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TUCoPS :: Radio :: lpbfaq.txt

Frequently Asked Questions about Low Power Broadcasting




Archive-name: radio/broadcasting/low-power-faq
Posting-frequency: sporadic
Last-modified: 1995.12.05

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.

Newbies often want to know how much territory they can cover with a given
antenna configuration and amount of power.  I have added this section on
predicting coverage in response to their shrill, strident queries...
 
On FM and TV 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.  (It is possible for FM and TV signals to 
travel greater distances by means of phenomena such as sporadic E-layer 
skip and tropospheric ducting; however, these propagation modes are 
intermittent, and are more likely to work for higher-power signals than 
for relatively low power broadcasters.)
 
If you know the amount of power being accepted by your antenna (i.e. the
transmitter output power, minus losses due to attenuation in the feedline,
minus losses due to impedance mis-matches), then you can use the following
BASIC program to calculate what the "free space" field strength would be
at any given distance from your antenna (the program assumes you are using
a half-wave dipole).  "Real world" field strengths are likely to be lower
than "free space" (theoretical) calculations...
 
100 INPUT "power accepted by antenna (in watts)"; PT
110 INPUT "distance (in miles)"; M
120 R = M * 1609
130 E = (30 * PT) ^ .5
140 E = E / R * 1000
150 D = E * 1.28
160 IF D < 100 THEN D = INT(D * 1000) / 1000
170 PRINT "free space field strength" D "millivolts per meter"
180 END
 
What's a useful field strength?  In the US, FM stations must provide at
least 3 millivolts per meter in their primary coverage area.  Even the
cheapest receivers will pick up a signal having 1 millivolt per meter of
intensity.  A fancy receiver with a good antenna can pick up a signal as
weak as .001 millivolt per meter, but reception will be noisy; and very
few listeners will tolerate any kind of static or fuzzy reception. 
 
Okay, so much for FM.  Signals in the AM broadcast band generally travel
by ground-wave (following the surface of the earth) in the daytime; at
night, the signals can also reflect from a layer of the upper atmosphere
and travel great distances.  The range of your ground-wave signal will
depend on your power level, the effectiveness of your antenna system, and
the moisture and mineral content of the soil in your part of the world, as
well as the frequency you have chosen -- lower frequencies have better
ground-wave propagation. 
 
On the shortwave broadcast bands, propagation is usually accomplished by
skipping the signal off the ionosphere; the angle at which the power
radiates from your antenna will determine how far away the first "hop" 
will be.  A dipole antenna's height above electrical ground will influence
its angle of radiation; see the _ARRL_Antenna_Book_ for more info. 
 

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

[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.  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.

a word about the BA1404 chip:

Many of the FM kits listed below use Rohm's BA1404 integrated circuit, 
which is esentially an FM transmitter in a single 18-pin chip.  The 
BA1404 has some limitations in sound quality.  The separation between 
the left and right channels and the overall audio distortion are not 
up to "broadcast standards."  To get a clear idea of how bad it is, 
obtain a studio reference CD (a.k.a. audio system test CD) that has a 
"sweep" on it -- a sweep is a tone that steadily rises in pitch from 
very low to very high -- and play the sweep through any BA1404-based 
transmitter, while listening on a high-quality receiver.  Blecccch!

------ sources of AM, FM, and TV 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 for the 
BA1404 which improves stability.  

Compared to Ramsey's FM-10A, the Stereocaster doesn't drift as much,
and I think the audio quality is slightly better; however the assembly 
instructions are not as clear as Ramsey's, the PC board layout is not
as elegant, and Ramsey's circuit puts out a little more power.

-----

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

Items listed in their ads 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)

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.

-----

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 of $62.50, their FM stereo trans-
mitter 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 specs, 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, 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 company has just introduced the FM-25 kit, which has PLL
tuning for greater frequency stability; the cost is about $129. 
Unfortunately, the audio section of the FM-25 is identical to the 
FM-10A; it uses a BA1404 with inadequate RF bypassing.

In a message dated 19 May 1995 15:46:14 GMT, William Conkright suggested
the following modifications for the FM25 circuit:
 
> First replace the .01 bypass caps with .001 's .  The .01s cause 
> unstability and are too high in value for this frequency.  2) carefully 
> check the 2sc xstr for the first rf buffer.  I have found that I needed
> to use a 47k resis vs 10k res here.  The 10K saturated, turned on the 
> xstr too hard.  [...]  Replace the 2 47 pf caps used around the 4 Mhz 
> ref osc, with mini ceramic pc mount 2-50 pf trimmers avail from DC 
> Electronics, Scottsdale AZ.  They enable one to fine tune the ref osc, so
> that you can get an exact (down to the 10 hz) range setting for your freq.
 
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.)

-----

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.

-----

Supercircuits
13552 Research Blvd
Austin TX 78750

A source of low-power TV transmitter kits and miniature cameras.

-----

Veronica FM
18 Victoria Street
Bradford
West Yorkshire BD13 1AR
United Kingdom
phone 01274 816200

Antennas, audio processors, and transmitters (fully assembled; no kits).

-----

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 Dorchester 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.

-----

sources of shortwave transmitters:

Shortwave pirates generally use "ham" radio gear that was designed for 
licensed radio amateurs.  Used vacuum-tube transmitters from the 1945 
to 1975 era are sold at swapmeets and hamfests; certain types are 
suitable for broadcasting music and speech.  (The Viking Challenger is 
especially popular for this purpose.)
 
------------------------------

[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)

world wide web --> http://www.crl.com/~clark/mycal/

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