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How Low (in Frequency) Can You Go?

Even though higher frequencies are the trend, these nine examples show that there is still life at the bottom of the radio-frequency spectrum.

Readers of Microwaves & RF generally expect to see articles, blogs, ads, and other information pieces about microwaves or millimeter waves. After all, that’s where all the action is these days. (Microwaves are greater than 1 GHz and millimeter waves are from 30 to 300 GHz). Most of us use these super-high frequencies every day, from GPS at 1,225 MHz through auto radar at 77 GHz with Bluetooth, Wi-Fi, 4G/5G cellular, and more in between.

The higher frequencies are dominant and lusted after simply because they provide the bandwidth for services like video and very high data rate information transfer. Nevertheless, keep in mind that there are lots of other lower frequency spectrum still in use. The U.S. government says that the lowest radio frequency is 3 kHz. Yikes, isn’t that an audio frequency? I suppose if it radiates, the FCC wants to regulate it.

It is no surprise that zero radio activity actually seems to take place below 9 kHz. But the spectrum up to 30 MHz is teeming with activity. Here is just a sampling of what’s happening down there:

60 kHz

Yes, 60 kHz. I am not making this up. U.S. station WWVB in Colorado transmits precise time measurement signals that anyone can use to synchronize their clocks. I have an “atomic” watch and a weather station that pick up this station and synchs to its signal to give me a dead-on accurate time.   Because of the high transmission power (50 kW to each of two transmitters) and the nature of the VLF ground wave propagation, full U.S. coverage is achieved. The U.S. station WWV also offers time tick services on 2.5, 5, 10, 15, and 20 MHz.

100 kHz

Before the satellite-based Global Positioning System (GPS) system came on board in the 1990s, most sea and some aviation navigation systems used the low frequencies. The popular and widely used Loran-C operated on 100 kHz.; it was phased out when GPS became the main navigation method. Today, the government is considering an enhanced version called eLORAN that would serve as a backup for GPS.

125/134 kHz

Radio Frequency Identification (RFID) tags use these two frequencies (and others) for tracking objects and assets like animals, inventory, goods in transit, and so on. Detection range is short, like from inches to no more than a few feet. Yet it is still useful. These passive tags are actually powered up by being in proximity to a reader that transmits a signal that is received by the tag, then converted to DC to turn on the tag. The tag then transmits its ID code.

160/190 kHz

This is a Part 15 unlicensed chunk of spectrum from 160 to 190 kHz for anyone to use. Maximum power of 1 watt and short antennas limit the range. There are no known applications, and it is used mainly for experimentation.

136/475.5 kHz

The FCC recently authorized two new bands for amateur (ham) radio operation. The 2,200 meter band is from 135.7 to 137.8 kHz. Ouch, only 2.1 kHz of bandwidth. The other is the 630 meter band from 472 to 479 kHz with a whopping 7 kHz bandwidth. Power is limited to 1 watt. Long-range operation is possible because of ground wave propagation.  The antenna lengths are ridiculous at these frequencies, from hundreds to thousands of feet, forcing hams to implement shortened wires or loop antennas. With restricted bandwidth, continuous wave (CW) operation with Morse code is the primary choice of communications mode, although digital modes like PSK31 and others have potential.

535/1,705 kHz

This is the frequency range you probably recognize as the AM radio broadcast band. It is still around and very active. Many of us listen to one of the 4,700 AM stations daily for their news, weather, sports, traffic, and commentary, and even for some music. To get a reasonable amount of coverage area and audience to sustain operations, high power is used during the day. The highest allowed U.S. power is 50 kW, which takes the signal out to several hundred miles. At night, a 50 kW clear-channel station can cover most of the U.S.

3/30 MHz

There is too much in this range to cover, but a great deal of ham radio activity takes place in the region. Propagation in this region takes place by sky wave where the signals are refracted (bounced) off the ionosphere, making it possible for the signal to travel around the world after a few skips. This frequency range is also still widely used by the military and state department for embassy communications. A considerable amount of international AM shortwave broadcasting also takes place in this HF spectrum.

2.7/10 MHz

This is the operating frequency range of the High Frequency Active Auroral Research Program (HAARP). Based in Alaska, this research program was initiated by the U.S. Air Force and is now run by the University of Alaska Fairbanks. It can radiate up to 3.6 MW of power by way of over 33 acres of 180 crossed dipoles into the atmosphere and ionosphere. Its main focus is research into the nature of the ionosphere for the purpose of improving radio communications in this frequency range. Research is not for weather control.

13.56 MHz

This is an ISM (industrial-scientific-medical) band frequency. Its big user is the short-range Near Field Communications (NFC) technology. NFC was slow to develop, but is now incorporated into most smartphones today. The main app is wireless touch payments. Who says that the low frequencies are no longer viable wireless design choices?

Want to Know More?

This is only a sample of all the stuff going on down below. For a more detailed look, go get NTIA’s great frequency spectrum chart. It will give you the big picture from 3 kHz to 300 GHz.

Better still, for even greater detail get yourself a copy of the U.S. Code of Federal Regulations (CFR) 47 parts 0-90. Part 2 covers frequency allocations. Interesting stuff and required reading. If you don’t already have these resources as a wireless engineer you should expedite both.

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