Dave's #60 Crystal Radio
I'm up to my #60 radio project! Only 40 more to go. :) Yes, I plan on making 100 of these sets. Then I will make more. This is another two piece radio. The two pieces allow me to separate the coils for optimum coupling. In my early tests, I am distancing the two coils about 1 foot (30 cm) apart.
There is another twist to this design. Both sides have a detector! This is not a ploy to receive stereo AM reception but to allow the user to use a one coil or two coil set. You could enter the annual crystal radio receiving contest in January in the hobby class and the open class with one radio. When entered in the hobby class, the detector board can be used as a wave trap. All this and you only have to buy your litz once! Each board has a link switch that disconnects the detector on the side that the detector is not being used. This insures that the detector isn't loading the tank circuits.
This is a DX radio but on a budget. I have stayed away from close wound cylinder coils in the past as they tended to be Q killers. A space wound coil puts a small amount of distance between each winding. I chose a fairly close distance on these coils because a larger spread between windings means that it takes more turns to arrive at the desired inductance. More wire, is more cost and higher losses. I chose 20 turns per inch ( 1,2 mm each turn) for is coil made with 165/46 litz wire. (100 feet (30m) will wind both coils.
The variable capacitors are some nice ones with a 2:1 vernier that I purchased from Leeds Radio. (Tell Richard that you saw Schmarder using them!) They are two gang, with each side having 270 pf maximum capacitance. You may use 360 pf capacitors too. I made the best use from these that I could. The capacitor in the antenna lead uses only one gang. 270 pf is quite enough capacitance. The higher this capacitor is, the more sensitive the radio is, but the selectivity is destroyed. The 2:1 vernier is enough for the antenna tuner portion of this set as the tuning isn't as critical as in the detector side.
The detector side is a different story. Adding a 6:1 vernier to the 2:1 ratio vernier in the capacitor gives you 6 full turns of the knob and a full turn on the dial. Very sweet!
On the detector board, I "trimmed" one of the 270 pf sections with a 270 pf fixed capacitor. That gives a capacitance range of just over 400 pf. Looking at how the dial is still only about 300 degrees used, this value can be reduced down to 220 pf. That fixed capacitor acts as a padder type, which means that changing the value has the most effect at the bottom of the band.
I used the famous FO-215 diode in this design. I feel this diode has some advantage in this design and the cost for improvement is low. A general purpose germanium diode, such as the 1N34A will also work. The small trimmer capacitor is used to unload the tank (higher Q, better selectivity). The 27 mh choke going to ground provides a dc return for the detector. The second choke provides additional rf isolation. It might be helpful if rf is coming in on your headphone leads. Besides I have 1000 of these chokes, and maybe I can get rid of them twice as fast. :)
The little object in the middle is a matching transformer for this little radio. There is a selector switch to allow quick changing from single to dual coil reception. The switch on the left selects the impedance match to the headphones. You can use the taps that go with what you own. It might even be good to skip taps for a more dramatic change in impedance. If you want to use a crystal earphone, use the 10k tap. A Bogen T725 is the transformer. It is Bogen's gift to crystal radio. There are some audio matchers on my audio matching page. Sound powered earphones are desirable too.
The base is built on a piece of 12x6 inch (60x30 cm) piece of Garolite®. I raised the detector board variable capacitor an inch (24 mm) so that I could make a bigger 4 inch (10 cm) dial. A Brother P-Touch was used to make the labels.
This radio works well and as expected. Better variable capacitors could be used for a boost in performance but it wouldn't be a big boost. You get the best performance for the money by keeping the quality of the components (coil and capacitor) about the same. I don't think I would use these variable on a 660 litz radio, just as I wouldn't use high performance variable capacitors on a small count litz radio.
This radio is easily built by the beginner but has enough "meat" for the seasoned builder / user. The use of ceramic spacers, black Garolite® and the brass thumb nuts in an open frame construction gives this radio the laboratory look. I wish you good luck building a version of this set.
Good DX! Dave - N2DS