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TUCoPS :: Radio :: lpbcfaq.txt

Low Power Transmissions FAQ (95/05/23)




Archive-name: radio/broadcasting/low-power-faq
Posting-frequency: sporadic
Last-modified: 1995.05.23

Low Power Broadcasting FAQ (Frequently Asked Questions)

contents:

[1] What equipment do I need to start a station?
[2] Is low power broadcasting legal?
[3] How much power do I need?
[4] Where can I get a transmitter?
[5] What kind of antenna should I use?
[6] How do I select a frequency?
[7] Where can I get more information?

----------------------------------------------------------------------

[1] What equipment do I need to start a station?

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.

When you are selecting audio equipment, try to get items that have metal
cases (not plastic or wood) and three-prong grounded electrical plugs.
This will reduce your chances of having problems with radio energy from
your transmitter getting into your audio gear and causing interference.

The most important item for a low power broadcaster is the _raison_d'etre_,
the reason for existing.  You won't have a high power signal, and you won't
have billboards and TV commercials announcing the existence of your station,
so listeners will have to put some effort into finding and receiving your
signal.  They probably won't make the effort unless you are offering
something unique and interesting.

------------------------------

[2] Is low power broadcasting legal?

This depends on what country you are in.  Here in the United States, legal
unlicensed broadcasting is limited to microscopic power levels.  For
example, the 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.

Violators who get caught are usually given a monetary fine, and sometimes
their equipment is confiscated.  The situation varies from country to
country.

------------------------------

[3] How much power do I need?

There is no simple answer to this question.  For starters, it depends
on whether you are broadcasting on the AM (medium wave) band, the FM
band, the international shortwave bands, or TV.  In order to reduce
interference to other radio services, it is always a good idea to use the
lowest amount of power that will serve your target audience.

On FM and TV frequencies, raising your antenna height and improving the
gain of your antenna system is generally a better way to increase your range
than using a more powerful transmitter.  If an FM or TV broadcast antenna
is only 12 feet (4 meters) above the ground, for example if it's in the
attic of a one-story building, then its range will always be limited to
a few kilometers and the signal will always be plagued by multipath
interference, even if you pump a million watts into it.

------------------------------

[4] Where can I get a transmitter?

Below is a brief list of companies selling low power transmitter kits.
Please send me information about any companies not listed, so that I can
include them in future versions of this list.

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, $14.95)
might be especially helpful to newbies.

When assembling radio circuit kits, I prefer to use narrow-diameter
silver-bearing solder (Radio Shack #64-013) and a 15 watt soldering iron
(64-2051).  You will need a more powerful soldering tool for making
antennas out of large-diameter wire, soldering really large connectors
to a printed circuit board, etc., but the 15 watt iron works fine for
assembling most kits and reduces the chances of over-heating transistors
and other heat-sensitive components.

-----

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 for the BA1404 which improves
stability.  Documentation is not quite as clear as Ramsey's.

-----

Free Radio Berkeley
1442 A Walnut St., #406
Berkeley, CA 94709
phone 510-464-3041 / 800-549-0732

Items listed in their advertisements include a 5 watt mono FM transmitter
kit ($55 plus shipping), a 1/2 to 1 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.

FRB is spear-heading an organized challenge to the FCC's regulations and
is trying to foster a low power broadcasting movement.  Contact them for
more info.  (Internet: frbspd@crl.com)

-----

North Country Radio
PO Box 53, Wykagyl Station
New Rochelle NY 10804-0053
phone 914-235-6611
(send $1 for catalog)

Many TV-related items: a 50 milliwatt UHF transmitter with crystal
controlled frequency ($78); a "video pallete" to create special effects;
a switcher that does cross-fades and wipes; and upconverters that will
take channel 3 video from a VCR and shift its frequency up to any UHF
channel 25 thru 70.  For licensed radio amateurs, they have a line of
more powerful UHF TV transmitters.

With a 100-foot range and a price tag of $62.50, their FM stereo
transmitter is not exactly competitive, but it is interesting from a
technical point of view.  Their stereo infrared transmitter and receiver
could be used to build a difficult-to-trace studio-to-transmitter link.

-----

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 REB-1 kit is a
100 milliwatt transmitter for the upper end of the AM band ($34.95 plus
shipping).  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 characteristics, can be combined
with their stereo generator to build a high-quality low power station
(> $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
1434 N. Mills Ave.
Claremont CA 91711

RF amplifiers, FM transmitters and stereo generators, components for RF
circuits 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.  These factors make Ramsey
kits a good choice for beginners, in my opinion.

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 company has just introduced the FM-25 kit, which has PLL
tuning for greater frequency stability; the cost is about $129.

Their AM transmitter kit (item #AM-1, $29.95) and their TV transmitter kit
(item #TV-6, $27.95) might also be of interest; however, there is much
room for improvement in the design of these two circuits.  (Robert Myers
of Ramsey Electronics tells me they do intend to release an improved
version of the AM-1 kit at some point in the future.)

-----

Supercircuits
13552 Research Blvd
Austin TX 78750

This company sells a low-power TV transmitter for channels 3 thru 6 which
appears to be of high quality ($49.95 plus $4.50 S & H).  For licensed
radio amateurs, they also sell some ham TV transmitter kits with 1 to 2
watts peak output power that can be adapted for use on UHF channels 14
thru 19, and a linear amp for boosting the output of these transmitters.

-----

Tentronix
3605 Broken Arrow
Coeur d'Alene ID 83814
phone 208-664-2312

Another BA1404-based FM stereo transmitter kit ($24).

-----

Xandi Electronics
Box 25647
Tempe AZ 85285
phone 800-336-7389 / 602-894-0992

The XFS108 kit ($41.95) is an FM stereo transmitter, probably based on
the BA1404.  Their advertisements give no specifics.

-----

In a message dated Nov 08 06:01:55 EST 1994, an22190@anon.penet.fi wrote:

>There is a company called "Spectrum Communications" in Dochester England
>that sells fm transmitters and associated gear.  A transmitter tunable from
>88-108Mhz (part CTX100V) with output of 0.5Watt is available for 135 pounds.
>This unit is synthesised. ... The phone number is 0305-262250.

------------------------------

[5] What kind of antenna should I use?

Antenna theory, design and construction is a very complex topic.  If you
really want to understand antennas, I recommend that you buy a copy of
_The_ARRL_Antenna_Book_ (published by ARRL, 225 Main St., Newington CT
06111 USA).  It is a large book and you might have to spend several weeks
studying it before it all begins to make sense.

Assuming you want to get on the air in a hurry, and then build a better
antenna system later on, I will describe the quickest and simplest options
available.  The systems described here are all less than optimum, but they
will get you on the air pronto.

WARNING: There are several ways you can get killed or injured while putting
up an antenna.  Never get within 10 feet of a power line, and never mount an
antenna where it could possibly fall onto a power line, or where a power
line could fall onto the antenna.  Avoid falling off of roofs and ladders.
Permanent outdoor antennas must be provided with a ground rod so that
lightning, if it happens to strike, will go into the ground instead of into
your equipment and your body.

For FM broadcasting, try Radio Shack's omni-directional FM antenna (catalog
#15-2164, price $12.99).  Don't forget the 75-to-300 ohm impedance matching
transformer (#15-1140 or 15-1143).  (A 50-to-300 ohm transformer would be
better, but you won't find those at Radio Shack.)  This antenna can be
mounted on a typical TV antenna mast, or a chimney, or hidden in the
attic.  Best results will be obtained when it's outdoors, away from trees
and other objects, and mounted several feet higher than the rooftops in
your neighborhood.

In AM broadcasting, a vertical section of TV antenna mast, 10 or 20 feet
long/high, makes a decent antenna.  The center conductor of the coaxial cable
from your transmitter is connected to the bottom of this vertical mast; the
base of the mast sits on an insulator which sits on the ground.  If the
vertical radiator is made of several sections of antenna mast, make sure the
sections are electrically connected -- try screwing some self-tapping sheet
metal screws into the joints.  The outer conductor (shield) of the coaxial
cable is connected to a set of "ground radials," which are pieces of copper
wire radiating out from the base of the antenna like spokes from the hub of
a wheel.  (The radials are not connected to the vertical radiator.)  The
radials can be buried a few inches below the surface for a permanent
installation.  "Beware the lawnmower."

For shortwave broadcasting, a horizontal dipole works well enough.  Cut two
pieces of un-insulated copper wire; the length of each piece will be 234
feet divided by your frequency in MHz.  Example: for 7385 kHz, each element
will be (234/7.385 =) 31.7 feet long, and you will need two trees or other
support structures about 63 feet apart.  Solder one element to the center
conductor of your coaxial feedline, and solder the other element to the
outer conductor (shield) of the co-ax.  (Note: the solder joints cannot bear
the weight of the cable; loop the cable once over an insulator and provide
some "strain relief".)  Make a little loop at the free end of one element,
and tie a long piece of string to that loop.  Tie a small, heavy object
(such as a lead fishing weight) to the other end of the string.  Throw the
weight up into the branches of a tree so that it goes over a branch and
comes back down to earth; then hoist up that half of your antenna.  Repeat
the process for the other element.

------------------------------

[6] How do I select a frequency?

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 1600.  (Some Travellers
Information Stations are licensed on 530, 1610, and 1620.  The channels
1610 through 1700 may soon be allocated to broadcast stations.)  In some
other countries, AM stations are spaced at 9 kHz intervals.  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 allocated to 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 there.

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.  In
some areas where the authorities have foolishly licensed both a channel 6
and a low-edge FM station, the stations often have to go to great lengths
to deal with interference complaints.

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.  Any other transmitter in your immediate neighborhood, whether
it's a cellular telephone system, an AM or FM broadcaster, or any other
service, might interact with your transmitter in unexpected, interference-
causing ways.  So, do some testing with a variety of receivers (including
cheap junk) before you make a final decision on your frequency.  In many
major cities where the FM band is quite crowded, you will find a few
conspicuous empty channels; in some cases, these channels have been kept
unused (or had to be evacuated) because of interference problems caused by
signals mixing together at the transmitters or in people's receivers.

  *How to define "often audible" is a matter of debate; opponents of
   radio freedom say that _every_ channel is occupied by a distant station
   that some DXer might be able to hear with his 50-foot antenna tower
   and $1,000 radio.

------------------------------

[7] Where can I get more information?

Introductory electronics textbooks are available at most bookstores and
libraries.  Magazines such as Electronics Now, Popular Electronics, 73,
QST, Communications Quarterly, 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.  The ACE, a monthly newsletter, covers shortwave pirates well
and occasionally contains data useful to AM and FM broadcasters (send
$2 for a sample copy to Box 11201, Shawnee Mission KS 66207).

Keep an eye on these Usenet newsgroups:
 alt.radio.pirate
 rec.radio.broadcasting
 rec.radio.amateur.antenna

Files of some relevance are available for ftp from these sites:

site            directory                       filename
====            =========                       ========
crl.com         /users/ro/frbspd                (several)

dg-rtp.dg.com   /fm10                           FM10-FAQ (& others)

netacsys.com    /pub/micro_radio                (several)

for WWW fans, a URL:ftp://netacsys.com/pub/web/mycal.html

----------------------------------------------------------------------
This text is copyright 1994-95 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.



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