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Path: utzoo!linus!philabs!cmcl2!seismo!harvard!talcott!wjh12!biomed!lagasse
From: lagasse@biomed.UUCP (Robert C. Lagasse)
Newsgroups: net.physics
Subject: freq. spectrum
Message-ID: <99@biomed.UUCP>
Date: Wed, 17-Jul-85 19:43:13 EDT
Article-I.D.: biomed.99
Posted: Wed Jul 17 19:43:13 1985
Date-Received: Sat, 20-Jul-85 16:28:14 EDT
Organization: Mass. Gen. Hosp. - Dept. Biomed. Engineering
Lines: 28


   I hope someone can help me with this one!!! :

Most radio transmitting equipment on this planet seems to transmit on
frequencies from about 400 kHz up to 1.5 gHz and do this by using an
oscillator, various stages of blah-blah-blah to an RF amp and then to a
resonant metal object (antenna).

  Now, if I want to transmit at a frequency of, say, 1 kHz which is
obviously in the audio (20 Hz-18 kHz) range, do I just take an audio
oscillator, feed it to an audio amp, and instead of driving a speaker
(please don't laugh) connect it to a 300 km long antenna (speed of light /
freq. = wavelength) with some sort of impedence matching network?  In other
words, how high up in frequency must one go before you can generate "RF", on
the other hand, how low in frequency can you go before you begin to affect
objects, people, etc.  I mean, if I build an RF transmitter for 10 Hertz and
connect it to the proper aerial (about 30 Mm long), would it shake every
object that would resonate at 10 Hertz even though it is "RF" and not audio,
or not??

   Another stupid question:  It seems that most low frequency (AM broadcast,
etc.) systems use enormous amounts of transmitting power as opposed to UHF
transmitters which use flea power in comparison.  Why?  How do the physics
of the propagation medium (dry air) affect how much power you need in
relation to where you are transmitting in the spectrum?


Thank You for all answers.