gifTuning Coils for Crystal Radio

Wiper Coil

The coil was wound on a strong cardboard mailing tube 3" and 3/16" in diameter. Tuning is achieved by abrading each coil in a narrow strip along its length, and employing two sliding contact mechanisms such as a “wiper arms”. The shorter coil C is for antenna tuning whilst the longer coil A is for radio tuning. Note: the coil is not yet varnished.

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Main Coil: A

Diameter: 3" 1/8"
Length: ≈ 7"

Wire diameter: 1mm (AWG 18)
Turns: 160

1.3 Ohms
L = 0.76 mH
814.0 kHz
Q = 5.5
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Gap : B
Length: ≈ 1/4" (five turns of wire).
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Antenna Coil: C

Diameter: 3" 1/8"
Length: ≈ 3" 2/8"

Wire diameter: 1mm (AWG 18)
Turns: 73

0.8 Ohms
L = 0.29 mH
1580 kHz
Q = 10.7
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Tube Diameter D: 3" and 3/16"
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Coil entry points:

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An example circuit for this coil is given below:

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Coil supports cut from scrap wood:

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Finished coil and stand ready for mounting on board:

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Coil mounted on base board; the coil ends will be fed underneath the board to brass thumb-screw terminals. The board itself was cut from a scrap ironing-board left outside someone’s house; but it took me over two years to remove the warp using weights!

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(Note: the white residue is not glue but Briwax furniture polish; the coil holders are fixed with screws in the base.).

To be continued...

Davey Long and Medium-wave coil with aerial and reaction windings

The design for this coil was first given in the January 1961 edition of “Boy’s Own Paper”. Subsequent editions described its use in transistor and one-valve sets. The coil has an aerial, medium-wave, long-wave and reaction windings. Unfortunately, medium-wave is pretty much dead in the United Kingdom, so I left space and included extra terminals for future windings:

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I first came across this particular type of coil at the Davey’s Radios web site, where they attribute the design to Davey, although I later found a very simililar design in “Practical Wireless”, April, 1947 (Vol 23, No. 490, edited by F.J. Camm). The author of the article is not cited, but it could be Davey himself for all I know...

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To be continued…