lagasse@biomed.UUCP (Robert C. Lagasse) (07/18/85)
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.
dsi@unccvax.UUCP (Dataspan Inc) (07/18/85)
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
.karsh@geowhiz.UUCP (Bruce Karsh) (07/19/85)
In article <99@biomed.UUCP> lagasse@biomed.UUCP (Robert C. Lagasse) writes: > > 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?? The military's project ELF (extremely low frequency) project in Clam Lake, Wisconsin broadcasts at 60 hz on huge (many miles long) antennas. The intent is to be able to communicate with submarines. (Skin depth in water is much bigger at lower frequencies.) There have been a lot of complaints, lawsuits, etc. about possible health and environmental effects. We did some seismic experiments up there. I understand that when ELF was broadcasting, our seismic cables went wild. -- Bruce Karsh | U. Wisc. Dept. Geology and Geophysics | 1215 W Dayton, Madison, WI 53706 | This space for rent. (608) 262-1697 | {ihnp4,seismo}!uwvax!geowhiz!karsh |
jp@lanl.ARPA (07/19/85)
> > 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
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (07/21/85)
> The military's project ELF (extremely low frequency) project in Clam Lake, > Wisconsin broadcasts at 60 hz on huge (many miles long) antennas. Hell, the whole country does this! It's a real problem (known as "cultural noise") in magnetotellurics.
dww@stl.UUCP (David Wright) (07/27/85)
<...eater...>
Re the discussion on how low can an AC signal go and still be radiated - no
20kHz is not the lowest ... here is an experiment:
Take a hi-fi amplifier with a sensitive input. Connect a piece of wire to
said input - a metre or two will do. Measure frequency of hum at output.
Result = 60Hz ==> you are probably in some reasonably advanced country (e.g.
the USA) outside Europe and are picking up RADIATION
at 60Hz from house and/or local grid wiring
Result = 50Hz ==> Ditto but in Europe
Result = 440Hz ==> Are you in an aircraft or what??
No result ==> You must be in the middle of the Sahara or Antartica or
somewhere ....
This is why audio wiring for low level signals uses screened cable. It is true
that this is also needed to prevent induced mains hum, and that hum can also
come from ground loops, but there's plenty of low-level radiation at 50/60 Hz!
[I thought everybody knew this, but as the discussion's been going on about
whether 10 or 20 or 50kHz was too low I've added my bit ... and yes there
is NO frequency so low that you cannot radiate it given sufficient power]hes@ecsvax.UUCP (Henry Schaffer) (07/30/85)
Another experiment at low (power) frequency. An electricians neon tester (a small neon glow lamp, e.g. NE-2 with a series resistor of about 200 kohms. note NE-2 is an obsolete but still used number), has one contact on the hot contact of a power line, and I hold the other contact while standing on a perfect insulator. (Wearing rubber soled shoes and standing on dry wood ought to be a good approximation.) The neon tester bulb will glow faintly (but visibly - unless you are in sunlight you should be able to see it.) Some reasonable number of microamperes are flowing through the neon lamp and ME. (Full brightness of the lamp takes about 400 microamps.) How is this power transmitted from the power line to me to ground. My theory is that this is capacitative coupling between me and ground. Does this count as EM radiation? Regardless, this certainly is a mechanism for injecting power line frequency hum into all sorts of equipment in the home and lab. NOTE: This involves the AC power line which can be hazardous, and can lead to a lethal accident. Don't mess with the power line without competent supervision, unless you are competent. --henry schaffer n c state univ
stpeters@steinmetz.UUCP (R L StPeters) (07/31/85)
> ... here is an experiment: > > Take a hi-fi amplifier with a sensitive input. Connect a piece of wire to > said input - a metre or two will do. ... > ... there's plenty of low-level radiation at 50/60 Hz! The field from an oscillating current consists of two components, a "near field" that only oscillates and a "radiated field" that propagates aways from the source. What this experiment detects is local induction from the "near field". Only when you detect energy transported many wavelengths from the source can you claim to have detected "radiation". See any good text on E&M theory, such as the one by Jackson. -- R. L. St.Peters (Dick) The "R" is for "Reptile". uucp: decvax!mcnc!ncsu!uvacs!edison!steinmetz!stpeters (uucp is forever) arpa: stpeters@ge-crd (federal express) "Any opinions expressed by my employer are probably not mine."