Welcome to my cylinder coil page. For most of my crystal radio career, I have focused on using spider coils. I still really like them, but other types of coils are fine too. This article will discuss some of my findings and examples of how I wound some of my coils. This page is likely to be revised as I build more cylinder coils.
3½ Inch Diameter Former
The basis of this former or coil form is the Genova Products S40130 styrene plastic drain pipe. This has been tested for low loss at MW frequencies. They are readily available (check your DIY home improvement center), and cheap! This is a perfect situation all around.
The pipe sizes available (inside diameter) are 2, 3 and 4 inches that we might be interested in using. The outside diameter is what we are interested in. The three inch is 3.5 inches (89 mm) on the outside. I chose the coupler size to match the wire size. That is, 165/46 and 100/45 wire nicely fill the coupler. Larger size litz will need the 4 inch coupler. The two inch coupler might work for smaller litz sizes, but I have not seen the dimensions of this piece yet.
I've found that only two coil inductance sizes are used for most radios. The nominal values are 240 and 150µH. The 150µH coil is used as a T-Tuner (Tuggle Tuner). This is a simple hookup usually using a dual 365pF type variable capacitor and the 150µH coil. The coil is connected from the rotor to a stator. This stator is also the radio ground connection. The other stator goes to your antenna. This provides overall good coupling, while maintaining a reasonable circuit Q across the entire band. This circuit is used in all my two coil dx crystal receivers.
The 240µH coil is used in the detector tuned circuit. The detector coil is inductively coupled to the antenna coil mentioned above. This coil will resonate the entire MW band without changing coil taps or adding an extra capacitor and the switch. The capacitor should be a 365-15pF type.
Other variable capacitors may be used, but the coil value may have to be adjusted. If the maximum to minimum capacitance ratio is low, very careful adjustment of the coil value is required. In some cases. this could mean half turn coil values.
The other coil that I will include is my favorite, the Contra Coil. This coil was brought to my attention by Ben Tongue at his website. It took me about two years before I realized the value of this type of coil. Now I am hooked. Since the contra coil is described in detail my contra coil page, I won't repeat it here.
The nominal value of my contra coils is 240µH series connected and 60µH parallel connected. As shown in my contra coil article, these values will differ depending on the value of the variable capacitor selected.
The reason for all this?
Why did the UPS guy give me that strange look when he brought my first 40 sewer or drain pipe couplers? (I don't know about you, but 3 inch would be too small for me.) Well, I'm always looking for the next way to build a crystal radio. I've built some pretty nice dx sets, including my #66 radio.
Some builders have become frustrated when attempting to look for the Holy Grail, super great capacitors. Then when they priced the big litz wire, I heard some fainted or were threatened with divorce by their spouse.
So to save the crystal radio builders from this disaster, I took it upon myself to design a budget dx receiver. My plan is to have all the components and design equally matched. That is, the coils, capacitors and matching transformer are designed to give approximately equal performance. I believe that this attention to these points will provide the biggest bang for the dx radio buck.
The preparation of this new receiver has started with the transformer, verniers, capacitors and now rests with the coil. More about this will be developed as the building progress. Please keep an eye on my main radios page for any news.
Some 3½ Inch Cylinder Coil Examples
Please note that the distances shown below are in inches. The mounting holes are 1/4 (6,3 mm) inch from each end.
The hole size to feed the 165/46 and 100/45 litz through is 5/64 inch ( 2 mm).
The measurements for the wire holes are based on a 3-1/4 inch (82,5 mm) long form.
The four center holes on the contra coil are 1/8 inch from the center.
The form is a Genova Products # S30130.
The inductance and Q measurements shown are with my HP model 4342A Q meter. Inasmuch as this is an old instrument with no knowledge of the former calibration history, you should take the measurements as that. But the relative readings between coils should be accurate.
L uH Litz Size Turns Coil Width Q-1.6MHz Outside Wire Holes 238 165/46 47 1-7/16 770 15/16 inch from each end 260 165/46 51 1- 5/8 840 13/16 inch from each end 148 165/46 35 1-1/16 800 1-1/8 inch from each end 235 165/46 25 * 7/8 950 3/4 inch from each end 247 100/45 46 1-3/16 710 1 inch from each end 286 100/45 51 1- 3/8 700 15/16 inch from each end ** 151 100/45 34 7/8 630 1-3/16 inch from each end 252 100/45 25 * 11/16 770 13/16 inch from each end * Contra Notes: 2x the windings listed. Q measured with leads tied in parallel. Hole distance between the two coils is 3/8 inches ** This is a special coil for use with 330pF variable caps.
Building Your Own Cylinder Coil
Building these coils aren't too difficult. First you have to remove that pesky UPC label. I found that some "Formula 409" cleaner would remove the sticky residue. You want to remove it all because dirt will cling to that and the coil will soon look crappy.
Next take a wide piece of masking tape and lay it across the form. This gives you a surface to mark on. By setting the form on a flat surface and using a carpenters square, you can mark three lines. One for the mounting holes, two, about a half inch apart for the wire holes. Each end of the wire needs two holes.
Then you measure from the edge of the form to make your holes location marker. The mark locations are shown above. I took a picture of a contra cylinder coil just after drilling the 10 holes. I marked the hole locations with a pen, but for this picture, after I drilled the holes, I used a black sharpie so the photo would pick up the lines.
The hole sizes are shown above too. Then peel off the masking tape and start winding your coil. If you would like to have a different inductance, then you should go to the Professor Coyle calculator.
Mystery Radio Coil
Here is my version of the famous Mystery Crystal Radio Coil. My last cylinder coil mystery radio was my #3 crystal radio set. I have always found the mystery set to be very interesting, operational wise. It is a very loud set too. Is it a dx radio? I have never though so. But we'll see how this one does!
After I got the design for the regular cylinder coils finished, I thought it was time to branch out to some new designs.
The mystery radio coil is made up of two windings. One is the tuned circuit and the other is the coupling winding for the detector. The main tuned winding has 48 turns of 165/46 litz wire, and the detector winding has 24 turns of 40/44 wire. Because I have a limited length of space on the coil form, I couldn't wind both windings with the larger litz. But I believe that there isn't too much difference by using a smaller wire for the detector.
First, the holes have to be drilled in the former. There are three sizes of holes. The distances mentioned are measured from each end. There are a total of 10 holes. Here are the measurements:
1/4 inch in from each end, two 9/64 holes (6mm 3,6 mm) These are the mounting holes.
5/8 inch in from each end, two 5/64 inch holes (14,25mm 2,0mm) for the 165/46 litz.
1-1/16 inch in from each end, two 1/16 inch holes (27mm 1,6mm) for the 40/44 litz.
The picture below clearly illustrates these holes.
I decided to wind my coil in a slightly different manner. I wound the full 48 turns of the main tuning coil using the 165/46 litz. I didn't tighten the ends real tight and using my thumbnail, I close spaced 12 turns at one end of the coil, exposing the 40/44 holes. I continued to space the wires down the coil, and then close spaced the last 12 turns.
Then I threaded the 40/44 wires through the pair of holes after the first 12 turns. Before I did this, I found it was handy to heat up the end with the soldering iron and add a little solder. That stiffened up the wire a bit and made the threading easier.
I wound 24 turns of 40/44 by placing the wire next to the 165/46 wire already wound on the former. When I wound the full 24 turns I then added some length for the lead wire and cut the litz. Again I heated the end so the threading would be easier. This type of side-by-side winding is called bi-filar wound. Or is it bifilar? In any event, my spell checker won't like the word.
After winding the coil, I tightened the ends by taking up the slack. Then I did my little freezer trick that I wrote about in the next section, and then tensioned the wires again. A couple pieces of masking tape on the inside of the former helps keep the wires tight as the temperature increases.
The inductance of the tuning coil is 214µH with an unloaded Q of 730. The detector winding measures 86µH with a Q of 140. I don't think these numbers are that meaningful as when the connections are made, the Q will be clobbered.
I will be putting this coil through further tests as time goes on. I expect that the litz will give the radio that this goes in much more zip.
A Cylinder Coil Tip
A problem I have always had with cylinder coils has always been trying to keep the wire taut. I know that this is a universal problem as others have discussed this with me.
Here may be a solution. When I made these coils, I wanted to see if placing the coil in a cold place would tighten up the windings. If so, I could place the coil in a heated area, then let the wire expand and then tighten it. Perhaps tightening my coils on the hottest day of the year would be a good activity.
However the opposite was true. The windings became very loose. Since I was sure that the copper litz wire would have contracted, I was surprised to find out that the styrene form shrunk to a greater degree!
I then tightened the windings on the cold coil. I then found that as the coils reached room temperature, the coil windings got very tight have remained so. I then placed the form outside again and when I brought it back in, the windings were only a little loose. As the coil warmed, the windings were taut again.
So my little tip is to place the coils in the freezer after you have finished winding them. If the wires become loose. Take the form out of the freezer and tighten the wires up.
When the wires are snug, I place a piece of masking tape on the inside of the coil over where the wires are fed through. At this point I am reluctant to put nail polish on the wire as I'm not sure what more adjustments I will have to do to the wire. It may be that each winter you will want to put your radio outside to get pensioned.
That's about it for now. Good luck with your coil project.