Time out!! You people are taking all this far to seriously. Just throw
an aerial out the window, feed it to your rig via a tuner, and enjoy
Amateur radio.
Butch Magee KF5DE
Tdonaly wrote:
Steve's info will get you a beginners understanding of circuit
theory which is based on a low-frequency, quasi-static simplification
of electromagnetic theory. Unfortunately, anything that has any
appreciable length, such as a transmission line or an antenna, or
a long coil of wire as Yuri and Cecil are arguing about,
can't be adequately explained by simple circuit theory; you have to
study wave mechanics to get any real idea of what is happening
in these situations. That isn't the end of it, though, since in order
to understand what is happening when an object radiates, you
have to understand Maxwell's equations. In order to understand
Maxwell's equations, you'd better know vector calculus. That isn't the
end, either, but it's as close as any *normal* human wants to go.
Whenever someone who was taught circuit theory tries to
apply its vocabulary and concepts to explain all electromagnetic
phenomena, that someone is going to run into trouble and
come up with a multitude of idiocies for which which he'll find no end of
people ready to criticize him.
This is the problem: Cecil and Yuri want to explain the current taper
through a long solenoidal coil using the vocabulary and concepts of
circuit theory rather than the difficult but more precise
language of electromagnetic theory. So far they've failed
miserably, not least because they don't even seem to have
a coherent idea of what they mean by "current flow." I
wish them luck, but I hope no one takes any of their
ideas seriously.
73,
Tom Donaly, KA6RUH
Steve wrote,
Unable to resist at least trying to provide the basis for some
understanding, Steve proceeds.
Jim, you know who you are...
Thank you. Here's a go at a start.
Apparently, because of the way the big bang occurred, when we put a voltage
across a resistor current flows in a manner that we discovered follows the
equation called Ohm's law. The resistor "resists", determines or limits the
current. Oh yea, resistors have this characteristic we call resistance
measured in "Ohms", just for using in the formulas.
Ohms law: I = V/R
Say... 12 Volts on a 50 ohm resistor results in 0.24 amps flowing.
For the same reason (big bang) this removes power from the circuit, or
"dissipates" it. Poof! Gone from the circuit. Resistors happen to turn this
power into heat. The value being discoverable by the power formula:
P=IxV and the variations P=E^2/R and P=I^2xR
It also turns out that ANYTHING else that removes power from a circuit looks
just like a resistor to the circuit (obeys Ohm's law), and *ONLY* things
that LOOK just like a resistor (behave or conduct current according to Ohm's
law) will so remove power from said ckt. IF you didn't catch this, there
are things that are not really resistors, yet act just like them as far as a
circuit is concerned.
Unfortunately, life is fraught with dangers and we have capacitors (C) and
inductors (L)(or things which behave just like them or combinations of
them). When we get into this realm, the "R" from above, just doesn't do it.
Things get all messed up.
These things also "resist" current flow. (or determine or limit it). We
call this form of resisting "Reactance", use the letter "X" to represent it
and it also is measured in "Ohms", just for using in the formulas. Oh yea,
we also use the little subscript letter to indicate if it is an inductive
(l) or capacitive (c)reactance.
Xc ("X" sub c) = 1/(2 x pi x f x C )
Xl ("X" sub L) = 2 x pi x f x L
When we want to talk about the effect or either an "L" or a "C" we simply
use the term "Reactance" It's like a good substitute for "he/she" (the
"wrong one being "they").
Because of (big bang again) the way the current in these (C & L)
corruptions, of our purely resistive world, work out to be 90 degrees out of
phase with the voltage (we are talking about AC now), we had to find a way
to account for them. I won't trouble you with just why now, but we use what
is called the "Series Representation". It looks like two numbers with a +
or - sign between them and all together we call this new kind of (corrupted)
resistance "Impedance". And use the letter "Z" to represent it. It has
some Resistance and some Reactance in there and it will have numbers on
ohms:
In general: It looks like this:
Z = R + jX
The "R" is the same kind of resistor as above, the "X" is one of the
reactances. The "j" helps the mathematicians do the math - like ohms law -
but with the reactance accounted for. In "math speak" the "R" is the "real"
part and the "X" is the "Imaginary" part of the impedance.
Impedance also resists current flow, but with the reactance in there, you
can't use Ohm's law like you used to.
SO... When I say "Impedance" or use "Z" I am talking about whatever happens
to be there. Since I don't know if it is only resistive, called also
"Purely resistive", or has some reactance in it, called "reactive", (or if I
am just too lazy to figure it out at the time), I use this word or symbol to
cover any situation.
Finally, since only the "resistive part" of a circuit dissipates any power,
we like to remove (somehow) all the reactance (or imaginary part) and
somehow make the real part (the resistive part) what we like best (for a
given situation). Doing this is the infinitely complex subject called
"impedance matching". When we make this happen on an antenna, the remaining
"resistive part" sucks power from the circuit (the transmitter circuit)
Poof! BUT converts it into radiated radio frequency energy (RF) also called
an electromagnetic field or wave.
Fortunately for us in this modern day and age, because if it didn't all the
receivers that we have would be useless and we would wonder why we built
them.
Help any???
73 Steve--
Steve N, K,9;d, c. i My email has no u's.
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