135 lines
7.0 KiB
Plaintext
135 lines
7.0 KiB
Plaintext
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How to Start a Very-Low-Power FM Station
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to Serve your Immediate Neighborhood
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based on information published in the Alternative Radio Handbook ($8 from
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R. K. Harrison, Box 547014, Orlando FL 32854) and in the forthcoming
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Free Radio Handbook, Volume 2.
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For starters, a little knowledge of electronics wouldn't hurt. You'll
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also need audio sources such as microphones, tape decks, CD players etc.,
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and a low-cost audio mixer, such as Radio Shack #32-1100.
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Next, obtain the stereo transmitter kit (catalog # FM-10) from Ramsey
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Electronics, 793 Canning Parkway, Victor NY 14564. Their phone number is
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716-924-4560. As of late 1990, the price of the kit was $29.95 plus $2
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shipping and handling.
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Similar kits are advertised by DC Electronics, P O Box 3203, Scottsdale AZ
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85271, and WJD Electronics, RD#2 Box 80A, Middleburg PA 17842.
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The Ramsey FM-10 is a straight-forward stereo FM transmitter kit that comes
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with a very clear set of instructions and all necessary parts (except for
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an enclosure and a 9V battery). It is designed around a BA-1404 integrated
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circuit. When assembled according to instructions, it complies with FCC
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Part 15 rules which allow unlicensed, low-power broadcasting on a non-
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interfering basis. The rules are explained very clearly in the manual.
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Realistic expectations for this kit are discussed later in this file.
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Several experimenters have assembled FM-10's and have come up with the
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following modifications to improve its performance.
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(1) Increase the value of R1 and R4 to 10K if you experience a problem
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with over-modulation. A value of 10K provides a better match to most
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mixers, CD players, and other household-type audio sources.
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(2) Resistors R3 and R6 govern the pre-emphasis curve. It is suggested that
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replacing them with 75K resistors (available from such sources as Mouser
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Electronics) would more closely match the pre-emphasis curve used in the
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USA, thus providing even better sound quality. 68K resistors (avaiable
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from Radio Shack) would suffice if 75K resistors are not available to you.
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(3) Frequency drift is often experienced with the FM-10 and is partly
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caused by the low quality of cermaic disc capacitor C16. Replace C16 with
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a silver-mica capacitor of the same value, or a temperature-compensated
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disc capacitor rated from N150 to N750.
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(4) Any change in the power supply will also cause frequency drift. Use
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of a filtered and regulated power supply is recommended. DC Electronics
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sells a reasonably-priced variable-voltage power supply kit. A 12-Volt
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supply will work okay and will produce a stronger output signal than a
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9-Volt battery.
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(5) Insert an RF choke in the positive power supply lead. Any value from
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3.3 microhenry to 1 millihenry can be used.
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(6) A home-made dipole antenna will provide better results than the built-in
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telescoping whip antenna. If you won't be using the whip, remove C21 (the
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capacitor which feeds RF to the whip).
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The output stage of the FM-10 matches the impedance of either 50-ohm or
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75-ohm coaxial cable, which is why it works well with a half-wave dipole.
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To use this kit with a commercially manufactured outdoor TV/FM antenna,
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it might be necessary to construct a 4-to-1 matching transformer, as
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described in the ARRL Antenna Handbook or the Alternative Radio Handbook.
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For best results your transmitting antenna should be outdoors and should
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be at least as high as the rooftops in your neighborhood.
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(7) It is often difficult to get the stereo subcarrier to work properly.
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One way to deal with this is to replace C7 with a 100 pF capacitor, and
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replace C8 with a 6 to 50 pF trimmer cap (Radio Shack # 272-1340). The
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RS trimmer won't fit the holes in the PC board; cut the leads off a spare
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resistor and solder them onto the legs of the trimmer to mount it on the
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component side of the PC board.
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(8) The power output can be increased by reducing the resistance of R9 to
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180 or even 150 ohms. Two warnings are in order if you make this modifi-
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cation: a) it might violate the power limit established by FCC Part 15
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rules, depending on what kind of antenna you're using, and b) feel
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transistor Q1 occasionally to make sure it doesn't overheat; if it gets
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scorchingly hot it will burn out (croak). Experience indicates that it
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is working near maximum efficiency when it is just a few degrees warmer
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than room temperature. This transisitor has a "maximum device dissipation"
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of 600 mW, so don't expect to get more than half a Watt of RF out of it
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(best case scenario).
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(9) For the advanced experimenter: Try replacing R8 with a 1K trim-pot
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and carefully adjust for the best balance between output power and sound
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quality. If you find that beefing up the power supply makes it harder to
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get the stereo subcarrier to work, try increasing the resistance of R12
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to 1 Megohm, 10 Megohms, or even remove R12 altogether. If you have a VHF
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field strength meter and lots of patience, you can experiment with using
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different values for C13; see what capacitance provides the best match to
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your particular antenna.
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realistic expectations:
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The sound quality of the FM-10 is great, considering that it only costs
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$30, and overall the kit is a much better value than the transmitter kits
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currently available from Panaxis.
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As mentioned in the manual, the signal from an unmodified FM-10 (or any
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FM transmitter that complies with FCC Part 15 rules) can be heard with
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a good receiver at a 1200-foot radius, assuming there are no obstructions
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between transmitting and receiving antennas. High quality stereo reception
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with a reasonable signal-to-noise ratio will be limited to a smaller
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area, depending on the quality of the receiver and its antenna.
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Although it's easy to increase the range of the FM-10, for example by
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connecting it to a directional "Yagi" antenna that provides some "gain",
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such a modification can easily violate the FCC's rules. If your broadcasts
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cause any kind of interference, or attract attention from the wrong people,
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it is likely that you will eventually be inspected by FCC agents. Rule-
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breakers are usually given a fine ranging from $750 to $1000 on their
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first offense.
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acknowledgements:
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Thanks to Rick Harrison, Rob Peebles, Dr. Laszlo Xasczkanuski, John
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Arthur, and Al Gorhythm for contributing to this research effort.
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23 Feb 1991
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~~~
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From Lunatic Labs UnLimited: 213-655-0691
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Press a key...
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