This is a complete archive of the old MWA website that was established
in October of 1997 and was terminated in March of 2004. Please keep
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[Medium Wave Alliance]

Part 15 low power license-free broadcasting FAQ

legal unlicensed broadcasting
as permitted in the US under the Part 15 rules
with an emphasis on the AM band

revised January 15, 2003
This page copyright 2003 by Medium Wave Alliance. All rights reserved.


Q: Can I legally engage in radio broadcasting without a license?

A: This depends on where you are located and how you define “broadcasting.” In the United States, Canada and some other countries, the law permits you to radiate very weak signals in the broadcast bands as long as you do not interfere with licensed stations. In other countries (England for example) no unlicensed transmissions are allowed.

I put the word “broadcasting” in quotation marks because the power levels allowed for unlicensed stations are very low and the signal will cover a very small territory. But that makes sense, you know. If there were no limits on unlicensed broadcasting, every channel on both AM and FM would already be filled with used car dealers, religious zealots and political extremists trying to drown out each others' signals– there would be no room for your signal.

Notice: The information on this page describes the situation in the United States only and was believed to be valid at the time of writing. However, the author is neither a telecommunications attorney nor a consulting engineer, therefore his opinions should be considered non-expert advice. It is the reader's obligation to independently research the current state of the regulations in his/her jurisdiction before emitting any radio signals. Because the author has no control over the ways in which this information might be used, he disclaims any liability with regard to the application or misapplication of the data herein.


Q: How powerful can an unlicensed FM signal be?

A: In the FM broadcast band (88 to 108 MHz), you are limited to a field strength no greater than 250 microvolts per meter at a distance of 3 meters from the transmitting antenna. This rule is written in an inconvenient way because the vast majority of people do not have access to a calibrated field strength meter. The best FM receivers will provide noise-free stereo reception down to the 5 uV/m level which would be reached at a distance of about 100 feet from a rule-obeying transmitter. In other words, the signal from a legal unlicensed FM stereo transmitter will start to fade into the background static at a distance of 100 feet and will be buried in the static by the time you get 1,000 feet away.

Anyone who tells you that it's legal to use 100 milliwatts (a tenth of a watt) on FM is either confused or is trying to sell you an illegal transmitter.


Q: What other frequency ranges are available for unlicensed broadcasting?

A: The FCC's “Part 15” rules allow unlicensed transmissions in the AM broadcast band (roughly 540 to 1700 kHz); a small slice of the longwave spectrum (160 to 190 kHz); a group of frequencies shared with CB radio (26.96 to 27.28 MHz); and other frequency ranges. These bands are not set aside specifically for unlicensed broadcasting but rather are open for all kinds of unlicensed activity including intercoms and baby monitors, experimental low power two-way communications, or whatever uses an experimenter can invent.


Q: Where can I find these FCC Part 15 rules?

A student should begin by studying the selected excerpts from the Part 15 rules available by following this link. After learning and understanding these sections, the student should read all of Part 15 at If you are planning to transmit, you should follow up by buying the hardcopy volume of 47 CFR from the Government Printing Office, studying it thoroughly, and keeping it handy in case a situation arises in which you need to discuss its details with FCC personnel.


Q: Are the rules for AM more liberal than for FM?

Yes. An AM station operating under the Part 15 rules can cover a larger area than its FM counterpart. The rest of this page deals exclusively with operations in the AM band.

There are 4 ways to broadcast without a license legally on AM radio in the USA. These four types of legal unlicensed transmissions are:

• 1. Tunnel radio. Inside a cave or tunnel you can transmit with any amount of power you like, as long as your signal does not leak out of the area enclosed by "naturally occurring earth or water."

• 2. Radiating cable. You can radiate a very weak signal from a special kind of cable called "leaky co-ax." This is useful for drive-in theatres, tourist attractions, school campuses and other situations where the audience is on private property and is not moving around much.

• 3. Carrier current. The signal is injected into the power lines instead of a conventional antenna, and radios near the power lines can receive the signal. Normally used on college campuses, and possibly able to cover a small town.

• 4. Antenna transmission. As per rule 15.219 anyone may transmit a signal from an antenna no more than 3 meters long, with no more than 0.1 watt (100 milliwatts) of power supplied to the final amplifier stage of the transmitter. If the station is located on an educational campus, it can operate under rule 15.221(b) which places no limits on transmitter wattage but does have other limitations.


Q: What kind of range can you get from a 100 milliwatt AM transmitter?

There are no simple answers to questions about range. A signal strength that produces good listening in a quiet rural area may be useless in a static-filled industrial city. Range depends on the receiver's sensitivity and on the quietness of your frequency and the frequencies nextdoor to it. In tech lingo, under the best conditions with the best available transmitter you might achieve a field strength of 1 mV/m at a radius of 1/4 mile.

This description from a back issue of Ionospheric Messenger describes what can be gotten from a typical installation of the ready-to-use (manufactured) Part 15 transmitters that are on the market:

"With a good car radio, you need to be within 100 to 200 yards of the transmitter to hear it clearly. The signal gradually disappears into the background noise as you drive farther away, becoming barely readable at 1/2 mile. Well-equipped radio hobbyists can hear it farther away."

If you ask around you will hear stories of Part 15 stations that had much smaller coverage areas than that, and legends of a few which had much larger coverage areas. Results are going to vary from one situation to another.


Q: Should I buy a manufactured transmitter, build a kit, or build from scratch?

A: Building a transmitter circuit from scratch is like building a car from scratch. Not too practical for most folks, although someone with the necessary parts and tools can do it. You at least want to get a kit, but it's even better if you can buy a manufactured unit.

If you are doing a very personal kind of "yardcasting" that will not attract public attention, such as feeding old-time radio programs to a collection of antique radios, or sending audio from your cable TV to the backyard so you can listen to CNBC while you sit under a shade-tree, you can safely use a home-made transmitter or one built from a kit. Just use the lowest power level and smallest antenna that covers the area you need to cover, turn off the rig when you're not using it, and you will not be hassled.

On the other hand, if you are actually going to try to reach an audience, you probably should use a manufactured FCC-certified transmitter. In theory a homebrew circuit should be able to pass muster, but if licensed high-power stations are asking the local FCC field office to find some way to shut you down, don't expect to be given any slack. FCC field agents are much more liberal when inspecting stations that use FCC-certified transmitters (and the antennas that come with them, without any modifications).

The FCC frowns on transmitters built from kits (as they frown on all non-certified transmitters) and the kits we know of do not have the range or signal quality of the manufactured units. For those reasons, anyone trying to reach an audience should not waste time with kits. On the other hand a kit might provide good enough results if you are just trying to cover your own property with a signal.

The FCC-certified manufactured transmitters cost $500 to $1500 or more. Seems like a lot, but think of it this way. If you can get 5 people to contribute $10 each per week, it only takes 3 months to gather up $600. Everybody can come up with $10/week, just by cutting back on cigarettes or anarchist literature or whatever other vices they may go for.

Here at the Medium Wave Alliance website, we maintain a list of transmitter sources.


Q: How do I pick a frequency?

A: Get a good radio with digital tuning and make a chart of all the stations you can hear during the day. Repeat the process at night. Start looking for gaps in the chart where you are not hearing any strong signals. Repeat this process over the course of several days, from a couple of different locations near your transmitter site, until you are very familiar with the AM band in your area.

AM signals travel farther at night. Paradoxically, this means that your low-power signal will have less range at night (because it will be affected by signals from powerful stations hundreds of miles away). It's easy to find a clear channel to use during the day, much harder to find one that remains useful after sunset.

The newly added channels (1610 to 1700) offer some hope because they are less crowded. Most new digital receivers will also tune the unused 1710 channel, but that is outside the legal frequency range for Part 15 unlicensed broadcasting (510 through 1705 kHz).

For broadcasting from an antenna, pick a frequency above 1500 kHz. For any given combination of transmitter power and antenna size, you will get more range at a higher frequency. Your range at 1600 kHz could be up to 10 times greater than your range at 530 kHz. (If you want to know how that was calculated, follow this link.)

On the other hand, if you are planning a carrier-current station, there are technical advantages to using frequencies in the middle or low end of the band.


Q: How can I reach more people?

multiple transmitters

Legend has it, there have been a couple of successful, profit-making Part 15 stations in the US. How do they get enough audience to attract advertisers? They use multiple transmitters scattered across the city. To use this method, you need to find an affordable way to feed audio from your studio to the various transmitters. In Union County, New Jersey there is a network of sixteen synchronized transmitters on 1620 kHz forming a Spanish Christian "station."

If the coverage areas of the multiple transmitters will overlap at the edges, they must either be on different frequencies or they must be synchronized. If two transmitters on the same frequency are not synchronized so that the signals are in phase with each other, they can cancel each other out creating a null in the overlap area, or they can create an annoying low-pitched interference tone that would be audible to the listeners.

go to the audience

Another way to increase potential audience size is to locate your transmitter near a place where traffic jams often occur and put up a sign inviting the bored, trapped motorists to tune in your fascinating program. A record company has succesfully used this method to promote new records in the northeast.

A Part 15 station is not required to remain in a fixed location. It should be possible to stuff all the needed gear into a backpack and then walk, bicycle or drive to campuses, festivals, office complexes, and other areas where people congregate.


How often do most people tune up and down the radio dial to see what signals are available at the moment? Almost never, I think. (I've heard a lot of talkshow callers say they never listened to AM until the Persian Gulf War or some other crisis, when their instincts told them to seek information on the AM dial.) Generally folks stay glued to their favorite stations and don't tune around unless a sign, bumper sticker, T-shirt, TV commercial or recommendation from a friend informs them of a worthwhile new station. You will have to publicize your station.

make contact

How can you find out if anyone is listening? Give out your phone number and e-mail address from time to time. Have a contest; offer to pay for a pizza delivery to the next person who calls in with a request. Say something controversial and ask people to call in with their opinions.

have a web presence

You can potentially enlarge your audience by making your station's audio available on the world wide web. Even if you can't afford to do streaming audio, you can certainly record highlights of your shows and make them available as downloadable wave or mp3 or realaudio files. But if you're trying to avoid legal troubles, be careful not to upload copyrighted music without permission or a license.


Q: Are there any loopholes in rule 15.219 that I can exploit to increase the range of my signal?

A: If you are concerned about being able to pass FCC inspections so that you can continue operating without interruption, you should use an FCC-certified transmitter and the antenna that comes with it, without making any modifications to either the transmitter or the antenna.

The ground lead is added into the 3-meter limit, but the actual ground stake or ground radials are not. A good ground system can enlarge the coverage area. For example, there is an add-on ground plane kit available for the Infomax transmitter that increases the range by 40% according to the dealer. A larger and especially well-designed ground system might produce even more gain. Some of the Part 15 stations with legendary range are installed on top of billboards, roadside signs, or metal poles; these stations are operating in a gray area of the rules (see our chapter on whip and mast antennas).

It is important to install a compressor/limiter in the audio line that is feeding your transmitter. A compressor/limiter is a device that keeps your sound level fairly constant so that there are no overly quiet periods or excessively loud fluctuations in your volume. There is much more energy in an AM signal when the modulation level is at 90% than when it is at 50%, so a compressor/limiter helps to boost your range by keeping your modulation level as high as possible for as much of the time as possible. Behringer comp’limiters are popular with low power AM broadcasters; they can be purchased from music stores and broadcast supply dealers.


Q: Are loading coils and capacitance hats permitted?

Among people who use homemade antennas there is a very old and apparently endless debate about 1) whether loading coils are legal, 2) whether loading coils should be legal, 3) whether capacitance hats count toward the three meter length limit, 4) whether they should count, 5) how the length of a helically wound antenna would be measured, etc. If you use an unmodified FCC-certified combination of antenna plus transmitter, this debate becomes irrelevant to you and you won't have to get involved in it. If you insist on wasting your energy by jumping into the quicksand of this debate, click here to read some of the best arguments on both sides.


Q: If I operate with more power than the Part 15 rules allow, how can I avoid getting busted?

A: This website does not discuss techniques for unlawful activity. However there are plenty of websites that do offer that kind of advice. If you don't have enough sense and determination to find those web pages, you probably don't have enough sense and determination to avoid getting busted.


Q: Will my flea-power station change the world?

With dozens of cable TV channels, zillions of web sites, plus videos and video-games and CDs, in addition to more high-power radio stations than ever before, people have a lot of choices for stuff to look at and listen to. Don't be too optimistic or too pessimistic about the effect your station might have. Over the years I've heard a lot of people tell stories about radio stations that they loved or particular broadcasts that really touched their souls. I've read eyewitness accounts of people driving into the neighborhoods of pirate and Part 15 stations and parking there, sitting in their cars and listening to the broadcasts. If you're doing something meaningful and unique, you might reach a few people.

On the other hand, only 20% of Americans listen to AM radio on a regular basis. Half of the population only listens to any kind of radio when they are driving around in their cars. But even during those periods of time when you are the only person listening to your station, it can still be a worthwhile exercise. You will learn about electronics and radio wave propagation, audio production, and other topics. An activity doesn't have to be earth-shattering or profitable to be worthwhile; consider the Japanese concept of tashinamu.

It is better to light a single candle than to sit and curse the darkness.



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