so it serves as a common ground, but the antenna and one of the headphone binding posts need to be insulated from it using little fiber washers to prevent those binding posts from making electrical contact with the panel.
The radio was originally sold either with or along side a pair of 4000 ohm Trimm Acme headphones. They were only middle of the road for sensitivity and a good old pair of Brandes Superior or Western Electric 509-W's would be much more sensitive.
Here is a shot of the main parts on the floor. The rotary switch, diode, and box are from Radio Shack. The variable caps were obtained from the Xtal Set Society. The coil is described above and I got the old mica capacitors from my junk box. I ended up using a different coil and no padder capacitors though.
One challenge was to tap a couple of the holes on the front face of the variable caps to allow them to be mounted to the front panel using 6-32 screws. Care must be taken to avoid damaging the capacitor and to keep the metal shavings out of the ball bearing mechanism.
OK, here is the completed CR-1 clone.
This first attempt has the binding posts spaced a bit too close. I didn't have an original to copy, just pictures. I also tried a couple of different coil configurations and the one I settled on has 45 turns of 24 GA wire for the tuned secondary, and the primary is tapped as noted above.
Inside it is pretty simple. Just the 2 variable caps, the rotary switch wired up to the toroidal coil, and the diode can be seen from the capacitor to the binding post on the far left.
I hooked it up and counted 6 local stations in the first minute in the daytime, and with a good pair of headphones got 7 locals and another 4 out of state stations on the first night.
If I ever get a hold of an original, I will post a comparison of the performance.
OK, I got a loan of the original Heathkit CR-1 and here is what I found.
The original CR-1 has a much more definite double tuned effect than my spinoff. The 2 knobs for tuning the ANT and DET circuits seem to track pretty closely with each other and both need to be tuned to the same frequency farily sharply to get reception. It is easy to miss a station by not having the tuners coordinated. I found it best to move the DET dial a little then sweep the ANT knob back and forth a little to zero in on the station if there was one there.
The circuit gurus emailed me and correctly predicted that my circuit would pretty much function as a single tuned circuit. The inductance tap switch mainly varied the selectivity and I found the 9 turn tap to be good for both sensitivity and selectivity. There was some noticeable tuning effect of the ANT variable cap at the high frequency end of the dial but the DET tuner did most of the tuning. It was, however, very sensitive and tuned very sharply anyway. In fact the volume was louder across the entire frequency range than the on the original CR-1, and I was able to get a couple of weaker stations that I couldn't reach on the real CR-1. So overall I am very happy with the performance but I will try another one to try and clone the coil and performance of the original a little more closely.
OK, I got a loan of the original Heathkit CR-1 and here is what I found.
The original CR-1 has a much more definite double tuned effect than my spinoff. The 2 knobs for tuning the ANT and DET circuits seem to track pretty closely with each other and both need to be tuned to the same frequency farily sharply to get reception. It is easy to miss a station by not having the tuners coordinated. I found it best to move the DET dial a little then sweep the ANT knob back and forth a little to zero in on the station if there was one there.
The circuit gurus emailed me and correctly predicted that my circuit would pretty much function as a single tuned circuit. The inductance tap switch mainly varied the selectivity and I found the 9 turn tap to be good for both sensitivity and selectivity. There was some noticeable tuning effect of the ANT variable cap at the high frequency end of the dial but the DET tuner did most of the tuning. It was, however, very sensitive and tuned very sharply anyway. In fact the volume was louder across the entire frequency range than the on the original CR-1, and I was able to get a couple of weaker stations that I couldn't reach on the real CR-1. So overall I am very happy with the performance but I will try another one to try and clone the coil and performance of the original a little more closely.
Next Version
After a lot of advice and many coils later, I think I got pretty close to the original performance. The secret is the coil. I am told the original has 2 small ferrite cores in it rather than 1 long one so, I experimented with different sizes of cores combined with different numbers of turns on the coil and then zeroed in on the correct spacing between the 2 cores. The spacing between the coils could be measured from outside but the stuff inside the cardboard tube was impossible to discover without damaging the coil. It looks like the core is about 1/4 inch diameter and extends just to the outer edges of the coil windings when you look into the ends of the tube. If anyone knows the exact original specifications please let me know. Anyway, I was able to pick up the same stations with this version as with the original.
After many attempts and examination of an original coil and X-ray here is what I ended up with. I used 21/44 Litz wire for the windings. The antenna coil was about 140 turns with a 9/16 inch slug inside that part for about 335 uH inductance. The space between the coils was 10 mm or about 7/16 inch. The detector coil had about 90 turns with a 9/16 inch long slug inside that and measured 240 uH. On some of the coils, I tried bringing out a tap about 30 turns from the stator end of the winding for the detector tap. Hard to say if this made and noticeable difference, not much if any. This coil seemed to duplicate the performance of the original in both sensitivity and selectivity. Any closer spacing of the coils and you get too much coupling and the tuning gets very broad.
After a lot of advice and many coils later, I think I got pretty close to the original performance. The secret is the coil. I am told the original has 2 small ferrite cores in it rather than 1 long one so, I experimented with different sizes of cores combined with different numbers of turns on the coil and then zeroed in on the correct spacing between the 2 cores. The spacing between the coils could be measured from outside but the stuff inside the cardboard tube was impossible to discover without damaging the coil. It looks like the core is about 1/4 inch diameter and extends just to the outer edges of the coil windings when you look into the ends of the tube. If anyone knows the exact original specifications please let me know. Anyway, I was able to pick up the same stations with this version as with the original.
After many attempts and examination of an original coil and X-ray here is what I ended up with. I used 21/44 Litz wire for the windings. The antenna coil was about 140 turns with a 9/16 inch slug inside that part for about 335 uH inductance. The space between the coils was 10 mm or about 7/16 inch. The detector coil had about 90 turns with a 9/16 inch long slug inside that and measured 240 uH. On some of the coils, I tried bringing out a tap about 30 turns from the stator end of the winding for the detector tap. Hard to say if this made and noticeable difference, not much if any. This coil seemed to duplicate the performance of the original in both sensitivity and selectivity. Any closer spacing of the coils and you get too much coupling and the tuning gets very broad.
Here is a front label that I made on some thin acetate plastic, printed the Heathkit logo on it, and glued on to the front of the aluminum panel. I sold a few of these clearly presented as home made clones, though I have occasionally seen my radios show up for sale on Ebay advertised as a genuine Heathkit, which should be obvious that they are not. Note the short flat Phillips head screws, the lettering which is clearly different than the original, and the coil inside is clearly hand wound as opposed to the tidy machine winding of the originals. Also note the round modern ceramic chip capacitors and the multiposition switch. If you run across one of my radios being passes off as an original, you can refer the seller to this page, but the radio itself, unless damaged, should work just as well as the real thing, and there were only a dozen or so ever made, so they are quite rare indeed!
Addendum 5/29/01
Unravelling the secrets of the CR-1 coil, or The CR-1 coil Exposed
This is an actual X-ray of a Heathkit CR-1 coil. As you see, it has the 2 coils each with its own separate ferrite slug. The best I can measure, each slug is about 13 mm or about 1/2 inch long by 1/4 inch diameter. The separation between the slugs is 7/16 inch. It looks like the slug in the larger (antenna) coil is not quite centered under the coil. I am not sure if this was intentional or not. Anyway it was helpful to know how it was made so one might get a better idea of how to make a duplicate.
The Amidon R61-025-400 core can be cut into the 1/2 inch lengths for use in constructing a coil like this one. I will post further information as I progress with this project. Thanks Mike!
Unravelling the secrets of the CR-1 coil, or The CR-1 coil Exposed
This is an actual X-ray of a Heathkit CR-1 coil. As you see, it has the 2 coils each with its own separate ferrite slug. The best I can measure, each slug is about 13 mm or about 1/2 inch long by 1/4 inch diameter. The separation between the slugs is 7/16 inch. It looks like the slug in the larger (antenna) coil is not quite centered under the coil. I am not sure if this was intentional or not. Anyway it was helpful to know how it was made so one might get a better idea of how to make a duplicate.
The Amidon R61-025-400 core can be cut into the 1/2 inch lengths for use in constructing a coil like this one. I will post further information as I progress with this project. Thanks Mike!
Link to Miller Tuner Page (sorry this linked site has been removed, but will try to relocate it) This is a link to a page with more information about another famous crystal radio, the Miller 565 / 595 Tuner. This was my next project. The modified schematic is shown below.
Parts List:
Part Description Source, Part#
C1 2 gang 365 pf tuning capacitor from Xtal Set Society
C2 0.1 uf capacitor Mouser 5989-250V.1
C3 0.05 uf capacitor Mouser 5989-250V.047
C4 200 pf capacitor Mouser 140-50P2-201K
C5 10 pf capacitor Mouser 140-50N2-100J
C6 15 pf capacitor Mouser 140-50N2-150J
R1 100K ohm resistor Mouser 293-100K
L1 Tuning coil 1 Core from Amidon FT 82-61, see text below for winding
L2 Tuning coil 2 Core from Amidon FT 82-61, see text below for winding
L3 Negative mutual coupling coil, see text below for construction
L4 270 uH peaking coil Mouser 43LR274
L5 (marked L in schematic) 2500 uH RF choke Mouser 542-70F253
D1 1N34A Germaniun Diode
Miscellaneous hardware:
Perforated Circuit Board Radio Shack 276-1396A
Solder Tabs Mouser 534-7312
Machine Screws 6-32
Washers size 6
Tuning Knob or Dial
Cabinet Radio Shack part #270-1807
Wire for the coils and hookup
Binding posts or Fahnstock clips
Parts List:
Part Description Source, Part#
C1 2 gang 365 pf tuning capacitor from Xtal Set Society
C2 0.1 uf capacitor Mouser 5989-250V.1
C3 0.05 uf capacitor Mouser 5989-250V.047
C4 200 pf capacitor Mouser 140-50P2-201K
C5 10 pf capacitor Mouser 140-50N2-100J
C6 15 pf capacitor Mouser 140-50N2-150J
R1 100K ohm resistor Mouser 293-100K
L1 Tuning coil 1 Core from Amidon FT 82-61, see text below for winding
L2 Tuning coil 2 Core from Amidon FT 82-61, see text below for winding
L3 Negative mutual coupling coil, see text below for construction
L4 270 uH peaking coil Mouser 43LR274
L5 (marked L in schematic) 2500 uH RF choke Mouser 542-70F253
D1 1N34A Germaniun Diode
Miscellaneous hardware:
Perforated Circuit Board Radio Shack 276-1396A
Solder Tabs Mouser 534-7312
Machine Screws 6-32
Washers size 6
Tuning Knob or Dial
Cabinet Radio Shack part #270-1807
Wire for the coils and hookup
Binding posts or Fahnstock clips
The secret of this set is in the construction of the coils.
L1 and L2 are straight ferrite rods wound with Litz wire. The best reproduction of this was to use an Amidon 2 inch by 1/4 inch diameter ferrite rod of material 61, and wind on 85 turns of Litz wire. I had some 21 strand by 44 GA wire and this worked well. I actually measured the inductance of the finished coils and adjusted the windings to get them exactly the same, to the uH, about 250 uH.
L3 is the real secret of this set. After trying several different core forms for this, the best seemed to be a ¼ inch hardwood dowel about 2 inches long. Wind 2 strands of the 30 GA wire onto the form together, side by side, close wound for somewhere between 20 and 31 turns and secure. Take the left end of strand A and the right end of strand B and join them together for the bypass to the grounded capacitor. The remaining free ends go to the points indicated on the circuit.
I am told you can get the actual flyer from the Miller Company.
This circuit and the construction details are published in the Newsletter of the Xtal Set Society in a 2 part article starting in Jan 2002. You can see the construction details and alignment instructions there.
Back to Scott's Crystal Radios
L1 and L2 are straight ferrite rods wound with Litz wire. The best reproduction of this was to use an Amidon 2 inch by 1/4 inch diameter ferrite rod of material 61, and wind on 85 turns of Litz wire. I had some 21 strand by 44 GA wire and this worked well. I actually measured the inductance of the finished coils and adjusted the windings to get them exactly the same, to the uH, about 250 uH.
L3 is the real secret of this set. After trying several different core forms for this, the best seemed to be a ¼ inch hardwood dowel about 2 inches long. Wind 2 strands of the 30 GA wire onto the form together, side by side, close wound for somewhere between 20 and 31 turns and secure. Take the left end of strand A and the right end of strand B and join them together for the bypass to the grounded capacitor. The remaining free ends go to the points indicated on the circuit.
I am told you can get the actual flyer from the Miller Company.
This circuit and the construction details are published in the Newsletter of the Xtal Set Society in a 2 part article starting in Jan 2002. You can see the construction details and alignment instructions there.
Back to Scott's Crystal Radios
Posted by Tim On December 26th, 2010 / 15 Comments
My father built this Heathkit CR-1 Crystal Radio for me for my birthday when I was in the 5th grade, circa 1959. I helped ;0)…..
It was a magical device and it got me seriously interested in radio at an early age. I didn’t even realize that the absence of a battery was itself quite an accomplishment. It worked great – keeping me up all hours of the night trying to discover its range limits. In the process I began to learn about what could be used for an antenna, the differences between 1N34’s and power supply rectifier diodes, and why speaking into the headphones would not produce a transmitted signal into the antenna. That should work, right?
Wrong….(I learned after looking through a Heathkit catalog that that feat would require a radio TRANSMITTER! I would drool over actual transmitters in the catalog that could do this – like the DX-100 which I would eventually own).
Note the markings on the right-hand “DET” dial. That was the Detector tuning capacitor. Those little CD (Civil Defense) markings were the frequencies set aside (640 and 1240 KC) in the AM broadcast band for CONELRAD alert messages. These markings were required on all radio receivers built between 1953 and 1963, including this one. COntrol of ELectromagnetic RADiation. Measures taken by the USG to prevent Soviet bombers from homing in on our AM broadcast transmitters as the Imperial Japanese Navy did during the attack on Pearl Harbor. The transmitting sites would change every several minutes as they broadcast emergency information messages. WGBB on Long Island was such a station. A crystal radio would have been very handy at that point.
My Dad installed a Philmore Aerial Kit with its shiny stranded copper wire along the eaves of our house, using TV twinlead standoff ‘s for end insulators. I think the kit originally had two transparent glass insulators for the antenna ends which we didn’t use for some reason. This kit had the porcelain “post” insulators to hold the lead-in in place and a cool conductive strap with Fahnstock connectors at each end to feed the signal under a closed wooden window. There was also a cool copper strap with its own Fahnstock clip which went around the hot water radiator pipe in my room for a ground.
Uninsulated, shiny, stranded copper wire antennas just work better than antennas made with “regular” wire !
Heathkit CR-1 Crystal Radio N6CC
Fun times – I still have the CR-1 and it still works great although the “chicken head” tuning knobs and headset are not original. I’ve built a lot of crystal radios but this one still works the best; due to careful antenna – diode – headset impedance matching.
The Fox Hole Radio
“If it’s stupid – but works – it isn’t stupid” Murphy’s Laws of Combat
During WWI, WWII and later conflicts, many GI’s built “Fox Hole Radios” to listen to news, music, Tokyo Rose, Axis Sally, Lord Haw Haw and probably Hanoi Hannah too. Although the enemy propaganda was laughable, those stations attracted GI’s with their popular music pumped out by powerful radio transmitters. The BBC, Radio Australia and US Armed Forces Radio Stations would carry news from home, music and the all-important sports scores. These Fox Hole radios did not require batteries – they were powered by the signal from the distant radio station as collected by the receiver antenna. They were built and used during the time spent in the “Wait” part of “Hurry up and Wait”. Often in a fox hole. If you were there, you know.
The radio could be built from basic scrap, with the exception of the earplug or earphones; those could be liberated from that enemy tank or aircraft wreckage. That perforated Messerschmitt or Betty could also be a source for the necessary wire for the antenna tuning coil and antenna. The radios were built by the young GI’s who just knew how to make such things. They were the equivalent of today’s techno-geeks and computer hackers (but not the modern kid who thinks “communications technology” is poking their finger at the screen of a Chinese-made iPhone. I’m not interested in those things, but I digress).
This was early 20th century technology and any self-respecting GI was up on it. The omnipresent Ham Radio operators in the Signal Corps probably helped, applying their skills of the day.
I built the Fox Hole Radio described below after a friend produced a WWII Red Cross package containing razor blades among other common items. These were the “Blue Blade” type that had a blue oxide finish, and that finish, in contact with an ordinary pencil “lead”, made a point-contact rectifier. This served the purpose of the “crystal” and “cats whisker” of early crystal radios; it permitted current coming in from the antenna to flow in only one direction. This served to “build up” the antenna signal to a level that could be heard in the earphones by a process known as “detection”.
An attached coil of enamel-insulated wire wound on a piece of bamboo acted along with the antenna wire capacitance to produce a tuned circuit, tuned approximately to the radio frequency band of interest. Experiment with the circuit – you can “short out” coil turns with the paper clip tuning tap; better yet, you can just connect the tap to the razor blade/earphone circuit instead, a more common configuration. This one is REAL basic – but impedance matching and having a healthy signal are the keys to performance.
All that was then required was a long piece of wire for an antenna (that barbed wire fence with wooden posts or a length of infantry telephone “commo wire” was perfect). Plus a ground connection consisting of a metal tent stake or bayonet stuck into the moist soil.
Field improvised ground connection
Above: A typical improvised “ground” connection. A metal tent stake pounded in the ground, kept moist. Here, grounding a PRC-25 radio set. You get the idea.
A few other bits such as a safety pin, a piece of bamboo, plastic water bottle, glass bottle or toilet paper cardboard tube for the tuning coil form. Then a piece of wood from that ammo crate for a chassis, a few screws, thumbtacks or nails and a paper clip to make a “tuner”. In the presence of a strong signal and/or a good antenna it is hard to make this type of circuit not work! The below photos illustrate the basic idea.
Above: The Fox Hole Radio built with a WWII-era “blued” razor blade and pencil lead for the detector. Note the bamboo coil form and the paper clip attached to a cork used to slide along the coil turns to tune the radio. The orange wire goes to the antenna, the black wire to ground. This is the “Deluxe Model” with spring-clips for the external connections.
Fox Hole Radio spring-loaded pencil lead N6CC
Above: Showing the detail of the safety pin used to hold the pencil lead against the razor blade. The copper wire connects the “lead” to the safety pin and the antenna circuit. This assembly functions as the “Cat’s Whisker” of classical crystal radios. My cat was especially appreciative of this alternate design.
Fox Hole Radio – pencil lead and razor blade connection N6CC
The radio is adjusted for best signal by moving the pencil point to different spots on the razor blade to find the “sweet spot” where rectification takes place. (The signal will sound louder). Then adjust the paper clip tuner along the coil turns (put on as many turns as you can – 100 +) to tune and optimize the signal you want to hear. It’s a bit trial-and-error to produce a good signal and it depends a lot upon the height and length of your antenna, the quality of your ground connection, the frequency and local strength of your target station. Or by just getting lucky. (“Chance favors only the prepared mind” – L. Pasteur)
You can also tune the radio by changing the inductance of the coil – make its inductance bigger for lower frequencies by inserting an iron bolt into the bamboo coil form. Or make it tune higher frequencies by reducing the coil inductance by inserting a piece of brass – that .50 Cal machine gun cartridge works just fine. Improvise, adapt, overcome. This radio will also hear local AM voice radio transmitters like those used throughout WWII and beyond for military communications. If they are close enough, you don’t even have to tune the radio! They come in perfectly clearly. These Fox Hole Radios radios were undoubtedly used clandestinely in POW Camps by very brave people. If you had a Fox Hole Radio, you were the Go-To GI for the straight skinny; you actually KNEW what was going on.
Heathkit Cr 1 Crystal Radio
A dog tag will also work instead of a blued razor blade but the blade seems to work better. Hmmm, I bet the blueing on that .45 caliber pistol magazine would work too. Semiconductor valence electron band-gap energy physics at work here. You need a high impedance earphone to convert the electrical signals to sound and the high impedance insures that the phones won’t load down the circuit. Eight ohm stereo phones or modern “ear buds” won’t work – but then again, they weren’t around in WWII…
The WWII Gillette company produced Blued razor blades for the war effort. Interestingly enough, if you look at one of those, it includes a small arrow printed on each side of the blade, ostensibly to indicate the way to insert it into the razor. (The blade used in this Fox Hole Radio was not made by Gillette). Gillette manufactured these blades and then voluntarily magnetized them for use by POW’s as a field-expedient magnetic compass. When suspended by a thread, the arrow pointed North. (“The Escape Factory”, Reference 6). That would be politically incorrect of today’s Gillette.
Emergency Radio Receiver:
Since crystal radios have minimal parts and the ones required can be very small, it is possible to build a really small emergency radio. It compromises “tunability” for compact size and of course, no batteries required. Actually, the largest part is the plastic spool that the antenna is wound on. That spool contains 50 feet of #34 enameled wire for the antenna. The longer the antenna, the better! (If your antenna stayed up all winter, it’s not big enough). The earphone is larger than the radio “set”. See below
The basic circuit:
The above circuit is very simple. It consists of a parallel LC tuned circuit where the inductor is a 1/2 inch diameter toroid wound with about 55 turns of small gauge “Kynar” wire-wrap solid copper wire. Enamel insulated magnet wire will work fine as well. With this particular toroid, 55 turns produces about 12 microhenries. The antenna is connected to one end, ground to the other. The parallel “tuning” capacitor is a random 0.005 microfarad disc capacitor found in my scrap pile. A 1N34A germanium diode (anode) is connected to the antenna end of this parallel combination. A high impedance crystal earplug is connected between the diode cathode and ground.
Since these crystal earplugs look like a very high resistance in parallel with a capacitance, this capacitance will charge up to the peak diode voltage and then stop the earplug current flow. (Not an issue with high impedance magnetic earphones commonly used. Eg: The Heathkit CR-1) To prevent this, place a 47K resistor across the earplug to drain off this charge. The resistor costs a little signal but it’s necessary; value not critical. A potential gremlin, easy to overlook. (The Razor Blade foxhole radio did not need this discharge resistor. I believe the “reverse leakage current” back through the improvised diode effectively discharged the earphone capacitance through the coil).
The LC circuit is resonant at around 650 KC with these values but I used a “junk box” toroid with unknown permeability. Experiment with the toroid, number of turns and capacitor to see what you have and how many turns are required to get to the broadcast band. Nothing is particularly critical. Just make sure you use a germanium diode like a 1N34A due to its lower forward voltage drop as compared with a typical silicon diode (improves sensitivity for weak signals). “Tuning” is very broad due to the overall simplicity and design compromises to make it as small as possible. The LC ratio is not remotely optimum, circuit “Q” is unknown but it is simple, built with what I had, and it works! The internet is full of crystal radio circuits – explore and experiment! Last time I searched for “Crystal Radio” Google returned 63,000,000 results!
Below is the same radio after being “field-hardened” by inserting it inside some heat-shrink tubing to protect it. The diode and resistor are tucked inside the toroid “hole”. The yellow disc capacitor is visible. You can see the small size and this is perfect for keeping inside my pocket emergency kit. Mine is fixed-tuned at about 650 KC; you could build it to prefer any frequency but it is really “broad band” by nature.
In the San Francisco bay area I can clearly hear KGO, KCBS and KNBR (810, 740 and 680 KC respectively) during the day and primarily KGO and KNBR at night as some stations must reduce transmitter power at sunset. Operation is simple: Unwind the antenna wire and toss it up into a nearby tree with a small weight on the end. Keeping it vertical works best since the AM transmitters are vertically polarized but it works in any position. Of course, performance depends upon your proximity to AM radio stations, their power output, their frequency separation from each other, the length of your antenna and your ground connection etc.
The antenna is connected to the red wire, the black wire is connected to “ground” (however switching those connections works just fine). Ground could be a metal tent stake or a knife stuck into wet soil, another piece of wire laying on the ground, the proverbial “cold water pipe”, that barbed wire or chain link fence – anything metal in contact with the Planet. In regions where the ground conductivity is very poor (desert, rocks) you can cut the antenna wire in half and connect half to the red wire, the other half to the black wire making a dipole with the radio in the middle. That’s it. Stick in the earphone and you are in business. Plus, you don’t have to “turn it off” when you fall asleep!
Making it tunable is easy but that requires a rather large variable capacitor and “knob” and even in that case, it is still pretty broad in its tuning range. I chose to keep it small and simple and use my ear-brain to do the tuning. Actually it is pretty easy once you are “tuned in” to the specific program you want to listen to. To Tune in a different one, just change your concentration! Works better than it “sounds”.
You could make it completely waterproof (less the earphone) by squirting a small blob of clear silicone “liquid nails” or hot glue into the ends of the heat shrink tubing to fully encapsulate it. Simple, no-frills and effective with design tradeoffs to minimize complexity and size. Perfect for emergencies when everything is “out”.
Another fun exercise is to demonstrate these radios for your local youth groups. In that case, you can also have them play with the basic Foxhole Radio circuit but with using alternate detector materials rather than the razor blade or a germanium diode.
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In the early days of crystal radios, galena (lead sulfide) crystals were found to be effective as a semiconductor when probed with the Cats Whisker. (I need a smaller Cat’s Whisker!)
Have the kids try various minerals from your handy-dandy minerals collection assortment or rocks they find locally. Some work, some won’t.
Crystal Radio Experiment with galena
Galena works well, or try iron pyrite (iron sulfide – or “fools gold”). That mineral also works.
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Experimenting with crystal radio detector minerals
After connecting the mineral to the Foxhole Radio tuning coil, have the kids probe the surface of the mineral with one wire from the earphone. When they hear a radio station, eyes LIGHT UP! WOW! Fun.
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Any future Engineer or Scientist worthy of the title will quickly reveal themselves via their ensuing curiosity.