Toronto AES Bulletin March 1997   Feature Article

At Last, A Portable Crystal Set

It all began in 1956, when selectivity improvement to my crystal set was top priority, (a real problem in Toronto with CFRB and CHUM being two 50 KW stations only kHz apart). But alas, after much effort I grudgingly concluded that improvement in selectivity could only be achieved at the expense of poor receiver sensitivity .

While quietly retreating to breadboards of transistor circuits, technology came to the rescue and provided two key ingredients to rekindle my enthusiasm for improving crystal set performance:

1. Flat ferrite bar antenna coils-for compact, very high Q tuned circuits.

2. Piezo-electric earphones-for very efficient, high quality sound reproduction while minimizing the load effects on tuned circuits due to their exceptionally high impedance.

In 1976 I performed many experiments to determine the most effective way to physically and electrically connect ferrite bar antennas. I made the following observations:

1. Maximum sensitivity for a multiple bar antenna is achieved when: a) all bars are physically aligned tightly end-to-end with each other, b) each coil is centered on its bar, c) each coil is closely wound at the centre of its bar opposed to distributing the winding over the entire length of the bar, d) the winding is made from Litz, multi-conductor wire, rather than from a solid, enamel coated, single strand conductor.

The above alignment substantially increases the total inductance of the multiple bar antenna beyond what would normally be anticipated. (Perhaps this is due to the fact that each coil, in effect, is wound on one giant bar.)

I. Maximum sensitivity for a multiple bar antenna is achieved when: A. all bars are physically aligned tightly end-to-end with each other, B. each coil is centered on its bar C. each coil is closely wound at the center of its bar as opposed to distributing the winding over the entire length of the bar D. the winding is made from Litz, multi-conductor wire, rather than from solid, enamel coated single strand conductor. II. The above alignment substantially increases the total inductance of the multiple bar antenna beyond what would normally be anticipated. (Perhaps this is due to the fact that each coil, in effect, is wound on one giant bar.)

The schematic shown in the diagram is the portable crystal set as I built it in 1976. It continues to work flawlessly and has survived two house moves and many demonstrations. A single 1N34 diode was originally used while an additional 1N34 and earphone were later added (as shown in dotted lines) to take advantage of the negative half of the rf cycle. This configuration works very well and is recommended rather than the standard voltage double as no load is placed on the tuned circuit when the earphones are disconnected. This then permits the use of the set as a highly sensitive, highly directive, tunable antenna, which is extremely effective when inductively coupled to other broadcast band radios. (Just align the giant bar antenna with the internal loopstick antenna of the radio.)

How well does it work? From my home in Mississaugua, I receive all metro Toronto stations without any external antenna or ground connections, and it does an admirable job of separating CFRB and CHUM due to its high directivity as well as selectivity.

Article and drawing originally appeared in The Xtal Set Society in 1994,
courtesy of Jim Hayward.

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