Store AC & Hertz's Spark Gap Transmitter

[LightFantastic]

How did Hertz do it with his spark gap transmitter?

 

[davenn]

hi there

you don't need to store AC ... the power for the transmitter comes from a battery(s)

why did you think that you had to ?

have you read the wiki info on spark gap transmitters ?

have a read of the link above and see if you still have questions

cheers
Dave

 

[LightFantastic]

Http://youtu.be/NoeuVxYoNio?t=6m15s

6:21 he generates an alternating current which runs alomg these rods and acoss a gap... Dang you, Jim! Generates an alternating current? Talk about misleading... more like a temporary AC from a DC source

 

[davenn]

there you go :)

what you may also not understand is that the spark itself is generating the AC current
The spark isn't just a single pulse, but rather it is an oscillation of current back and forward across the gap
ie AC
and its that oscillating current that generates the radio waves that are emitted

cheers
Dave

 

[LightFantastic]

Http://youtu.be/NoeuVxYoNio?t=6m15s

6:21 he generates an alternating current which runs alomg these rods and acoss a gap...


Dang you, Jim! Generates an alternating current? Talk about misleading... more like a temporary AC from a DC source

Well I've never quoted myself before. Isn't this incorrect? Current in a DC battery never changes direction, therefore it is not AC in nature. Is there a better term for this?

Also,

If I tap the leads of a 9V, not only will current flow, but I will also get radiation. This radiation will seize to radiate when the current IS flowing.

Why?

Because once set in motion, the electrons can argue that they are at rest, and it is everything else that is moving.

Question

If I disconnect the leads from the 9V, I should get radiation again, correct?


How should I explain to someone the source of radiation? Movement of charge? I think that is misleading. How about when charge changes its state of motion?

 

[sophiecentaur]

Well I've never quoted myself before. Isn't this incorrect? Current in a DC battery never changes direction, therefore it is not AC in nature. Is there a better term for this?

Also,

If I tap the leads of a 9V, not only will current flow, but I will also get radiation. This radiation will seize to radiate when the current IS flowing.

Why?

Because once set in motion, the electrons can argue that they are at rest, and it is everything else that is moving.

Question

If I disconnect the leads from the 9V, I should get radiation again, correct?


How should I explain to someone the source of radiation? Movement of charge? I think that is misleading. How about when charge changes its state of motion?

You will get a bigger spark when you disconnect than when you connected, in fact - so more RF energy on disconnection. Similarly, in 'Old' motor car ignition systems, the sparks were generated when the contact breaker was opened.
The reason you radiate RF power is because magnetic energy has been stored in the inductor and this energy is radiated because there is a LC circuit that resonates (by design) at the wanted Radio Frequency. Current flows back and forward around the resonant circuit until the energy has all been radiated away.

The example of a Spark Transmitter is not the best one to use to get an understanding of how RF power can be radiated. There are too many added complications, compared with a modern transmitter that uses an oscillator and amplifier.

 

[LightFantastic]

The example of a Spark Transmitter is not the best one to use to get an understanding of how RF power can be radiated

That is why I tried breaking it down to something so simple as shorting out a 9V. I see no reason for Hertz to use a gap in his transmitter to emit RF power. Can you not emit a photon without a spark? I believe it is possible to emit a photon without a spark. Suppose you were to short out the poles of a loudspeaker. Now take your hand, and move the diaphragm (cone) in and out. If you were to connect the leads from an oscilloscope to the terminals of a loudspeaker as the diaphragm was being pushed and pulled, you would see voltage fluctuating in a sinusoidal fashion on the oscilloscope. Current of course, would move in one direction, then in the other. It is my understanding that this very act will radiate RF power. The important thing to note here is that there is no spark involved.

And thus my confusion with Hertz and his gap. I see no reason for it.

Back to the battery. I do not know whether or not every atom within that length of conducting wire (the one shorting out the poles of a battery) are emitting photons or are all photons radiated from the spark itself.

 

[sophiecentaur]

Hertz did it that way because he had no RF equipment. To radiate RF energy, you need a varying signal. It is a truly horrible thing to produce RF with a spark gap.
If you want to make a Loudspeaker produce a sound, you need an alternating voltage. You can produce this AC mechanically (vibrating or rotating something) but you cannot produce an alternating voltage at a high enough frequency to launch an EM wave of any significant signal power.

You have connected two different issues in your OP - the idea of radiating EM waves and the methods of producing AC. If you want to discuss RD radiators then that is best done in the context of conventional electronic RF generators and amplifiers. If you want to discuss why a spark generates radio frequency signals then that is best discussed separately. The two things don't go together very well, which is why no one uses spark transmitters any more. In fact, they are illegal, because they produce such a high amount of interference.

And thus my confusion with Hertz and his gap. I see no reason for it.

I think you are confused because you are trying to understand something that is too far down the line from your present state of knowledge. It has already been mentioned that the act of interrupting the flow of Direct Current in a circuit will produce a high voltage spike, The energy in this spike happens to contain non-DC components (RF), which is what can be fed to an antenna, when you use a suitable Inductor and Capacitor. You just have to accept this unless you want to learn some basics of circuits and circuit components. Just look at all the images that a google search of "spark gap transmitter" will show you. There is more to the circuit than just a battery with a switch.