The Power Of Directional Antennas

AM broadcasting had a big problem, but usually only at night. During the day the AM signals had limited range, but at night they could travel across the country. With simple wire antennas, any two stations on the same frequency would interfere with each other. Because of this, the FCC required most radio stations to shut down or reduce power at night leaving just a handful of “clear channel” stations for nighttime programming. However, creating directional antennas allowed more stations to share channels and that’s the subject of a recent post by [John Schneider].

When it comes to antennas, ham radio operators often think bigger is better. After all, hams typically want to work stations far away, not some specific location. That’s not true in the commercial world, though. The big breakthrough that led to for example cell phones was the realization that making smaller antennas with lower power at higher frequencies would allow for reuse of channels. In those areas the focus is on making cells smaller and smaller to accommodate more people. You can think of AM broadcasting as using the same idea, except with relatively large cells.

A lot of innovation occurs when someone is angry about something as was the case here. Two cities in Florida shared a common radio transmitter in 1927 to promote tourism in the area. Because of the wide area coverage, people from far away could hear the benefits of a Florida vacation. However, a reorganization of the AM band in 1929 caused the Florida transmitter to interfere with a station in Wisconsin who complained.

As a result of that complaint, Florida had to cut their nighttime power and as their whole goal was to attract tourists from other states they were not happy. The Florida stations — WFLA and WSUN — contacted a consulting engineer named Craven to solve the problem.

What follows is an interesting study in RF problem solving, and while the answer might seem obvious today, it was pretty amazing back in the early 1930s. Without directional antennas, the 15,000 or so full-power AM radio stations in the United States at its peak would not have been possible.

If the AM band ever goes completely away we’ll be sad because it is the easiest thing to receive with a crystal radio. Of course, then you could just broadcast yourself, we suppose.

32 thoughts on “The Power Of Directional Antennas

  1. Great article with some basics and history that I’m thinking inspired or led the way to more advance beam forming mechanically and maybe I’m thinking regarding phase… electronically. To think there are far more directional antennas now days that are electronically steered or flat out ?ased/mased/lased.

    Though figure with those earlier AM designs (that’s about all there was other than Bose and a few others work until more advanced transmitters capabilities came about main stream)… adding elements like a director and reflector made more gains and then adding different perpendicular elements changes the polarity and then thinking perpendicular elements at specific spacing makes polarity changes to be circular or I guess elliptical also.

    Let’s see more of these antenna articles. Neato science even if seems old school!

    Beam forming is very interesting… especially when you get into higher frequencies and optics seems to be the ways to optimize beam form. However, lenses can be made down to the audio range even.

    Really neat studying the history and looking at beam patterns.

      1. “I would love to see some articles on audio beam-forming.”

        Me too, the last reference below is the only presentation I’ve seen before using a lens in the audio range directly. Other methods may use a modulated carrier wave effect system (e.g. acoustic hailing device like https://hackaday.com/2019/02/14/creating-coherent-sound-beams-easily/ or https://hackaday.com/2018/03/24/us-military-developing-laser-plasma-speakers/ ) or pulse train system ( ??? good example at the moment ) or a heterodyne system (Hypersound – https://en.wikipedia.org/wiki/Sound_from_ultrasound#HyperSonic_Sound).

        Sound Waves in Air 1961 PSSC; Richard Bolt, MIT; Acoustic Lens Demonstration (I save a copy just in case since I’ve seen these demos disappear once already):
        https://youtu.be/IQYU9H41Eik?t=1760

        1. Acoustic Doppler Wind Profilers are a commercial off-the-shelf (COTS) thing. They come in the “big dish” versions and the “array of transducers” version. They make tightly focussed beams of sound and listen to the doppler-shifted echo to give you wind speed and direction as a function of altitude. They’re loud and annoying. :-)

          There are COTS microphone arrays for listening only for sounds coming from a specific direction. There was one for sale that looked like a picture hanging on a wall – the “beam” could be “steered” to record a couple of people talking in a hotel room while not being able to hear the TV blasting at full volume. This was made by “Digital Recorders, Inc”, but they seem to have sold their security/surveillance products and kept only their transit bus products. I’m sure there are others sold now.

      2. You’ll disappear into hi-fi-ism pretty quickly if you try to google it. Acoustic design cargo cult stuff

        Beamforming works better in ultrasound, and there are some applications where you use the ultrasound as a carrier, and cross two beams. Any object at the intersection of the beam causes the beams to interfere with each other and that demodulates the signal down to the audible range. That makes it possible to “cast” sound onto a distant object, or make it audible only to a person standing in a particular spot (and to those right nearby).

        1. @Luke, et.al. “Any object at the intersection of the beam causes the beams to interfere with each other and that demodulates the signal down to the audible range.”

          I was under the impression there are multiple ways for this phenomena whether ultrasonic, or another higher frequency range, to work. Please correct me if I’m wrong since I’m still learning and trying to comprehend the methods to the madness.

          One is the carrier method where the audio is modulated onto the carrier and demodulated when comes into contact with certain materials like the human head or maybe even air to a certain extent.

          The second method is heterodyne, which is like a bi-neural beat, where at the intersection point of the two frequencies the difference (or I guess sum potentially) frequencies are in the audible range.

          A third method is like using any frequency from one source (though two can be used for a more complex as noted above type one method) where the source is pulse modulated in the audio range (or maybe other frequencies ranges the body can detect as sound) on the target. This is more like the “microwave hearing method” however, if I understand… can be “non-thermal” effect transmission also where like bone conducting or pressure modulation and maybe photo-acoustic effect also.

          Now, pure audio source beam forming is a little different and more challenging on smaller scale. Phased arrays are one method, “whisper dish” parabolic dish/trough is another method, or other audio-phile methods that are more typical can create directionality by case, port, cone and even lens design.

  2. “ham radio operators often think bigger is better. After all, hams typically want to work stations far away, not some specific location.”

    You make hams sound stupid. Amateur radio operators who want to work far-away stations know that directional antennas give them GAIN; instead of broadcasting to the whole world and spreading their limited power over areas of no immediate interest, directional antennas allow them to focus their power on the area they’re trying to reach.

    This is an aspect you don’t cover at all in the article – that by using a directional antenna, a broadcast station can achieve the desired field strength in the areas they are most interested in, using less transmitter power.

      1. The statement about bigger is better was taken as intended. Recall the phrase “If your antenna stayed up last winter, it wasn’t big enough”- I think attributed to Sam Harris W1FZJ of Arecibo fame. All us amateur radio operators felt that way. A big antenna usually paid off in better directivity and less QRM from directions that you didn’t want to hear from.
        Now with antenna restrictions, etc, we have to make do with less “visible” options.
        But we will succeed!

  3. “During the day the AM signals had limited range, but at night they could travel across the country.” this part is deceiving, it’s not about modulation type (AM) but these effects happen at certain frequencies (which were used at that time).

    1. It happens for many frequencies, even ones used today. You can often pick up terrestrial DTV from hundreds of miles away when the weather is just right.

      The important bit about AM is that it “mixes on the air”: the other stations can be heard on top of each other, whereas with FM interference sounds more like random unintelligible noise. Since the center frequencies of the stations aren’t ever exactly the same, you can often tune out the offending station by de-tuning your receiver slightly.

  4. Nice to see mention of WLW – lived in Cincinnati and every time I saw that tower I was impressed – I could hear them halfway across the country – farthest was in texas… Clear channel was great

      1. Nope. CDMA would be the modern (well, 1957) innovation that allows many stations to broadcast on the same channel. It’s just not used in the context of commercial radio stations, except on DAB which was generally a failure.

        It works by a pseudo-random code. The idea is that while everyone’s using the same band, all the simultaneous transmissions look like random noise, but if you know a pattern behind that noise you can extract a correlation to that particular pattern over time. The pattern runs at multiple times the actual data rate, and by a clever use of an encoding scheme that allows the signal to degrade gracefully, things like cellphones can exist on the same frequency by basically just shouting on top of each other.

        1. DAB (Digital Audio Broadcasting) is not a CDMA system, DAB and DAB+ use OFDM. On frequency-‘reuse’, DAB+ can be used in a single-frequency network, even with solely opensource tools – see http://opendigitalradio.org

          With regard to CDMA (Code Division Multiple Access) Wifi @ 1 mbps/preamble uses it (11 bit barker code), and so does GPS and the EU navigation solution Galileo (sharing the same E1/L1 band, although different rates). See http://navipedia.org/

          The Apollo 11 radio ranging system (USB) also used CDMA. https://en.wikipedia.org/wiki/Unified_S-band

        2. CDMA for uncoordinated radio station co-existence would not work. The coding gain of most CDMA systems is very low, around 5 to 20dB. This would mean that a mobile receiver would frequently not be able to pick out the desired station.

  5. BIGGER IS BETTER HUH? WELL I don’t know all that there is to know about radio or antennas, I’ve only been a RF Engineer specializing in Antennas since the 1970’s and a Ham since 1966 so my time on board is insignificant in the scheme of things. However most people involved in any facit of radio know that when FM broadcast is mentioned the 100mHz spectrum is thought of and when AM broadcast is mentioned the 1mHz spectrum is being referenced. It was pointed out above that the modulation format makes little difference and for the most part that is true. Lower Frequencies require larger antenna structures, and higher frequencies dictate smaller antennas. So coverage is more affected by heights than size of antenna. That said directional antennas are always always larger than single element (DiPole type) antennas. Having only been a Ham for a little over 50 years I know that directional or steerable arrays are double edge swords that cut both ways. When they increase the signal in one location it is at the expense of signal loss in another area. This can be used as benifit to eliminate signal that is perceived as interference and to enhance desired signal coverage in another specific area. This is exactly what the Signal Engineer mentioned did. I know this because two of the people who worked with him in doing this told me of this story many years ago when I was learning RF and Antenna technology. Even today many AM Broadcast stations use multiple antennas to control their signal coverage. All FM and TV transmitters and virtually all TV reciever antennas are directional in one way or another so that they can achieve the maximum functionality in the desired area.
    Laurin Cavender WB4IVG

    1. Anyone in the RF business for as long as you are should know that the length of the dipole for 1mHz (1miliHertz=1/1000Hz) would be bigger than diameter of the Earth.
      Either your experience is not as big as you claim to be, or you’re just a sloppy engineer, who pays no attention to those silly units used to measure physical values.

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