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From: jp@lanl.ARPA
Newsgroups: net.physics
Subject: Re: freq. spectrum
Message-ID: <28494@lanl.ARPA>
Date: Fri, 19-Jul-85 11:46:41 EDT
Article-I.D.: lanl.28494
Posted: Fri Jul 19 11:46:41 1985
Date-Received: Sat, 20-Jul-85 15:08:36 EDT
References: <99@biomed.UUCP>
Organization: Los Alamos National Laboratory
Lines: 70

> 
>    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??
> 
Yes, you can radiate electromagnetic energy at arbitrary frequencies.  The 
Navy uses very low frequencies (10-15kHz) for underwater communications.
The antenna does not need to be a halfwavelength long in order to radiate, but
a bigger antenna will, generally speaking, radiate more effectively.  One
problem that is worthy of consideration is that the bandwidth of the antenna
may limit the speed of data transmission.  (The lower the frequency the
narrower the bandwidth for a given Q.)  I believe the Navy uses slow speed
Morse code for this reason.  The usefulness of low frequency is the depth
of penetration of the waves into the seawater, which is essentially a
conductor.  The depth of penetration is proportional to the square root of
the resisitivity (reciprocal conductivity) and the square root of the 
reciprocal of the frequency.

Electomagnetic radiation is an oscillating electric and magnet field that
can heat conductors and interact with charged particles but should not
shake objects.  Sound is a mechanical vibration propagating through a
massive medium (air, the earth, etc) that indeed can shake things around.
(e.g. earthquakes, the famous Tacoma Narrows bridge(I believe) that was 
destroyed by a resonant excitation from the wind.  (Moral, build low Q
bridges.)

>    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?
> 
AM broadcast stations use relatively inefficient antennas, almost always
significantly shorter than a quarter wavelength.  Furthermore, the radio on
which you receive them has a very inefficient antenna, it is usually just
a coil of wire on a ferrite rod that makes up the inductance of the
input tuned circuit.  In contrast, television and fm broadcasters have
arrays of resonant antennas designed to focus their radiation parallel to
the earth.  Likewise, you have a fancy antenna on top of your house to
receive them, unless they are very near.   In the broadcast world,
AM stations use 250W to 50kW, while most TV stations are in the 50KW
ballpark. When a TV station quotes numbers like 1 Megawatt they are talking
about effective radiated power (ERP) which is antenna gain times transmitter
power.  

The actual power required for communications (with a given signal to noise
ratio) depends on antenna gains, receiver noise figure, and bandwidth.  And,
also on the conductivity of the medium.  Air can be quite lossy at some 
frequencies (like 10GHz).
> 
> Thank You for all answers.

Glad to be of assistance.

Jim Potter  jp@lanl.arpa