From: SFPhysics@aol.com
Date: Tue, 18 May 1999 21:53:49 EDT
Subject: AM LOOP Crystal Set Questions
To: pinhole@exploratorium.edu
>The volume is OK but I can pick up only one station using
>a 365 Variable capacitor. Do you have any suggestions on increasing the
>volume and selectivity or the radio??? SHould I add an inductor of some
>sort??? A ground???? Lengthen the antenna or the frame??? Also the
>signal sometimes disappears for a while and then reappears. How can
>this be expained???
>Check out the plans for the project at...
>http://www.alphalink.com.au/~parkerp/crystal.gif
Thanks in advance < :
Tim Ostrom < :
OUSD < :
Hi Tom -
First off, let me say that the broadcast bands in Australia are different
from those of the U.S. The plans are from down under so the number of turns
in the antenna coil will be different. You might want to tap the turns every
few turns and then try the taps to see which one works best. This is where
neat coil winding becomes advantageous.
Second, if you are not using crystal headphones with high impedance, the
earphones will load down the circuit so that only a high-power local station
will have enough energy to drive them. The old magnetic earphones (2kOhms)
work well but not the new low impedance modern stereo ones.
A ground is always a good idea for a crystal set as is an antenna. However,
your set should work fine standing alone as it is a parallel LC (tank
circuit). I would surmise that having a ground and an antenna to feed each
side of the tank circuit should help sensitivity but in a low Q circuit such
as a coil 1 meter square selectivity will not be.
As for the signal dropping out then coming back, the AM frequencies tend to
hug the ground. That is why AM transmitters put antennas up on mud flats in
the San Francisco Bay. If a large iron or metallic object gets in between
the ground wave the wave will drop in amplitude. Are you between the
transmitter and a shipping channel? Wave reflection, reinforcement, and
cancellation all work with even low electromagnetic frequencies.
Have fun -
Al Sefl
SFPhysics@aol.com
