A double-tuned four band crystal radio set (DT4BCS) can be created by coupling together two Version ‘b’ single tuned crystal radio sets (VbST4BCS). See Article #22.
Selectivity and sensitivity are essentially constant over the entire AM broadcast band because of the use of ‘Venodyne’ constant bandwidth antenna coupling, resonant impedance control and band splitting. The coupling coefficient of the resultant double tuned circuit is easily adjustable from greater than critical coupling to approximately zero. Insertion power loss varies from about 4.5 to 6.5 dB at the center of each of the four bands (including the diode detector loss but excluding audio transformer loss). Selectivity is quite sharp: -3 dB bandwidth is 6-7 kHz and -20 dB bandwidth varies from 16 to 22 kHz. The average ratio of the -20 dB to the -3 dB bandwidth is 3.0. This is the same as the theoretical value shown on a graph in Terman’s Radio Engineer’s handbook, page 160, for two critically coupled circuits resonant at the same frequency and having a Q ratio of unity. Operation can be quickly changed to that of a single tuned crystal radio set accompanied by a sharp and deep tunable trap.
The tank inductor in a VbST4BCS is mounted so that its axis makes a 30 degree angle with the front panel. This orientation enables the magnetic coupling between two VbST4BCS tuned circuits to be easily varied from above critical coupling down to about zero. Incidentally, if one uses two identical coils, wound in the same direction and positioned as recommended in Article #22, the capacity coupling between the coils will be phased such as to partially oppose the magnetic coupling. To create a DT4BCS, two VbST4BCS are placed side-by-side next to each other with baseboards touching each other. The coupling between the two tuned circuits can be varied by sliding one VbST4BCS forward or back compared with the other.
If one is starting from scratch and is only interested in having a double-tuned crystal radio set, unnecessary parts may be removed from each of the VbSt4BCS units as follows:
- Antenna tuner unit: Eliminate switches SW4 through SW7 and all other parts to the right of SW4 as shown in Fig. 5 of Article #22.
- Detector unit: Eliminate C7 and all parts to the left of it as shown in Fig. 5 of Article #22.
If one is interested in having a single-tuned crystal radio set as well as a double tuned one, one full VbST4BCS may be used with either of the reduced parts count units #1 or #2 above.
Operation of a DT4BCS when two VbST4BCS units are used
Connect antenna and ground to unit #1. Position SW1 and SW2 at their 0 dB settings. Make sure no diode is connected to either the Diode #1 or Diode #2 terminals of unit #1. Connect phones to unit #2. Connect a detector diode to either of the two diode terminal pairs of unit #2 and switch it into the circuit using SW6.
In set #2, set SW1, SW2 to activate their attenuation, and C7 to minimum capacitance. Select one of the four bands for listening and set SW3, SW4 and SW5 in set #2 appropriately, as described in Table 1 in Article #22. Set SW3 in unit #1 to its up position when using bands LoLo and HiLo and its down position when and for using bands LoHi and HiHi.
Listening is usually done using critical coupling between the two tuned circuits. This occurs in the Lo band when the front panel of unit #2 is placed about 2 7/8″ back from that of unit #1. The figure for the Hi band is about 3 3/4 “. Almost complete cancellation of coupling occurs when the front panel of unit #2 is pushed about 5 3/8” back from that of unit #1.
Tune in a station using C7 and C8 of unit #1 and C8 of unit #2. Greater selectivity is possible if needed. See Part 5 of Article #22 for more info on this. If interference is a problem, try reducing coupling below critical by moving set #2 further back. Very important: Optimum operation and best selectivity occurs when the loaded tank Q of unit #1 equals that of unit #2. A way to check for for this is to tune in a station and lightly place a finger on the stator (point A in Fig. 5 of Article #22) of C8 of unit #1, then on C8 of unit #2. If similar decreases of volume occur, the Qs are about equal. If the volume is reduced more by touching C8 of unit #1 than unit #2, increase C7 of unit #1 somewhat, restore tuning with C8 and try again. If the volume is reduced more touching C8 of unit #2 than unit #1, decrease C7 somewhat, restore tuning with C8 and try again for equal effects.
If a strong local station seems to ‘bleed through’ the tuned circuits, that may be because the inductor of unit #2 is acting as a loop antenna and picking it up. One way to reduce this problem is to rotate the unit #1-unit #2 assembly about a vertical axis and attempt to null out the pickup. Another approach could be to make physically smaller coils while still maintaining (or increasing) Q by using litz wire or by adding a trap.
Measured Performance of DT4BCS at an Output Audio Power of -70 dBW (not including Audio Transformer Loss), using the Method described in Article #11.
Note: The diode rectified DC voltage at the power levels used in the measurements above is 0.5 volts (Rheostat R3 set to 350 k ohms). At this power level, a SPICE simulation of the detector shows a theoretical diode detector insertion power loss of about 1.4 dB.