Not in the biz just a hobbyist. So why do some AM stations have like 2 or 3 towers next to each other? What about 4 in a sort of diamond shape? Why are some towers shorter than the others? Finally, why do some have weird shaped things on the top?
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Why do some AM stations have as many as 4 towers?
- Thread starter Jaycifer666
- Start date
Short answer: multiple towers make the coverage pattern directional. A single tower sends equal amounts of power in all directions and a map of the stations coverage will lshow more or less a circle of coverage.
The radio signal is sent to the various towers so that in individual waves are a bit later than the main tower and when the waves from the towers mix together, the signal is strong in some directions, weak in others.
Strange sha[ped things at the top of AM towers tend to be devices to make the power output a bit more or make it focused.
Some towers have FM antennas near the top. Do you know those when you see them?
The radio signal is sent to the various towers so that in individual waves are a bit later than the main tower and when the waves from the towers mix together, the signal is strong in some directions, weak in others.
Strange sha[ped things at the top of AM towers tend to be devices to make the power output a bit more or make it focused.
Some towers have FM antennas near the top. Do you know those when you see them?
Jaycifer666 said:
Stations have multiple towers to create a directional pattern. For example, WLAC here in Nashville is allowed to run 50,000 watts during the day. However, if they ran 50,000 watts at night, they would interfere with WWZN Boston and KGA Spokane on the same frequency. So, WLAC must use a directional antenna to reduce the power radiated towards Boston and Spokane.
To do this, they installed two extra towers which are only active at night. By adjusting the amount of power fed to these extra towers and delaying VERY slightly the signal fed to them, WLAC actually interferes with itself at night -- knocking down the amount of signal beamed towards WWZN and KGA.
The complexity of the directional pattern (the number of other stations which must be protected) determines the number of towers. The most I know of are 12 at a station in Dallas. The arrangement of the towers (whether they're in a diamond pattern, or a square, or a row) depends on the "layout" of the necessary pattern.
Someone else will have to answer why some towers in an array are taller than others. I think in many cases it's because the taller tower supports the antenna of an FM station which could use the extra elevation.
There are a variety of things that can appear on top of an AM tower:
- Lights. Towers more than 200' tall must be lit to warn aircraft.
- Antennas for FM stations. If the AM station also owns an FM they often put the FM antenna on top of (or on the side of) the AM tower. Less often these days but there are still plenty of stations doing it. On rare occasions, antennas for TV stations are mounted on AM towers.
- Non-broadcast antennas. Maybe a cuing link, or a two-way radio for dispatching news crews. Or, they lease space on their tower to two-way companies.
- Studio-transmitter link antennas. (dishes)
- Hawks & eagles!
Jaycifer666 said:
Licensed AM broadcast stations using more than one tower at a time do so to meet the requirements of their FCC license -- which defines the amount of power they can radiate in various compass directions at various times of day.
Multiple radiators for an AM station can produce a large range of net radiation patterns depending on their geometric arrangement, the r-f phases and amount of r-f power applied to each radiator, and the presence/absence of "weird" things present at the top of one or more of such radiators .
Some AM broadcast stations are licensed as non-directional for daytime operation, but directional at night. Some of those could use a single (omnidirectional), taller antenna during the day, and add one or more shorter tower(s) at night when they are required by the FCC to have a directional radiation pattern. The taller tower used for daytime operations would provide a greater coverage radius for the daytime power permitted.
The purpose of all of this is to serve the general public by limiting the amount of interference that such stations will generate to other stations on the same, and adjacent frequencies.
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w9wi said:
Please read the posts carefully...power out into a DA system depends on the system to determine the direction and intensity of the radiated signal...I.E,
50 KW NON directional can be totally different from 50KW directional, even at the same TPO (transmitter power out).
50 KW NON directional can be totally different from 50KW directional, even at the same TPO (transmitter power out).
Yes, More than one tower indicates that at least for part of the broadcast day, these AM stations broadcast with a directional signal. I believe my Nashville colleague is correct about the 12 tower array in Texas having the most. A station I used to work for had 11, but in two different locations, 6 for day at one location, and 5 for night in another location. That is done when the signal can only go in one direction and the people are in various directions in relation to the towers. The towers are placed at an extreme of the population...say to the south of the city, and all of the signal is directed north. The station to the south gets no interference, and the people to the north get the signal.
The various heights of towers is often done to control the azimuth (upward angle) of the skywave radiation. Accurate calculations can be made, for example, not radiate toward the north at a 45 degree upward angle, because we know how far away that signal would bounce back to earth after encountering the ionashpere. The ionasphere becomes a radio wave mirror at night. In this manner we can put some ground wave energy toward the population in our town, and lessen or eliminate the sky waves that would interfere with a far off stations listeners. Sometimes the shorter towers actually have nothing to do with the AM system, they are simply utility towers for other communications antennas the station may need to use.
As others have said, the attachments to AM towers can be microwave dishes, or FM antennas, even backup FM antennas. Usually the funny looking things right at the top are devices to bleed off static charges on the tower and make it less likely to be the target of a lightning strike.
The various heights of towers is often done to control the azimuth (upward angle) of the skywave radiation. Accurate calculations can be made, for example, not radiate toward the north at a 45 degree upward angle, because we know how far away that signal would bounce back to earth after encountering the ionashpere. The ionasphere becomes a radio wave mirror at night. In this manner we can put some ground wave energy toward the population in our town, and lessen or eliminate the sky waves that would interfere with a far off stations listeners. Sometimes the shorter towers actually have nothing to do with the AM system, they are simply utility towers for other communications antennas the station may need to use.
As others have said, the attachments to AM towers can be microwave dishes, or FM antennas, even backup FM antennas. Usually the funny looking things right at the top are devices to bleed off static charges on the tower and make it less likely to be the target of a lightning strike.
Couldn't this also be a "hat" like the KFI tower was supposed to have to top-load the tower to make it look electrically taller?Jaycifer666 said:
A capacitive hat would be good guess for sure! Another thought might be something like a Static disapation device that has tons of sharp points towards the sky to let the static drain off preventing many lightning bolts from blasting a tower. Ours looked sorta like a thorny ring hat on top of the tower. (BTW they DO help!) http://www.roger-russell.com/lightning/static1.jpg and http://www.mwpersons.com/nott-dissipaters.html
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gHz said:
That would be 1190 the former (and famous) KLIF where Top40 was King in the 60s and 70s...Actually 1190 has 16 towers....but 4 are at the daytime site in S. Irving....50KW daytime / 5 KW night...the night time pattern from the site in Rockwall is VERY narrow....people in N Dallas cannot hear it because the tightness in the pattern. The day site covers the DFW area quite well (and a good chunk of N Texas as well).....The night site is two rows of 6 towers.....the pattern is broadside to the array (like a rectangle with a straight line coming off one side 90 deg to the rectangle sides) looking straight thru Dallas, out toward South side of Ft Worth and beyond....The array with the MOST METAL in the air I BELIEVE is 1070 in Houston....it is 9 (or 11??) towers but they are taller than 1190s (except the last short one) but because of the height of the towers, the 1070 site has more metal overall.....but in number of structures, 1190 in Dallas wins with its night site (been there, touched it, maintained it)
The array with the MOST METAL in the air I BELIEVE is 1070 in Houston....it is 9 (or 11??) towers
11 towers are used for KNTH's daytime pattern, and 9 are used for their night pattern.
http://www.fccinfo.com/CMDProEngine.php?sCurrentService=AM&tabSearchType=Appl&sAppIDNumber=58118&sHours=D
http://www.fccinfo.com/CMDProEngine.php?sCurrentService=AM&tabSearchType=Appl&sAppIDNumber=58118&sHours=N
11 towers are used for KNTH's daytime pattern, and 9 are used for their night pattern.
http://www.fccinfo.com/CMDProEngine.php?sCurrentService=AM&tabSearchType=Appl&sAppIDNumber=58118&sHours=D
http://www.fccinfo.com/CMDProEngine.php?sCurrentService=AM&tabSearchType=Appl&sAppIDNumber=58118&sHours=N
MW_FM_DT_DXer said:
You'll note that towers 10 and 11, used only in the day pattern, have an electrical height that's less than half that of all the others. Virtually none of their nighttime signal is going to the north-northwest (toward adjacent channel KRLD 1080 in Dallas) but they boom into Galveston, over 60 miles away down I-45. Maintaining such an array is a daunting task; I've taken meter readings for it and that was enough!
What actually made me think of this is KYCY 1550 with 3 towers and KNBR 680 with one tall one with a Spiderman looking thing on top of the main tower and their short back up. KYCY site is in Belmont and KNBR is in Redwood Shores and both are San Francisco stations. They are located near the bay for ground conductivity. I don't think the thing on top of KNBR is for FM although when it was an NBC station they did have their FM facilities in the same place. Now SF FMs transmit from southern Marin County, MT. Sutro or San Bruno Mountain. So I doubt the thing on KNBR's tower is for FM.
great topic!
I'd say that the most tower-per-station city is probably Detroit, and the frequency with the most high power directionals in the least area is 1130, with Minneapolis, Milwaukee and Detroit all on.
Here's a photo of the KNBR site (courtesy Scott Fybush), and I suspect it is a capacitive hat as I mentioned earlier. The tower is under 600 feet, which is too short for a station on 680.
1550 KYCY runs 10kW full-time, DA-2 (two different directional patterns). Both patterns send most of those 10kW out to sea. There is a 5kW drop-in near LA that it has to protect, along with KXEX in Fresno. Probably other stations too, like KXTO in Reno. Lots of stations on 1550
1550 KYCY runs 10kW full-time, DA-2 (two different directional patterns). Both patterns send most of those 10kW out to sea. There is a 5kW drop-in near LA that it has to protect, along with KXEX in Fresno. Probably other stations too, like KXTO in Reno. Lots of stations on 1550
AM stations with multiple towers are required to restrict radiation in directions toward other stations they might interfere with on the same channel or even adjacent channels. The number and configuration of the towers along with their respective relative phases and currents, determine where the signal minimums and maximums are. These are based on FCC rules and regulations that establish the protection requirements of a stations particular class of operation and relative location. Details regarding this are beyond the scope of this forum and delve heavily into geometric math. The higher the tower number count, the more complex the pattern. Some stations are directional in the daytime as well as night, but the interference protection concept is different in the day than at night. In the day, the station is protecting a stations ground wave contour and is calculated using a simple azimuth angle. A station that is directional at night when sky wave is at play, is protecting an azimuth but also an elevation angle, known as “theta” towards the ionosphere who’s geometry would result in interference towards a distant station. This take-off angle is also impacted by the electrical height of the transmitting antenna, so this is primarily why you see some towers taller than others in a directional array. Where the principal is similar, no two facilities are identical. As an example, WLAC in Nashville protects KGA in Spokane, WA and another station in Boston, Ma. The equivalent signal towards Spokane is about 75 watts, towards Boston is about 2500 watts, these are referred to as nulls. As a result of restricting the signal in these null areas, WLAC enjoys a gain in other directions, also known as lobes. Towards the southwest, the equivalent signal is about 140 kW and south east about 85 kW. It all averages out to 50 kW if you take into count the entire 360 degree circle.
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PTBoardOp94 said:
Ehhh, try a ~350ft tower on 560 kHz (no top loading either)!!! One of my 1st stations (KLVI) was like that (still is!).....omni during the day, 4 tower inline directional at night (all 4 towers same height...66 degrees electrical height tall where a 1/4wave or 90deg tower would be 450+ ft)...the #1/omni tower becoming NEGATIVE Z in the array (in other words, it doesnt "radiate"; it feeds signal back into the phasor for correction)...it was a PITA with open wire feed..poor WQAM in Miami would often call if we didnt switch in time
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