Crystal Radio #68 DX Radio
Seeing double? Well, not exactly. I built two radios at once! That's pretty good for someone that can't chew gum and walk at the same time. But enough about our political leaders. I have come here to talk about crystal radios. This will be a long story, so it is split between this page and my #Sixty Nine radio page. It is recommended that you start here and continue to the next page for the rest of the story.
The Big Idea
The idea of the two radios is so that I would have one to play with! Someone had a desire for my last dx set, so that one is gone. I was without a crystal set for a little while. In looking though my parts inventory (junk box), I found that I had two Holy Grail tuning capacitors that were alike, as well as a pair of premium quality ganged capacitors, also alike. It is off to the races now.
I decided that this radio would be the highest quality set that I could build, using the best features from previous sets. Some of my earlier dx sets had some extra features built in that were part of my research but maybe not of big practical value to most users. Then I thought of features that I felt would be useful to the crystal set user. Most of my research is now over, so I am concentrating on building efficient and easy to use radios.
The new feature of this radio is a way to switch in and out a small Selectivity Enhancement Circuit (similar to the Hobbydyne ®) This gives you the option of better sensitivity or better selectivity. Sorry, but you can't have it both ways at once.
Ok, this will be the big section. Some of you can skip this section, unless you like to listen to me pontificate. I have noticed in the past, that on some of my radio pages, I might gloss over something, because it was discussed in a recent page. I hate to repeat myself too much because that is boring to the reader. That is assuming that all my visitors have read every page. (A few wrote me to tell me that they have.) But in this case, I will go from antenna to headphones.
An antenna and ground are required to make this radio work. Since there are no amplifiers, the audio output is dependant on it's power from the transmitter. To think of this is amazing, that the station half way across the country is providing power to my radio. Go Tesla!
To efficiently couple the antenna to the detector, I use a popular circuit, as old as radio itself. It is now called a Tuggle Tuner. Simply stated, most radios that do well in the receiving contests, use this arrangement to couple their detectors to the antenna and ground. It is a simple circuit, using one coil and one dual gang capacitor. You see that I used a 4 gang capacitor on mine but two gangs are connected.
There are actually three capacitors in this circuit. The one you don't see is
the antenna - ground capacitance. So if you add the capacitance of the capacitor directly
across the coil, plus the series capacitances of the other gang with the antenna - ground
capacitance, that is the total capacitance across the coil, making a LC resonant circuit.
Some use two separate capacitors in their front ends, but that makes a third knob to turn. By adjusting the two capacitors, you can end up with a better sensitivity vs. selectivity outcome. The series capacitor can control this, while the parallel capacitor keeps the circuit tuned.
The coil is about 150 micro henries. If the combined capacitance of the ganged capacitor is at least 700 pf, then you can tune the entire band with most antennas.
In the case of my radios shown here, a 500 pf per gang, or total of 1000 pf are used. I did reduce coil winding by one turn and added a fixed 10 pf capacitor across the one capacitor in an effort to increase the dial spread. It is desirable to have as much space on the dial dedicated to frequencies that will be tuned. I do this by adjusting the coil value as well as placing a small amount of parallel capacitance across the variable that is directly across the coil. My 10 pf capacitor is really too small, but I was not unhappy with the dial spread that I already had. This is something that you can experiment with at your leasure. Variables with less than 400 pf per section will not need this tweak.
Now on to the detector board. Before I start, I went to mention the reason for making my radios in two sections. It is important to have the right amount of coupling between the two coils. Too little coupling and your reception is weak. If there is too much coupling, then the coil Q is degraded, making the selectivity poor. Over coupling also does not help the sensitivity.
The centerpiece feature of this radio is the Contra Wound Coil. I have a good description with practical examples on my ContraCoil page. I gave special detail to one of these coils in the second part of this article. Further detailed technical information is found at Ben Tongue's site. He developed this for the dx crystal set builder. The Contra Wound Coil is a practical way to enhance the performance of a crystal set at the high end of the band. There is no advantage at the low end over a regular wound coil, but it's the high end that is the crystal radio problem child.
Please look below at the pictorial diagram. The coil is switched to a series or parallel configuration with the 4 pole, 4 position rotary switch. Only two poles (section A and B) are being used to do the actual coil switching. The other two poles will be discussed later. Notice that positions 1 and 2 are tied together as well as positions 3 and 4. Positions 1 and 2 select a series coil configuration for tuning the low end of the band (to around 1000 khz). Positions 3 and 4 are for configuring the coil connections parallel for tuning the high end of the band.
When you make your contra coil, pay special attention how it is wound. That is discussed on my contra coil page too, as well as the next page of this article. If it isn't exactly right, the circuit won't work!
There are a couple of capacitors across the coil to tune the radio. First is the main tuning capacitor, in my case, 350 pf. (Contra coil systems are real happy with capacitors in the 330-375 pf range.) There is also a 100 pf air trimmer capacitor directly across the main capacitor. A contra coil circuit does not need a real large difference in tuning capacitance. If you use a high ratio tuning capacitor (350 to 15 pf is a 23:1 ratio), then there will be a very wide tuning range. This is ok except the desired frequency range is squeezed into a smaller portion of the dial. Adding a 75 pf capacitor across the 350 makes a 425:90 or about a 4.7:1 ratio. This, along with adjusting the inductance of the coil gives you a useful frequency range covers nearly the entire dial.
Still with me? Good.
Getting back to that 4 pole switch, if you look at sections C and D, notice that positions 1 and 4 are tied together as well as positions 2 and 3. When combined with section A, you get a total of 4 selections. That is Low band, sharp - Low band, broad - High band, sharp - and High band, broad.
The output of the contra coil is goes to the common of section C and the diode input is wired to the common of section D. Positions 2 and 3 are wired together in a bypass circuit.
Positions 1 and 4 are the sharp settings. The enhanced selectivity is made possible with the 3-20 pf variable capacitor plus the 27 millihenry rf choke. The series capacitor reduces the diode load on the tank circuit. This raises the Q, which improves the selectivity. In certain instances, having the right value capacitor setting, will improve the sensitivity too. So careful adjustment of this capacitor is recommended.
The detector is a germanium diode. I use a FO-215 old time diode that was discovered, had special high impedance characteristics not found in most diodes. This provides a better match to the tank circuit. (The "Tank Circuit" is a way of expressing the main resonant coil and capacitor.) A 100 pf mica capacitor is on the output of the diode is for RF bypassing. The value isn't critical as long as it doesn't roll of the high audio component of the detected signal.
The next goodie is that little R-C combination of a 180k ohm resistor and a .1uf capacitor. This is known in the crystal radio circles as a Benny. Ben taught us that when a strong signal is present at the diode, severe tank loading and distortion can occur. This is because the diode becomes much lower impedance and the dc resistance of the transformer affects the tank. This effect is reduced by using that little R-C network. It provides the high dc resistance that won't load the tank, while the capacitor easily passes the audio to the transformer.
The transformer is a specially built unit just for the crystal radio hobbyist. It is called The Überformer. It is only available from 1n34a.com. After the transformer, comes the headphones. A single pole, 5 position selects the tap on the Überformer that best matches the headphone impedance. Sound powered or balanced armature headphones provide the best sensitivity for good reception.
Ok, you did well. If you understood some of that, it is time to go to the second part. Don't smoke em even if you have em, and look for this to be continued on the next page.
73 and good crystal DX. Dave - N2DS