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Contra Wound Coils

Hi Friends. Welcome to my contra coil page. The coils that I am showing on this page are used as the main tank coils for crystal radios (and tube radios too). I believe that this improved type of tank coil will be one that you will want to incorporate in your next crystal set.

I owe the ideas on this page to Ben Tongue. A while ago he published this very interesting article on his web site. The article describes a radio he built that has a constant receiving bandwidth across the MW broadcast band. Reading this article is very worthwhile.

My goal isn't to have constant bandwidth, but improved performance where it is really needed, at the top end of the band. There are several difficulties in tuning the high end of the band and I believe the contra coil will improve your dx reception.

The contra coil in its simplest form is two equal coils wound on the same coil form, but in opposite directions. The coils are connected in series to tune the low end of the band and in parallel for the high end. How the coil is wound and connected is where the secret lies. With the windings wound as they are, the losses are low and the Q is high. The variable capacitor also operates in the sweet spot, further cutting losses. On the high end of the band, the litz strands are doubled with the parallel connection. That's got to be good!

Ben's coils have extra taps for the diode connection. The tap used is dependant on the part of the band that is tuned, along with connecting the coils in series or parallel. He splits the band into 4 segments. For now, I'm leaving off the taps and going with a hobbydyne type circuit that will allow for variable tank loading and matching.

Cylinder Coil ~ A Good Place to Start!

The first coil shown is my test prototype cylinder coil. I felt I should build one cylinder type to get the feel of how this coil will operate. The coil form is a styrene sewer pipe coupler. I bought it at Home Depot. The outside diameter is 4-1/2 inches (11,5 cm). There are 22 turns of 165/46 litz wire on each winding. Each coil calculates out to 27 feet (8,25 m), including 6 inch (15 cm) wire leads. The start (s) windings are in the center, and the finish (f) windings are towards the form edges. When winding the coil, start at the center. Wind the first coil. Then when winding the second coil, beginning again at the center. The two start windings will wind in the same direction around the form. That makes the coil wound reverse, or contra wound around the form.

To connect the coils in series connect the start of one coil to the finish of the other coil. The other leads are connected to the rest of the circuit. For parallel operation, the two start wires are connected together as the two finish wire are connected together. The connections to the rest of the radio are from these points.

When the coils are in series, the inductances add (plus the mutual coupling). This is because the coils are in phase. It is just that the wiring connections jump to different positions on the forms. When the coils are in parallel, they are also still in phase, but at 1/4 the original inductance value.

The inductance is approximately 240 µh in series and 60 µh in parallel.

The Switch

It is likely that you will want to use some kind of switch to go from series to parallel. Try to find a low loss switch, such as a ceramic rotary switch, or use thumb nuts, and brass links mounted on low loss materials. The circuit is shown below. It is important that the coil and switch be wired exactly as shown. If your radio doesn't work, check the wiring first. I included the physical wiring pictoral below. This is from my #64 contra radio. The picture of the switch is the top view, while the pictoral is how it is wired from the bottom.

Design Your Own Contra Coil

Now how do you design your own contra coil? Here is the place to start. You may want to skip this part for now and look below at the pre-designed coils. If your tuning capacitor matches one of the situations below, you don't need this section.

You will need a few things before you start. First, bookmark this page on crystalradio.net. This link sends you to Dan Petersen's Professor Coyle calculator. This takes nearly all the math out of designing your own coil. You want to select the cylinder coil calculator as this one has the resonance calculator.

If you build a coil, you should have an L/C meter. I use the one produced by AADE. It is important to be able to balance the inductances of the two coils. If you don't have one, you can just shoot for the best by using the designs that I have made.

If you want to test the actual tuning ranges, you will need an accurate signal generator, a capacitance meter and a scope or other rf level indicator. If you are building one coil, you can just put it in your crystal set and check out the tuning range by listening for the stations.

It is best now to go over some of the basic design criteria, map out what we have and discover a few truths and speculate on some assumptions. So here we go:

Here is an example: To start, you have to measure and add the capacitances for when the tuning cap is at minimum and maximum. Add 25 pF for the extra radio capacitances. Also include a starting value of 10 pF for the trimmer setting. The values are reached by rocking the values back and forth until a suitable value for the coil is found.

Let us assume a 15-350 variable capacitor plus 35 pF for the extra capacitances described above. You can see them in the schematic shown above too. This means that the circuit capacitance ranges from 50 to 385 pF. These are starting values, and the minimum and maximum values are likely to be more like 80 to 415 pF as the trials go on. The starting value for the inductor is 240 and 60µh.

Starting with the low end, plug in a value of 240 µH and 385 pF into Professor Coyle. That is a pretty close 524 khz. Lucky, hµH? Ok, now use the 240µh coil and plug in the minimum total capacitance. That comes to 1453 khz. That is too wide of a frequency spread. Instead of going further, we will change the maximum and minimum capacitor value and start over.

Now plug in a higher value of maximum capacitance and adjust the coil value to a 520-530 khz range. Let's try 415 pF. That drops it down to 504 khz with a 240µh coil. Let's reduce the value of the coil. 225µh brings the frequency in at 521 khz. Now let's try the minimum value of 80 pF with a 225µh coil. That sets it at 1186 khz.

Ok, it is time to try the high range. Since the low range is 225µh, the high coil will be about 56 µH. The low end with 415 pF now tunes 1044 khz. We have more than enough overlap, but the parallel coil low end tuning range is a tad high.

So let's go back and turn up the trimmer 20 more pF. This will give us a capacitance range of 100 pF to 435 pF. 225µh with that capacitance tunes 1061 to 509 khz. The 56µh coil and 435 pF tunes to 1020 khz. That is a 41 khz overlap. This looks like a good value to go, but let's make one more tweak.

How about raising the inductance to 232µh. The tuning range is pretty good but the low to high band split is a little higher than I like. The capacitance at 100 to 435 pF is good. Remember that the trimmer will take care of the inaccuracies.

So 230 µH and 435 pF tune to 501 khz and at 100 pF tunes to 1045 khz. The parallel coil is now 58µh. That tunes down to 1002 khz. That is a 43 khz overlap. The trimmer will let you adjust to 1041 to 998 or 5 channel overlap.

Now a couple of things: First, you won't get the coil that close, but if it is plus or minus 5µh, that is fine. The trimmer will iron out the glitches.

Notice that we didn't talk about the high end of the band in the parallel coil configuration. This isn't important as it will always fall above 1700 khz.

You will have good dial spread with these values. The actual dial spread will depend on the shape of the capacitor plates (straight line capacitance, or straight line frequency). You will have better dial spread than with regular wound coil.

If you are unsure, wind the coil on the large side. It can be taken apart and adjusted if you are real far off. It is better to have less dial spread than not being able to tune the band.

This works with a cylinder wound coil or spider coil.

So to recap, we found a 232/58µh coil would be good with a 15-350 pF capacitor with a 60 pF trimmer (with the fixed 25 pF for the radio and coil capacitance).

Recently Jeff Welty whipped up this page for calculating spider contra coil. It is very complete, from entering the data to printing a coil form template.

So here is the calculator. I'm sure this will take the misery and doubt out of your coil building.

I wish you the best success with your coil. (and luck too) :)

Dial Spread

There is a special feature that comes with the contra coils. That is, the tuning range on each band is somewhat wide. This means that a capacitor with a big capacitance ratio isn't really needed. But what happens is that the dial spread is not as good as it could be. Take a look at the two pictures. They are the same radio, kind of a before and after shot.

The top picture shows the tuning covers a much narrower part of the dial scale as the bottom picture. You have to look close at the numbers as the ranges are a little different. The left side is the low band (530 - 1000) and the right side is the high band (900 - 1700).

The difference is that I placed a small trimmer capacitor (about 75 pF maximum) across the main variable capacitor. Now the capacitance ratio of my variable is about 6:1 (90-550pF). Before the ratio was about 23:1 on this capacitor (20-475pF).

I did try a more aggressive type of band spreading using both a trimmer capacitor along with a padder type. But since I am stuck with a fixed 4:1 coil ratio, I had some trouble getting the dial spread to work correctly on both ranges. However with the way I did the dial spread, I get about a total of 2 turns of the knob in each range. A regular coil set, such as my #63 is just under 3 turns of the knob to cover the entire band. Another contravantage.

Your actual situation will be different, depending on the actual components you use. But after you build your set with a contra coil, investigate using a trimmer to widen your dial spread.

My first contra set is finished. It is my set #64. It is a set with a dual contra coil, one for the antenna tuning unit and the other in the detector. I am very pleased with this set. If I wasn't, it wouldn't have received the coveted set number. :)

Q

I took more unloaded LC tank Q measurements, this time taking more care. Below are the numbers, and a picture of my test setup. Q measurements have not been a high priority for me or this web site. I once sent the same coil to several people and the results reported varied greatly. These people were in possession of professional equipment.

My tests involve measuring the Q of an unloaded LC tank circuit. This type of measurement doesn't take into account other factors that adjust the Q, such as distributed capacitance corrections. Even in my test setup, I found the numbers could vary a lot, depending on how the coil was tested. One thing that was pointed out to me is that a coil has a fixed and specific Q value. Finding that true value isn't always so easy.

So please remember that everyone's unloaded L/C Q measurement will likely measure differently. But since I used the same techniques for each of these coils under test, my numbers can be used to compare one against the other. Other than that, I am not making any Q accuracy claims here or anywhere else on my web site.

                       1600 khz        1000 khz        1000 khz        600 khz
                       Parallel        Parallel         Series         Series
     660/46 234/58 µH    670             910             625            750
     165/46 266/66 µH    484             588             384            428
     100/44 260/65 µH    347             476             344            400	
      40/44 340/85 µH    258             306             217            240
	  Cylinder Coil
     165/16 260/65 µH    410             454             357            375	  
      	  

My Winding Experiences
or DIY With The Cookie Cutter Approach!

Below is a table of some of my winding experiences. I allow for a 50khz minimum overlap, with a reasonable dial spread. These are not "aggressive" coil designs. That is, I am not cutting the parameters so close that success is not assured if something is a little off. Variable capacitors down to 280 pF can be used but it is recommended that you try a 330 pF or higher variable. The design can be tight for the lower values.

L is the total series inductance. (The parallel inductance is 1/4 that value)
OD is the outside diameter of the spider (flat) coil form in inches.
L1 is the number of turns on the inside coil.
L2 is the number of turns on the outside coil.
LITZ is the litz wire size, number of strands and gauge of each strand.
CL is the variable capacitor minimum value.
CH is the variable capacitor maximum value.
CT is the trimmer value, (approximately)

A 20 pF detector capacitance is assumed. The trimmer will fix this inaccuracy.

The hub diameter is always 2 inches (50 mm).

The form material is .125 inch (3mm) thick HDPE (High Density PolyEthylene).

  L   OD   L1  L2   LITZ    CL    CH   CT
 250   5   29  22  165/46   15   365   60
  
 204   6   27  20  330/46   15   420   80 
 
 187   7   26  18  660/46   15   475   85  
 218   7   28  19  660/46   15   400   75   
 240   7   30  20  660/46   15   350   75
 290   7   32  21  660/46   15   280   40
  

I have noted after doing about a dozen of these contra coils is the inside to outside ratio is not constant! As you add an extra turn to the inside winding, only about a quarter turn is added on the outside. I believe this is due to the outside diameter is getting a lot bigger than the break point of the coil. When I started all this, I used two equal lengths of litz. That gets you pretty close. Just add a little extra on the outside to trim.

There is something else I noticed. The inductance of one of the coils is approximately equal to 1/3 of the total inductance when the windings are in series. If I find this to be true, then the inner winding could be wound a little high and then trimmed before adding the second winding. But somehow I don't think that will be necessary.

Spider 165/46 Contra Coil

You know me: I never met a spider coil that I didn't like.

I do believe that the best crystal sets are made with spider coils, or rook coils. Once I tested the cylinder version of the contra coil, I knew I had to port the design over to a spider.

The problem was how would I wind the coil without making a mess or wasting a lot of litz? The two coils have to be fairly close in inductance. If the inner coil were to be too large, that would mean unwinding the coil and trying again. If the inductance is too small, then I have to use more litz to wind a new coil. Since this coil is much like a regular spider coil, I went with the amount of wire that I would have used if the coil was a regular one, plus some extra for the wire leads.

Here is where I did the work, so you don't. I decided on 9 inch leads for the coil. The total amount of wire needed is 46 feet of 165/46 litz. The coil form has a hub of 2 inches (51 mm). The outside diameter need is at least 5 inches (12,7 cm). Start winding the form with the first coil. At 29 turns, after allowing for the wire lead, cut the litz.

Skip one slot and start the next coil. Wind another 22 turns, but in the opposite direction. 22 turns will give you a little more inductance than the first coil. If you don't have a LC meter, then make the last winding 21.5 turns. If you do have some way of measuring inductance, then you can adjust the outside winding to match the inside winding inductance exactly.

Once you have wound both coils, space the wires using an object of your choice. Make it all neat and pretty before doing the next step. Measure the two inductors. It is likely that the outside coil has a higher inductance. Remove a little of the wire of the form, maybe a half turn or so. When the inductances are equal, then trim the wire and tin the end.

As with the cylinder contra coil, making the right connections is important. The two connections in the middle of the coil are the start(s) wires. The inside and outside wires are the finish(f) wires. I think it is best to have the wire that comes off the start winding of L1 be the ground side of the variable capacitor. Please refer to the diagram above for the connections.

Having a low loss switch is important for maintaining the high Q of this circuit. A ceramic insulated rotary switch is excellent. Phenolic insulators tend to be lossy. In my example, I built a switch using some brass links and a piece of styrene. The switching is not as easy as a regular type of switch but it is good enough for my prototype.

This is the close up view of the switch. The switch is shown in the parallel coil configuration. The link pivots on the middle screws. To switch to the series coil configuration, move the top link 180 degrees and open the bottom link.

Spider 40/44 Contra Coil

Not everyone has the big money for the 660 litz. Here is a coil using 40/44 litz and with a little twist. See that big variable capacitor in the picture above, in the Q section? You drop that on your toes and you are off to the hospital. :) Besides weighing a ton, it has some odd features. The big capacitor sections are coupled to each other by the small variables in between. This was a band pass capacitor. The sections come in at 285 pF each (after I pulled some of the small sections out.) The bad thing about this capacitor, besides not being a 365 or higher is the minimum capacitance is a high 25 pF. In most cases this is a disaster. But this is a premium quality silver plated plates, ceramic insulators and superior wiper arms. We have to use this puppy!

I don't think you will have to pull out small capacitors as this will be used as a single section stand alone capacitor. I used only the middle section.

If you connect this capacitor to this special contra wound coil, you will be able to tune the whole BC band in two band sections. The low end starts around 500 khz and goes to 1200. The high end starts at 1000 and easily goes to 1700 khz.

To make this work, I altered the inductance values of the contra coil. This coil has a series inductance of 340 µh and a parallel wired inductance of 85 µh. The inside winding has 33 turns and the outside winding is 27 turns. The litz I used is 40/44. The hub diameter of the coil is 2 inches (5,1 cm) and the outside is 4 inches (10 cm). Wind your coils as described above and you will be all set. This arrangement will work for any capacitor you have that is 250 pF or above. If you have a 350 or higher capacitor, use 29 turns on the inside winding and 23 on the outside.

If you are going to use that big capacitor with larger litz, you may want to wind the coil with the regular 240 - 260 inductance and put two capacitor sections in parallel. You widen the frequency range which makes the tuning more touchy. No matter which way you go, add a vernier dial drive.

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