Dave's Full Detection Crystal Radio
An Experiment in Efficient Radio Reception.
Hello fellow radio builders! Instead of watching my tomato plants grow, I thought I would take my mind off that and make another radio. It had to be a big effort as this may be the radio that I use in the upcoming crystal radio receiving contest. BTW, this is not really a stereo crystal set, but a radio that is trying to make full use of the rf energy that comes it's way.
This DX radio took me almost 40 hours to build and I am a fast and experienced radio constructioner. Please do not attempt this as a first set, but more of one to look towards. Your first sets should be graded as easy on my main crystal radios page.
At this point, I have to put the "experimental" tag on this project. Before you got to see this project, I've had several friends evaluate this set. The feeling is that the set will give louder volume but may not be more sensitive. After hearing their line of thinking I am apt to agree with this.
There are several new ideas presented in this radio that have never been seen on my pages. Some of the ideas are experimental. These ideas are not original. One was published in 1926, but in a slightly different way from my presentation. Another first for my radios is the ultra slow detector tuning system. There are two vernier drives connected together to provide a very slow station tuning system. Friends have done this and have reported good results. The other development is a somewhat different selectivity enhancement circuit. This one uses a selector switch to get the right selectivity vs. sensitivity balance that we all look for.
Many of the ideas have been described in previous articles and projects on this web site. These include: contra coil, DX Notes, my #66, #68, #69 and my #71 radios. If you don't understand something on this page, I may have discussed it on one of these other pages.
The radio starts out as a typical two unit, two coil radio. The ATU or Antenna Tuning Unit is shown above on the left. This is where the antenna and ground connect, and the signal is then radiated to the detector unit.
This detector unit is complicated, but becomes easy if divided into different sections. The resonant or tank circuit consists of a contra coil with the associated high / low frequency range switch. This radio tunes from 500 to 1000 kHz and then from 1000 to 2000 kHz. Also there is a main tuning capacitor and a trimming capacitor as part of the tank circuit.
As it turns out, when you cut the inductance to 1/4 value, you double the frequency. I used this to my advantage to have a simple arrangement for the dial scale markings. This ratio of double the frequency isn't exact due to the distributed capacitance of the coil, but it will get you close enough to a good frequency reading.
The coil is carefully calculated to cover the exact frequency range that I wanted and with little overlap. My coils were made very aggressively to accomplish this. I also wanted the breakpoint to be 1000 kHz, so it was necessary to have the low range resonate at 500 kHz.
The detection comes next. I used two diodes, to each detect ½ of the rf
energy present at the tank. My hopes is to do this without a lot of extra loading on the
tank. There is some loading due to the reverse resistance on the diode, but I am hoping
that the loading isn't much and the increased power delivered to the phones makes it worthwhile.
If this is too whacky (or costly) for you, just take out one diode and one transformer.
I believe there can be a 3 db improvement in a perfect world with this arrangement. Besides that 3db improvement, I feel there is another 3db improvement because each detector is driving one earphone element instead of two. This is why I decided to "take-a-flier" on something like this. This circuit was originally called "take-a-flier".
Another part of this circuit is the "selectivity enhancement circuit" also called a Hobbydyne TM circuit. Instead of an expensive differential capacitor, I decided to do it with a series of fixed capacitors. The diodes are switched to various points on this series string of capacitors. It is like having several coil taps, but with capacitors. The highest position is the diode connected to the top of the tank. When switched to this position, the 27 mH dc load inductor is switched out of the circuit.
Each side of the detectors have their own dc balancing network or "benny" connecting to the top of an Überformer autotransformer. These are custom made transformers specifically built for diode to headphone applications. There is a 2 pole, 5 position switch that simultaneously switches the taps on the transformers to supply the headphones with energy.
My radio is constructed on two wooden bases, about 12 x 7 inches (30 x 17,75 cm). there are four 4 inch spacers that hold the chassis up. These can be longer if you desire to move the coil further from the desk. I used some 2 inch spacers to move the coils higher off the chassis.
The dual vernier system was tricky because to have a readout, the dial had to be attached to the vernier closest to the variable capacitor. I've seen several ways of doing this and they are all fine. But I wanted a nice looking detector unit, built in the way I had built my other dx crystal sets. I decided not to extend everything out the front, but to move the main tuning capacitor back some to make room for the extra vernier.
Next I had to figure out the slot for the dial. I decided that a 4-¾ inch (12 cm) dial was the largest I could use without scraping the wood base. I drew the mechanics of all this on paper and found that a 4 inch slit for the dial would be just right. There is a lot of measuring and figuring that goes into making all this. If you are not confident, try it on some scrap material first.
Once the capacitor is mounted, the rest of the pieces can be mounted. The coil is next, making sure it is symmetrical with the coil on the ATU (for looks). Then the 4 pillars can be located where they aren't in the way. The switches and jacks can be laid out next. Finally the transformers can be placed where there is room. It is a matter of mounting the most critical parts first.
Not shown in the pictures at this time is an additional SPST switch, which is labeled as S4 on the diagram. This disconnects the second diode from this circuit,making it into a more regular radio.
The switch should be wired to the diode which eventually connects to the ring terminal of the stereo headphone jack. This will allow the use of the more common mono headphone hookup when single diode reception is desired.
After your radio is all put together and working, then it is time to calibrate the radio. A good signal generator is essential for this task. Make sure your antenna and ground are connected when you calibrate your ATU. Loosely couple the generator to the antenna and ground terminals. Use your detector to tune the signals. Mark the dial at each 100 kHz points down to 800 kHz and 50 kHz points below that. Once your ATU is calibrated, it is time for the detector unit.
Set the frequency range selector to the high band. The selectivity enhancement can be switched to the off position or the next one from the top. Then starting at 1000 kHz, calibrate the dial all the way up to 2000 kHz. When switched to the low band, the readings can be cut in half. I marked my dial with both numbers.
To change the frequency range, the brass links have to be moved from one set of thumb nuts to the other. Since 5 connections are used, I left one of the screws / thumb nuts off. It is a pain in the butt to change ranges but the losses are lower with this kind of switch.
The selectivity enhancement circuit should be set to the wide position for the low frequency range and maybe at the second position down on the high range. You can go from there, depending on the strength of adjacent frequency signals.
The output impedance switch does control the tonal quality somewhat, so you can select what sounds the best to you.
I enjoyed making this radio very much. I do like sharing ideas and plans with all of you out there in the internet land. As I said in the beginning, this project is for the experienced builder. Build a couple of easier sets shown on my pages first before moving up. A big thank you to Peebles Originals for making some of the special parts for this project. Best wishes and good luck with your radio. Dave, N2DS