Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Simple radio signal



One demonstration I've done was developed by Phil Morrison at MIT and
distributed as part of the ZAP! lab set. These are labs for the E&M half of the
MIT introductory course that are designed to be done at home.

The transmitter is a spark-gap transmitter constructed from a wooden clothespin
with thumbtacks pressed into the jaws and a bolt and nut in the handles to
adjust the spark gap. Stiff copper wires, each about 2" long are attached to
each thumbtack. A capacitor (probably a few hundred pf) is also across them.
It is powered by a 2-kV source of dc (also homebuilt from an oscillator and a
step-up transformer made from a choke coil and a few-turn primary.

The receiver is simply a diode with 2" wires as antennae that is connected to a
small VOM.

You adjust the spark gap until you see a rather continuous purple glow between
the thumbtacks. The transmitter works over a distance of about 25 cm.
Polarization is obvious when you change relative orientations of the two
antennae or by using a comb of Al foil strips. You can reflect the signal off
Al foil (and show polarization by reflection). You can also show interference
between the direct and reflected signals.

=================

The "Stray Cats" (Japanese high school teachers) demonstrate EM waves by using
the speaker output of a boom box for the transmitter. The receiver is an audio
amplifier of any kind with the input the microphone jack. A pair of soda cans
attached to the leads (either transmitter or receiver) is an electric field
transmitter/detector. A coil of wire shows that you can also generate/receive
magnetic fields. Any combination works. Of course, these are VERY low
frequency waves (1 kHz). Does the VMI demo book (Rae Carpenter) show this one?

Paul Zitzewitz


"Daniel L. MacIsaac" wrote:

The easiest way to get a simple radio reciever is to buy the cheapest
transistor AM receiver.
ck


DEFINITELY. Now how to we get a cheap transmitter that will send a
resonably polarized signal and how to we make a cheap AM receiver
into a polarized receiver?

Dan M

Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
danmac@nau.edu http://purcell.phy.nau.edu PHYS-L list owner
begin:vcard
n:Zitzewitz;Paul
tel;fax:(313) 593-4937
tel;work:(313) 593-5277
x-mozilla-html:FALSE
org:Department of Natural Sciences;University of Michigan-Dearborn
version:2.1
email;internet:pwz@umich.edu
title:Professor of Physics and Chair
adr;quoted-printable:;;4901 Evergreen Road=0D=0ADearborn, MI 48128;;;;
fn:Paul Zitzewitz
end:vcard