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Path: utzoo!linus!decvax!mcnc!unccvax!dsi
From: dsi@unccvax.UUCP (Dataspan Inc)
Newsgroups: net.physics
Subject: Re: freq. spectrum
Message-ID: <232@unccvax.UUCP>
Date: Thu, 18-Jul-85 09:13:43 EDT
Article-I.D.: unccvax.232
Posted: Thu Jul 18 09:13:43 1985
Date-Received: Tue, 23-Jul-85 05:30:40 EDT
References: <99@biomed.UUCP>
Organization: UNC-Charlotte
Lines: 57


     In response to question #1: There is probably RF usage down to 20 kc/s
or so. . .while I don't think there is anything theoretically impossible 
about constructing a 300 km transmitting element, a single vertical radiator
is going to be mighty interesting to build; the horizontal case will be 
very inefficient for the same reasons.

     The US Navy operates a transmitter at 60 kHz (Naval Observatory time) 
but our lab has so much computer equipment in it that 60 kHz is unusable.
Is this true in most of the US (high man-made noise)

     In response to question #2: Inverse distance is inverse distance is
inverse distance, from a point source (blah blah blah, usual electromagnetics
stuff).  However, the communications being done determines the amount of
power/tower required.  For example, I can sit on our living room couch and
hear all kinds of neat air traffic control stuff from the aircraft to the
FSS's (or whatever); but I can't get the Charlotte tower at all.  The former
very much approaches 'inverse distance propagation,' the latter behaves
like your usual FM broadcasting station which suffers from terrain diffraction
and outright blockage.

     I think the transmitters in aircraft are 4 watts.

     There are other factors which come into play.  I, and every daytime AM
station in the United States, would love to get a 9 dB power 'gain' out of
their radiating system!!!! This is done (commonly at VHF/UHF) by stacking
a whole bunch of the elements in the correct phase and amplitude relationship
so that maximum directionalisation is obtained in the plane which the
elements are normal to.  For example, you can buy a 60 kw FM transmitter for
broadcasting, but your FCC says that the power output from the antenna is
EFFECTIVE radiated power; so why not buy as much antenna gain as you can
afford and a 20 kw transmitter? 

     Similarly, there is power gain at TV stations, too; but the trend is
towards CBR (tm) - cavity backed radiators - which generate circularly
polarised signals, a debatable antighosting measure. These antennas are
notoriously inefficient.

     At UHF frequencies, the radiating elements are extremely efficient -
you no doubt have a UHF TV station who has an effective radiated power of
5000 kw visual, 500 kw aural -- but the transmitter is most likely 60 kw.
(These guys may be using 'fleapower' but talk to a UHF station manager
about his power bills!!!! Some of those old UHF klystron transmitters are
10-15% efficient! By comparison, a Continental 312C2 50kw AM transmitter
can reach 80% efficiency at 100% modulation!) 

     Hope this answers your questions.  The service being performed has
a lot to do with the power and radiating system designed, but the fundamental
propagation problems are pretty much the same. (pleeeze, no flames from
you EM engineers !!! )


David Anthony
Chief Development Engineer
DataSpan, Inc

.