An improved design
As we have seen that the circuit below has several drawbacks.
1. The aerial forms a significant part of the total tuning capacitance and so the tuning range available by varying C1 will be small
2. A consequence of the aerial capacitance is that the upper frequency that can be tuned is quite limited.
3. The circuit is not particularly efficient and tuning will be quite 'flat'.
4. The detector also adds capacitance and it's loading effect worsens efficiency
To help overcome these difficulties we need to:
a) Not directly connect the aerial (this will improve the tuning range and improve efficiency)
b) Tune the aerial separately (tuning not as sharp as the main tuning but the two together can give good selectivity)
c) Buffer the loading effect of the detector.
Fig. 2 below shows a practical arrangement that yields these improvements
Fig. 2 - Improved crystal set design
Referring to Fig. 2, Amplitude Modulated (AM) radio signals are picked up on the aerial connected to terminal marked ‘A’. The aerial has a certain natural capacitance to earth and L1 + L2 react with this natural capacitance to form a series tuned circuit. L1 is adjustable so that the aerial can be tuned to any desired frequency in the Medium Wave band. Tuning of L1 is accomplished by sliding the ferrite-rod core in or out of the coil.
L1 and L2 form an auto-transformer, with a proportion of the tuned aerial signal being developed across L2. This signal is loosely coupled across to L3 by transformer action. The degree of coupling may be varied by adjusting the position of L2 relative to L3. This variable coupling can help improve selectivity but at the expense of sensitivity.
L3 is tuned by main tuning capacitor C1. C1 is adjusted until the level of the desired signal developed across L3 is at a maximum.
D1 is the detector. It acts in combination with the low pass filter R1, C2 to recover the original audio from the radio signal. The recovered audio is passed to the ‘Phones where it may be heard.
You will need to provide a reasonably good aerial and earth in order for the set to work adequately.
Building the set
The following instructions will allow you to build this simple but efficient set using simple tools. I can supply any of the parts listed and I can also supply you with a kit that contains all the electrical parts required, leaving you free to to deal with the cabinet.
R1 = 82k 20%
C1 = 35 to 365pF air dielectric variable capacitor
C2 = 100pF 20% ceramic
D1 = OA90 or OA47 or similar Germanium diode
L1 = see winding details below
L2 = see winding details below
L3 = see winding details below
Phones = crystal earpiece or high impedance headphones (e.g. 2000 Ohm resistance coils)
Screw terminals qty 4 for Aerial, Earth and the two Phone connections
Qty 4 pan head wood screws and washers for baseboard connections
Base and front panel or cabinet
Means of fixing and adjusting L1, L2 and L3.
Coil Winding Details:
L1 consists of 75 turns of 0.193mm dia (36 swg) ecw* or Litz wire wound in a single layer on a card tube which is a slide fit over the ferrite rod core. Alternatively L1 can be made from Maplin LB12N ferrite rod aerial; discard the LW section and link the two remaining MW windings in series.
L2 consists of 10 turns of 0.508mm dia. (25 swg) ecw* wound counter clockwise on a pre-prepared CD substrate, starting from centre (see preparation details below).
L3 consists of 48 turns total with tap at 33 turns from start, 0.508mm dia. (25 swg) ecw* wound counter-clockwise on a pre-prepared CD substrate, starting at the centre (see preparation details below). For reference, start of winding = 1, tap = 2 and finish = 3.
*ecw = Enamelled Copper Wire or it's modern equivalent, polyurethane covered wire
CD substrate preparation:
Take an ordinary CD which is no longer required for it's original purpose. Turn the CD so that the printed side is uppermost. Using P400 grit abrasive paper abrade the surface to remove the printing and the aluminium silvering. Avoid breathing the dust. You should be left with a translucent acrylic substrate free of all silvering. Wash to remove dust. Using the template below (pdf document), mark out using a soft pencil the position of 13 slots running radially from a baseline 25mm from the centre of the disc. Using stout scissors, cut out the 13 slots. Having made 2 radial cuts per slot you will need to sharply bend the waste to remove cleanly from the base of the slot. With all 13 slots cut, use the same abrasive paper to de-burr the substrate, wash again and dry. Make 2 of these, one for L2 and one for L3.
For CD substrate template click HERE.
Refer to the Assembly Drawing Fig. 3 below for guidance on the general arrangement.
Fig. 3 - Assembly drawing
You will need to provide a base board and front panel upon which to mount the components. The base and panel may be constructed from dry wood. Size is not too important but a base of some 12” wide x 9” deep x ½” thick and a front panel of 12” wide x 6” high x ¼” thick should suffice. Keeping it open, i.e. not in a cabinet, is probably best. If enclosed I suggest you provide a lift up lid.
Drill the front panel to accept the 4 terminal posts, C1 the Main Tuning capacitor and any fixing holes required. Once you have attached the front panel to the base you can start fitting the components.
Keep the connections between terminal ‘A’, L1, L2 and terminal ‘E’ fairly short.
You will need to fix the coil former of L1 firmly to the base, perhaps by glueing, and arrange for the ferrite-rod core to be slid in and out of the former for aerial tuning purposes. A couple of plastic pipe saddle clips with felt lining might provide a means of retaining the ferrite-rod whilst allowing a smooth sliding adjustment. The ferrite rod is very brittle and if stressed or dropped will break, so do be careful. The good news is that it is not expensive to replace!
Ideally L2 and L3 should be mounted so that the separation between the two can be adjusted. The Assembly Drawing gives the outline of one way of achieving this. Another very simple way is to fit the coils into a CD jewel case and vary the coupling by opening and closing the case. If you don’t want to be bothered with this refinement you can simply set the two coils at a fixed distance apart. Start with them just a few millimetres apart.
Base board connections are made with pan head woodscrews and washers. Use one screw and two washers per connection point, and trap the wires between the two washers.
Wire all the components up as per the circuit and the assembly drawing and the set is complete.
WARNING – DO NOT ERECT AN AERIAL ABOVE OR BELOW OR CLOSE TO ANY ELECTRICITY POWER LINE(S).
I suggest 50 feet of stranded insulated wire for the aerial. Suspend this as high as possible/reasonable and insulate both ends from the supports (tree, house or whatever) by means of some plastic insulator (plastic pipe or similar). Bring the end nearest the set into the house without touching windows, doors, walls etc. for a permanent installation the wire might be guided in through a narrow duct (perhaps a small bore plastic pipe. Alternatively you could simply have the wire dangling near a door or window and then temporarily guide it in with the door or window open.
One of the following might provide a good enough earth connection:
A metal cold water (mains) pipe
A metal pipe of a central heating system
A metal object such as a biscuit tin buried in damp earth
I have successfully used the earth connection from a mains outlet but if you want to try this you do so entirely at your own risk.
Connect the aerial to the terminal marked ‘A’ and the earth to the terminal marked ‘E’.
Connect the ‘phones or the crystal earpiece to the terminals marked ‘P’.
Set the ferrite rod halfway into L1 and bring coil L2 and L3 close together. Rotate the main tuning knob and you should hear radio transmissions. When you have tuned into a station, use a combination of L1 adjustment and main tuning adjustment to maximize the received volume. Note that adjusting L1 will require a slight re-adjustment of the main tuning. With a little practice it should be fairly easy to tune to any desired station within range. The lower frequency end of the MW band is tuned in with C1 towards its fully meshed position (spindle rotated clockwise), whereas the higher frequency end of the band will be tuned with C1 towards its fully open position. For lower frequencies L1 will need its ferrite rod pushed further in and for the higher frequencies L1 will need its ferrite rod pulled further out.
If you have made the spacing between L2 and L3 adjustable then you will find that bringing them close together makes signals stronger with moderate selectivity. Separating the coils tends to make for better selectivity but does reduce signal strength; it’s a compromise.
Good luck and I do hope you have fun building and operating the set. You might like to try modifying the circuit to tune to the shorter wavelengths. The short wave bands really 'come alive' after dark when the propagation of the radio waves improves. You will need to adjust the inductance of your coils to suit and it would be convenient to make up sets of coils to cover the various wavelengths and perhaps even make them interchangeable with a plug and socket arrangement.
Copyright Ó 2004 Lorne Clark