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"Dave Pitzer" wrote in message
... First of all, I'm a novice where taransmitting antennas are concerned. It seems to me that many AM "broadcast band" antennas are four-footed towers. Do these towers merely serve to hold the actual antenna (a "wire") off the ground vertically. The "wire" runs up the center of the tower? FM and TV (and some AM) towers tend to be slim "guyed" towers with some sort of antenna element(s) at the top. Is this correct? Is the actual tower ever a radiating element? The tower _IS_ the wire. Some towers have insulators at the base, some don't, but the metal in the tower is what radiates. The guy wires might be a part of the radiation system, but usually they're just anchors. Also... Some tower structures (AM, FM & TV) tend to be cited on hight ground -- mountains or atop high buildings. Other antenna structures tend to be intentionally located on "low" flat ground. Why the difference? FM and TV signals use frequencies that travel best in a straight "line of sight" from the transmitting to the receiving antenna; ergo, they're high up so that more receiving antenna can "see" them. AM stations benefit from being near water, and since the frequencies used for AM don't need to be in a line of sight from transmitter to receiver, they can be low down where the water is. If a station is using its AM tower to hold an FM antenna, the location is a compromise between height (for FM) and proximity to water (for AM). Thanks, Dave Pitzer You're welcome. Please tell your teacher that r.r.a.a. sends its regards. ;-J Bill |
#2
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Hi Dave, (and remember- There are never DUMB QUESTIONS!) The towers you see
(even at low elevations) can be used for more than AM broadcast (and ath the frequencies for am, the WHOLE tower is used for the radiator- not the "wire" inside it. in fact, under OSHA regulations, the "WIRE" you see (and probably associated with a ladder, tho not necessarily, may be "FALL PROTECTION" for those that must climb it to do repairs- what you DONT see is a device that will allow free travel one way (up), but unless a catch is depressed, will not travel down (in case a maintainence worker slips). You may also see smaller antennas (normally towards the top), for fm, 2 way, pageing, and even microwave-- occasionly you will even see metal plates, pich at a 45 degree angle- periscope reflectors for microwave- the dish feeding them sits on the base of the antenna. As stated, for AM, the antenna is USUALLY insulated from ground (the "counterpoise" can consist of wire netting, or individual radials running on top, or just below the surface of the soil surrounding the area.) . When was going to college, worked at a fm station in Portland, Or- that station had something like 8 - F.M stations, and 5 - two way radio stations on a 350 foot tower (Healey Heights) and the tower was probably 30-40 foot at the base! (coax for each of the broadcast antennas was about 8 inches in Diameter!!). Tho because of security problems in todays world, some of the Engineers at these instalations are quite proud to give a Guided Tour- might find it interesting to see just what makes them tick! Jim NN7K " First of all, I'm a novice where taransmitting antennas are concerned. It seems to me that many AM "broadcast band" antennas are four-footed towers. Do these towers merely serve to hold the actual antenna (a "wire") off the ground vertically. The "wire" runs up the center of the tower? FM and TV (and some AM) towers tend to be slim "guyed" towers with some sort of antenna element(s) at the top. Is this correct? Is the actual tower ever a radiating element? The tower _IS_ the wire. Some towers have insulators at the base, some don't, but the metal in the tower is what radiates. The guy wires might be a part of the radiation system, but usually they're just anchors. Also... Some tower structures (AM, FM & TV) tend to be cited on hight ground -- mountains or atop high buildings. Other antenna structures tend to be intentionally located on "low" flat ground. Why the difference? FM and TV signals use frequencies that travel best in a straight "line of sight" from the transmitting to the receiving antenna; ergo, they're high up so that more receiving antenna can "see" them. AM stations benefit from being near water, and since the frequencies used for AM don't need to be in a line of sight from transmitter to receiver, they can be low down where the water is. If a station is using its AM tower to hold an FM antenna, the location is a compromise between height (for FM) and proximity to water (for AM). Thanks, Dave Pitzer You're welcome. Please tell your teacher that r.r.a.a. sends its regards. ;-J Bill |
#3
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OOPS-- Tho because of security problems, many stations are off limits, but
Some of these engineers are quite proud to give you a guided tour to show how they "tick"!! Jim |
#4
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Another point is since the entire tower is RF hot not only must it stand on
an insulator at the bottom it also needs a way to get the tower lighting voltage to it. This is done w/ "Austin Rings" basicly a pair of interlocked toroids or rings. In addition, for lightning protection they have "johnny balls" a pair of air spaced balls to form a spark gap. Do a google search for these and you should find pictures. http://hawkins.pair.com/atcitynj.shtml is one such site, just scroll to near the bottom of the page. "Jim" wrote in message ... OOPS-- Tho because of security problems, many stations are off limits, but Some of these engineers are quite proud to give you a guided tour to show how they "tick"!! Jim |
#5
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The Masked Marvel wrote:
Another point is since the entire tower is RF hot not only must it stand on an insulator at the bottom ... I don't know much about commercial AM transmitting systems. Why not feed the towers with a gamma match? -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#6
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"Cecil Moore" wrote
I don't know much about commercial AM transmitting systems. Why not feed the towers with a gamma match? ________________________ Radiation patterns are not as omnidirectional using this method as they are when a tower is series-fed. The FCC frowns on them, especially for use in directional arrays. The following link may be of interest: http://www.dlr.com/dlrweb/papers/nabpaper/nabpaper.htm RF Visit http://rfry.org for FM broadcast RF system papers. |
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