Question about radio basics, spark gap transmitters etc

[jedispork]

Studying for my general right now. I bought the license manual since it was said to have more explanation behind the simple q&a books yet none of them try very hard to explain the basics of radio which I very much want to understand.

The description of a simple inductor and capacitor oscillator being like a pendelum on a clock was helpful. The wiki also has a animation of how the current actually flows through the circuit and that was good as well. I get this ... I think.

Please correct me as as needed but I'm trying grasp some basics here. Can a EM wave also be created by simply switching a dc circuit on and off? When the current is shut off then the magnetic field collaspes and brings about a electric field and this keeps going back and forth and propogating in space? And this would be called a square wave because the electricity used to create it doesn't alternate but is simply switched on and off?

It seems like electricity always has to flow in a closed circuit so when they show you a wire for the antenna only attached on one side its confusing how it could work. So the current can still move up and down in the wire even though its not in a closed loop circuit? If you take a magnet and move it back and forth over a wire electrons will still move even if its not connected in a loop? In a transformer does it induce voltage into the secondary coil as the magnetic field is established or collaspses?

I thought it would make things easier if I went back to basics and tried to understand how a spark gap transmitter works. I was confused how a dc voltage could maintain current across the secondary windings to sustain the spark. After much research I finailly found that they use something called a interrupter so the dc current is switched on and off. Is the spark direct current that creates em by erratic starting and stoping of the current in various archs? or does the electricty alternate back and forth?

When they add a oscillator at the end of the diagram it gets even more confusing for me.

Also whats annoying is that I tend to think of current flowing as the electrons do but diagrams are drawn for the flow of holes and this gets confusing for me too.

thanks for the help

 

[WA0CBW]

The holes and electrons use to confuse me. Here is another look. Assume you have a tube the size of marbles (electrons). Hold the tube at a slight angle and remove a marble from the bottom of the tube and move it to the top. What did you see? You probably saw the marbles (electrons) roll down the tube and fill the space where you removed the marble. I saw the empty space where you removed the marble move up to the top of the tube and was filled with the marble you removed from the bottom. The direction of the flow of electrical current depends on weather you are watching the holes or the electrons. So current flows in both directions depending on if you are watching electrons or holes (the absence of electrons).
BB

 

[jedispork]

thank you,

and with a antenna those marbles can move back and forth pushing and pulling on your receiver to give it a signal which is how I think it works. I found this article that I need to take my time reading and absorb all the information. If anyone else is interested in how a antenna radiates its here

http://www.arrl.org/files/file/Technology/tis/Why an Antenna Radiates.pdf

 

[zz0468]

Studying for my general right now. I bought the license manual since it was said to have more explanation behind the simple q&a books yet none of them try very hard to explain the basics of radio which I very much want to understand.

Which specific books have you bought? In the ARRL books that I've seen, the basic electricity sections are actually pretty good. But that doesn't mean that by just reading it, everyone will walk away having absorbed it. Being able to ask questions helps.

The description of a simple inductor and capacitor oscillator being like a pendelum on a clock was helpful. The wiki also has a animation of how the current actually flows through the circuit and that was good as well. I get this ... I think.

Remember that a capacitor and an inductor are storage devices. A capacitor stores an electric field, and an inductor stores a magnetic field. Also remember, electricity and magnetism are both manifestations of the same physical force - the electromagnetic force.

Please correct me as as needed but I'm trying grasp some basics here. Can a EM wave also be created by simply switching a dc circuit on and off?

Yes, but what's happened is, by turning the dc source on or off, for that brief moment, it's an alternating source, not a direct current source. Once the current flow is stabilized at a fixed value, then it's dc and it's field doesn't propagate.

When the current is shut off then the magnetic field collaspes and brings about a electric field and this keeps going back and forth and propogating in space?

Not back and forth so much, as it propagates. The collapsing magnetic field in the conductor induces an electric field. The electric field induces a magnetic field, which induces an electric field, which induces... and so on. That's how it propagates through space.

And this would be called a square wave because the electricity used to create it doesn't alternate but is simply switched on and off?

No. That's more of a pulse. But a pulse with a fast rise or fall time contains higher frequency components, and those are what actually radiates. You're taking this WAY beyond basic electricity now.

It seems like electricity always has to flow in a closed circuit so when they show you a wire for the antenna only attached on one side its confusing how it could work.

Don't allow yourself to think of antennas in terms of DC theory, and your confusion will go away. Master DC electrical concepts first. Then go on to AC electrical concepts. One you've mastered AC electrical theory, then you can begin to understand what happens in an antenna.

So the current can still move up and down in the wire even though its not in a closed loop circuit?

In a word, yes. But it's ac, not dc. At the risk of jumping ahead of where your level of knowledge is, here's a couple of tidbits that will be counter-intuitive to one who doesn't have a grasp of ac theory: A capacitor, which has two plates that don't physically or electrically touch, will pass an ac current, but block dc. An inductor, a coil, will pass DC but can block an ac current.

So, as soon as you start thinking of ac and rf behaving in terms of how dc behaves, you'll be wrong, and confused.

If you take a magnet and move it back and forth over a wire electrons will still move even if its not connected in a loop?

Yes.

In a transformer does it induce voltage into the secondary coil as the magnetic field is established or collaspses?

Yes to both. It's the act of the magnetic field CHANGING, not just being established or collapsing.

I thought it would make things easier if I went back to basics and tried to understand how a spark gap transmitter works.

I'm not so sure that delving in to spark gap transmitters will help you understand basics.

When they add a oscillator at the end of the diagram it gets even more confusing for me.

The oscillator merely represents an alternating current source.

Also whats annoying is that I tend to think of current flowing as the electrons do but diagrams are drawn for the flow of holes and this gets confusing for me too.

I'm not sure I understand what diagram you're referring to. The "holes" are merely places in an atom's electron shell where an electron could be placed. If a current of electrons is flowing in a conductor, the electrons are jumping from one atomic "hole" to another. I suppose it could be a useful concept to think of the holes "moving" in the opposite direction, but at this level, I think it would just add confusion.

 

[jedispork]

Which specific books have you bought? In the ARRL books that I've seen, the basic electricity sections are actually pretty good. But that doesn't mean that by just reading it, everyone will walk away having absorbed it. Being able to ask questions helps.

Its just the arrl general license manual which has slightly more information than one of the quick study guides. They have componenets, equations, and logic gates all in one chapter. The book itself admits its more of a summary and had suggestions for other books to study. It suggested a book called Basic Radio and the companion book Basic Electronics. With the radio book some reviewers claimed it has a lot of assumed knowledge and you need to read the electronics book first. What I do know I learned from a green book radio shack used to sell but I can't find it now.

In a word, yes. But it's ac, not dc. At the risk of jumping ahead of where your level of knowledge is, here's a couple of tidbits that will be counter-intuitive to one who doesn't have a grasp of ac theory: A capacitor, which has two plates that don't physically or electrically touch, will pass an ac current, but block dc. An inductor, a coil, will pass DC but can block an ac current.

Potential is held between the plates however electrons cannot cross because its insulated. So once that capacity is full no more current flows blocking dc. I understand inductors pass dc and like to resist changes in current by using their stored(storing) energy?

I'm not so sure that delving in to spark gap transmitters will help you understand basics.

Your probably right and I will leave that alone for a while. However I might understand what it means that they put out a damped wave. In the models that have a oscillator the surge across the spark gap charges the capacitor which sets in motion oscillation between the cap and inductor. The damped wave is from the energy fizzling out in the oscillator before the spark can fire again?

The "holes" are merely places in an atom's electron shell where an electron could be placed. If a current of electrons is flowing in a conductor, the electrons are jumping from one atomic "hole" to another. I suppose it could be a useful concept to think of the holes "moving" in the opposite direction, but at this level, I think it would just add confusion.

Things like a npn transistor with the arrow pointing in the oppostie direction of electron flow is sort of confusing but I'm getting over it. I'm going to study up on some basic books and do some simple projects on a breadboard. I might study up on algebra as well although I am able to go through the equations.

thanks for taking the time to help me with my questions. I'm going to keep working at it after I pass general.

 

[zz0468]

Its just the arrl general license manual which has slightly more information than one of the quick study guides. They have componenets, equations, and logic gates all in one chapter.

Why don't you get a copy of the ARRL Amateur Handbook. It has separate chapters for DC and AC theory. It's not written at a 3rd grade level, so you'll need to be awake when you read it, but if you're up to it, it's the best amateur oriented treatise on basic electrical theory that I've encountered.

Your probably right and I will leave that alone for a while. However I might understand what it means that they put out a damped wave. In the models that have a oscillator the surge across the spark gap charges the capacitor which sets in motion oscillation between the cap and inductor. The damped wave is from the energy fizzling out in the oscillator before the spark can fire again?

If you insist on covering spark gap transmitters as a way to understand basic electrical theory, the best I can suggest is "Practical Wireless Telegraphy" by Elmer E. Bucher (Wireless Press, 1917). Pretty sure it's out of print by now, but I do see it up on Amazon. It's got a pretty thorough description on a variety of spark and motor-generator transmitters. It reads like a collection of application notes from the American Marconi Corporation, kind of like the old HP application notes.

Things like a npn transistor with the arrow pointing in the oppostie direction of electron flow is sort of confusing but I'm getting over it.

There are some funny conventions in regards to how electron and current flow is depicted. Some of it is of historical/legacy significance, and probably dates back to prior to the discovery of the electron (1897).

I'm going to study up on some basic books and do some simple projects on a breadboard. I might study up on algebra as well although I am able to go through the equations.

thanks for taking the time to help me with my questions. I'm going to keep working at it after I pass general.

Good. You're asking some good questions.

 

[KC4RAF]

zz0468 has given you some good leads.

The best being the ARRL Amateur Handbook; a little pricey but a wealth of information. If you follow the laws in algebra operations you'll do fine. Also, transistors are PNP or NPN and an easy way to remember is NPN never Points iN whereas PNP the arrow always Points iN, The arrow tells you quickly whether it's NPN or PNP. "Hole flow" is from positive to negative and "electron flow" is from negative to positive. You more than likely knew this stuff, but I just threw it out there just in case.
My problem with electronic way back when was the capacitor. It had a nasty habit of confusing me! I studied when tubes were the in thing and transistors were just the NKOTB (New Kid On The Block).

 

[jedispork]

The best being the ARRL Amateur Handbook; a little pricey but a wealth of information. If you follow the laws in algebra operations you'll do fine.

Its on my list of books to purchase. Once I pass the general I'm going to read some proper books and not something that just gives you a summary to pass the test.

I was looking at the book basic radio which was suggested from my license manual. Aftering reading some reviews I was directed again to another book called Electronic Communication by Robert Shrader. Its out of print but it looks like a good read as well. Most other beginner books all have the same complaints. Its easy to put all the pieces together for a project but we need a better explanation of how the circuit actually works and why a certain componenet is where it is. I like the animated videos on youtube where they show how the current moves through the circuit.

thanks for the suggestions from everyone.

 

[KC4RAF]

You are the type that wants the

answers about how IT works, not something you memorize. You're going to do just fine. If you should have questions that you need answers to, by all means ask them here at RR. There's also a couple of ham sites that are friendly to the nkotb.
That book by Robert Shrader will be a good companion to the ARRL book too (if you can find it).