The Low-Down On Long-Wave: Unlicensed Experimental Radio

In the 125 years since Marconi made his first radio transmissions, the spectrum has been divvied up into ranges and bands, most of which are reserved for governments and large telecom companies. Amidst all of the corporate greed, the “little guys” managed to carve out their own small corner of the spectrum, with the help of organizations like the American Radio Relay League (ARRL). Since 1914, the ARRL has represented the interests of us amateur radio enthusiasts and helped to protect the bands set aside for amateur use. To actually take advantage of the wonderful opportunity to transmit on these bands, you need a license, issued by the FCC. The licenses really aren’t hard to get, and you should get one, but what if you don’t feel like taking a test? Or if you’re just too impatient?

Well, fear not because there’s some space on the radio spectrum for you, too.

Welcome to the wonderful world of (legal!) unlicensed radio experimentation, where anything goes. Okay, not anything  but the possibilities are wide open. There are a few experimental radio bands, known as LowFER, MedFER, and HiFER where anyone is welcome to play around. And of the three, LowFER seems the most promising.

Gettin’ The Band Back Together

Before we dive into what the LowFER, MedFER, and HiFER bands actually are, it’s worth noting that these rules apply in the US only. That’s not to say that these bands are illegal elsewhere, but be sure to check your local frequency allocations before firing up a transmitter.

Ground wave radio propagation along the surface of the Earth. Courtesy of

LowFER, as the name would suggest, contains the lowest frequency range of the three, falling between 160 kHz and 190 kHz, with a whopping wavelength of around one mile. Also known as the 1750-meter band, this frequency range is well-suited for long transmission paths through ground wave propagation, a mode in which the radio signals move across the surface of the earth. This can easily carry even low-power signals hundreds of miles, and occasionally through some atmospheric black magic, signals have been known to travel thousands of miles. These ground wave signals also travel well across bodies of water, especially salt water.

MedFER is the medium frequency experimental band, specifically running from 510 kHz to 1,705 kHz. Now that range may sound similar, and it should because it’s also known as the AM Broadcast band! That’s right, you can listen in on this one with your old AM radio. There’s a catch though — amateur experimenters are limited to 0.1 W of transmit power, and can only use a three-meter long antenna. While that’s fine for playing around, there’s little chance of being heard very far away over the 500 W  professional stations with massive antennas that dominate the band.

And then there’s HiFER, the high-frequency experimental band. Much narrower than the others at only 14 kHz wide, it sits centered on 13.56 MHz. This band is commonly used for many RFID applications, including keycards, public transportation payments, and Nintendo Amiibo. Experimentation on this band is limited to extremely low power levels, and at such power levels signals only travel a few inches, which is perfect for RFID.

While there’s a lot that can be done on any of these bands, LowFER seems to be the one that yields itself to some serious fun. MidFER and HiFER both restrict power used so low that you’re not reaching outside of your house, or even arm’s length, respectively.

Low Frequencies, High Expectations

Like the other bands, LowFER does have some restrictions — but they’re much less limiting. First and foremost, the power into the last change of the transmitter can’t exceed 1 W. That’s still fairly low power, but there are some digital modes, such as WSPR, that are known to propagate around the world at 1 W on some frequencies. Antenna lengths are also limited to 15 meters– which seems awfully short compared to the nearly-two-kilometer wavelength. Generally, the length of such a wire antenna should be tuned to a fraction of a wavelength — 1, ½, ¼, etc. for maximum efficiency. In this case, “antenna length” also includes the transmission line between the radio and the antenna. For this reason, it’s common to connect antennas directly to LowFER radios to maximize the radiating length of the antenna.

As you may imagine, because the frequencies we’re dealing with here are so low, there are few commercially available solutions that let you get on-the-air with LowFER– but when has that ever stopped the hacker and amateur radio communities? Even with these limitations, we’ve seen some wonderful kHz-range projects, like this Altoids Tin Beacon and this Arduino-based transmitter. If you want to start out by listening in, there are a number of beacons on the air right now.

Bandwidth is obviously an issue down low, so LowFER applications probably want a microcontroller- or computer-based solution driving them, so there’s nothing to stop you from keeping the link running 24/7. The long antennas required also favor fixed operation. Intra-Hackerspace low-bitrate data networks?

How Low Can You Go?

So, now it’s your turn. What will you do with LowFER? Build a tiny transmitter and try to talk to a far-away friend? Send some waterfall art out into the æther, hoping some distant hacker sees it? Maybe even just engage in some good-old fashion CW. Although LowFER has been around for a while, we feel that there’s still a ton of untapped potential here for some crazy hacker fun. Just make sure to check (and obey!) your local laws, and tell us about anything awesome you do!


67 thoughts on “The Low-Down On Long-Wave: Unlicensed Experimental Radio

  1. Here’s the fun part.. You can drive your PA with up to a watt. It doesn’t say what your final output should be.
    “15.217 Operation in the band 160–190kHz.
    (a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt. “

    1. Efficiency defines the output power. The more efficient that output stage, the better.

      More people are likely to have a meter to accurately measure voltage and current to that output stage, than to have an accurate RF power meter. And it beats measuring field strength, a certain strength at a certain distance, since accurate field strength meters are rarer for hobbyists than RF power meters.

        1. “one watt input drive to the final PA”

          What sense would it make to specify the maximum drive to the PA? The “final radio frequency stage” -is- the PA and the inclusion of “exclusive of filament or heater power” indicates they are referring to the total power used by the PA and not the RF drive into the PA.

          1. It’s the easiest way to limit the power you put out. You can read current off a power supply. Not everybody has an RF power meter that reads down there at VLF. There’s only a few dB difference between Class A, C and D, although I’d be sure to run an efficient PA.

      1. There’s plenty of documentation about the intent and interpretation of the regulation. Very old handbooks even included worked examples for determining if a design was compliant. It’s the last *active* (has gain) stage that is relevant. It was a very common and expected approach during the tube era, but tends to confuse people used to working with modular components.

        The intent was to limit the maximum power available as rf (heater power doesnt contribute any RF energy), but also to drive experimentation and to reward anyone who came up with more efficient designs.

        The bigger restriction is the antenna. A compliant antenna will have very low radiation resistance (very little power “lost” as transmitted signal). Improvements in antenna will matter much more than transmitter power for purposes of extending range… And “antenna and feedline combined must be less than 50 ft” is hard to “reinterpret” and really hard to disguise.

          1. You can do that for sure. Although note that a feedline is specifically the cable or line that feeds the antenna. In this context, this is the line between the antenna tuning coil and the antenna. So you could mount your transmitter indoors and have your tuner close to the antenna (which has been standard practice on these bands since the beginning of radio). In order to meet the intent of the rules, you’d want to make sure that whatever feeds the antenna tuner doesn’t radiate any significant RF.

    1. weak and fading signals, I use the app for android HFPager. Unlike radio amateur programs, it allows you to transmit any text.I continue to upgrade this program, which allows you to transfer not only geographic coordinates, but also transfer text messages through gateways. UA3AHM Evgeny

  2. ” While that’s fine for playing around, there’s little chance of being heard very far away over the 500 W professional stations with massive antennas that dominate the band.”

    I’m thinking you’ve dropped a kilo somewhere…

      1. 500 kilowatt is about right for true coast to coast AM commercial entertainment in the AM broadcast band, although it has been a long time since any US station operated at such power. Clear-channel stations usually operate at 50,000 watts in the US. Regional treaties allow up to 100kw, but I believe the FCC has rarely issued authorizations for more than 50kw.

        1. Oh yeah, they were up to a million watts at one point. You could hear them in the background noise of wired phones in the Cincinnati area and reports of people tuning in as far as South America to broadcasts.

        1. I later looked it up.
          250 W is the FCC’s legal minimum for AM broadcast.

          (aside) We had a 250 Watt AM transmitter in Electronics Lab (school) which we would run and read voltages. It used a large incandescent light bulb for a dummy load.

    1. As noted, only off by a factor of 100. 50,000 W is the current limit for AM broadcast. Crosley’s WLW, of course, tried for years (in the ’30s/’40s) to get permanent authority for 500,000 W but never got more than an STA (Special Temporary Authority). My understanding is that there were one or two other “super power” stations with STAs but I don’t know callsigns.

      And yet, do not underestimate the DX possible at 100 mW. There are better bands than the “Standard Broadcast” band but even there, if you choose your frequency and based on your location, you might be surprised.

  3. Bit what’s the end game here? Experimenting with equioment, or talking on the radio?

    27MHz was license free for 100mW input transmitters. Endless walkie talkies, some with better receivers. I don’t know what the US rules say nowadays. There were plenty of projects for that, sort of a grey area about building. Electronics Illustrated even had some projects where the transmitter would be placed high with the short antenna, for DXing. Nothing against long disgance or sending CW, except equipment limitations. International Crystal even offered a very expensive commervial version briefly.

    Then in the seventies, license free was moved to 49MHz. I can’t remember if that banned new 27MHz license free units. Probably more rigid rules, but better receivers some of the time, and get up.high and range increases. And the band can ooen up.

    And of course FRS around 450MHz.

    For a long time, the AM Broadcast band was seen as broadcasting, set up a 100mW transmitter and spin some records for the neighbourhood to hear. I don’t recall much talk of two way or experimental till the seventies or a bit later.

    160KHz got projects every few years. The low frequency meant building your receiver too, unless you found a surplus receiver. But it seemed mostly novelty, you can do this, but very femw doing it so little chance of reaching someone else.

    That also seemed to change in the seventies, or maybe better organization. Ken Cornell published a book on the topic, I’d say the first serious work.

    About 30 years ago one magazine published a receiver article for under the noise reception, something, I’d previously seen in ham magazines only applied to moonbounce. But that’s taking it to another level. It’s not “talking on the air” as an end, and is way above playing with simple circuits.

    One key thing. The antenna length for transmitter is very limited, and specified, but you can use a separate receiving antenna, and most do.

    And in recent years, some ham bands have been added at LF and MF, for the first time since 1912. Limitations there too, but not as bad. And likely to improve equipment for the license free operations.

    1. The end game is “having fun”… making your equipment *and* playing with it. My mentor (“elmer”) was making Synchronous CW contacts at 179KHz) in the 1970s at a range of about 9 miles (14 KM). Timing reference was WWV. I was working AM from dorm to dorm on 174 in the late 80s. :-) I scored a surplus pair of crystals that were matched closely enough for a direct conversion receiver. Come to think of it, I still have them…

      1. I was gonna suggest synchronous operation. Cool that someone thought of that. These days you’d sync to GPS for even better sensitivity, since the propagation phase errors are better compensated vs WWV.

        In the lowFER band, if it were to become popular, the preferred mode would be code division like in GPS. I haven’t read legalese about whether that mode is allowed. That way a couple stations in one city could coexist and transmit at the same time.

    1. Very low, or maybe extremely low frequencies. They can penetrate water, souseful to eend messages to subs. They have to surface, or at least get close enough to get an antenna on the surface, to respond.

      I seem to recall it’s very slow speed sending.

      And a giant transmitting antenna.

      1. Here’s what wikipedia said about VLF data rates.

        “Therefore, only text data can be transmitted, at low bit rates. In military networks frequency shift keying (FSK) modulation is used to transmit radioteletype data using 5 bit ITA2 or 8 bit ASCII character codes. A small frequency shift of 30-50 hertz is used due to the small bandwidth of the antenna.”

        So I guess no VLF Netflix.

  4. Someone was selling a batch of talking house transmitters on Craigslist locally a few year ago. Realtors were into this a few years ago before smartphones.

    In our local radio scene starved for culture Purdue helped Purdue Student Radio get on the air. Yeah? 1710 AM it lasted less than a year in 2011. They were going to get an actual FM license, instead we got yet another preaching station. Now we’ll never get anything, it’s too late in the radio age. Little Attica 20 miles away got a community station but last week high winds caused a lot of damage in the town and no FEMA funds are coming. Sadly the tower is down and he might not keep it on the air.

    1. Some universities used wired wireless. Connect a transmitter properly to the AC line, and it distributes the signal, but it dimishes fast away from the AC wiring.So not transmitting into an antenna.

  5. The above article only applies to the USA.

    Other countries, I would urge people to go check with their local radio frequency regulation authority (ACMA in Australia, Ofcom in the UK… etc) about the status of these frequencies as USA band allocations are NOT world-wide!

      1. Exactly, and it’s worth noting that the 135.7-137.8kHz allocation (2200 metre band) is NOT unlicensed spectrum. Far from it: you need an Advanced (not Standard or Foundation) license to be allowed to transmit.

        As for that LIPD class… you’ll find if you make a lot of noise on that band, you’ll upset every customer and merchant utilising contactless payment services. My guess is that band has duty cycle limits so you may not transmit continuously.

        1. Interestingly, I’m guessing, with a practical sized antenna at 2200m, gain might be of the order of -30dB or more, so 1000W final stage will sure outdo the vast majority of final stages driven by a piddling little 1W.

          And no, no mention of any duty-cycle limit in the LIPD legislation. I think that your 100mW at 5cm will way outdo my 100mW into my half-wave dipole at >20m. We could do some sums if we cared enough.

    1. The Alexanderson transmitter station in Sweden with call sign SAQ is going on the air for UN Day October 24, 2021, frequency 17.2 Kc. The transmitter was designed at GE and has a lot of US parts from the 1920s. No vacuum tubes – a 500 HP motor drives a high frequency alternator through a step-up gear box. The output to the antenna system can be 200 KW, but due to the age of the equipment, the UN Day message will probably be around 80 KW to avoid too much mechanical stress. The antenna loading coil at the end of the feed line has Litz wire as thick as you wrist. Neat stuff.

  6. If you want to see something impressive in long wave, look up Jim Creek in the state of Washington. It is the one Megawatt station the US Navy uses to communicate with submarines.

    1. There’s one in NW Australia, and one in Hawai’i. There’s another in Cutler, Maine (unless they’ve turned it off recently). I have one of those Heathkit digital SWL receivers and received it off my TV antenna.

      I’ve also used it to do ground penetration experiments. I picked it up 170 feet down a mine through quartzite and soil, 300 miles away from Cutler.

      I bid on a job to make an antenna tuner that would retune the antenna every time it shifted frequency. And it uses FSK, so that’s all the time at the message rate. Pretty easy to do, unless you’re transmitting a megawatt. Didn’t get the job, though it was fun to design a tuner.

  7. Also look up the Alexanderson station in Sweden. It is an alternator which puts out up to 200 KW at 17.2 Kc. There will be a transmission on Oct. 24 in honor of UN Day, but at 80 KW to limit stress on the century old equipment.

    1. Good that you mentioned this!

      They’re testing the machine on the 22nd, for those who need to shake down their own gear.

      Build a simple receiver:

      And for everyone else — they’re streaming on YouTube.


    2. I always try to have a listen to the SAQ transmissions on 17.2 kHz. My recording from July 2020 is on

      There is quite a bit of activity now from countries where the spectrum below 9 kHz is unregulated, with transmissions on 8.27 kHz using EbNaut and similar protocols to get down to signal to noise ratios that are breathtakingly low, very close to the physical/mathematical limits, in microHertz bandwidths. OK, so it takes many hours to send a 5 character message, but hey…

  8. Looks like these low power bands are accessable using the modulation inputs to a function generator. Might try listning on a frequency band before running test equipment and if anything’s heard, see if it gets wiped out. Obviously shouldn’t transmit by accident. Would be good to check…

  9. Most cave voice radios work in the LowFER range. It’s low enough frequency to get good penetration through rock, but high enough bandwidth to get voice transmission.

    Morse or beeping cave radios often work in the unlicensed 3khz-10khz frequency range for superior ground penetration. But they don’t have enough bandwidth for voice transmission.

  10. The antenna + feedline limit seems a little silly. This is so much shorter than the wavelength, it’s really just a way of limiting the antenna gain. So they make the rules easy to enforce, but otherwise nonsense. You allow 1 Watt, but due to the low antenna gain, you won’t sse anywhere near that on the air.

    Seems like a way of dealing with this would be to use a magnetic loop antenna. When I was in Alaska, we had a VLF antenna on our roof, which was a loop about 1/2 meter in diameter, that was used to relay messages to submarines all over the world. Probably a little more than a Watt, though.

  11. Don’t forget, most folks that have, or are, on LF don’t even know it. Especially, if they use those NBFM Intercoms that use the house wiring on a Carrier-Current technique. Most are around the FREQ of 175kHz. There’s actually a lot of used ones out there on ebay. This could be the easiest way to get on LF for RX / TX experiments. Just remember, they are coupled directly to the AC power lines.

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