Crystal Radio Coil Sizing (4K) 

The most common crystal radio question I get is about the size of the coil. We go over several pipe / wire sizes and discuss the tradeoffs.
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Q: Does my coil have to be exactly like yours?
A: Yes and No. For this style of radio it should have the same inductance of around 531 uH (measured resonance at 1MHz)
The following will get us to 531 uH (all equivalent, but efficiencies differ, bigger is better)
Bigger diameter is more efficient, but shorter.
Shorter coil means less tuning/fewer stations.
Longer coil is harder to make a wiper and has resistance.
Bigger wire is better for tuning, BUT you need more turns, longer coil. (Fine wire can be a bear to tune)
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These are different ways to get 531 uH by changing tube/wire sizes.
The wire I use is .036" or 0.91mm (19 AWG), which is bigger and easier to tune.
Toilet paper tube (weak): 46.7mm OD diameter (measured), 100mm long wire diameter INCLUDES the enamel coating.
19 AWG = .036", 0.91mm === length: 8.65" or 219.6mm and 244 turns (rounded up)
22 gauge = 0.6438mm === length: 4.74" or 120mm and 187 turns
24 gauge = 0.5106mm === length: 3.2" or 81.2mm and 159 turns
One & a half inch PVC plumbing pipe (blue): actual = 1.9" or 48.26mm OD
19 AWG = 0.036", 0.91mm === length: 8.4" or 211.5mm and 235 turns (rounded up)
22 AWG = 0.6438mm === length: 4.5" or 114mm and 178 turns
24 AWG = 0.5106mm === length: 3.1" or 77.4mm and 152 turns
Two Inch PVC plumbing pipe (blue): actual = 2.375" 60.325mm OD
19 AWG = 0.036", 0.91mm === length: 5.74" or 145.7mm and 160 turns (rounded up)
22 AWG = 0.6438mm === length: 3.22" or 81.8mm and 127 turns
24 AWG = 0.5106mm === length: 2.25" or 57 mm and 112 turns
Two & a half inch PVC plumbing pipe (blue): actual = 2.875" 73.025mm OD
19 AWG = 0.036", 0.91mm === length: 4.3" or 109mm and 120 turns (rounded up)
22 AWG = 0.6438mm === length: 2.5" or 63.6mm and 99 turns
24 AWG = 0.5106mm === length: 1.79" or 45.5mm and 89 turns
Three Inch PVC plumbing pipe (blue): actual = 3.5" or 89.8mm OD
19 AWG = 0.036", 0.91mm === length: 3.25" or 82.5mm and 91 turns (rounded up)
22 AWG = 0.6438mm === length: 1.97" or 50.1mm and 78 turns
24 AWG = 0.5106mm === length: 1.44" or 36.7mm and 72 turns (too short!)
The Cub Scout Book from 1963 says, "TUNING COIL Wind wire tightly around a dry shellacked cardboard tube about 1 1/2" across and 6" to 7" long Shellac all over again. When dry, fasten it to baseboard at both ends. Put in washers or blocks to keep it from touching the board." The diagram says "22 gauge enameled wire". With these specifications, they are targeting between 1250 uH (6") and 1470 uH of inductance.