Foundations of Amateur Radio

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A nifty idea looking for a purpose in 1947 changes the world as we know
it...

Posted: 28 Jan 2017 09:00 AM PST


Foundations of Amateur Radio If you have the need to switch something on
and off, a likely first candidate is to get a switch from the local
hardware store. The principle is pretty straightforward. You put the switch
into the power supply lead and by pushing it on, the two halves of the
switch make contact with each other, completing a circuit, and the thing
you're switching turns on. It's a lot like having two bits of bare wire
that you can touch the ends together. What if you want to remove the human
touch from the equation, that is, switch something without having to
actually push a switch? A potential candidate for this is a relay. In
essence it's exactly the same as a manual switch, except the pushing is
done by an electromagnet. The way it works is that you send a current
through a coil that is wound around a metal core which results in a
magnetic force. This force is used to push or pull the switch open or
closed. Now both a manual switch and a relay have moving parts. That means
that there is a limit to how fast you can switch something on and off. You
can probably push a manual switch several times a second, lets say 10. So
the switching speed is called 10 Hz. A relay can likely get you switching
around 100 times per second, or 100 Hz, but if you want to switch something
at a much higher speed, say at 1000 Hz or more, some other form of
switching comes into play. At its simplest, a transistor is like a relay
without any moving parts. There's no actual switch, no coil, no
electromagnet, none of that. Without going into the physics of how this all
works, let's look at an analogy. Imagine a water-pipe with a valve on it.
You can open or close the valve and water flows or not. In a transistor,
the same principle applies. There are three legs, two of them act as the
water pipe, the third one acts as the valve. You open or close the valve by
putting a current onto the valve - or base - leg and a current flows
between the other two, the collector leg and emitter leg. Now, so far I've
just told you that you can open or close a transistor with a current, but
it's actually more nifty than that. You don't need to have it all on or all
off. In our water pipe you can set the valve to any setting and control how
much water flows. In a transistor you can do the same by changing how much
current you put onto the switching or base leg. You might have heard a
description that says that a transistor is both a switch and an amplifier.
If you haven't don't fret, I'll explain. Let's go back to water for a
moment. Imagine a huge water pipe connected to a dam. Lots of water all
pushing into our water pipe. The valve we have can be controlled by you
blowing water through a straw into the valve. The more water you blow into
the valve, the more water flows out of the dam through the pipe. If you
blow hard into the straw, the result is a wide open valve and lots of water
from the dam, if you blow softly, less water. In essence your little water
flow from your straw is being amplified by the dam. A transistor works
just like that. As I said, you don't have to use a transistor just to
switch something completely on, or completely off. If you vary the current
into the base, you can vary the amount of flow between the collector leg
and emitter leg. The current you use to control the flow is tiny, so you
can use a really weak signal to control the thing. In essence that's how a
transistor radio works. The small signal that we use to control the flow is
the tiny one coming from an antenna, the dam is the battery and the speaker
is connected to the output. So, a small electrical current coming from the
antenna controls the transistor which in turn controls the amount of
current coming from the battery onto the speaker. A hearing aid also works
in the same way. A small current coming from a microphone controls the
transistor which in turn controls the amount of output from the battery to
a speaker. The reason I mention transistor radios...
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3