Crystal Radio #66,"David's DX Driller"
Hola Amigos, and welcome to another one of my crystal radio project pages. The name The DX Driller Thriller was suggested to my by a member of the RadioBoard. The person to blame for this is member: OldOhioIn. It was a good name for two reasons. First, it looks like an ocean oil drilling platform, and second it is as expensive as oil to build!
You regular visitors here know that I build all kinds of radio projects, from the simple ones that can built in an hour or two to the big contest grade dx sets. This is one of the latter. The expense wasn't spared in the technical workings of this radio.
A few newer features of this set starts with a Contra Coil in the detector unit. This is a dual winding coil that promises and delivers better performance at the top end of the band. Also, a trimmer capacitor network is added for improved dial spread. The numbers aren't crunched together on this set. Finally a new custom wound audio autotransformer, which I have named the Überformer provides a good match between the detector and the headphones.
Little extras, such as a 5 position headphone impedance selector switch,
and terminals to allow for single coil operation (with a wavetrap) were added for good measure.
This radio was constructed mostly with HDPE (high density polyethylene). This material was found to have excellent low loss rf properties. The idea behind the raised deck is to move the coil further from objects that might detract from it's performance and to bring down the size a little.
The main panel is 1/4 inch (6mm) thick HDPE. This provides a very sturdy platform to mount the coils on top and the rest of the components on the bottom. The panel is raised 4 inches (10cm) by four plastic standoffs. Both halves of the radio look nearly alike. It is important that your two variable capacitors have the same distance between the shaft and the base. If not, the height of the tuning knobs will be different. You can also place spacers under the capacitor that is too short.
The front panels are made with Garolite®. I didn't want an all white radio. The panels were cut in a decorative manner. An oak base makes a nice base platform for the threaded spacers to attach. As I was building all this, I knew that it was going to be an unusual design.
The Antenna Tuning Unit (ATU) has a 150 µh coil. This is made on a 6 inch (15 cm) form made with 1/8 inch (3mm) thick HDPE. There are 39 turns of 660/46 high performance litz wire wound on this form. The hub diameter is 2 inches (50mm).
I had a very nice 3 gang variable capacitor in my junk box just waiting for a radio like this. Only a 2 gang variable capacitor is necessary. But since I had the three gangs, I had to do something with them all. Otherwise you might think that I just use anything I have. Well I do. But to use all three gangs, I came up with a little circuit to adjust the series antenna capacitance. The 200 pf section connects directly to the antenna, while one of the 475 pf sections is placed in series with a 140 pf air trimmer capacitor. By adjusting the trimmer value, anyone can finely adjust the antenna tuner to their antenna. The radio can be tuned to the bottom of the band, and with the antenna connected, a signal generator at 530 khz can be used and the trimmer adjusted so the dial matches the generator. But you aren't likely to have this capacitor available, so use any dual gang that adds up to a total of at least 750 pf.
There is a 6:1 vernier that is coupled to the variable capacitor. There
must be isolation between the capacitor and the vernier. I used a ceramic coupler.
Also important is that the vernier drive itself be grounded. You can use a solder
lug under the nut, or just solder a wire to the vernier.
There is an extra set of terminals across the coil that may connect to an external detector. This is if you want to have a single coil radio. The output is louder, but the selectivity is less. This makes this radio more versatile. There are matching connections on the detector unit, or a separate experimental detector system can be used.
The detector board is where the big money goes. Start with the contra wound coil, holy grail
variable capacitor, differential capacitor, right out to the audio transformer.
If you have to economize, do it on the ATU. The selectivity heavy
lifting is done on the detector board. Using smaller litz or lesser capacitors on
the ATU won't hurt you as much as cutting corners in the detector board.
The detector unit is built on a piece of HDPE that is 11 inches (28cm) by 7-1/4 inches (18,5cm), the same size as the ATU. The width was barely enough to mount the band switch and selectivity control. Make sure that you have enough room for everything.
The band switch should be of good quality. A ceramic switch is essential to maintain the low losses. A switch made with two metal links and five thumb nuts can also be used. My #64 radio has this kind of a switch. Make sure to check your wiring connections carefully when wiring. It is very easy to get this mixed up. Make drawings and check them twice. If your set doesn't tune right, you either made the coil incorrectly or the switch is wired wrong.
By looking at the under view, you can see the main tuning capacitor in the center. This capacitor is connected directly to a 6:1 vernier drive. I cut a 4 inch styrene disc to attach to the front of the vernier for a calibrated dial. It is recommended that you wait and do the calibration last. Make sure the radio tunes right and everything is tightened down before doing this fairly long task. Since a low loss material is used against the variable capacitor, no ceramic standoffs are needed.
There is a vertical piece of HDPE that is placed about 5-1/2 inches (14 cm) behind the front panel. The band switch, differential, tracking and trimmer capacitors are attached to this sub panel. Long insulated shafts are used to bring the controls out to the front panel. This eliminates the pesky hand capacitance effects.
All the other components are mounted or soldered on the bottom of the main board. This keeps it all neat and tidy. A link switch is used to disconnect the main LC circuit from the detector. This is done at the differential capacitor connection to the main tank. Two thumb nut terminals are provided to connect an external tuning circuit to the detector. It is provided to mate with the corresponding connections on the ATU. Other LC circuits may be used too.
Part of the planning is the placement of the litz coils. The parts mounted on the top must not interfere with the bottom mounted parts. There is a lot of latitude on where the coils can be mounted. I mounted my coils as close to the middle of the main board as I could. I had a little trouble with the ATU because of the variable capacitor, so I moved the coil. The distance between coils is 9 inches (23cm), which is not too far away for good coupling. The best coupling is usually obtained at about 15 inches (38cm).
The differential capacitor is used in a selectivity enhancement circuit. As the capacitances lessens between the tank and diode, the selectivity improves. But there are losses across the differential. A good balance can be found and except for the extreme dx digging, this control can be left alone. If you don't have a differential capacitor, don't worry. Just use a 3-30 pf variable capacitor.
The reason for a differential capacitor is to compensate for the frequency shift due to capacitance change caused by change in the selectivity enhancement circuit capacitor value. By placing one of the differential capacitor leads to ground, reduces the shift. But not being a perfect world, an additional tracking adjust trimmer is needed.
To adjust this circuit, after the radio is completed, adjust the differential so the maximum capacitance favors the diode. Then listen to a station at the top of the band. Then adjust the differential to minimum capacitance to the diode side. Adjust the tracking capacitor so you get the signal peak again. Go back and forth until the circuit tracks. This will take some experimenting and fiddling. You may not be able to track over the entire range of the differential. In that case, try for tracking over 120 degrees of the rotation.
The next adjustment is the frequency trimmer. The higher the value of the trimmer, the lower the tuning frequency and the lower the frequency tuning range width (higher dial spread). A high dial spread is sought, but also there must be some frequency overlap between the high and low band ranges. My overlap is from 1020 to 1050 khz. If you increase the trimmer too much, you may have no overlap at all, or worse yet, a tuning gap!
A large 290µh coil was required to do the job. This is because of the fairly low 280 pf variable capacitor. A recommended value of capacitor is over 300 pf. a 330 to 360 pf is optimum. Capacitors of higher value can be used but other changes will be needed. Please visit my contra coils page and look at the contra coil design section. Don't start this project unless you, or the person that makes your coil understands the principles of this section.
Each time I make a dx radio, I learn more and make improvements. I think they are improvements. The older sets also worked very well. This radio builds on what I learned about contra coils this summer. I was impressed with my #64 radio. Someone else was also impressed and managed to get it from me (I hate to say no). I was radioless, so it was time to make another. Now I have a radio for the next contest.
73 and good crystal DX. Dave - N2DS