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How do broadcast towers work?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N6MST, Jan 14, 2019.

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  1. N6MST

    N6MST Ham Member QRZ Page

    Are there some professional RF folks in here or even someone who just knows this, but would someone clue me in to how broadcast antenna towers work? The ones I am thinking of are the tall red and white painted towers, a few hundred feet tall at least, usually located on large (other than the tower and associated equipment buildings) vacant lots, guyed out like crazy, flashing red light at the top, but with obviously visible antennas. To complicate the question, I have no idea what station or frequency the ones I see every day operate on, so I apologize for the vague question. Is the tower itself the antenna, shunt-fed or some other way of feeding from the transmitter(s)? I admit I am clueless on this subject, figured someone here has got to know.

  2. NH7RO

    NH7RO Ham Member QRZ Page

    They are Marconi antennas as a general rule (1/4 wave ground planes).
    KP4SX likes this.
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Most are just base-fed Marconis, but they're all various "lengths." The KFI-AM tower here in L.A. is 1/2-WL on 640 kHz so it's very tall. Some are shorter, some are even longer in terms of degrees (percentage of wavelength).

    The "matching network" at the bottom of the tower does a lot of work.:p
    WA7PRC likes this.
  4. WB5WPA

    WB5WPA Ham Member QRZ Page


    AM or FM? The guys above are describing AM broadcast antennas or 'structures' as some in the biz refer to them.

    FM uses antennas more like 2-way radio antennas, that is, 'gain arrays' are mounted up a tower and fed via coax ...
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Broadcast towers for the AM band (or Medium Wave but they're not as common) are the actual antenna itself and are paired with a network of ground radials. As posted above they're often not actually a quarter wavelength long nor resonant so a matching network is used at the base to feed them. A full quarter wavelength antenna can be very tall and expensive and not really necessary so antenna systems are designed taking into account height and cost constraints. It's not much different in concept than feeding a non-resonant ham vertical like a 33' or 43' ham vertical but easier in the sense that the antenna only has to support a single frequency though of course at much higher power levels.

    But broadcast towers are also used to hold up FM radio and TV transmit antennas in which case they're just mechanical support towers and the actual antennas are bolted onto the tower. So it's hard to know which you were looking at but if it's out in the middle of a big field or out in a marshy wetland and there aren't a lot of visible antennas bolted on the tower then pretty good chance it's AM radio and the tower itself is the antenna.
    K7MH, WA7PRC, NH7RO and 1 other person like this.
  6. AG6QR

    AG6QR Subscriber QRZ Page

    Right. And how do you tell the difference?

    In California, where the original poster lives, AM broadcast antennas are usually located on flat ground, often near water, with tall towers. FM antennas are usually located on mountaintops, with relatively short towers, since the mountain is doing most of the work in elevating the antenna.

    For FM, height is king. For AM, ground conductivity is more important, but the tower needs to be tall because of the long wavelength involved. If the tower can be close to a quarter wavelength long, the matching network doesn't have so much work to do.
    WA7PRC and NH7RO like this.
  7. N1BCG

    N1BCG Ham Member QRZ Page

    In Europe, it's common for the "mast" to merely be a support for a set of vertical wires that are the direct radiators for AM or LW transmissions...


    The mast is typically grounded at the base where an antenna tuning unit (ATU) contains matching circuitry to couple the RF from the feedline to the wires...


    The diagram below shows the elaborate center feeding of the 747 and 1008 masts * making them extremely large vertical dipoles...


    The practice of adding wires to AM broadcast towers in the U.S. has become a popular way to broaden the bandwidth of the tower by reducing the "Q" since it electrically appears "wider". We did this with several AMs since the 80s. The base insulators were strapped across, making the tower solidly grounded and eliminating the need for isocouplers previously needed to connect coaxial feedlines for FM or repeater antennas as well as the tower lighting wiring.

    * Sadly, these masts in The Netherlands were recently imploded as both stations have been decommissioned.
    Last edited: Jan 14, 2019
    WB2UAQ, N4UP and N0TZU like this.
  8. N6MST

    N6MST Ham Member QRZ Page

    I messed up my original post, every one. I meant to type "with NO obviously visible antennas" but I think you guys answered my question. They must be AM Marconi antennas, but now I am confused about the matching units at the tower bases. Is there a need for matching at a 1/4 wave vertical? The one's I've built (20 and 40 meters, 100 watts MAX) have never had one and they did ok. Anecdotal at best, I know :)
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That transmission line is AWESOME!
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    They all have matching units.

    A 1/4-WL Marconi over a very, very good ground system has a feedpoint impedance around 30 Ohms. A 1/2-wave can be thousands of Ohms. Anything in between, or between 1/2-WL and 3/4-WL will be something in between and will be reactive.

    The KFI-AM site locally has a building next to the main tower containing the matching system.

    Inside, the T-matching network looks about like this.

    The lightning discharge system at the tower base.

    The tower itself is 654 feet tall and is somewhat top loaded by a 50' diameter top loading "hat."
    KE5OFJ, KI5WW, N4DJT and 6 others like this.

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