Revised 1997.01.04
Some individuals and small groups operate low power radio stations as a
hobby, or as a way of spreading some commercial, religious or political message.
These stations are often short-lived, sometimes portable, and generally
unpredictable.
This depends on what country you are in. The situation varies from place to place, so you should check with a lawyer or with the agency that regulates broadcasting in your part of the world before turning on any transmitter. Don't rely on web pages or usenet newsgroups for such critical information.
Here in the United States, legal unlicensed broadcasting is limited to microscopic power levels, and it must be done on a frequency that does not interfere with reception of licensed stations. The power limit for unlicensed FM transmissions is 250 microvolts per meter, measured 3 meters from the transmitting antenna; at this power level, stereo reception with a good signal to noise ratio is only possible within a 100 foot radius, and an average car radio can barely detect the signal at a distance of 200 meters. On the AM band, the limit is 0.1 watt and an antenna system no more than 3 meters long; with an optimized antenna, this provides a range of 3 or 4 city blocks for cheap receivers, farther for high-quality radios. Obviously a network of such low-power stations would be needed if you wanted to legally cover any significant amount of territory.
Other legal options include carrier current transmission (using the power lines as an antenna system, legal only on educational campuses); cable FM broadcasting (working in conjunction with your local cable TV system); and burying special "leaky" coaxial cable to use as a transmitting antenna on a piece of private property such as a tourist attraction. The booklets available from Panaxis cover these options in greater detail. (You can look up the rules in title 47 of the Code of Federal Regulations, Part 15.) The US regulations also allow microscopic power levels on the shortwave broadcast bands, but they do not allow any unlicensed emissions on television broadcast frequencies.
It is a good idea to obey the rules, give or take a fraction of a watt.
People who seriously break the rules often have to pay large fines and their
equipment is sometimes confiscated. Many people feel that the rules are too
restrictive, and a court battle against the federal government is underway;
see the freeradio.org site for more
information about this.
You will need audio sources (tape players, CD players, microphones,
etc.), an audio mixer, a transmitter, a coaxial cable (usually RG-8 or
RG-58/U) to carry the signal from your transmitter to your antenna,
and an antenna.
If you are in the USA and you obey the signal strength limitations imposed by the federal government, your range will normally be limited to a radius of 1/4 mile or less. However, people who make a hobby of listening to low power signals have sometimes heard 0.1 watt AM signals at distances of several hundred miles. They accomplish this by using sensitive (expensive) receivers and large antennas located in rural areas away from urban static, and they listen when atmospheric conditions are good.
On FM broadcast frequencies, antenna height puts an upper limit on your range, regardless of power levels. The distance from your antenna to the radio horizon is determined by this formula:
distance_in_miles = 1.415 * square_root_of_(antenna_height_in_feet)
You can add the transmit antenna's range to the receive antenna's range to predict whether reception will be possible. Example: a transmit antenna is 25 feet high; the square root of 25 is 5; 1.415 * 5 = 7.075 miles to the radio horizon. If the receive antenna is also 25 feet high, if there are no major obstructions between antennas, and if power is high enough (or receiver sensitivity is high enough), then reception at a distance of 14 miles could be achieved.
In summary, the range you can reach depends on your transmitter power,
antenna height and effectiveness, and the sensitivity of the listener's receiver.
Below is a brief list of companies selling low power transmitters and transmitter kits.
Note to newcomers: to assemble these kits, you must be able to solder components onto a circuit board, and it helps if you know the difference between a resistor and a capacitor. If you haven't reached this stage of electronic know-how yet, consider buying some of the educational kits available from C & S Sales, 1245 Rosewood, Deerfield IL 60015, telephone 800-292-7711. Their electronic components course (item #ECK-10) might be especially helpful to newbies.
sources of transmitters:
DC Electronics
P O Box 3203
Scottsdale AZ 85271
phone 800-423-0070
The Improved Stereocaster is an FM stereo transmitter based on the BA1404 chip with a few milliwatts of output power ($29.95 plus $3.50 S & H). It has a smooth fine-tuning control which makes it easy to get on the exact frequency you want, and a voltage regulator which improves stability. It has been reported that the power output can be increased by replacing the supplied output transistor with an MRF581.
Free Radio Berkeley
1442 A Walnut St., #406
Berkeley, CA 94709
phone 510-464-3041
e-mail frbspd@crl.com
Items listed in their ads include a 1/2 watt stereo FM transmitter kit ($50), an FM transmitter with phase locked loop (PLL) frequency control ($95), plus kits for output filters, dummy loads, RF amplifiers, and antennas.
Some people have posted messages in alt.radio.pirate indicating that FRB sometimes takes several weeks or months to respond to orders. Some have said that the instructions are not very helpful.
Panaxis Productions
P O Box 130
Paradise CA 95967-0130
(send $1 for catalog, or $2 if you're in a hurry)
This company offers many interesting books and kits. The FMO kit ($75) is a high fidelity stereo FM transmitter kit with 2 to 20 milliwatts of output power. The FME-500, a half-watt mono FM transmitter with excellent technical specs, can be combined with their stereo generator to build a high-quality low power station (more than $200 for the two kits). Panaxis kits might not be suitable for absolute beginners; you should have some experience in circuit assembly before you tackle these.
Progressive Concepts
phone 708-736-9822
fax 708-736-0353
RF amplifiers, FM transmitters and stereo generators, components for RF circuits, high-quality antennas for low-power FM broadcasters, and more.
Ramsey Electronics
793 Canning Pkwy
Victor NY 14564
phone 716-924-4560
Ramsey kits have well-written instruction manuals, and most of the circuit boards have lots of wide-open space which makes modifications easy. The company also has a good reputation for service. On the negative side, they only offer plastic cases for their broadcasting kits (transmitter circuits generally perform better in metal cases).
The FM-10A is an FM stereo transmitter kit ($34.95 plus shipping) with a few milliwatts of output power; it is based on the BA1404 integrated circuit. The FM-25 kit, which has PLL tuning for greater frequency stability, costs about $129.
Ramsey also offers a low-budget AM transmitter and a more expensive AM transmitter that has PLL frequency control.
Check out the Ramsey web site for more information.
Scott Communications
6974 Larkspur Rd. RR-3
Sooke, B.C., Canada VOS-1NO
phone 604-642-2859
e-mail: kscott@pinc.com
This company offers a 3-watt mono FM transmitter which they say has good sound quality and frequency stability. Kits cost $90 plus $5 shipping; fully assembled and tested $129 + $7. They have recently added a stereo FM transmitter with PLL frequency control to their product line; contact them for details.
Veronica FM
18 Victoria Street
Bradford
West Yorkshire BD13 1AR
United Kingdom
phone 01274 816200
Antennas, audio processors, and transmitters (fully assembled; no kits).
The L D Brewer company sells
transmitter kits, antennas and supplies from a variety of companies, and
will assemble a kit for you if you don't know how to do it.
Receivers with digital tuning will only lock onto signals that are on standard broadcast frequencies. In the US, AM stations are at 10 kHz intervals, ranging from 540, 550, 560 ... to 1700. FM stations are spaced at 0.2 MHz intervals, ranging from 88.1, 88.3 ... to 107.9 MHz.
Do not use an out-of-band frequency; they are reserved for other services. (For example, the frequencies just below 88 MHz are used for TV broadcasts, and the frequencies just above 108 MHz are used for aircraft communication.)
Make a survey of the band you are planning to use. Get some graph paper or notebook paper and make a list of all the channels. Listen during the day and at night, making a note of what station(s) you can hear on each channel. Use a good receiver with digital tuning and a decent antenna, not some cheap piece of junk clock-radio or dime-store pocket radio. Repeat this band-scanning process several times during the course of a couple of weeks. (If you really want to be thorough, get a list of all the licensed stations in a 150-mile radius. You can extract this data from the Broadcasting Yearbook [a trade publication] or the FCC database [available on computer disks from several vendors]). If you know any DXers (people who make a hobby of listening to distant and unusual signals), ask them for a copy of their "log."
Now, sit down with your data and search for an appropriate channel. Don't start with any prejudices or pre-conceived notions: don't plunk your signal onto 99.9 MHz just because you think it's cute ("666" upside down) or onto 1000 kHz because you think it's an easy number to remember. Don't try to wedge your signal into the non-commercial part of the FM band (88 to 92 MHz) if there isn't an appropriate opening.
An appropriate channel for low power broadcasting is one that is not occupied by a local station, or by an often-audible distant station. The adjacent channels -- the next channel above and the next channel below the one you're considering -- also must not be occupied by local stations, because they will "splatter" onto your signal (and they will claim that you are splattering onto them).
There are a couple of other things you must keep in mind when selecting an FM broadcast frequency. First, if there is a TV station broadcasting on channel 6 in your area, it is unwise to operate on 88.1, 88.3, or 88.5 MHz. TV receivers have broadband tuning circuits (a TV channel is 6 MHz wide, enough spectrum to hold 30 FM stations), so broadcasts at the "low edge" of the FM band can easily interfere with reception of channel 6.
Another thing for FMers to consider is the mixing of signals that can occur in a listener's receiver. Most FM radios use an intermediate frequency of 10.7 MHz; in other words, whatever frequency you're tuned to is converted down to 10.7 MHz before the sound waves are extracted from the radio waves. As a result, a strong signal can interfere with reception of stations that are on a frequency 10.6 or 10.8 MHz above or below it. For example, if you transmit on 92.3 MHz, some listeners who are located near your transmitter will have trouble hearing a station on 102.9 or 103.1 MHz (92.3 + 10.7 = 103.0). The interference might take the form of an "image" of your signal being heard on the other frequency, or vice versa; or a mixture of the two signals might be heard on blank spots and on top of weak signals all over the band. So, do the math and avoid picking a channel that is 10.6 or 10.8 MHz away from a strong local signal.
In some major cities, there is no room in the FM band for any more signals.
Introductory electronics textbooks are available at most bookstores and libraries. Magazines such as Electronics Now, Popular Electronics, and Nuts & Volts sometimes have articles and advertisements of interest to low power broadcasters. Monitoring Times and Popular Communications carry relevant news items from time to time.
Keep an eye on these Usenet newsgroups:
alt.radio.pirate
rec.radio.amateur.antenna
rec.radio.broadcasting
alt.radio.broadcasting
Relevant world wide web sites:
http://www.clandjop.com/~jcruzan/frn.html (FRN)
http://www.rootsworld.com/rw/feature/rrb.html
This text is copyright 1994-96 by Rick Harrison. Permission is hereby granted for unlimited distribution of this text via Usenet newsgroups, Internet file servers, and computer bulletin boards. Any publication of this text in semi-permanent form (such as hardcopy or CD-ROM) requires the author's prior permission.