Two Radios in one — My #24 Crystal Radio
Hi there! This is another of my recently revised radios. Not that I built them wrong to begin with, but I come across new and better materials to make my sets. I don't mind going back and revisiting a set now and again.
Building on the success of my last radio, I proudly offer this crystal set for consideration for your next construction project. This is another one of my dual circuit crystal radios. The idea for this set comes from Mike Tuggle's Lyonodyne 17 and experience that I have gotten from my past sets. I found that with a very simple switching system, I can make this radio's circuit act like two different ones. I have wanted to build a Lyonodyne type set for a while but couldn't resist adding the extra switch.
This radio features the use of three variable capacitors. While at first, this is quite a chore to tune, after a while you would get used to where the dial is to be set for certain stations. You also have a choice of two radio circuits in one radio. One is the single coil that works very well and is featured in my #12 and other sets. A hidden feature of this radio circuit is that the unused coil / capacitor becomes a wave trap! This wave trap can be used to reduce the signal of a loud interfering blowtorch station, allowing you to hear other, weaker stations that are close in frequency.
The two coil receiver is similar to the Lyonodyne circuit. The series tuning capacitor has been transposed so it is in the antenna line, rather than the ground line. I also decided to use two capacitors, rather than one capacitor. This increases the tuning complexity of this radio. The circuit was explained in some forums, but I just couldn't wrap my arms around using one ganged capacitor, so I used two. Since this is my radio, I built it my own way. It is kind of a "German" in me.
The antenna coil is the original one I used in this set. I could have used 660/46 litz on both coils, but this works dandy the way it is. The best inductance in the antenna coil will depend on your antenna. It is cut and try but you have leeway. My antenna is a single wire about 75 feet long.
The detector coil is made from 660 strand 46 gauge litz wire. This is the first coil I wound with the big litz. It came up short from the 240 µH that I wanted for my other project but will work pretty well in my single / double coil arrangement. The worst case is that the low end of the band can't be tuned without an extra capacitor.
The coils are mounted using dowel rods and 1/2 inch square wood blocks. I have used this scheme in my #23 and #16 sets with good results. I made it so the antenna coil could slide to adjust the amount of coupling used. Most of the time I use them fully apart for the best selectivity. If you experience real weak reception, you can move the coils closer together.
My revision included changing the detector circuit to match the extra high performance of the detector coil. This included using the 100k to 1.5k ohm audio transformer. You can pick out the transformer that meets your headphones requirements, or build a matching box. The coil and the transformer turn this set into a high performance radio. If you can, try to find good quality variable capacitors. The ones commonly available really aren't that good in high performance radios. The capacitors I selected have a ceramic insulation on the stator.
The circuit details is where it starts to get interesting. Look at the circuit below while I rattle on about this set. When the switch is set to double, you see an antenna input, series capacitor connecting to a coil / capacitor parallel circuit. If you set the switch to single, you see an antenna input, series capacitor also connecting to a coil / capacitor parallel circuit. In the case of the double switch position, there are two tuned circuits, compared to a single tuned circuit in the single position. I just found that highly interesting and also helped me in how I should tune this set.
Here is how I tune this radio. There are three variable capacitors, the series (in series with the antenna), the antenna tuning (across the antenna coil) and the detector tuning (across the tapped coil). Tuning to maximum means that the capacitor plates are fully meshed and minimum means the plates are open. Ok? Here we go.
Single position tuning instructions: Tuning starts at the high end of the band and works downwards. Set the detector and series capacitors fully open. Fully mesh the antenna tune capacitor ( if you are near the low end of the band, this capacitor should be fully open as it is a wave trap). Rotate the series capacitor as you listen for stations. If you hear a strong station, and want to increase the selectivity (at the cost of some sensitivity), mesh the plates of the detector capacitor a little and then peak the series capacitor to maximize the signal. After you make your first pass, set the detector capacitor at a third of rotation and start tuning again. Do this until you have reached the bottom of the band. The antenna capacitor can be tuned to tune out a loud station so you can hear others. Try it.
Double position tuning instructions: Tuning is a lot like in the single position. Set the antenna capacitor at minimum. Rotate the detector capacitor a little at a time while sweeping the series capacitor. You will be tuning starting at the high end of the band. When you hear a signal, then start peaking each capacitor starting with the antenna, then detector, then back to series. Keep repeating that until the signal is at full strength. Having numbered knobs and keeping a log of the settings helps finding the station again.
As a little "extra", I have included a little circuit to use if your sets don't tune the full range. It is a padder arrangement. Instead of hooking two gangs of a capacitor together for full range tuning, adding a padder capacitor will allow you to tune only to the bottom of the band. That way, you can spread out your tuning range to fit the full rotation of the capacitor. It does require the use of a 2 gang capacitor.
I haven't talked too much about construction details on this page. I have a new construction hints page on my site. Also, take a look back through my previous sets for little nuggets of hints. If you have a question or two, or would like to chat about crystal sets, I am only an e-mail away.
Best wishes and good DX. Dave - N2DS