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The most common earth is a connection to your (steel) water pipe where it enters the ground. The dirt around your house has resistance and this can be improved
(a) Use three spikes (steel, copper or copper-clad steel) driven a metre in to the ground and a metre apart in a triangle configuration, connected together and to the set.
(b) Add chemicals to the ground, the commonly recommended one is ammonium chloride solution around the earth spike. However this is very corrosive and you may need to replace the earth spike every few years.
(c) Use a counterpoise, this is a mat of wire laid out under the antenna. This can be a series of radial wires joined at one earth point to the set. Alternatively the wires can be in a grid joined (or soldered) at each crossing point with the earth connection at a suitable place near the aerial downlead. The counterpoise can be buried if you prefer, use a smooth edge garden edger tool to make a thin slot in the ground and push the wire in to it.
Here is another circuit to try:-
Use the coil already would for the previous circuits for the left-hand side (aerial input), for the detector side wind up to 100 turns of 0.4 mm wire on a tube which will just fit over the other coil. This could be a PVC or cardboard tube. Tap this coil every 10 turns at the top and every 5 turns at the bottom. Experiment with the coils side by side and inside each other as well as with different tapings.
We now have a loaded antenna with 2 coupled tuned circuits. The coupling on the LH side is tight the second coil has variable coupling; tight if the coils are inside one another and loose if side by side. This arrangement improves selectivity) at the expense of sensitivity). The looser the coupling the better the selectivity.
Another source of loss is the wire used in a crystal set. The wire in the 150 turn coil (26 gauge wire) in a crystal set has a DC resistance of about 2 ohms However Broadcast Band waves only travel on the surface of the wire, ‘the skin effect’ BC waves penetrate to a depth of about 0.05 mm to 0.09 mm. The wire is 0.4mm diameter its resistance at BC frequencies is about 3 to 5 ohms which is not significant. If you built a Short Wave crystal set, this resistance could be significant as the resistance increases with frequency. Some purists like to use Litz, which consists of a number of insulated strands of very fine wire. eg. Antique Electronics of America sell one litz wire which consists of 54 strands of 38 gauge wire (0.1mm), and another one which is 25 strands of 41 gauge wire (0.07mm) The 38 gauge litz is ideal at BC frequencies , The 41 gauge litz will not change resistance noticeably until the frequency is above 3.5 mHz
The biggest loss obviously is in the detector circuit, the average resistance of a crystal used in 1920 was about 2000 ohms at BC frequencies, earphones of 2000 ohms were recommended because maximum power is available to the earphones if the resistance of diode and earphones are equal.
Fig 8 is a variation of the circuit in fig 7 using the same coils.
The two coils are kept well separated, preferably mutually at right angles
The top tuned circuit is configured as a wave trap. It is tuned to a station you wish to reject, it will look like a high resistance to that frequency. The rest of the circuit is the same as the improved version of fig 6.
Peter Hadgraft