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Set #64, The Contra Coil Radio

Dave's Homemade Crystal Set

Dave's Homemade Crystal Set

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Hi Gang! Yes, I know it's been a while since my last crystal set. But you know how us artists are - undependable. I hope you feel it was worth the wait with my new Contracoil radio. This radio is the next step forward in dx radios.

This step is a modification of the two big coils. I got the idea from one of Ben Tongue's articles published on his web site. Besides reading his article (#26), I invite to read what I wrote on the contra coil.

My #64 radio uses a popular front end antenna tuning circuit. It involves using a variable capacitor wired in series between the antenna and top of the tank coil. There is also a variable capacitor across the coil that serves to resonate the circuit to the desired frequency. Both of these variables interact. I used some 270 pf capacitors I had, but you can also use a 365 pf variable. My capacitors have a plastic shaft that sticks out the front panel. If your doesn't you should use a shaft insulator. Otherwise your hand being near the front panel will de tune the radio. Not a pleasant experience. The capacitors are mounted on a piece of HDPE. Styrene can be used too.

Due to the nature of the contra coil switching, and after some experimenting, I decided that this kind of coil should be used with two separate variables. This allows specific control of the selectivity and sensitivity of the radio. Increase the value of the antenna coupling capacitor and you increase the sensitivity at the cost of selectivity. Increasing the capacitor past a certain point tends to reduce both the sensitivity and selectivity. After fiddling with the controls for a short time, you will be an expert in the adjustment of your crystal set.

I used a litz coil in the antenna tuning unit made of 165/46 litz. I am finding that using the smaller litz (1/3 the cost) doesn't seem to affect the sensitivity. I expect there is a selectivity downgrade. I'm counting on the detector board to do the heavy lifting in that department. If you want to go best of breed, then you can substitute 660 litz.

I chose to go a little lighter on the inductance of this coil. I used a 225/56 µh coil, rather than the higher 260/65 coil. This coil has 27 turns on the inside winding and 20.5 on the outside. The hub diameter of the coil is 2 inches (5,1 cm) with the outer diameter of the coil at 5 inches (12,7 cm).

Both radio sections have a D.P.D.T. switch made from some brass thumb nuts, and screws all mounted on a small piece of styrene. I made some brass links to short between the screws. This selects if the coil is wired in series or parallel. Low loss is key here! I have a wiring drawing showing the physical wiring of my link switch. It is shown below. You can also use a ceramic wafer rotary switch. If you do, it is a good idea to have the switch next to the coil and use a long shaft to the front panel.

The detector board uses a litz coil made with 660/46 wire. The hub is the same 2 inches, but the outside diameter is 7 inches. This is a big coil. The coil is connected to a switch as described in the paragraph above. The inside winding (L1) has 28.5 turns and the outside winding is 20 turns.

Both coils are "contra wound". Wind the inside coil as you would for a normal coil. The outside winding is started by going in the opposite direction. The inside coil is called L1 and the outside coil is L2. The two wires in the center of the coil are the start (s) wires, and inside and outside wires are the finish (f) of the winding. It is important to connect these to the switch properly.

The variable capacitor is a 475 pf top of the line, holy grail type variable capacitor. Silver plates, ceramic insulators and great wipers. Here (along with the big coil) is where the money is. One of the dual gang 270 pf capacitors like I used in the antenna tuning unit will work fairly well. A 365 pf capacitor will let you tune the band with ease.

Between the detector coil and diode is a little circuit that is a cousin to the Hobbydyne circuit. Instead of the hard to find differential capacitor, I used a small 20 pf variable. I used a butterfly type as I had a bunch of them in my junk box. This little circuit reduces the loading on the tank circuit as well as providing a match to the diode on it's way to the output transformer.

The output transformer is a Bogen T725, a very popular item among the crystal radio builders community. A single pole, 10 position switch selects the taps of the transformer. The diagram only shows 6 points, but all the wires, except the black and white are connected. I did place a phone jack that I can connect in a higher performance transformer if needed. You should try to find a transformer with a 100k - 200k primary.

The bases are made with oak boards about 6x15 inches (15,25x38 cm). I used 3/16 inch thick (4,75 mm) Garolite® for the front panels. The main detector tuning dial is a styrene disc with labels made with a Brother P-touch. You can see a better picture of this on my contra wound coil page. The dial has two sets of numbers. On the left is the numbering when the coil is in series (low end) and the right side has the parallel setting (high end).

The antenna tuning unit capacitors have a built in 2:1 vernier drive. A big vernier drive on each of these capacitors isn't necessary. The tuning rate is ok here. However, make sure to put in at least a 6:1 vernier drive in the detector tuning system. If you don't, you will miss a lot of stations. Even at 6:1, the tuning is very sharp.

I built the detector in the same modular fashion as in my #63 set. It worked out so well that I continued the trend. Try to use ceramic or styrene insulators for the hot connection points to the main variable capacitor, the hobbydyne selectivity capacitor and the diode. As in my last set, you can build modules for any type of detector that you want. Tube, transistor or anything in between. The module of your choice can be attached to either board, making it a one coil or two coil radio.

Good luck building your set. Good DX! Dave - N2DS

Dave's Homemade Crystal Set, variable capacitor view

Dave's Homemade Crystal Set, close up of the link switch

Dave's Homemade Crystal Set, looking at the detector module

Dave's Homemade Crystal Set, looking at the detector module


The Schematics

Closeout Radio Parts Available