AM Radio

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    I’m trying to understand how does AM radio work in terms of antenna height, how many towers, directional vs. non directional, 1/4 wave, 1/2 wave, vs 1/8 wave. Are there any classes or books out there for me to understand about AM antennas and waves height? FM seems to be less complicated than AM.


    I’ll answer the easy one. Non-directional: One tower, unless we’re talking about an old classic flat top design.


    Here’s a good FREE resource for download:

    The book was written in 1952, but AM broadcasting hasn’t changed much since then. The basic principles are the same.

    Andy Brown

    An old edition of the ARRL Handbook has proven to be very useful for me, as well as an old edition of the NAB Engineering Handbook I’ve had since the 80’s. The TV section is mostly obsoleted now, but as mv/m said AM engineering hasn’t changed all that much.


    ..and while we’re on the subject:

    I long for any GOOD/ENTERTAINING commercial AM radio ‘anywhere’ – today…


    In many Canadian markets, an AM commercial station is #1 rated, so there are obviously a lot of people who listen, presumably finding it “GOOD/ENTERTAINING”.

    Back to AM transmitters and such: the only reason that FM is “so simple” is because there is not the need to create the incredibly complex directional patterns that all but the oldest clear channel AM stations need to broadcast at night with any amount of power.

    From what I’ve read, a directional FM station — and, yes, there are quite a few now — simply has a null in one specific direction. And that null is created simply by sticking a grounded piece of pipe in the direction of the null, not very far from the transmitter antenna “element”.

    Of course, the reason is that FM doesn’t normally do the long distance skip that AM is famous for at night.

    By the way, that would be an interesting topic for discussion sometime: what is the record for number of towers for an AM station? Not so simple anymore since we are now seeing multi-station sites for directional AMs. That really starts to add up to a lot of towers.


    KNRK’s a directional signal with some huge nulls in East Vancouver. W7PAT tells me it’s to protect KLYK. Once in a really great long while, under the right conditions, I can null out KNRK’s sideband on 94.9 and faintly hear KIOK.

    Andy Brown

    “simply has a null in one specific direction. And that null is created simply by sticking a grounded piece of pipe”

    Not exactly, although that is one way to achieve a single null.

    FM directional antennae can have multiple nulls, more easily done the lower the power.

    Two nulls are common, more than that isn’t usually found except in lower power service

    where they can be achieved by stacking yagi’s or log periodics to achieve the ultimate pattern,

    which to be approved must fill out 85% of the pattern as filed with the FCC. You can stack

    higher power elements on different azimuths as well, but I haven’t seen that much.

    Spacing of elements in any FM broadcast antenna array effects gain. Certain kinds

    of mounting have directional results and compensation can be built in, often with

    parasitics and odd additional stubs on the element.

    Antenna design is not simple.

    Parasitic elements are more complex than “sticking a grounded piece of pipe” in the design.

    Their shape and size and placement are critical, and are usually tested by building a scale

    model and testing it on a range. This is why all but the most simple off the shelf solutions

    in the DA business are so expensive and the multiple or deep nulls more so.

    I have an old DOS program that lets you design a pattern using some stock low power antennae patterns.

    It’s way cool but it can take hours to get the relative azimuth of multiple antennae and the power

    split just right. Been there.


    1/4 wave, 1/2 wave, vs 1/8 wave

    The vertical radiation patterns of these antennas look roughly like this: (note: it is 5/8 wave, not 1/8)

    The 5/8 wave antenna produces the strongest groundwave (that is, the signal hugs the ground). However, as you can see, there is also a minor lobe going up at a high angle when this type of an antenna is used. Even though a 5/8 wave tower is theoretically better than a 1/2 wave tower, broadcasters overwhelmingly prefer 1/2 wave because the minor lobe causes interference to the groundwave signal at night. (By the way, somewhere on the Internet, there is a story about how WLW fine-tuned the height of its tower to minimize this type of interference. Many years later, when they added a co-located FM station, the extra height added by the FM antenna atop the AM tower worsened nighttime groundwave reception in moderately fringe areas. I grew up about 110 miles from the WLW tower, and I recall that at night there was a lot of selective fading because I could simultaneously receive groundwave and skywave.)


    There’s something in the 500KW WLW logs, indicating interference to WOR so they put in a directional antenna and the amount of signal toward the null was 50KW!


    The KNRK one-bay antenna has a null at 345 degrees (NNW).

    No nulls to East Vancouver.

    Radio Lover

    Thanks for the information everyone.


    WOW Kent, 16 total towers for that Dallas station! No wonder CC is 21B in debt- they have to keep all those lights lit. 😀

    What do you guys/ gals feel is the most complicated array in the Portland market? I am guessing the messiest one up here in Seattle just died, to be replaced by 31 McMansions. It was the 6 towers shared between 1460 (Doomsday Harold Camping station, RIP) and 1250 Disney night only- also likely doomed.

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