[Lowfer] 8820 Hz question - maybe OT

[Michael Sapp]

John, Mike & All: I'll toss my 2 cents in...

>The one absolute condition for radiating an electromagnetic wave at radio 
>frequencies is being able to accelerate charge carriers.  We do this by 
>pushing RF current through a conductor (the antenna), causing electrons to 
>accelerate first in one direction, then the other, repeatedly, for the 
>duration of the transmission.  That's pretty much it.

    While this statement is correct, it can be easily misinterpreted.  For 
years I had the incorrect assumption that
a series of electron flowed from A to B (in an alternating fashion) from one 
end of the antenna to the other as the energy is radiated. This incorrect 
assumption was due to
taking the "water flow in a pipe analogy" a bit too seriously.

  The short answer is that with direct current, electrons travel at a pace 
know as "drift velocity" in a material.  With alternating current, the 
electrons in a material stay "in place," but they oscillate at the AC 
frequency.

    This link explains current and drift velocity 
http://pfnicholls.com/physics/current.html

    This link explains the difference between DC and AC in terms of 
velocity.      http://en.wikipedia.org/wiki/Speed_of_electricity

    Note this statement

    "The drift velocity deals with the average velocity that a particle, 
such as an electron, gets due to an electric field. In general, an electron 
will 'rattle around' randomly in a conductor at the Fermi velocity.[2] Free 
electrons in a conductor vibrate randomly, but without the presence of an 
electric field there is no net velocity. When a DC voltage is applied the 
electrons will increase in speed proportional to the strength of the 
electric field. These speeds are on the order of millimeters per hour. AC 
voltages cause no net movement; the electrons oscillate back and forth in 
response to the alternating electric field (over a distance of a few 
micrometers - see example calculation).[3]"

Which takes one to the  example calculation link :

http://en.wikipedia.org/wiki/Drift_velocity#Numerical_example

"In the case of alternating current, the direction of electron drift 
switches with the frequency of the current. In the example above, if the 
current were to alternate with the frequency of F = 60 Hz, drift velocity 
would likewise vary in a sine-wave pattern, and electrons would fluctuate 
about their initial positions with the amplitude of:

A = \frac{1}{2}F \frac{2\sqrt{2}}{\pi} |v| = 2.1\times10^{-6} \text{m} "

    Understanding the nature of  AC and  drift velocity also leads to a 
better understanding of how RF energy is transported in a coaxial 
cable...with that whole velocity factor thing.....

    73, Mike wa3tts

















----- Original Message ----- 
From: "JD" <listread at lwca.org>
To: "Discussion of the Lowfer (US, European, & UK) and MedFer bands" 
<lowfer at mailman.qth.net>
Sent: Wednesday, December 03, 2014 12:51 AM
Subject: Re: [Lowfer] 8820 Hz question - maybe OT


>>>> what are the conditions or parameters for a signal like 8820 Hz to 
>>>> change from being contained in the magnetism of a transformer or 
>>>> speaker voice coil to being radiated from an antenna?
>
> You are correct that there is no magic frequency for EM radiation to 
> occur, as illlustrated by the example Doug gave.
>
> The one absolute condition for radiating an electromagnetic wave at radio 
> frequencies is being able to accelerate charge carriers.  We do this by 
> pushing RF current through a conductor (the antenna), causing electrons to 
> accelerate first in one direction, then the other, repeatedly, for the 
> duration of the transmission.  That's pretty much it.
>
> Of course, that's also the challenge...getting enough of that current to 
> flow, and not letting the current be balanced by another current 
> accelerating electrons in the opposite direction within the same plane. 
> The lower the frequency, the harder it is as a practical matter to get 
> current to flow at all in a short Marconi antenna because of all the 
> reactance that must be cancelled; or to achieve sufficient separation of 
> the sides of a loop antenna to prevent most of the signal from cancelling 
> itself.
>
> John
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