A D V E N T U R E S   in   C Y B E R S O U N D

A 'Sensitive' Crystal Radio from Gnome Technologies (see also a matching 'Selective' Set)

This radio is as simple as they get to still give good performance. In reality there's just one inductor, with the antenna and detector tapped near the bottom to maximize selectivity.

better-radio.jpg


The coil is 80 turns (approx 3") of #24 wire on a 2.25" PVC form. The tap is 6 turns from the bottom, giving antenna-coupling and tuning inductances of 10 uH and 180 uH respectively.

Given how simple the circuit is, I decided to work through the math of the frequency response. Some simplifying assumptions:

    The antenna can be treated as a resistor (around 70 ohms for an end-fed long wire antenna) in series with a small capacitor (about 160 pF, estimated from my last radio design)

    The detector, filter-capacitor and headphones appear to the rest of the circuit as simply a 1K resistor


We set the tuning capacitor to 100 pF and compute the reactance for the set as:

    w = 6.28 * freq

    V in= 100*10^-3 (100 mV input signal typical)

    Z antenna = 70 + 1/(i*w*160*10^-12)

    Z tap = i*w*10*10^-6

    Z tuner = 1 / (1/1000 + 1/(i*w*180*10^-6 + 1/(i*w*100*10^-12)))

    Z radio = Z antenna+1/(1/Z tap+1/Z tuner)

    I radio = V in/ Z radio

    V phones = I radio*1000


The detector is a piece of fused silicon with point-contact being made on a sharpened 3H pencil lead.

Sensitivity and selectivity are impressive.


Source: Gnome Technologies http://www.cool.mb.ca/~gnome

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